set value and profiling

Variable Speed Drives in Lift Systems Lutfi Al-Sharif B.Sc., M.Sc., Ph.D., C.Eng., M.I.E.E., D.B.A. Team Delivery Manager, E&M Assets, Infraco JNP Ltd. 30 The South Colonnade, Canary Wharf, London, E14 5EU United Kingdom

Published in Lift Report and Elevatori in 2001 and Elevator World (September 2001). ABSTRACT This paper provides a general overview of the use and application of variable speed drives in lift systems, discussing both current systems and recent advances in this area. Lift systems must provide good stopping accuracy and excellent levels of riding comfort. They are also characterised by their high number of starts and stops and the need for special braking systems and speed feedback mechanisms. Some general concepts in drive systems are first discussed, including the load and drive speed/torque curves and their relationship to stability; generating the reference curve; profiling of the speed curve of a lift system to achieve a smooth journey; and the use of the double cage rotors for lift system to provide better starting characteristics. A number of drive systems are discussed and analysed outlining their relative advantages and disadvantages. These drives include (in historical order): The Ward-Leonard DC system, the DC-SCR drive, the variable voltage AC system and the variable voltage variable frequency system. The methods of braking in lift systems are of particular importance. Three different methods of braking are discussed. Recent advances in lift drive systems are discussed. These include the use of slim-disk permanent magnet synchronous motors, which have eliminated the need for machine rooms for lifts. A brief overview of the futuristic linear induction motor drive is also given. Keywords: Lifts, variable speed drives, VVVF, DC, motors, linear motors, braking. 1. INTRODUCTION Variable speed drives when applied to a lift system have to satisfy a number of requirements. Characteristics of lift drives are: • Riding comfort: The movement of the lift car needs to be smooth, with limits on the values of the acceleration and jerk. The value of jerk, which is the rate of change of acceleration is particularly important, as it relates to the smoothness of the ride and its comfort. • Starting and stopping: Lift systems are characterised by the high number of starts and stops, as some systems could be rated for up to 240 starts per hour. This places special demands on the heat generated by the motor. • Leveling accuracy: As a lift stops at a landing, high precision in the accuracy of the stopping is needed. This could be as little at 1-2 mm leveling accuracy.

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• Speed feedback: Invariably, all modern systems will employ speed and position feedback to achieve good speed control, good riding comfort and good leveling accuracy. • Electrical braking: In order to control the speed accurately, electrical braking needs to be incorporated in the drive system. Various methods of braking are used in lift systems. This is in view of the fact that mechanical braking is only used as a parking brake. 2. GENERAL PRINCIPLES OF LIFT DRIVES Before introducing the types of drive systems used in lifts, there are a number of general principles that apply to lift drives, which need to be discussed. Regardless of the method of implementing a variable speed drive, these general guidelines and principles have to be followed to ensure successful operation. These principles are discussed in this section. 2.1 LOAD SPEED-TORQUE CHARACTERISTICS A speed torque characteristic is a curve which describes the relationship for a load or a drive between the speed and torque at various points. It effectively answers the question: “if the load (or the drive) is running at a certain speed, what would be the required (or supplied) torque?” Speed torque characteristics are necessary to be used in matching the load and the drive. Figure 1 shows a block diagram of a drive supplying a load, with the speed-torque curves for both shown. The drive is made up of the electric section and the motor, and is coupled mechanically to the load. Thus, the speed and torque of both should always be equal. However, there are certain conditions for stability.

Power supply

TL

TD

Electric Drive S

S

Drive

Load

Motor

Figure 1: Block diagram of the drive and the load, with s-T curves. When the s-T curve for the load is placed on the same diagram as the s-T curve of the drive, the following rules apply: • The operating point will be the intersection point of the two curves, at which the speed of both the load and the drive are equal, and the torques of both are equal.

