Chapter 16
Lifts, escalators and paternosters
Introduction
The general design of lifts is very well established, and in this country at least, nearly all
the reputable lift manufacturers will design and supply a satisfactory lift as a matter of
routine if given the details and the size of building. Nevertheless, the designer of the
building electrical services must be able to advise the client about the lifts, to negotiate
with the lift suppliers and to compare competing tenders. The designer must, therefore,
know something about the technical details of lifts and we shall accordingly devote this
chapter to a brief outline of the subject.
First, we can note that there are three categories of lifts. Passenger lifts are designed
primarily for passenger use; goods lifts are mainly for goods but can on occasion carry
passengers; and service lifts are for goods only and are of such a size that passengers
cannot enter into the car. Lift speeds are determined by the number of floors served and
the quality of service required. They vary from 0.5ms−1 to 10ms−1 in high office blocks.
In deciding the size of car one can allow 0.2m2 for each passenger, and when
determining the load the average weight of a passenger can be taken as 75kg. It must,
however, be remembered that in many buildings the lift will be used for moving in
furniture and the car must be big enough for the bulkiest piece of furniture likely to be
needed. The author has made measurements of domestic furniture and has concluded that
the most awkward item to manoeuvre is a double bed, which can be up to 1670mm wide
by 1900mm long and 360mm high. In flats it is unfortunately also necessary to make sure
that stretchers and coffins can be carried in the lift. To accommodate these, a depth of
2.5m is required. The whole car can be made this depth or it can be shallower but have a
collapsible extension which can be opened out at the back when the need arises. The lift
well must, of course, be deep enough to allow the extension to be opened. In hospitals
some of the lifts must take stretchers on trolleys and also hospital beds and these lifts
must be the full depth of a complete bed.
Grade of service
The quality of service is a measure of the speed with which passengers can be taken to
their destination. It is the sum of the time which the average passenger has to wait for a
lift and of the travelling time once in the lift. The maximum time a person may have to
wait is called the waiting time (WI) and is the interval between the arrival of successive
cars. It depends on the round trip time (RTT) of each lift and on the number of lifts.
The average time a person has to wait is WI/2. The average time travelling is RTT/4.
The sum of these, WI/2=RTT/4, is called the grade of service. If there are N lifts, then
WI=RTT/N, and grade of service becomes WI(2+N)/4. It is usual to classify the grade of
service as excellent if WI(2+N)/4 is less than 45s, good if it is between 45 and 55s, fair if
it is between 55 and 65s and casual if it is more than 65s.
The use of a building will often enable a designer to estimate the probable number of
stops during each trip. If this is difficult, then a formula can be developed by probability
theory, and is:
where
Sn=probable number of stops
n=number of floors served above ground floor
N=number of passengers entering lift at ground floor on each trip
P=total population on all floors
Pa, Pb,…pn=population on 1st, 2nd…nth floor.
Three to four seconds must be allowed for opening and closing the doors at each stop.
A further 1 to 11.5s have to be allowed for each passenger to enter the lift and 1 to 2s for
each passenger to leave.
The travelling time is made up of periods of acceleration, constant speed and
retardation. Figure 16.1 gives the time versus distance curves for the acceleration
normally associated with various lift speeds. On each curve, the point marked X indicates
the end of acceleration and start of constant velocity. The retardation is generally taken to
be equal in magnitude to the acceleration. Providing the distance between stops is long
enough for the lift to reach steady speed before starting to slow, the total travelling time
of a round trip is given by:
Design of electrical services for buildings 236
Figure 16.1 Acceleration curves for
lifts
where
t=total travelling time
d=distance during which acceleration takes place
D=distance between ground and top floors
Sn=number of stops between ground and top floors
V=lift speed.
The best way of showing how all this data is used to assess the grade of service is by
means of an example. Let us assume we are dealing with an office block with eight
floors. The heaviest traffic will occur in the morning when people are arriving at work,
and we shall assume that we know enough about the occupancy of the building to have
been able to estimate that 75 per cent of the work force will arrive in one particular half
hour. For estimating the probable number of stops, traffic to the first floor can be ignored
and we can set out the number of people requiring service as follows:
Lifts, escalators and paternosters 237
Table 16.1 Lift service comparison
A B C D
Load persons 10 20 10 15
Speed ms−1 1.5 1.5 2.0 2.0
Probable no. of stops per trip (Sn) 5.23 6.35 5.23 5.99
Accelerating distance (d) m 2.60 2.60 2.20 2.20
d×Sn 13.60 16.50 11.50 13.20
Distance between ground and top floors (D) m 25.00 25.00 25.00 25.00
(dSn+D+d) 41.20 44.10 38.70 40.40
55.00 59.00 38.00 40.00
Door opening time (s) 21.00 28.00 21.00 24.00
Passengers entering and leaving (s) 25.00 50.00 25.00 37.00
Total travelling time 101 137 84 101
10% margin 10 13 8 10
RTT(s) 111 150 92 111
No. of lifts 4 3 4 3
No. of trips per lift in 30min 16 12 19 16
No. of persons per lift in 30min 160 240 190 240
Total no. of persons carried in 30min 640 720 760 720
WI(s) 28 50 23 37
42 62 35 46
Grade of service Excellent Fair Excellent Good
Calculation of Sn P=662
n=7
i Pi P−Pi
2 36 626 0.95 0.60 0.46 0.36
3 93 569 0.86 0.22 0.10 0.05
4 160 502 0.76 0.06 0.01 0.00
5 85 577 0.58 0.22 0.10 0.05
Design of electrical services for buildings 238
6 120 542 0.82 0.12 0.04 0.02
7 105 557 0.84 0.18 0.08 0.03
8 63 599 0.905 0.37 0.22 0.14
Σ 1.77 1.01 0.65
Sn=7−Σ 5.23 5.99 6.35
Floor 2 3 4 5 6 7 8 Total
No. of persons requiring service 36 93 160 85 120 105 63 662
The figures in the second line are 75 per cent of the floor populations, which we assume
we have either been given or can guess. The distance between the ground and eighth floor
is 25m.
The round trip time can be calculated and hence it is possible to calculate the number
of lifts needed to carry 662 people in 30mm. From this, the grade of service can be
obtained. If the calculation is set out in tabular form, different combinations can be easily
compared. This has been done in Table 16.1. A 10 per cent margin is added to the
calculated total time to allow for irregularities in the time interval between different lifts
in the bank of lifts.
It can be seen that in this example the most satisfactory arrangement is four lifts each
taking 10 persons at a speed of 1.5ms−1. A speed of 2.0ms−1 would be unnecessarily
extravagant.
It will be found that where the service is not so concentrated lower speeds are
sufficient. For this reason, it should not be necessary to use speeds of more than 0.75 or
1.0 ms−1 in blocks of flats.
Accommodation
The machine room for the lifting gear is normally at the top of the lift shaft or well. It can
be at the bottom or even beside the well, and in the latter case it can be at any height, but
from these positions the ropes must pass over more pulleys so that the overall
arrangement becomes more complicated. It is, therefore, better to provide space for the
machine room at the top of the well. Room must also be left for buffers and for inspection
at the bottom, or pit, of the well. The sizes of the machine and pit rooms must ultimately
be agreed with the lift manufacturers, but for preliminary planning before a manufacturer
has submitted a quotation the dimensions in Table 16.2 may be taken as a guide.
Drive
Nearly all lifts use a traction drive. In this, the ropes pass from the lift car around a cast
iron or steel grooved sheave and then to the counterweight. The sheave is secured to a
steel shaft which is turned by the driving motor. The drive from the motor to the shaft is
usually through a worm gear. The force needed to raise or lower the lift car is provided
Lifts, escalators and paternosters 239
by the friction between the ropes and the sheave grooves. The main advantage of the
traction drive is that if either the car or counterweight comes into contact with the buffers
the drive ceases and there is no danger of the car being wound into the overhead
structure. Other advantages are cheapness and simplicity.