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• The slope of the load s-T (speed-torque) curve at the intersection point should be more than the slope of the drive curve, to ensure stability. Otherwise, if the slope of the load is less than that of the drive, then any slight increase in speed, would raise the drive torque above the load torque causing the system to run-away, and any slight decrease in the speed, would lower the drive torque below the load torque and cause the system to stall. • For ideal operation, the intersection should be as near orthogonal as possible (i.e., take place at right angles). • To achieve good regulation, the drive s-T curve should be as near to the vertical as possible. This ensures that any changes in the load torque are met without any significant change in speed. An example of a stable system is shown in Figure 2, where the slope of the drive s-T curve is less than the slope of the load s-T curve at the point of intersection. A slight increase in speed will cause a reduction in drive torque and an increase in load torque which will slow the system back to its original operating point. A slight decrease in speed, would cause the drive torque to rise above the load torque and thus speed up the system and return it back to its operating point. On the other hand, Figure 3 shows an example of an unstable system, because the slope of the drive’s s-T curve is larger than that of the load at the point of intersection (TD refers to the torque supplied by the drive, and TL refers to the torque needed by the load). A slight increase in the speed, would cause the drive torque to exceed the load torque. This difference in torque will cause the system to speed up, causing a further rise in drive torque against load torque...and so on. This will cause the drive to run-away, until it arrives at a stable point. In the same way, a small reduction in speed would cause the drive torque to drop below the load torque, which will cause the system to slow down, which will further increase the difference leading to further slowing down...and so on. The system will eventually stall.

T

Operating point

T Unstable operating point

T

D

TL

T

L

T

D

S

Figure 2: Stable operating point.

S

Figure 3: Unstable operating point.

Figure 4 shows a good s-T curve set-up for drive and load, for a lift. The curves intersect at right angles, and the drive s-T curves are nearly vertical. The drive changes the s-T curve needed to achieve the required speed (s1, s2 and s3).

3

T TD2

TD1

TL

TD3 TL

Full Load

c No Load

N S1

S2

S3

S

Figure 4: Good system s-T curve for load and drive for an elevator.

Figure 5: Load speed-torque curve for a hoist, lift or escalator.

For a more detailed discussion of speed-torque curves for various loads see reference [1]. 2.2 SET VALUE (REFFERENCE VALUE) The variable speed drive is a closed loop feedback system. It will monitor the value of the speed of the lift, by monitoring the speed of the motor. It will then compare the actual value of the speed to the required value (sometimes called the reference value) of the speed, and it will drive or brake the motor according to the relative value of these two signals. In order to know what the speed should be at any point in time, a profile of the desired speed of the lift has either to be available or generated. This profile represents the value of the speed at which the lift should travel at any point in time; it does not necessarily mean that that is the speed at which it is actually traveling. The actual value from the speed feedback device represents the speed at which the lift is actually traveling. In an ideal situation, the two profiles should be identical. Inside any variable speed lift drive is a section called the reference value generator (or pattern generator). This is responsible for evaluating the reference speed that the motor (and thus the lift) has to run at, at any point in the cycle. In the earlier designs, the set value generator was situated inside the variable speed drive itself. However, in many of the more advanced systems, and in any integrated logic controller/speed controller system, the set value generator is situated within the logic controller itself. These two set-ups are discussed in more detail in the following two subsections. (a) Internal set value systems In the internal set value systems, the reference value generator is situated within the variable speed drive itself, and generates the profile based on external information. In order to achieve that, the reference value generator section has to receive signals from the shaft (or from the lift logic controller) about the various stages in the lift cycle. The rest of the data necessary to achieve the speed profile is received from the manual adjustments of the top speed, leveling speed and values of acceleration and deceleration.

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VariablespeedLift drive Lift logiccontroller