Table 16.2 Lift dimensions
Passenger lifts
Well Machine room
Load
persons
Speed
(ms−1)
Width
(m)
Depth
(m)
Width
(m)
Length
(m)
Height
(m)
Top landing to
MIC room
floor (m)
Pit
depth
(m)
General purpose passenger lifts
8 1.0 1.80 1.90 3.10 4.80 2.60 4.00 1.60
10 0.75 2.00 1.90 3.10 5.00 2.60 4.00 1.60
10 1.0 2.00 1.90 3.10 5.00 2.60 4.00 1.70
10 1.5 2.00 1.90 3.10 5.00 2.60 4.20 1.70
16 0.75 2.60 2.20 3.50 5.30 2.70 4.10 1.70
16 1.00 2.60 2.20 3.50 5.30 2.70 4.20 1.90
16 1.50 2.60 2.20 3.50 5.30 2.70 4.30 1.90
20 0.75 2.60 2.50 3.50 5.60 2.70 4.10 1.70
20 1.00 2.60 2.50 3.50 5.60 2.70 4.20 1.90
20 1.50 2.60 2.50 3.50 5.60 2.70 4.30 1.90
High speed passenger lifts
12 2.5 2.20 2.20 3.20 7.50 2.70 6.80 2.80
16 2.5 2.60 2.30 3.20 8.00 2.70 6.80 2.80
16 3.5 2.60 2.30 3.20 8.00 3.50 6.90 3.40
20 2.5 2.60 2.60 3.20 8.30 3.50 6.20 2.80
20 3.5 2.60 2.60 3.20 8.30 3.50 7.10 3.40
20 5.0 2.60 2.60 3.20 8.30 3.50 8.20 5.10
Goods lifts
Well Machine room
Load
(kg)
Speed
(ms−1)
Width
(m)
Depth
(m)
Width
(m)
Length
(m)
Height
(m)
Top landing to
M/C room
floor (m)
Pit
depth
(m)
General purpose goods lifts
Design of electrical services for buildings 240
500 0.5 1.80 1.50 2.00 3.70 2.40 3.80 1.40
1000 0.25 2.10 2.10 2.10 4.30 2.40 3.80 1.50
1000 0.50 2.10 2.10 2.10 4.30 2.40 3.80 1.50
1000 0.75 2.10 2.10 2.10 4.30 2.40 3.80 1.50
1500 0.25 2.50 2.30 2.50 4.50 2.70 4.00 1.50
1500 0.75 2.50 2.30 2.50 4.50 2.70 4.20 1.80
1500 1.00 2.50 2.30 2.50 4.50 2.70 4.20 1.80
2000 0.25 2.80 2.40 2.80 4.70 2.90 4.10 1.50
2000 0.75 2.80 2.40 2.80 4.70 2.90 4.50 1.80
2000 1.00 2.80 2.40 2.80 4.70 2.90 4.50 1.80
3000 0.25 3.50 2.70 3.50 5.00 2.90 4.20 1.50
3000 0.50 3.50 2.70 3.50 5.00 2.90 4.40 1.70
3000 0.75 3.50 2.70 3.50 5.00 2.90 4.50 1.80
Well Machine room
Load
(kg)
Speed
(ms–1)
Width
(m)
Depth
(m)
Width
(m)
Length
(m)
Height
(m)
Top landing to
M/C room floor
(m)
Pit
depth
(m)
Heavy duty goods lift
1500 0.50 2.60 2.40 2.60 4.80 2.70 4.80 1.70
1500 0.75 2.60 2.40 2.60 4.80 2.70 4.80 1.80
1500 1.00 2.60 2.40 2.60 4.80 2.70 4.80 1.80
2000 0.50 2.90 2.50 2.90 5.00 2.90 4.80 1.70
2000 0.75 2.90 2.50 2.90 5.00 2.90 4.80 1.80
2000 1.00 2.90 2.50 2.90 5.00 2.90 4.80 1.80
3000 0.50 3.50 2.80 3.50 5.30 2.90 4.80 1.70
3000 0.75 3.50 2.80 3.50 5.30 2.90 4.80 1.80
3000 1.00 3.50 2.80 3.50 5.30 2.90 4.80 1.80
4000 0.50 3.50 3.40 4.00 6.20 2.90 5.20 1.70
4000 0.75 3.50 3.40 4.00 6.20 2.90 5.20 1.80
5000 0.50 3.60 4.00 4.00 6.80 2.90 5.20 1.70
5000 0.75 3.60 4.00 4.00 6.80 2.90 5.20 1.80
Figure 16.2 shows a geared traction unit has a drive motor permanently coupled to the
gear by a vee rope drive. There is a brake working on a brake disc on the end of the worm
Lifts, escalators and paternosters 241
shaft and the grooved sheave is easily discernible at the side of the machine. The control
panel is in the foreground
The only other kind of drive is the drum drive. In this case, one end of the car ropes
and one end of the counterweight ropes are securely fastened by
Figure 16.2 Geared drive unit
(Courtesy of Schindler Lifts Ltd)
clamps on the inside of a cast iron or steel drum, the other ends being fastened to the car
and counterweight respectively. One set of ropes is wrapped clockwise around the drum
and the other set anti-clockwise, so that one set is winding up as the other set is
unwinding from the drum. As the car travels, the ropes move along the drum in spiral
grooves on its periphery. The drum drive suffers from the disadvantage that as the height
of travel increases the drum becomes large and unwieldy. It has been almost entirely
superseded by the traction drive.
Figure 16.3 shows various arrangements of ropes for traction drives. Figure 16.3b
shows a double wrap drive, in which each rope passes over the sheave
Design of electrical services for buildings 242
Figure 16.4 Compensating ropes
twice. The increased length of contact between rope and sheave increases the maximum
available lifting force, or alternatively permits a lower coefficient of friction for the same
lifting force. Figure 16.3c shows a double wrap two-to-one system in which the speed of
the car is half the peripheral speed of the sheave. Figures 16.3d, e and f correspond to
Figures 16.3a, b and c but with the winding machine at the bottom of the well.
Compensating ropes are sometimes fitted on long travel lifts in order to make the load
on the motor constant by eliminating the effect of the weight of the ropes. A simple
method of doing this is shown in Figure 16.4.
Motors
A lift motor should have a starting torque equal to at least twice the full load torque; it
should be quiet and it should have a low kinetic energy. The last requirement is necessary
for rapid acceleration and deceleration and also for low wear in the brakes. The
theoretical power needed can be calculated from the lifting speed and the greatest
difference between the weights of car plus load and counterweight. The actual power will
depend on the mechanical efficiency of the drive which can be anything from 30 per cent
to 60 per cent. Suitable motor sizes for various lifts are given in Table 16.3. The
acceleration is settled by the torque-speed characteristic of the motor and the ratio of
motor speed to lift speed.
Design of electrical services for buildings 244
Types of motor
In most cases, in the UK at least, a three phase a.c. supply is available in a building which
is to have a lift installation. For lift speeds up to about 0.5ms−1 a single-speed squirrel
cage motor is suitable, although it has a high starting
Table 16.3 Approximate lift motor ratings (motor
ratings in kW)
Low efficiency geared lifts
Contract load (kg)
Car speed (ms−1) 250 500 750 1000 1500 2000
0.25 2 3.5 5 5 8 10
0.50 3 5.0 8 10 15 20
0.75 4 8.0 10 15 20 25
1.00 5 10 12 18 25 35
1.25 6 12 15 20 30 45
1.50 7 15 20 25 40 50
High efficiency geared goods lifts
Contract load (kg)
Car speed (ms–1) 250 500 750 1000 1500 2000 3000
0.25 1 2.0 3 4.0 6.0 8 10
0.50 3 4.0 6 7.5 10.0 14 20
0.75 5.0 8 10.0 12.5 20 30
1.00 7.5 10 14.0 20.0 25
High efficiency geared passenger lifts
No. of passengers
Car speed (ms−1) 4 6 8 10 15 20
0.50 3 3 4.0 5.0
0.75 5 7.5 7.5 12 15
1.0 10 10 15 18
1.5 12 17.5 22
current and tends to overheat on duties requiring more than 100 starts an hour.
Lifts, escalators and paternosters 245
For speeds between about 0.5ms−1 and 1.25ms−1, it becomes necessary to use a twospeed
motor in order to have a low landing speed. The squirrel cage motor can be wound
to give two combinations of poles, thus giving two speeds. A two-speed system is not a
variable-speed drive; it is variable in the sense that it has two speeds. These systems were
used widely in the 1970s and 1980s. They effectively employed direct on-line starting
and thus needed flywheels attached to the motor to smooth the movement and reduce
jerk. The use of a flywheel makes it inefficient
DC silicon-controlled rectifier (SCR)
Using power electronics, a DC motor can be used to vary the speed of the lift by varying
the armature voltage. This method is the most widely used in DC drives, in the form of a
controlled three-phase rectifier. This can be implemented in two forms. One form is a
fully controlled bridge rectifier, which allows two-quadrant operation.
The other form uses two bridges in parallel, each connected to drive the motor in the
opposite direction of the other. By using both bridges, the motor can be operated in both
driving and braking modes, in forward and reverse directions (i.e. four-quadrant
operation).
AC variable voltage (ACVV)
These systems were widely used in the mid-1980s and early 1990s. They are very simple
in the method of operation. They rely on three pairs of back-to-back thyristors for varying
the stator AC voltage on a double-cage squirrel cage motor. By varying the firing angle,
the stator voltage is varied and a new speed torque curve results.
Variable voltage variable frequency (VVVF)
The most widely used system today is the VVVF system, usually referred to as an
inverter drive. The principle of operation relies on a rectifier to produce DC into the socalled
DC link and an inverter, which produces sinusoidal current into the windings. By
changing the frequency of the inverted signal, the synchronous frequency and hence the
speed torque curve is moved to the desired profile. The supply is fed to a servo motor.
Servo motors can be programmed to give the required S curve profile for travel
between floors.
For speeds above 1.5ms–1 the same types of motor could be used as for the lower
speeds, but in fact they very rarely are. At the higher speeds, it is possible to design a d.c.
motor to run at a speed which makes reduction gearing to the drive unnecessary. The a.c.
supply is therefore used to drive a variable voltage motor generator set which supplies the
d.c. to the lift driving motor.
If only a single phase supply is available, a repulsion-induction motor can be used for
speeds up to 0.5ms−1. For higher speeds it is better to use the a.c. to drive a motor
generator and have a d.c. lift machine.
When d.c. is used for speeds less than 0.5ms−1, a single speed shunt or compound
wound motor is employed. When the lift is decelerating the machine runs as a generator,
and in the case of a compound wound motor this makes special arrangements necessary.
Design of electrical services for buildings 246
For speeds between 0.5ms−1 and 1.25ms−1 two-speed shunt motors are used, so that a
lower speed is available
Figure 16.5 Gearless drive unit
(Courtesy of Schindler Lifts Ltd)
for good levelling at the landings. The increase from low to high speed is obtained by the
insertion of resistance in the shunt field. For speeds of 1.5ms–1 and above, a d.c. shunt
wound motor running at between 50 and 120 rpm is employed. At these speeds the motor
can be coupled directly to the driving sheave without any gearing. Because of the size of
the motor it is not possible to vary the speed by more than 1.5 to 1 by field control, and
speed was usually controlled by the Ward Leonard method. The absence of gearing
increases the overall efficiency, improves acceleration and results in smoother travelling.
A picture of a gearless machine is shown in Figure 16.5.
External set value systems
The more modern approach has been to integrate a logic controller and the drive system.
This has the advantage that the precise speed can be selected at any point in the journey,
levelling speed can be eliminated, and different speeds can be selected depending on the
expected length of journey.
This integration has taken the form of feeding the required speed value directly into
the variable speed drive from the logic controller. Thus, the logic controller, which is
monitoring the position of the lift car and its destination and its exact position with
respect to the floor and landings, generates the required speed profile and sends it either
as an analogue signal or a digital signal to the drive. The analogue signal is usually sent
to the drive via an isolation amplifier, in order to eliminate any interference or faults in
Lifts, escalators and paternosters 247
one system affecting the other. If sent in a digital format, an opto-coupler is usually used
to isolate the two systems.