Start signal Deceleratesignal Stopsignal

Reference Value Generator Speedreference Valuepattern Topspeed

Manual Adjustment

Levellingspeed Deceleration Acceleration

Figure 6: Role of the speed pattern generator in the drive/logic controller set-up. A block diagram of the relationship between the reference value generator, the drive, the lift logic controller and the human adjuster is shown in Figure 6. Prior to running the lift, the human adjuster has to set the required parameters via a manual adjustment (potentiometers have been shown in the diagram, but any other method could be used, e.g., DIL switches). When all values have been set and the lift is ready to start, the lift logic controller will apply the main contactors, lift the mechanical brake and send a start signal to the drive, into the reference value generator. The reference value generator will then start to generate a ramp, the slope of which corresponds to the value of the set value of acceleration. When it approaches the top speed, it will keep the value of speed constant. When, the lift is near the destination floor, the logic controller sends a “decelerate signal” to the reference value generator, which in turn starts to ramp down the reference value, in accordance with the set value of deceleration, until the set value is equal to the leveling speed. The speed then stays constant while the lift is leveling and approaching the landing. When the landing switch is activated, the logic controller send a “stop signal” to the reference value generator, which in turn starts ramping down from the leveling speed to zero speed also in accordance with the value of set deceleration, until the speed of zero is reached. When the zero speed is detected, the drive usually sends a signal to the logic controller to apply the mechanical brake and release the main driving contactors.

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Decelerate signal High speed adjustment Stop signal Dec. adjustment

Acc. adjustment Start signal

Levelling speed adjustment

Figure 7: The effect of signals and adjustments on the final speed pattern generated. The effect of the signals from the logic controller and the manual adjustment are shown in Figure 7. To summarise, the shape of the curve in general is decided by the manual adjustments, while the points in time at which changes in the curve happen are decided by signals from the logic controller, which in turn are generated from feedback from the shaft switches. The signals received from the logic controller are usually implemented via volt free contacts, especially if the logic controller and the variable voltage drive have been supplied by different manufacturers. Using volt free contacts ensures good electrical isolation, and eliminates problems of interface compatibility. (b) External set value systems The internal set value systems discussed in the last sub-section, have independent logic controller and drive system. The more modern approach has been to integrate the two systems: the logic controller and the drive system. This has the advantage that the precise speed can be selected at any point in the journey, leveling 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 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 one system affecting the other. If sent in a digital format, an optocoupler 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 or the motor indicating the exact position of the lift car in the shaft. Based on this information, a value in a table is fetched which provides 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. 2.3 PROFILING As discussed earlier, lift drives have to meet strict riding comfort requirements. To achieve this, the profile of the speed curve needs to be smooth, providing acceptable levels of acceleration and deceleration, and limiting the value of jerk. Whether the speed profile is generated internally within the drive, or externally within the logic controller, there are certain

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rules and criteria which have to be adhered to in generating the profile. These are outlined in an example. The relationships between a fictitious speed profile, the distance covered, the acceleration and the jerk are shown in Figure 8. These values are for an 8 metre trip, at a top speed of 1.6 m/s. These curves are fictitious because of the sharp edges of the speed profile, which would generate high values of jerk as shown. High values of jerk are not acceptable in modern systems, as they detrimentally affect riding comfort. However, jerk is not controlled directly; but it is controlled indirectly by ensuring that the speed profile curve is smooth and filleted. Distance, speed, acceleration and jerk profiles 15

2 Speed 1.5

Acceleration

1

5 0.5

6.6

6.4

6

6.2

5.8

5.6

5.4

5

5.2

4.8

4.6

4.4

4

4.2

3.8

3.6

3.4

3

3.2

2.8

2.6

2.4

2

2.2

1.8

1.6

1.4

1

1.2

0.8

0.6

0.4

0 0

0 0.2

Distance (m) and jerk (m/s^3)

Distance

-0.5 -5 -1

Speed (m/s) and acceleration (m/s^2)

10

-10 -1.5

jerk

-15

-2 Time (seconds)

Figure 8: Relationships between speed, distance covered, acceleration and jerk for a trip (no levelling phase shown). The “filleting” effect is the responsibility of the set value generator. It is desirable to have a large radius of the speed profile curve when changing from rest to acceleration; or from acceleration to constant top speed; or from constant top speed to deceleration; or from deceleration to standstill. This is shown for the speed profile in Figure 9, where all the transitions have been filleted. The larger the radii of these transition, the lower the value of jerk and the better the riding comfort. The limiting case is when an “S” or an “inverted S” shape forms when the two curves of two consecutive transitions meet.