The set value generator in this case will reside inside the lift logic controller, which is
invariably implemented using a microprocessor-based system. The logic controller would
receive pulses from the shaft encoder or the motor indicating the exact position of the lift
car in the shaft. Based on this information, a value in a table is provided, giving the
required value of speed at that point. This value is sent to the variable-speed drive, which
then controls the speed accordingly. These systems are always position-dependent
systems, as the speed always follows the position, regardless of time
Brakes
Lift brakes are usually electromagnetic. In the majority of cases, they are placed between
the motor and the gearbox; in a gearless machine the brake is keyed to the sheave. The
shoes are cperated by springs and released by an electromagnet the armature of which
acts either directly or through a system of links. A typical brake is shown in Figure 16.6.
The brake is used only when the car is parked. To slow the car down, several methods are
employed. Plugging is reversing the phase sequence as the motor is running; the
synchronous magnetic field reverses direction, causing the motor to slow
Figure 16.6 Lift brake
Design of electrical services for buildings 248
down rapidly. Eddy current braking uses an aluminium disc on the end of the drive motor
shaft. A magnetic field is applied to the disc and currents are induced in the disc which
act to slow the shaft down. The braking effect is proportional to the speed of rotation of
the shaft. Another method is to inject a DC current into the motor winding. Injecting DC
in a motor winding will tend to try to stall the rotor. This is because the magnetic field set
up inside the motor is a stationary constant field always pointing in one direction.
Lift cars
Passenger cars should be at least 2.00m high, and preferably 2.15m or more. They can be
made to almost any specification, but most manufacturers have certain standard finishes
from which the client should choose.
Lift cars consist of two separate units, namely the sling and the car proper. The sling is
constructed of steel angles or channels and the car is held within the sling. The sling also
carries the guide shoes and the safety gear. The car is sometimes insulated from the sling
frame by anti-vibration mountings. Goods cars are of rougher construction than passenger
cars but otherwise follow the same principles.
Except for very small installations it is now almost universal practice to have an
emergency telephone in the car. It can be connected either as a direct line to the public
telephone network or as an extension of the private branch exchange in the building. It is
generally fitted in a recess in the wall of the car with a hinged door over it.
All electrical connections to a car are made through a multi-core hanging flexible
cable. One end of this is connected to a terminal box under the car, and the other end to a
terminal box on the wall of the well approximately half-way down. A separate hanging
cable may be needed for the telephone.
Counterweights
A counterweight is provided to balance the load being carried. As the load carried varies,
the counterweight cannot always balance it exactly; it is usual for the counterweight to
balance the weight of the car plus 50 per cent of the maximum load to be taken in the car.
A typical counterweight frame is shown in Figure 16.7. It contains cast-iron sections held
in the steel framework and rigidly bolted together by tie rods. The lifting ropes are
attached to eye bolts which pass through the top piece of the frame.
Guides
Both the car and the counterweight must be guided in the well so that they do not swing
about as they travel up and down. Continuous vertical guides are provided for this
purpose. They are most commonly made of steel tees,
Lifts, escalators and paternosters 249
Figure 16.7 Counterweight frame
and there are standard tees made especially for use as lift guides. The guides are fastened
to steel plates by iron clamps at intervals of about 2m and these plates are secured to the
sides of the well. They may be secured by bolts passing through the wall of the well and
held by back plates on the other side or by being attached to angle irons or channels
which are in turn built into the wall. The latter is the usual practice with concrete building
construction.
Guide shoes are fitted on the car and on the counterweight and run smoothly on the
guides. Figure 16.8 shows a shoe on a guide. For smooth
Design of electrical services for buildings 250
Figure 16.8 Shoe of lift guide
running, the guides must be lubricated and various types of automatic lubricators have
been designed for lift guides. The commonest kind makes use of travelling lubricators
mounted on the car and counterweight. More recently unlubricated guides have been used
with shoes lined with carbon or PTFE.
Doors
Solid doors have now entirely superseded collapsible mesh gates. They are quieter,
stronger and safer. It is usual now for the car and landing doors to be operated together. If
the entrance to the car is not to be much narrower than the car itself then in the open
position the door must overlap the car. To accommodate this, the well must be wider than
the car. This will be clear from the plan of a typical lift installation shown in Figure 16.9.
Doors can be opened and closed manually, but it is more usual to have them power
operated. In order not to injure passengers caught by closing doors, the drive has to be
arranged to slip or reverse if the doors meet an obstruction. Every lift car door must have
an interlock which cuts off the supply to the lift controller when the door is open. This
can be a contactor which is pushed closed by the door and falls open by gravity or spring
action when the door opens.
The landing door must be locked so that it cannot be opened unless the car is in line
with the landing. The most usual way of doing this is by means of a lock which combines
a mechanical lock and an electrical interlock. The electrical interlock ensures that there is
no supply to the controller unless the gate is locked. The mechanical part can be unlocked
only when a cam on the car presses a roller arm on the lock; thus the landing door can
only be opened when the car is at the landing. The controls withdraw the cam when the
car is in motion and return it only as the car approaches a floor at which it is to stop. This
makes it impossible for anyone to open a landing door as the car passes the landing if the
car is not stopping there.
Lifts, escalators and paternosters 251
Figure 16.9 Plan of typical lift
arrangement
Indicators
Indicators are available for showing when the car is in motion, the direction of travel and
the position of the car in the well. A position indicator may be installed in the car, and in
many cases also at each landing. Direction indicators are provided at the landings, and a
common arrangement is to have a position an direction indicator in the car and at the
ground floor with direction indicators at the other landings.
The car positional controllers previously described may be interfaced with an
electronic system which then sends the car positional information to LCD displays at
each floor level and in the car itself. It would be beyond the scope of this book to
describe the circuitry required to do this.
Safety devices
Every lift car must have a safety gear which will stop it if its speed increases above a safe
level. The motor and brake circuits should be opened at the same time as the safety gear
operates. If the lift travel is more than about 9m the safety gear should be operated by an
overspeed governor in the machine room.
Design of electrical services for buildings 252
For speeds up to 0.8ms−1 instantaneous gear is generally used. This consists of a pair
of cams just clear of each guide, one on each side of the guide. The cams have serrated
edges and are held away from the guide by springs. A safety flyrope passes from the
safety gear over a top idler wheel to the counterweight. Tension on the safety rope causes
the cams to come into instantaneous contact with the guides and they then clamp the car
to the guides. The safety rope comes into tension if the lifting ropes break. Alternatively,
it can be connected to an overspeed governor. One type of governor is shown in Figure
16.10. It has a pulley driven from the car by a steel rope. Flyweights are mounted on the
pulley and linked together to ensure that they move simultaneously and equally. The
flyweights move against a spring which can be adjusted to give the required tripping
speed. As the speed increases, the weights move out against the spring force and at the
tripping speed they cause a jaw to grip the rope, which produces the tension necessary to
operate the cams.
For lifts at higher speeds, gradual wedge clamp safety gear is used. This also works by
clamping the car to the guides, but the clamps are forced against the guides gradually and
so bring the car to rest more smoothly. The clamps can be brought into play by screw
motion or by a spring.
Figure 16.10 Lift governor
Lifts, escalators and paternosters 253
Figure 16.11 Flexible guide clamp
Another type of safety gear used on high speed lifts is the flexible guide clamp, an
example of which is illustrated in Figure 16.11. It consists of two jaw assemblies, one for
each guide, mounted on a common channel under the car. The jaw assembly has a pair of
jaws with a gib in each jaw. Tension on the governor rope resulting from operation of the
governor pulls the operating lever and causes the gibs to move up the jaws. The
consequent wedging action of the gibs between the jaws and the guides compresses the
jaw spring to produce a gripping force on the guides which gives a constant retardation.
A lift must also have upper and lower terminal switches to stop the car if it overruns
either the top or bottom floor. These can take the form of a switch on the car worked by a
ramp in the well, or they may consist of a switch in the well worked by a ramp on the car.
There should be a normal stopping switch and a fixed stopping switch at each end of the
travel.
Clearance must be allowed for the car at the top and bottom of the well to give it room
to stop if the normal terminal switch fails and is passed and the terminal switch operates.
The clearances can be as given in Table 16.5. The bottom clearance given in the table
includes the buffer compression.
The final safety device consists of buffers in the well under the car and under the
counterweight. For low speed lifts they can be made as volute or helical springs, but for
high speed lifts oil buffers are used.
Design of electrical services for buildings 254
Table 16.4 Lift clearances
Lift speed (ms−1) Bottom clearance (m) Top clearance (m)
0−0.5 0.330 0.455
0.5−1.0 0.410 0.610
1.0−1.5 0.510 0.760
Landing
As it stops, the car must be brought to the exact level of the landing. With an automatic
lift, this depends on the accuracy with which the slowing and stopping devices cut off the
motor current and apply the brake. Levelling is affected by the load being carried; a full
load travels further than a light load when coming to rest from a given speed on the
downward trip and less far on the upward trip. To overcome this, it is desirable that the
car should travel faster when carrying a full load up than when travelling up empty. A
motor with a rising characteristic would be unstable, but the desired effect can be easily
achieved with variable voltage control. The rising characteristic is needed only at the
levelling speed, which is from about 1/6 to 1/20 of the maximum speed.
Type of control
An automatic control system has a single call button at each landing and a button for each
floor in the car. A passenger presses the car button for the desired floor and the lift
automatically travels there. Calls made from landings while the car is in motion are
stored in the controller memory. With Automatic Collective Control, each landing has
both an UP and a DOWN button, and there is a set of floor buttons in the car. Every
button pressed registers a call, and up and down calls are answered during up and down
journeys respectively, in the order in which the floors are reached. The order in which the
buttons are pressed does not affect the sequence in which the car stops at the various
floors, and all calls made are stored in the system until they have been answered. Down
calls made while the lift is travelling up are kept until after the up journey is finished, and
up calls made while the lift is moving down are similarly kept until that trip is finished.
The system can be modified to work as a collective system in the down direction and
as a simple automatic system in the up direction. It is then known as Down Collective.
This version is sometimes used in blocks of flats and is based on the assumption that
occupants and their visitors travelling up like to go straight to their own floors, but that
everyone going down wants to get off at the ground floor. Thus upward travellers should
be able to go straight to their own floor without interference, while downward travellers
are less likely to be irritated by intermediate stops to pick up other passengers going to
the same destination. This reasoning ignores milkmen, postmen and other delivery
workers, and the author of this book finds it unconvincing. Nevertheless, it appears to be
popular with many authorities.