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Speed (m/s)

6.9

6.6

6

6.3

5.7

5.4

0

5.1

0

4.8

0.2

4.5

1

4.2

0.4

3.9

2

3.6

0.6

3

3

3.3

0.8

2.7

4

2.4

1

2.1

5

1.8

1.2

1.5

6

1.2

1.4

0.9

7

0.6

1.6

0.3

8

0

Distance (m)

Smoothed speed profile and distance

Time (seconds)

Figure 9: Speed profile including the filleting effect. The speed profiles in this sub-section have been shown for profiles which do not have a leveling phase; however, the same principles apply in these profiles as well. 2.4 DOUBLE CAGE INDUCTION MOTORS FOR LIFTS Lifts require high starting torques, and for this reason the standard squirrel cage motor is not suitable. The speed torque characteristic of a standard induction motor can be improved by using two cages, one with high rotor resistance and one with low rotor resistance. By adding the two speed torque characteristics of both, a high starting torque characteristic results, as shown in Figure 10. All AC lift motors today are double cage rotors. The use of variable voltage variable frequency drives might change the need for double cage rotors, as these drives provide good starting torque characteristics even from standard motors. Speed torque characteristic for double cage rotor induction motor

Total speed torque

Torque

Low resistance cage

High resistance cage

Speed

Figure 10: Speed-torque characteristic for a double cage rotor induction motor. 8

3. OVERVIEW OF DRIVE SYSTEMS This section provides an overview of the main types of drive systems used today. A more detailed overview of variable speed lift drives is given in [2]. 3.1 TWO SPEED SYSTEMS Although 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 1970’s and 1980’s. They effectively employed direct on line starting, and thus needed flywheels attached to the motor to smooth the movement and reduce jerk. Two speed systems rely on the fact that the motor has two sets of windings, embedded in the stator, both interacting with the same rotor. The ratio of speeds in these systems is usually either 4:1 or 6:1. Most motors have the following pole number combinations: 6:24, 4:16; 4:24. On a 50 Hz supply, these would correspond to synchronous speeds of: 4 poles: 1500 rpm 6 poles: 1000 rpm 16 poles: 250 rpm. 24 poles: 167 rpm These in turn correspond to the ratio of 4:1 or 6:1, thus achieving low speeds 25% or 16.6% of the top speed. 3.2 DC WARD LEONARD SYSTEMS The first type of variable speed drive ever used on lifts was the so-called Ward-Leonard system. Ward-Leonard systems employed a prime mover, originally in the form of a DC motor, when the supplies were DC. As power supplies became AC, the prime movers have become invariably AC motors. The prime mover is mechanically coupled to a DC generator. This combination is called the motor-generator set (or MG set for short). The MG set can either be one integrated unit (with one mechanical frame), or two completely separate machines, connected via a rigid or flexible coupling (for more details see [3] and [4]). A block diagram of a Ward-Leonard system is shown in Figure 11, which has an AC motor as a prime mover controlled by a star-delta starter. This is mechanically coupled to the DC generator, which in turn is electrically coupled to the DC hoist motor. The generator has a shunt field, which is usually controlled by the variable speed controller, and the motor also has a shunt field, which usually has fixed excitation.

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R

S

T

Motor field controller

Generator field controller Star-delta starter

M

G

M

Coupling

AC

DC

Figure 11: Block diagram of a Ward-Leonard control system. As shown in Figure 11, the left side of the diagram is basically AC based, and the right side of the diagram is DC based. However, the field excitation for both DC machines, will be rectified from an AC source, for convenience. At the heart of the system lies the generator field controller, which is effectively a variable speed drive controller. 3.3 DC SCR By abolishing the use of an MG 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 2 quadrant operation. An example of this configuration is shown in Figure 12.

R S T

M

Figure 12: Fully controlled bridge rectifier. The other form, uses two bridges in parallel, each connected to drive the motor in the opposite direction to the other. By using both bridges, the motor can be operated in both driving and

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braking modes, in forward and reverse directions (i.e., four quadrant operation). This set-up is shown in Figure 13.

R S T

M

Driving bridge

Braking bridge

Figure 13: Double bridge converter.