Lifts, escalators and paternosters 255
Duplex control is used when two lifts are installed in adjacent wells. The landing
buttons serve both lifts. Landing calls are stored and allotted to the cars one at a time as
the cars finish journeys already in progress. For three lifts working together Triplex
Control is used.
It is also possible to arrange lifts with dual control so that they can be used either with
or without an attendant.
Electronic systems have made more sophisticated forms of control possible and these
have proved particularly valuable in tall office buildings with a group of three or more
lifts in each service core. The term ‘home floor’ is used to designate the floor to which a
car returns when it is not in use. It is not necessary for all the cars in a group to have the
same home floor. Programmed control of groups of high speed lifts is a system in which
cars are despatched from their home floor in a predetermined sequence so that even if
they are not in use they are already moving through the building when the first call is
made. Providing the programme is suitable for the pattern of traffic, this will ensure that
when a call is made there is a car nearer than the home floor to the floor at which the call
was made.
In a ‘Balanced’ programme cars are dispatched at set intervals from both terminal
floors and each car makes a complete round trip. An ‘Up Peak’ programme dispatches
cars from the lower terminal only and reverses each car when it is empty. A ‘Down Peak’
programme despatches them from the upper terminal at set intervals and returns each car
from the lower terminal as soon as it is empty. In an ‘Intermittent’ programme cars can
be taken out of service in a pre-arranged sequence.
When there is traffic in both directions but more going up than down a ‘Heavier Up’
programme will dispatch cars from both terminals at intervals which are automatically
adjusted so that the cars are equally spaced. In the reverse situation cars stop more
frequently when going down and a ‘Heavier Down’ programme adjusts the dispatch
times accordingly.
A traffic analyser measures the rate at which calls are made and answered and in doing
so distinguishes between up and down calls. It can be used to change operation from one
control scheme to another. This is valuable in office blocks where the pattern of traffic
changes throughout the day. ‘Up Peak’ and ‘Down Peak’ programmes are useful at the
beginning and end of the working day with the ‘Heavier Up’ and ‘Heavier Down’
programmes making a contribution during the lunch period.
Weighing contacts under the car floor can detect the load and this can be used to make
a car which is already fully loaded pass a landing without stopping even though there is a
call waiting at that landing in the direction in which the car is travelling.
Every lift manufacturing company has its own particular system. This means that the
specifier must discuss the scheme with manufacturers and if it is desired to put the lift
installation out to competitive tender the description of the controls must be loose enough
to allow some variation.
Controllers
The conventional lift motors are started, stopped and reversed by contactors. The
operating coils of the contactors are energized at the appropriate times by relays which
Design of electrical services for buildings 256
are connected in a circuit to give the required scheme of operation. The assembly of
contactors, relays and associated wiring forms the controller, which is usually placed in
the motor room, which may be the physical size of the panel shown in Figure 16.5.
Sufficient space must be left round the controller for maintenance, and it must also be
placed so that a maintenance technician working at it cannot accidentally touch a moving
part of the lift machine.
A controller circuit is necessarily complex because it contains many interconnected
relays; but each relay performs only one function and they are arranged to energize each
other in a logical sequence to achieve the operation required, an also interlocked to
prevent up and down functions to be selected at the same time. The resulting wiring
diagram may perhaps be described as involved but not complicated. In a book which
deals with electrical services as a whole and devotes only one chapter to lifts, it is not
practicable to give a full description of circuits for all the possible modes of control, and
we shall do no more than explain the working of a simple automatic system. It will have
to serve as a model illustrating principles which can be extended and adapted for other
systems.
Servo motors are controlled by electronic servo drives, and do not require relay. The
motor commands are supplied to the motor from the servo drive. The motor position thus
the car position can be controlled to a fraction of a degree of rotation, by utilising the
digital encoder fitted to the motor shaft.
Escalators
An electrical services engineer should also know something about escalators. These are
moving staircases. They consist essentially of pivoted steps linked to each other and
pulled along by an endless chain. The steps have guide pins which move in tracks on
either side of the tread arranged so that the steps come out of the concealed section
horizontally, are then pulled or pivoted into the shape of a staircase and finally return to
the horizontal before returning back into the concealed section. The steps complete their
circuit within the concealed section, and on this part of their travel they are flat. This is
illustrated in Figure 16.12.
The surface of each tread has grooves parallel to the direction of motion. The stationary
platforms at the top and bottom of the escalator have fixed combs which mesh with the
grooves in the treads in order to ensure a smooth run in and out of the treads under the
fixed floor. The stationary platform is actually a floor plate over the recess under the
moving staircase and covers the working mechanism at the top and bottom landings. It
has an extension known as a combplate and the combplate carries the projecting comb
teeth.
An escalator also has a balustrade with a handrail, and the handrail moves on an
endless chain in step with the stairs. There are separate chains for the handrail and the
steps but they are both driven through a gearbox from the same motor. The usual speed of
an escalator is 0.5ms−1 and it ought never to exceed 0.75ms−1. The inclination varies
between 27° and 35° to the horizontal.
Lifts, escalators and paternosters 257
Figure 16.12 Escalator
The drive and transmission have to carry the total load on the escalator. Since people
do not stand at even and regular intervals on the whole staircase the load averaged over
the whole length of the escalator is less than the maximum load on individual treads. The
peak load on each tread is of concern to the structural designer but the electrical engineer
concerned with the power requirements can use the average passenger load taken over the
total area of exposed treads. This average can be taken as 290kgm−2.
An escalator must have a brake which has to fail safe if there is an interruption to the
electrical supply. The brake is therefore applied by a spring or a hydraulic force and is
held off against the mechanical force by an electrically energized solenoid. As is the case
with lifts, there is also provision for releasing the brake manually and handwinding the
escalator.
Since an escalator is in continuous operation there are no passenger controls, but there
must be an on/off switch which can be worked by a responsible person in charge of the
premises. It would be dangerous for the escalator to be started or stopped by someone
who could not see the people on it and the switch must be in a place from which the
escalator can be seen. Such a place is almost inevitably in reach of the public using the
escalator, who ought not to be able to work the switch, and so the switch must be a key
operated one. The British Standard on Escalators (BS 2655) requires a key operated
starting switch to be provided at both ends of the escalator.
Emergency stop switches are provided in the machinery spaces under the escalator and
also in positions accessible to the public at the top and bottom of the escalator. The
operation of any of these switches disconnects the electrical supply from both the driving
machine and the brake. The removal of the supply from the brake allows the mechanical
force to apply the brake and bring the escalator to a halt. Some escalators are fitted with a
speed governor which similarly disconnects the electrical supply from the drive and
Design of electrical services for buildings 258
brake. There are further safety devices to disconnect the supply if one of the driving
chains breaks.
Most escalators are reversible. The driving motor is a squirrel cage induction motor
and the drive is reversed by contactors which change the phase sequence of the supply to
the motor.
A travellator differs from an escalator in being either horizontal or having a very small
slope not exceeding 12°. This makes it unnecessary for it to form steps and passengers
are conveyed on a continuous platform. The upper surface of the platform must have
grooves parallel to the direction of motion which mesh with the combplates. In other
respects a travellator or moving walkway is designed in exactly the same way as an
escalator.
Paternosters
Whilst paternosters are a type of lift they also have similarities with escalators and it is
more convenient to discuss them after the latter. A paternoster is a lift which has a series
of. small cars running continuously in a closed loop. It is difficult to explain this clearly
in words but it should be clear from Figure 16.13. The cars are open at the front and
move slowly enough for people to step in and out of them whilst they are in motion, just
as they step on and off an escalator. In fact a paternoster can perhaps be thought of as a
vertical escalator. To make it safe for people to get on and off whilst the cars are in
motion the speed must be less than 0.4ms−1.
The cars are constructed in the same way as ordinary lift cars but do not have doors and
are not large enough to take more than one person each. In practice this means that the
cars are less than 1.0m×1.0m in plan. They must of course be of normal height. The front
of the floor of each car is made as a hinged flap. This ensures that if a person has one foot
in the car and one on the landing he will not be thrown off balance as the car moves up.
Since the cars move in a continuous loop they provide their own counterweight and no
additional counterweight is needed. Rigid guides are provided for the cars which have
shoes similar to those of ordinary lift cars.
In the space between cars there is a protective screen level with the front of the cars.
This prevents people stepping into the shaft in between cars. It is still necessary to make
sure that the landings and entrances are well illuminated. The cars are carried on a
continuous steel link chain. The driving machinery is similar to that of an escalator and is
always placed above the well. It includes a brake which is applied mechanically and held
off electrically, so that the paternoster is braked if the electrical supply fails. As in the
case of both lifts and escalators there is provision for handwinding.
A paternoster is started by a key-operated switch, either at the ground floor or at the
main floor if this is other than the ground floor. There are emergency stop buttons at each
floor, in the pit and in the machinery space.
Lifts, escalators and paternosters 259
Figure 16.13 Paternoster
Although they have advantages, paternosters are not used very often. They take up
rather less space than escalators but have a lower carrying capacity. We can show this by
considering an escalator with a slope of 350 and a vertical rise between treads of 230mm.
The distance along the slope between succeeding steps is 230/sin 35°=400mm or 0.4m. If
the speed of the escalator is 0.5ms−1, the steps follow each other at intervals of
0.4/0.5=0.8s. As each step can take one passenger, the carrying capacity is one person in
every 0.8s, or 75 persons per minute.
The vertical speed of a paternoster is at most 0.4s−1’ and the car height cannot be less
than 2.2m. Neglecting any gap between the cars we see that the cars follow each other at
intervals of 2.2/0.4=5.5s, and each car can take only one passenger. The carrying capacity
is thus one person every 5.5s, or 11 persons per minute.
However well constructed and carefully operated it is, a paternoster cannot help being
more of a hazard than an escalator to the elderly, the infirm and above all to small
children. This practically restricts its applications to industrial premises not needing a
high carrying capacity.