3.4 VARIABLE VOLTAGE ACVV These systems were widely used in the mid 80’s early 90’s. They are very simple in the method of operation. They rely on the a three pairs of back to back thyristors for varying the stator AC voltage, as shown in Figure 14. A detailed description of this type of drive is discussed in [5], [6], [7], [8], [9], [10], [11], [12] and [13]. R

4

S

1

6 3

T

2

5

Figure 14: The use of three pairs of back to back thyristors to control the voltage to the induction motor. The principle of operation of the AC variable voltage (ACVV) variable speed drive is best understood by examining Figure 15. It assumes the use of a double cage motor. By varying the firing angle the stator voltage is varied and a new speed torque curve results.

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Speed torque characteristic for a lift motor at various values of voltage. 2.1 2 1.9 1.8 1.7

380V

Ratio of torque to rated torque

1.6 1.5 1.4

320V

1.3 1.2 1.1 1

280V

0.9 0.8 0.7 0.6

225V

0.5 180V

0.4 0.3 0.2

140V

0.1 1

0.95

0.9

0.85

0.8

0.75

0.7

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0

0

Ratio of speed to synchronous speed

Figure 15: Speed torque characteristics for a lift motor (note lines showing possible range of speed control at 0.4 rated torque). By inspecting Figure 15, it can be seen how this method of speed control works successfully with this speed-torque characteristic. Let us say the torque required was 40% of rated torque (this could be because the car is only 20% full of passengers, which is not an unusual occurrence). If we draw a horizontal line through the 40% torque point, this intersects 5 of the curves, all at negative sloping points. This achieves speeds of 0.47, 0.85, 0.94, 0.96, 0.98 of synchronous speed, which is a wide range of control. However, it has to be borne in mind that when low speeds are required at low torques, this is not possible and the system becomes unstable. For these reasons, the performance of variable voltage systems at leveling is not as good and stable as more advanced systems (e.g., variable voltage variable frequency or DC). 3.5 VARIABLE VOLTAGE VARIABLE FREQUENCY (VVVF) The most widely used system today is the variable voltage variable frequency system, usually referred to as an inverter drive. The principle of operation relies on a rectifier to produce DC into the so-called 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. This is illustrated in Figure 16.

12

T

o

r q

uf 3e

f

2

f

1

f

0

S

p

e

e

d

Figure 16: Principle of speed control in the variable frequency drives.

3.6 DISCUSSION ON DRIVES The two speed system is rarely used these days. Although it is simple to operate, it is not smooth in operation, and its leveling is very load dependent. The use of a flywheel makes it inefficient. Hyrdaulic systems (which have not been discussed here) are used for smaller installations. If a DC drive is needed, then DC SCR are usually used (as opposed to the bulky MG set). DC drives have good speed torque characteristics, and are stable at high loads. However, DC motors are maintenance intensive, as the brushes need special attention. The variable voltage AC system, although widely used in the 80’s and early 90’s, is now much less used in favour of the VVVF systems. VVVF systems offer much better speed and torque control, and their prices have come down significantly. Generally, the cost of DC motors is nearly 50% more than AC motors, and in view of the increased maintenance costs of DC motors, they are used less in favour of AC motors. Thus the predominant drive nowadays is the VVVF drive. As for energy efficiency and energy consumption, the VVVF drive is the most efficient, while the two speed drive is the least efficient, due to the losses in the flywheel. For methods of calculating the energy consumption of lift drives, see [14].

4. BRAKING METHODS In traditional vehicle applications, braking is achieved by using a hydraulically operated mechanical brake (e.g., as in a car). In lifts, electrical braking is used on the motor, and the mechanical brake is only used as a parking brake. In this section, some methods of electrical braking in AC variable voltage lift drives are discussed, as an example. 4.1 PLUGGING The second most widely used method of braking is plugging, which involves applying the reverse phase sequence to the winding, as if trying to reverse the motor rotation. A typical set-up is shown in Figure 17. The motor will act as a generator and will push energy back into the main supply while slowing down. This is quite an effective method, especially that it can be applied to a single winding motor. The braking energy is returned to the mains and is not dissipated as heat. However, care has to be taken not to switch on any of the reverse sequence

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thyristors until the forward sequence thyristors have ceased to conduct; otherwise a short circuit will result, which would damage the thyristors and trip the electrical protection. For these reasons, plugging systems will invariably have a zero current detector fitted in the path of the motor current to check that the current has dropped to zero before reversing the phase sequence. The plugging method suffers from the disadvantage that it can inject high values of current in the rotor, and rotor bars have been known to rupture due to the high currents induced in the rotor bars. The main advantage of this method is that it can achieve a braking torque even at standstill, as opposed to the DC injection braking method.

R

4' 1'

S

4 1 6 3 2

T

5 2' 5'

Figure 17: The use of 5 pairs of back to back thyristors to drive and brake the motor.

4.2 EDDY CURRENT BRAKING Another method of braking which is not widely used is the so-called Eddy current braking method [9]. The term eddy current braking is sometimes incorrectly used to describe DC injection braking, which confuses it with this method. “This method is used to obtain braking torque from the eddy current brake which is attached on one end of the motor shaft. It has the characteristics of the DC dynamic brake. The braking torque, which is substantially good enough at longer speed ranges, becomes zero when the speed becomes zero [9].” This method needs a complicated non-standard motor, cannot produce any torque at standstill and dissipates all the heat in the machine, rather than returning it to the supply.

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4.3 DC INJECTION BRAKING The most widely used method to achieve electrical braking is to inject a DC voltage into the low speed winding of the motor, as shown in Figure 18. This method has mainly developed because many of the variable voltage system, when they first appeared where retrofitted on sites which had two speed AC motors on them. It was easy then to use the low speed windings for braking, by injecting a DC current in the low speed 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 which is always pointing in one direction. This will have a braking effect on the moving rotor, trying to bring it to the synchronous speed of DC (i.e., which is 0 rpm). The disadvantages with this method is that the energy is dissipated as heat in the windings, and that braking torque approaches zero at the speed approaches zero, making it difficult to brake the motor at standstill.

R

4

S

T

1 6 3 2

5

Figure 18: Diagram of a variable voltage variable speed system using a two speed induction motor and DC injection braking in the low speed winding. 5. FUTURE DEVELOPMENTS Several developments are taking place in the field of lift drives. The three main developments are the linear induction motor drive, the compact disk drive and the Odyssey system.

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5.1 LINEAR INDUCTION MOTOR The concept of the linear induction motor has been applied in trains and other transport systems. The principle of the linear motor is based on the normal induction motor. If the stator and rotor of an induction motor are cut and opened up, then a linear motor results, as shown in Figure 19.

Linear rotor Rotor Linear stator

Stator

Figure 19: Principle of operation of a linear induction motor. The first prototype appeared in France, by Otis, and has a tubular induction motor mounted on the counterweight. This has the advantage of eliminating the machine room. The rope travels at the top of the shaft over an idler sheave. Figure 20 shows the arrangement of a linear induction motor mounted on the counterweight. Figure 21 shows an arrangement of a indirect acting linear induction motor.

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Idler sheave

Fixed rotor

Counterweight & Stator

Car

Stator

Fixed rotor

Car

Figure 21: Indirect acting linear induction motor.

Figure 20: The use of a linear induction motor mounted on the counterweight.

If the linear motor can be mounted on the car itself, then the counterweight can be dispensed with as well as the ropes. This becomes a truly ropeless drive. This opens up the possibility of moving all lifts in one shaft in the up direction, and in another shaft for the down direction. Cars, when arriving at the top, move horizontally to the top of the other shaft. One of the main problems with linear motor drives, is maintaining the gap between the rotor and the stator, in the face of the enormous attractive forces between them. This effect leads to two problems [15]: • The gap is not maintained at a constant value, leading to changes in performance and speed. • The attractive forces leads gradually to the loosening of the fixing bolts. This problem can be overcome by placing rollers on the counterweight which run on the reaction plate. It is also possible to use a tubular motor on the counterweight instead of a flat motor.