Standards relevant to this chapter are:
Design of electrical services for buildings 260
Lifts, escalators and paternosters
Introduction
The general design of lifts is very well established, and in this country at least, nearly all
the reputable lift manufacturers will design and supply a satisfactory lift as a matter of
routine if given the details and the size of building. Nevertheless, the designer of the
building electrical services must be able to advise the client about the lifts, to negotiate
with the lift suppliers and to compare competing tenders. The designer must, therefore,
know something about the technical details of lifts and we shall accordingly devote this
chapter to a brief outline of the subject.
First, we can note that there are three categories of lifts. Passenger lifts are designed
primarily for passenger use; goods lifts are mainly for goods but can on occasion carry
passengers; and service lifts are for goods only and are of such a size that passengers
cannot enter into the car. Lift speeds are determined by the number of floors served and
the quality of service required. They vary from 0.5ms−1 to 10ms−1 in high office blocks.
In deciding the size of car one can allow 0.2m2 for each passenger, and when
determining the load the average weight of a passenger can be taken as 75kg. It must,
however, be remembered that in many buildings the lift will be used for moving in
furniture and the car must be big enough for the bulkiest piece of furniture likely to be
needed. The author has made measurements of domestic furniture and has concluded that
the most awkward item to manoeuvre is a double bed, which can be up to 1670mm wide
by 1900mm long and 360mm high. In flats it is unfortunately also necessary to make sure
that stretchers and coffins can be carried in the lift. To accommodate these, a depth of
2.5m is required. The whole car can be made this depth or it can be shallower but have a
collapsible extension which can be opened out at the back when the need arises. The lift
well must, of course, be deep enough to allow the extension to be opened. In hospitals
some of the lifts must take stretchers on trolleys and also hospital beds and these lifts
must be the full depth of a complete bed.
Grade of service
The quality of service is a measure of the speed with which passengers can be taken to
their destination. It is the sum of the time which the average passenger has to wait for a
lift and of the travelling time once in the lift. The maximum time a person may have to
wait is called the waiting time (WI) and is the interval between the arrival of successive
cars. It depends on the round trip time (RTT) of each lift and on the number of lifts.
The average time a person has to wait is WI/2. The average time travelling is RTT/4.
The sum of these, WI/2=RTT/4, is called the grade of service. If there are N lifts, then
WI=RTT/N, and grade of service becomes WI(2+N)/4. It is usual to classify the grade of
service as excellent if WI(2+N)/4 is less than 45s, good if it is between 45 and 55s, fair if
it is between 55 and 65s and casual if it is more than 65s.
The use of a building will often enable a designer to estimate the probable number of
stops during each trip. If this is difficult, then a formula can be developed by probability
theory, and is:
where
Sn=probable number of stops
n=number of floors served above ground floor
N=number of passengers entering lift at ground floor on each trip
P=total population on all floors
Pa, Pb,…pn=population on 1st, 2nd…nth floor.
Three to four seconds must be allowed for opening and closing the doors at each stop.
A further 1 to 11.5s have to be allowed for each passenger to enter the lift and 1 to 2s for
each passenger to leave.
The travelling time is made up of periods of acceleration, constant speed and
retardation. Figure 16.1 gives the time versus distance curves for the acceleration
normally associated with various lift speeds. On each curve, the point marked X indicates
the end of acceleration and start of constant velocity. The retardation is generally taken to
be equal in magnitude to the acceleration. Providing the distance between stops is long
enough for the lift to reach steady speed before starting to slow, the total travelling time
of a round trip is given by:
Design of electrical services for buildings 236
Figure 16.1 Acceleration curves for
lifts
where
t=total travelling time
d=distance during which acceleration takes place
D=distance between ground and top floors
Sn=number of stops between ground and top floors
V=lift speed.
The best way of showing how all this data is used to assess the grade of service is by
means of an example. Let us assume we are dealing with an office block with eight
floors. The heaviest traffic will occur in the morning when people are arriving at work,
and we shall assume that we know enough about the occupancy of the building to have
been able to estimate that 75 per cent of the work force will arrive in one particular half
hour. For estimating the probable number of stops, traffic to the first floor can be ignored
and we can set out the number of people requiring service as follows:
Lifts, escalators and paternosters 237
Table 16.1 Lift service comparison
A B C D
Load persons 10 20 10 15
Speed ms−1 1.5 1.5 2.0 2.0
Probable no. of stops per trip (Sn) 5.23 6.35 5.23 5.99
Accelerating distance (d) m 2.60 2.60 2.20 2.20
d×Sn 13.60 16.50 11.50 13.20
Distance between ground and top floors (D) m 25.00 25.00 25.00 25.00
(dSn+D+d) 41.20 44.10 38.70 40.40
55.00 59.00 38.00 40.00
Door opening time (s) 21.00 28.00 21.00 24.00
Passengers entering and leaving (s) 25.00 50.00 25.00 37.00
Total travelling time 101 137 84 101
10% margin 10 13 8 10
RTT(s) 111 150 92 111
No. of lifts 4 3 4 3
No. of trips per lift in 30min 16 12 19 16
No. of persons per lift in 30min 160 240 190 240
Total no. of persons carried in 30min 640 720 760 720
WI(s) 28 50 23 37
42 62 35 46
Grade of service Excellent Fair Excellent Good
Calculation of Sn P=662
n=7
i Pi P−Pi
2 36 626 0.95 0.60 0.46 0.36
3 93 569 0.86 0.22 0.10 0.05
4 160 502 0.76 0.06 0.01 0.00
5 85 577 0.58 0.22 0.10 0.05
Design of electrical services for buildings 238
6 120 542 0.82 0.12 0.04 0.02
7 105 557 0.84 0.18 0.08 0.03
8 63 599 0.905 0.37 0.22 0.14
Σ 1.77 1.01 0.65
Sn=7−Σ 5.23 5.99 6.35
Floor 2 3 4 5 6 7 8 Total
No. of persons requiring service 36 93 160 85 120 105 63 662
The figures in the second line are 75 per cent of the floor populations, which we assume
we have either been given or can guess. The distance between the ground and eighth floor
is 25m.
The round trip time can be calculated and hence it is possible to calculate the number
of lifts needed to carry 662 people in 30mm. From this, the grade of service can be
obtained. If the calculation is set out in tabular form, different combinations can be easily
compared. This has been done in Table 16.1. A 10 per cent margin is added to the
calculated total time to allow for irregularities in the time interval between different lifts
in the bank of lifts.
It can be seen that in this example the most satisfactory arrangement is four lifts each
taking 10 persons at a speed of 1.5ms−1. A speed of 2.0ms−1 would be unnecessarily
extravagant.
It will be found that where the service is not so concentrated lower speeds are
sufficient. For this reason, it should not be necessary to use speeds of more than 0.75 or
1.0 ms−1 in blocks of flats.
Accommodation
The machine room for the lifting gear is normally at the top of the lift shaft or well. It can
be at the bottom or even beside the well, and in the latter case it can be at any height, but
from these positions the ropes must pass over more pulleys so that the overall
arrangement becomes more complicated. It is, therefore, better to provide space for the
machine room at the top of the well. Room must also be left for buffers and for inspection
at the bottom, or pit, of the well. The sizes of the machine and pit rooms must ultimately
be agreed with the lift manufacturers, but for preliminary planning before a manufacturer
has submitted a quotation the dimensions in Table 16.2 may be taken as a guide.
Drive
Nearly all lifts use a traction drive. In this, the ropes pass from the lift car around a cast
iron or steel grooved sheave and then to the counterweight. The sheave is secured to a
steel shaft which is turned by the driving motor. The drive from the motor to the shaft is
usually through a worm gear. The force needed to raise or lower the lift car is provided
Lifts, escalators and paternosters 239
by the friction between the ropes and the sheave grooves. The main advantage of the
traction drive is that if either the car or counterweight comes into contact with the buffers
the drive ceases and there is no danger of the car being wound into the overhead
structure. Other advantages are cheapness and simplicity.
Table 16.2 Lift dimensions
Passenger lifts
Well Machine room
Load
persons
Speed
(ms−1)
Width
(m)
Depth
(m)
Width
(m)
Length
(m)
Height
(m)
Top landing to
MIC room
floor (m)
Pit
depth
(m)
General purpose passenger lifts
8 1.0 1.80 1.90 3.10 4.80 2.60 4.00 1.60
10 0.75 2.00 1.90 3.10 5.00 2.60 4.00 1.60
10 1.0 2.00 1.90 3.10 5.00 2.60 4.00 1.70
10 1.5 2.00 1.90 3.10 5.00 2.60 4.20 1.70
16 0.75 2.60 2.20 3.50 5.30 2.70 4.10 1.70
16 1.00 2.60 2.20 3.50 5.30 2.70 4.20 1.90
16 1.50 2.60 2.20 3.50 5.30 2.70 4.30 1.90
20 0.75 2.60 2.50 3.50 5.60 2.70 4.10 1.70
20 1.00 2.60 2.50 3.50 5.60 2.70 4.20 1.90
20 1.50 2.60 2.50 3.50 5.60 2.70 4.30 1.90
High speed passenger lifts
12 2.5 2.20 2.20 3.20 7.50 2.70 6.80 2.80
16 2.5 2.60 2.30 3.20 8.00 2.70 6.80 2.80
16 3.5 2.60 2.30 3.20 8.00 3.50 6.90 3.40
20 2.5 2.60 2.60 3.20 8.30 3.50 6.20 2.80
20 3.5 2.60 2.60 3.20 8.30 3.50 7.10 3.40
20 5.0 2.60 2.60 3.20 8.30 3.50 8.20 5.10
Goods lifts
Well Machine room
Load
(kg)
Speed
(ms−1)
Width
(m)
Depth
(m)
Width
(m)
Length
(m)
Height
(m)
Top landing to
M/C room
floor (m)
Pit
depth
(m)
General purpose goods lifts
Design of electrical services for buildings 240
500 0.5 1.80 1.50 2.00 3.70 2.40 3.80 1.40
1000 0.25 2.10 2.10 2.10 4.30 2.40 3.80 1.50
1000 0.50 2.10 2.10 2.10 4.30 2.40 3.80 1.50
1000 0.75 2.10 2.10 2.10 4.30 2.40 3.80 1.50
1500 0.25 2.50 2.30 2.50 4.50 2.70 4.00 1.50
1500 0.75 2.50 2.30 2.50 4.50 2.70 4.20 1.80
1500 1.00 2.50 2.30 2.50 4.50 2.70 4.20 1.80
2000 0.25 2.80 2.40 2.80 4.70 2.90 4.10 1.50
2000 0.75 2.80 2.40 2.80 4.70 2.90 4.50 1.80
2000 1.00 2.80 2.40 2.80 4.70 2.90 4.50 1.80
3000 0.25 3.50 2.70 3.50 5.00 2.90 4.20 1.50
3000 0.50 3.50 2.70 3.50 5.00 2.90 4.40 1.70
3000 0.75 3.50 2.70 3.50 5.00 2.90 4.50 1.80
Well Machine room
Load
(kg)
Speed
(ms–1)
Width
(m)
Depth
(m)
Width
(m)
Length
(m)
Height
(m)
Top landing to
M/C room floor
(m)
Pit
depth
(m)
Heavy duty goods lift
1500 0.50 2.60 2.40 2.60 4.80 2.70 4.80 1.70
1500 0.75 2.60 2.40 2.60 4.80 2.70 4.80 1.80
1500 1.00 2.60 2.40 2.60 4.80 2.70 4.80 1.80
2000 0.50 2.90 2.50 2.90 5.00 2.90 4.80 1.70
2000 0.75 2.90 2.50 2.90 5.00 2.90 4.80 1.80
2000 1.00 2.90 2.50 2.90 5.00 2.90 4.80 1.80
3000 0.50 3.50 2.80 3.50 5.30 2.90 4.80 1.70
3000 0.75 3.50 2.80 3.50 5.30 2.90 4.80 1.80
3000 1.00 3.50 2.80 3.50 5.30 2.90 4.80 1.80
4000 0.50 3.50 3.40 4.00 6.20 2.90 5.20 1.70
4000 0.75 3.50 3.40 4.00 6.20 2.90 5.20 1.80
5000 0.50 3.60 4.00 4.00 6.80 2.90 5.20 1.70
5000 0.75 3.60 4.00 4.00 6.80 2.90 5.20 1.80
Figure 16.2 shows a geared traction unit has a drive motor permanently coupled to the
gear by a vee rope drive. There is a brake working on a brake disc on the end of the worm
Lifts, escalators and paternosters 241
shaft and the grooved sheave is easily discernible at the side of the machine. The control
panel is in the foreground
The only other kind of drive is the drum drive. In this case, one end of the car ropes
and one end of the counterweight ropes are securely fastened by
Figure 16.2 Geared drive unit
(Courtesy of Schindler Lifts Ltd)
clamps on the inside of a cast iron or steel drum, the other ends being fastened to the car
and counterweight respectively. One set of ropes is wrapped clockwise around the drum
and the other set anti-clockwise, so that one set is winding up as the other set is
unwinding from the drum. As the car travels, the ropes move along the drum in spiral
grooves on its periphery. The drum drive suffers from the disadvantage that as the height
of travel increases the drum becomes large and unwieldy. It has been almost entirely
superseded by the traction drive.
Figure 16.3 shows various arrangements of ropes for traction drives. Figure 16.3b
shows a double wrap drive, in which each rope passes over the sheave
Design of electrical services for buildings 242
Figure 16.4 Compensating ropes
twice. The increased length of contact between rope and sheave increases the maximum
available lifting force, or alternatively permits a lower coefficient of friction for the same
lifting force. Figure 16.3c shows a double wrap two-to-one system in which the speed of
the car is half the peripheral speed of the sheave. Figures 16.3d, e and f correspond to
Figures 16.3a, b and c but with the winding machine at the bottom of the well.
Compensating ropes are sometimes fitted on long travel lifts in order to make the load
on the motor constant by eliminating the effect of the weight of the ropes. A simple
method of doing this is shown in Figure 16.4.
Motors
A lift motor should have a starting torque equal to at least twice the full load torque; it
should be quiet and it should have a low kinetic energy. The last requirement is necessary
for rapid acceleration and deceleration and also for low wear in the brakes. The
theoretical power needed can be calculated from the lifting speed and the greatest
difference between the weights of car plus load and counterweight. The actual power will
depend on the mechanical efficiency of the drive which can be anything from 30 per cent
to 60 per cent. Suitable motor sizes for various lifts are given in Table 16.3. The
acceleration is settled by the torque-speed characteristic of the motor and the ratio of
motor speed to lift speed.
Design of electrical services for buildings 244
Types of motor
In most cases, in the UK at least, a three phase a.c. supply is available in a building which
is to have a lift installation. For lift speeds up to about 0.5ms−1 a single-speed squirrel
cage motor is suitable, although it has a high starting
Table 16.3 Approximate lift motor ratings (motor
ratings in kW)
Low efficiency geared lifts
Contract load (kg)
Car speed (ms−1) 250 500 750 1000 1500 2000
0.25 2 3.5 5 5 8 10
0.50 3 5.0 8 10 15 20
0.75 4 8.0 10 15 20 25
1.00 5 10 12 18 25 35
1.25 6 12 15 20 30 45
1.50 7 15 20 25 40 50
High efficiency geared goods lifts
Contract load (kg)
Car speed (ms–1) 250 500 750 1000 1500 2000 3000
0.25 1 2.0 3 4.0 6.0 8 10
0.50 3 4.0 6 7.5 10.0 14 20
0.75 5.0 8 10.0 12.5 20 30
1.00 7.5 10 14.0 20.0 25
High efficiency geared passenger lifts
No. of passengers
Car speed (ms−1) 4 6 8 10 15 20
0.50 3 3 4.0 5.0
0.75 5 7.5 7.5 12 15
1.0 10 10 15 18
1.5 12 17.5 22
current and tends to overheat on duties requiring more than 100 starts an hour.
Lifts, escalators and paternosters 245
For speeds between about 0.5ms−1 and 1.25ms−1, it becomes necessary to use a twospeed
motor in order to have a low landing speed. The squirrel cage motor can be wound
to give two combinations of poles, thus giving two speeds. A two-speed system is not a
variable-speed drive; it is variable in the sense that it has two speeds. These systems were
used widely in the 1970s and 1980s. They effectively employed direct on-line starting
and thus needed flywheels attached to the motor to smooth the movement and reduce
jerk. The use of a flywheel makes it inefficient
DC silicon-controlled rectifier (SCR)
Using power electronics, a DC motor can be used to vary the speed of the lift by varying
the armature voltage. This method is the most widely used in DC drives, in the form of a
controlled three-phase rectifier. This can be implemented in two forms. One form is a
fully controlled bridge rectifier, which allows two-quadrant operation.
The other form uses two bridges in parallel, each connected to drive the motor in the
opposite direction of the other. By using both bridges, the motor can be operated in both
driving and braking modes, in forward and reverse directions (i.e. four-quadrant
operation).
AC variable voltage (ACVV)
These systems were widely used in the mid-1980s and early 1990s. They are very simple
in the method of operation. They rely on three pairs of back-to-back thyristors for varying
the stator AC voltage on a double-cage squirrel cage motor. By varying the firing angle,
the stator voltage is varied and a new speed torque curve results.
Variable voltage variable frequency (VVVF)
The most widely used system today is the VVVF system, usually referred to as an
inverter drive. The principle of operation relies on a rectifier to produce DC into the socalled
DC link and an inverter, which produces sinusoidal current into the windings. By
changing the frequency of the inverted signal, the synchronous frequency and hence the
speed torque curve is moved to the desired profile. The supply is fed to a servo motor.
Servo motors can be programmed to give the required S curve profile for travel
between floors.
For speeds above 1.5ms–1 the same types of motor could be used as for the lower
speeds, but in fact they very rarely are. At the higher speeds, it is possible to design a d.c.
motor to run at a speed which makes reduction gearing to the drive unnecessary. The a.c.
supply is therefore used to drive a variable voltage motor generator set which supplies the
d.c. to the lift driving motor.
If only a single phase supply is available, a repulsion-induction motor can be used for
speeds up to 0.5ms−1. For higher speeds it is better to use the a.c. to drive a motor
generator and have a d.c. lift machine.
When d.c. is used for speeds less than 0.5ms−1, a single speed shunt or compound
wound motor is employed. When the lift is decelerating the machine runs as a generator,
and in the case of a compound wound motor this makes special arrangements necessary.
Design of electrical services for buildings 246
For speeds between 0.5ms−1 and 1.25ms−1 two-speed shunt motors are used, so that a
lower speed is available
Figure 16.5 Gearless drive unit
(Courtesy of Schindler Lifts Ltd)
for good levelling at the landings. The increase from low to high speed is obtained by the
insertion of resistance in the shunt field. For speeds of 1.5ms–1 and above, a d.c. shunt
wound motor running at between 50 and 120 rpm is employed. At these speeds the motor
can be coupled directly to the driving sheave without any gearing. Because of the size of
the motor it is not possible to vary the speed by more than 1.5 to 1 by field control, and
speed was usually controlled by the Ward Leonard method. The absence of gearing
increases the overall efficiency, improves acceleration and results in smoother travelling.
A picture of a gearless machine is shown in Figure 16.5.
External set value systems
The more modern approach has been to integrate a logic controller and the drive system.
This has the advantage that the precise speed can be selected at any point in the journey,
levelling speed can be eliminated, and different speeds can be selected depending on the
expected length of journey.
This integration has taken the form of feeding the required speed value directly into
the variable speed drive from the logic controller. Thus, the logic controller, which is
monitoring the position of the lift car and its destination and its exact position with
respect to the floor and landings, generates the required speed profile and sends it either
as an analogue signal or a digital signal to the drive. The analogue signal is usually sent
to the drive via an isolation amplifier, in order to eliminate any interference or faults in
Lifts, escalators and paternosters 247
one system affecting the other. If sent in a digital format, an opto-coupler is usually used
to isolate the two systems.
The set value generator in this case will reside inside the lift logic controller, which is
invariably implemented using a microprocessor-based system. The logic controller would
receive pulses from the shaft encoder or the motor indicating the exact position of the lift
car in the shaft. Based on this information, a value in a table is provided, giving the
required value of speed at that point. This value is sent to the variable-speed drive, which
then controls the speed accordingly. These systems are always position-dependent
systems, as the speed always follows the position, regardless of time
Brakes
Lift brakes are usually electromagnetic. In the majority of cases, they are placed between
the motor and the gearbox; in a gearless machine the brake is keyed to the sheave. The
shoes are cperated by springs and released by an electromagnet the armature of which
acts either directly or through a system of links. A typical brake is shown in Figure 16.6.
The brake is used only when the car is parked. To slow the car down, several methods are
employed. Plugging is reversing the phase sequence as the motor is running; the
synchronous magnetic field reverses direction, causing the motor to slow
Figure 16.6 Lift brake
Design of electrical services for buildings 248
down rapidly. Eddy current braking uses an aluminium disc on the end of the drive motor
shaft. A magnetic field is applied to the disc and currents are induced in the disc which
act to slow the shaft down. The braking effect is proportional to the speed of rotation of
the shaft. Another method is to inject a DC current into the motor winding. Injecting DC
in a motor winding will tend to try to stall the rotor. This is because the magnetic field set
up inside the motor is a stationary constant field always pointing in one direction.
Lift cars
Passenger cars should be at least 2.00m high, and preferably 2.15m or more. They can be
made to almost any specification, but most manufacturers have certain standard finishes
from which the client should choose.
Lift cars consist of two separate units, namely the sling and the car proper. The sling is
constructed of steel angles or channels and the car is held within the sling. The sling also
carries the guide shoes and the safety gear. The car is sometimes insulated from the sling
frame by anti-vibration mountings. Goods cars are of rougher construction than passenger
cars but otherwise follow the same principles.
Except for very small installations it is now almost universal practice to have an
emergency telephone in the car. It can be connected either as a direct line to the public
telephone network or as an extension of the private branch exchange in the building. It is
generally fitted in a recess in the wall of the car with a hinged door over it.
All electrical connections to a car are made through a multi-core hanging flexible
cable. One end of this is connected to a terminal box under the car, and the other end to a
terminal box on the wall of the well approximately half-way down. A separate hanging
cable may be needed for the telephone.
Counterweights
A counterweight is provided to balance the load being carried. As the load carried varies,
the counterweight cannot always balance it exactly; it is usual for the counterweight to
balance the weight of the car plus 50 per cent of the maximum load to be taken in the car.
A typical counterweight frame is shown in Figure 16.7. It contains cast-iron sections held
in the steel framework and rigidly bolted together by tie rods. The lifting ropes are
attached to eye bolts which pass through the top piece of the frame.
Guides
Both the car and the counterweight must be guided in the well so that they do not swing
about as they travel up and down. Continuous vertical guides are provided for this
purpose. They are most commonly made of steel tees,
Lifts, escalators and paternosters 249
Figure 16.7 Counterweight frame
and there are standard tees made especially for use as lift guides. The guides are fastened
to steel plates by iron clamps at intervals of about 2m and these plates are secured to the
sides of the well. They may be secured by bolts passing through the wall of the well and
held by back plates on the other side or by being attached to angle irons or channels
which are in turn built into the wall. The latter is the usual practice with concrete building
construction.
Guide shoes are fitted on the car and on the counterweight and run smoothly on the
guides. Figure 16.8 shows a shoe on a guide. For smooth
Design of electrical services for buildings 250
Figure 16.8 Shoe of lift guide
running, the guides must be lubricated and various types of automatic lubricators have
been designed for lift guides. The commonest kind makes use of travelling lubricators
mounted on the car and counterweight. More recently unlubricated guides have been used
with shoes lined with carbon or PTFE.
Doors
Solid doors have now entirely superseded collapsible mesh gates. They are quieter,
stronger and safer. It is usual now for the car and landing doors to be operated together. If
the entrance to the car is not to be much narrower than the car itself then in the open
position the door must overlap the car. To accommodate this, the well must be wider than
the car. This will be clear from the plan of a typical lift installation shown in Figure 16.9.
Doors can be opened and closed manually, but it is more usual to have them power
operated. In order not to injure passengers caught by closing doors, the drive has to be
arranged to slip or reverse if the doors meet an obstruction. Every lift car door must have
an interlock which cuts off the supply to the lift controller when the door is open. This
can be a contactor which is pushed closed by the door and falls open by gravity or spring
action when the door opens.
The landing door must be locked so that it cannot be opened unless the car is in line
with the landing. The most usual way of doing this is by means of a lock which combines
a mechanical lock and an electrical interlock. The electrical interlock ensures that there is
no supply to the controller unless the gate is locked. The mechanical part can be unlocked
only when a cam on the car presses a roller arm on the lock; thus the landing door can
only be opened when the car is at the landing. The controls withdraw the cam when the
car is in motion and return it only as the car approaches a floor at which it is to stop. This
makes it impossible for anyone to open a landing door as the car passes the landing if the
car is not stopping there.
Lifts, escalators and paternosters 251
Figure 16.9 Plan of typical lift
arrangement
Indicators
Indicators are available for showing when the car is in motion, the direction of travel and
the position of the car in the well. A position indicator may be installed in the car, and in
many cases also at each landing. Direction indicators are provided at the landings, and a
common arrangement is to have a position an direction indicator in the car and at the
ground floor with direction indicators at the other landings.
The car positional controllers previously described may be interfaced with an
electronic system which then sends the car positional information to LCD displays at
each floor level and in the car itself. It would be beyond the scope of this book to
describe the circuitry required to do this.
Safety devices
Every lift car must have a safety gear which will stop it if its speed increases above a safe
level. The motor and brake circuits should be opened at the same time as the safety gear
operates. If the lift travel is more than about 9m the safety gear should be operated by an
overspeed governor in the machine room.
Design of electrical services for buildings 252
For speeds up to 0.8ms−1 instantaneous gear is generally used. This consists of a pair
of cams just clear of each guide, one on each side of the guide. The cams have serrated
edges and are held away from the guide by springs. A safety flyrope passes from the
safety gear over a top idler wheel to the counterweight. Tension on the safety rope causes
the cams to come into instantaneous contact with the guides and they then clamp the car
to the guides. The safety rope comes into tension if the lifting ropes break. Alternatively,
it can be connected to an overspeed governor. One type of governor is shown in Figure
16.10. It has a pulley driven from the car by a steel rope. Flyweights are mounted on the
pulley and linked together to ensure that they move simultaneously and equally. The
flyweights move against a spring which can be adjusted to give the required tripping
speed. As the speed increases, the weights move out against the spring force and at the
tripping speed they cause a jaw to grip the rope, which produces the tension necessary to
operate the cams.
For lifts at higher speeds, gradual wedge clamp safety gear is used. This also works by
clamping the car to the guides, but the clamps are forced against the guides gradually and
so bring the car to rest more smoothly. The clamps can be brought into play by screw
motion or by a spring.
Figure 16.10 Lift governor
Lifts, escalators and paternosters 253
Figure 16.11 Flexible guide clamp
Another type of safety gear used on high speed lifts is the flexible guide clamp, an
example of which is illustrated in Figure 16.11. It consists of two jaw assemblies, one for
each guide, mounted on a common channel under the car. The jaw assembly has a pair of
jaws with a gib in each jaw. Tension on the governor rope resulting from operation of the
governor pulls the operating lever and causes the gibs to move up the jaws. The
consequent wedging action of the gibs between the jaws and the guides compresses the
jaw spring to produce a gripping force on the guides which gives a constant retardation.
A lift must also have upper and lower terminal switches to stop the car if it overruns
either the top or bottom floor. These can take the form of a switch on the car worked by a
ramp in the well, or they may consist of a switch in the well worked by a ramp on the car.
There should be a normal stopping switch and a fixed stopping switch at each end of the
travel.
Clearance must be allowed for the car at the top and bottom of the well to give it room
to stop if the normal terminal switch fails and is passed and the terminal switch operates.
The clearances can be as given in Table 16.5. The bottom clearance given in the table
includes the buffer compression.
The final safety device consists of buffers in the well under the car and under the
counterweight. For low speed lifts they can be made as volute or helical springs, but for
high speed lifts oil buffers are used.
Design of electrical services for buildings 254
Table 16.4 Lift clearances
Lift speed (ms−1) Bottom clearance (m) Top clearance (m)
0−0.5 0.330 0.455
0.5−1.0 0.410 0.610
1.0−1.5 0.510 0.760
Landing
As it stops, the car must be brought to the exact level of the landing. With an automatic
lift, this depends on the accuracy with which the slowing and stopping devices cut off the
motor current and apply the brake. Levelling is affected by the load being carried; a full
load travels further than a light load when coming to rest from a given speed on the
downward trip and less far on the upward trip. To overcome this, it is desirable that the
car should travel faster when carrying a full load up than when travelling up empty. A
motor with a rising characteristic would be unstable, but the desired effect can be easily
achieved with variable voltage control. The rising characteristic is needed only at the
levelling speed, which is from about 1/6 to 1/20 of the maximum speed.
Type of control
An automatic control system has a single call button at each landing and a button for each
floor in the car. A passenger presses the car button for the desired floor and the lift
automatically travels there. Calls made from landings while the car is in motion are
stored in the controller memory. With Automatic Collective Control, each landing has
both an UP and a DOWN button, and there is a set of floor buttons in the car. Every
button pressed registers a call, and up and down calls are answered during up and down
journeys respectively, in the order in which the floors are reached. The order in which the
buttons are pressed does not affect the sequence in which the car stops at the various
floors, and all calls made are stored in the system until they have been answered. Down
calls made while the lift is travelling up are kept until after the up journey is finished, and
up calls made while the lift is moving down are similarly kept until that trip is finished.
The system can be modified to work as a collective system in the down direction and
as a simple automatic system in the up direction. It is then known as Down Collective.
This version is sometimes used in blocks of flats and is based on the assumption that
occupants and their visitors travelling up like to go straight to their own floors, but that
everyone going down wants to get off at the ground floor. Thus upward travellers should
be able to go straight to their own floor without interference, while downward travellers
are less likely to be irritated by intermediate stops to pick up other passengers going to
the same destination. This reasoning ignores milkmen, postmen and other delivery
workers, and the author of this book finds it unconvincing. Nevertheless, it appears to be
popular with many authorities.
Lifts, escalators and paternosters 255
Duplex control is used when two lifts are installed in adjacent wells. The landing
buttons serve both lifts. Landing calls are stored and allotted to the cars one at a time as
the cars finish journeys already in progress. For three lifts working together Triplex
Control is used.
It is also possible to arrange lifts with dual control so that they can be used either with
or without an attendant.
Electronic systems have made more sophisticated forms of control possible and these
have proved particularly valuable in tall office buildings with a group of three or more
lifts in each service core. The term ‘home floor’ is used to designate the floor to which a
car returns when it is not in use. It is not necessary for all the cars in a group to have the
same home floor. Programmed control of groups of high speed lifts is a system in which
cars are despatched from their home floor in a predetermined sequence so that even if
they are not in use they are already moving through the building when the first call is
made. Providing the programme is suitable for the pattern of traffic, this will ensure that
when a call is made there is a car nearer than the home floor to the floor at which the call
was made.
In a ‘Balanced’ programme cars are dispatched at set intervals from both terminal
floors and each car makes a complete round trip. An ‘Up Peak’ programme dispatches
cars from the lower terminal only and reverses each car when it is empty. A ‘Down Peak’
programme despatches them from the upper terminal at set intervals and returns each car
from the lower terminal as soon as it is empty. In an ‘Intermittent’ programme cars can
be taken out of service in a pre-arranged sequence.
When there is traffic in both directions but more going up than down a ‘Heavier Up’
programme will dispatch cars from both terminals at intervals which are automatically
adjusted so that the cars are equally spaced. In the reverse situation cars stop more
frequently when going down and a ‘Heavier Down’ programme adjusts the dispatch
times accordingly.
A traffic analyser measures the rate at which calls are made and answered and in doing
so distinguishes between up and down calls. It can be used to change operation from one
control scheme to another. This is valuable in office blocks where the pattern of traffic
changes throughout the day. ‘Up Peak’ and ‘Down Peak’ programmes are useful at the
beginning and end of the working day with the ‘Heavier Up’ and ‘Heavier Down’
programmes making a contribution during the lunch period.
Weighing contacts under the car floor can detect the load and this can be used to make
a car which is already fully loaded pass a landing without stopping even though there is a
call waiting at that landing in the direction in which the car is travelling.
Every lift manufacturing company has its own particular system. This means that the
specifier must discuss the scheme with manufacturers and if it is desired to put the lift
installation out to competitive tender the description of the controls must be loose enough
to allow some variation.
Controllers
The conventional lift motors are started, stopped and reversed by contactors. The
operating coils of the contactors are energized at the appropriate times by relays which
Design of electrical services for buildings 256
are connected in a circuit to give the required scheme of operation. The assembly of
contactors, relays and associated wiring forms the controller, which is usually placed in
the motor room, which may be the physical size of the panel shown in Figure 16.5.
Sufficient space must be left round the controller for maintenance, and it must also be
placed so that a maintenance technician working at it cannot accidentally touch a moving
part of the lift machine.
A controller circuit is necessarily complex because it contains many interconnected
relays; but each relay performs only one function and they are arranged to energize each
other in a logical sequence to achieve the operation required, an also interlocked to
prevent up and down functions to be selected at the same time. The resulting wiring
diagram may perhaps be described as involved but not complicated. In a book which
deals with electrical services as a whole and devotes only one chapter to lifts, it is not
practicable to give a full description of circuits for all the possible modes of control, and
we shall do no more than explain the working of a simple automatic system. It will have
to serve as a model illustrating principles which can be extended and adapted for other
systems.
Servo motors are controlled by electronic servo drives, and do not require relay. The
motor commands are supplied to the motor from the servo drive. The motor position thus
the car position can be controlled to a fraction of a degree of rotation, by utilising the
digital encoder fitted to the motor shaft.
Escalators
An electrical services engineer should also know something about escalators. These are
moving staircases. They consist essentially of pivoted steps linked to each other and
pulled along by an endless chain. The steps have guide pins which move in tracks on
either side of the tread arranged so that the steps come out of the concealed section
horizontally, are then pulled or pivoted into the shape of a staircase and finally return to
the horizontal before returning back into the concealed section. The steps complete their
circuit within the concealed section, and on this part of their travel they are flat. This is
illustrated in Figure 16.12.
The surface of each tread has grooves parallel to the direction of motion. The stationary
platforms at the top and bottom of the escalator have fixed combs which mesh with the
grooves in the treads in order to ensure a smooth run in and out of the treads under the
fixed floor. The stationary platform is actually a floor plate over the recess under the
moving staircase and covers the working mechanism at the top and bottom landings. It
has an extension known as a combplate and the combplate carries the projecting comb
teeth.
An escalator also has a balustrade with a handrail, and the handrail moves on an
endless chain in step with the stairs. There are separate chains for the handrail and the
steps but they are both driven through a gearbox from the same motor. The usual speed of
an escalator is 0.5ms−1 and it ought never to exceed 0.75ms−1. The inclination varies
between 27° and 35° to the horizontal.
Lifts, escalators and paternosters 257
Figure 16.12 Escalator
The drive and transmission have to carry the total load on the escalator. Since people
do not stand at even and regular intervals on the whole staircase the load averaged over
the whole length of the escalator is less than the maximum load on individual treads. The
peak load on each tread is of concern to the structural designer but the electrical engineer
concerned with the power requirements can use the average passenger load taken over the
total area of exposed treads. This average can be taken as 290kgm−2.
An escalator must have a brake which has to fail safe if there is an interruption to the
electrical supply. The brake is therefore applied by a spring or a hydraulic force and is
held off against the mechanical force by an electrically energized solenoid. As is the case
with lifts, there is also provision for releasing the brake manually and handwinding the
escalator.
Since an escalator is in continuous operation there are no passenger controls, but there
must be an on/off switch which can be worked by a responsible person in charge of the
premises. It would be dangerous for the escalator to be started or stopped by someone
who could not see the people on it and the switch must be in a place from which the
escalator can be seen. Such a place is almost inevitably in reach of the public using the
escalator, who ought not to be able to work the switch, and so the switch must be a key
operated one. The British Standard on Escalators (BS 2655) requires a key operated
starting switch to be provided at both ends of the escalator.
Emergency stop switches are provided in the machinery spaces under the escalator and
also in positions accessible to the public at the top and bottom of the escalator. The
operation of any of these switches disconnects the electrical supply from both the driving
machine and the brake. The removal of the supply from the brake allows the mechanical
force to apply the brake and bring the escalator to a halt. Some escalators are fitted with a
speed governor which similarly disconnects the electrical supply from the drive and
Design of electrical services for buildings 258
brake. There are further safety devices to disconnect the supply if one of the driving
chains breaks.
Most escalators are reversible. The driving motor is a squirrel cage induction motor
and the drive is reversed by contactors which change the phase sequence of the supply to
the motor.
A travellator differs from an escalator in being either horizontal or having a very small
slope not exceeding 12°. This makes it unnecessary for it to form steps and passengers
are conveyed on a continuous platform. The upper surface of the platform must have
grooves parallel to the direction of motion which mesh with the combplates. In other
respects a travellator or moving walkway is designed in exactly the same way as an
escalator.
Paternosters
Whilst paternosters are a type of lift they also have similarities with escalators and it is
more convenient to discuss them after the latter. A paternoster is a lift which has a series
of. small cars running continuously in a closed loop. It is difficult to explain this clearly
in words but it should be clear from Figure 16.13. The cars are open at the front and
move slowly enough for people to step in and out of them whilst they are in motion, just
as they step on and off an escalator. In fact a paternoster can perhaps be thought of as a
vertical escalator. To make it safe for people to get on and off whilst the cars are in
motion the speed must be less than 0.4ms−1.
The cars are constructed in the same way as ordinary lift cars but do not have doors and
are not large enough to take more than one person each. In practice this means that the
cars are less than 1.0m×1.0m in plan. They must of course be of normal height. The front
of the floor of each car is made as a hinged flap. This ensures that if a person has one foot
in the car and one on the landing he will not be thrown off balance as the car moves up.
Since the cars move in a continuous loop they provide their own counterweight and no
additional counterweight is needed. Rigid guides are provided for the cars which have
shoes similar to those of ordinary lift cars.
In the space between cars there is a protective screen level with the front of the cars.
This prevents people stepping into the shaft in between cars. It is still necessary to make
sure that the landings and entrances are well illuminated. The cars are carried on a
continuous steel link chain. The driving machinery is similar to that of an escalator and is
always placed above the well. It includes a brake which is applied mechanically and held
off electrically, so that the paternoster is braked if the electrical supply fails. As in the
case of both lifts and escalators there is provision for handwinding.
A paternoster is started by a key-operated switch, either at the ground floor or at the
main floor if this is other than the ground floor. There are emergency stop buttons at each
floor, in the pit and in the machinery space.
Lifts, escalators and paternosters 259
Figure 16.13 Paternoster
Although they have advantages, paternosters are not used very often. They take up
rather less space than escalators but have a lower carrying capacity. We can show this by
considering an escalator with a slope of 350 and a vertical rise between treads of 230mm.
The distance along the slope between succeeding steps is 230/sin 35°=400mm or 0.4m. If
the speed of the escalator is 0.5ms−1, the steps follow each other at intervals of
0.4/0.5=0.8s. As each step can take one passenger, the carrying capacity is one person in
every 0.8s, or 75 persons per minute.
The vertical speed of a paternoster is at most 0.4s−1’ and the car height cannot be less
than 2.2m. Neglecting any gap between the cars we see that the cars follow each other at
intervals of 2.2/0.4=5.5s, and each car can take only one passenger. The carrying capacity
is thus one person every 5.5s, or 11 persons per minute.
However well constructed and carefully operated it is, a paternoster cannot help being
more of a hazard than an escalator to the elderly, the infirm and above all to small
children. This practically restricts its applications to industrial premises not needing a
high carrying capacity.
Standards relevant to this chapter are:
Design of electrical services for buildings 260
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