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5.2 THE COMPACT DISK DRIVE Although not a linear drive, this drive represents an important development. KONE were the first company to develop a system which they called the Eco-Disk. This is effectively a compact drive, which does not use a gearbox, and which is so slim, that it can be fitted in the space between the guide rail and the wall of the lift shaft. Since then, other companies have developed similar drives. This motor is very similar in operation to the synchronous machine. A permanent magnet is used to provide a constant excitation magnetic field for the rotor. The stator produces a constant rotating magnetic field. The rotor attempts to align itself with the rotating magnetic field. It runs at exactly the same speed as that of the rotating magnetic field, hence the name of the motor. Depending on the output torque from the motor, an angle of lag forms between the rotor magnetic field and the direction of the stator rotating magnetic field. This principle of operation can also be compared to the DC motor. However, this form of permanent magnet DC motor is in the shape of a disk, as shown in Figure 22. Permanent magnets are affixed to the rotor, while the stator has a toroidal winding wound around a set of laminations. The sheave is part of the rotor. Annular stator laminated core Stator windings Rotor Sheave Stator

Permananent magnet(s)

Figure 22: Principle of operation of KONE's Eco-Disk. The compact disk can be fixed to the rails. It can either be fitted at the top of the shaft (as shown in Figure 23) or at the bottom of the shaft (as shown in Figure 24). More details can be found in [16], [17] and [18]. The system is described in detail in the European Patent [19].

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Compact Disk

Counterweight Car

Counterweight Car

Guude Rail

Compact Disk

Guude Rail

Figure 24: Lower driving arrangement for KONE eco-disk drive.

Figure 23: Upper driving arrangement for KONE's Eco-Disk.

5.3 THE ODYSSEY The Odyssey is a new system developed by OTIS. It is based on a concept which addresses the horizontal as well as the vertical transportation of building occupants. Lifts move in the shaft sideways as well as up and down. The system is described in more detail in [20]. 6. CONCLUSIONS AND DISCUSSION Variable speed lift drives require smooth movement to provide riding comfort, good leveling accuracy, and a high number of starts and stops. For these reasons, they generally employ speed and position feedback, and they require special electrical braking systems. These requirements dictate the design of these systems. Four general principles have been discussed. The speed-torque curves for the load and the drive have to be matched to achieve stability at all speeds. The reference value is either generated inside the variable speed drive of a lift system, or is fed externally from the lift logic controller. Whichever method is used, the profile has to be smoothed, to ensure that the level of jerk (rate of change of acceleration) is kept to a minimum value. Double cage rotor induction motors are invariably used in lift systems to achieve good starting torques and enable good speed control using variable voltage systems. Two speed systems use an induction motor with two sets of windings, which have a suitable ratio to achieve high and low speeds. These are not used anymore, due to the rough ride and bad leveling accuracy. DC systems used to employ a motor-generator set feeding a DC hoist motor, with the controller controlling the generator field excitation. These system are being replaced by DC SCR drives, which do away with the motor-generator sets, and vary

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the motor armature using power electronics. AC variable voltage drives employ three pairs of back to back thyristors to vary the stator voltage of an AC induction motor, but these systems are being replaced by the more modern variable voltage variable frequency (VVVF) systems which provide much better torque and speed control and which are becoming more competitively priced. Three methods of electrical braking in AC variable voltage systems are the methods of plugging, DC injection braking and Eddy current braking. Future developments in drives feature the use of linear motors. These are used in roped configurations, and promise a future for lifts with no ropes nor machine rooms. The compact disk lift is a very slim disk which can drive the lift car without the need for a machine room or a gearbox. And the Odyssey system is a transport system which attempts to integrate vertical and horizontal transport in a building. REFERENCES [1] Shepherd, W., Hulley, L.N. & Liang, D.T.W., 1995, “Power electronics and motor control”, Second Edition, Cambridge University Press. [2] Barney, G.C. & Loher, A.G., 1990, “Elevator Electric Drives: Concepts and principles, control and practice”, Ellis Horwood. [3] Leroy Somer, 1989, “Motor-Generators”, 31/10/1989. [4] Bull Electric Elevator Systems, “Motor Generator sets”, Data sheet number MG P511/2863 APS. [5] Al-Sharif, L.R., 1999, “Variable Voltage AC drive systems”, Lift Report, November/December 1999. [6] Fukuda, T., 1978, “AC feedback control in Japan: Part I”, Elevator World, Apr. 1978. [7] Fukuda, T., 1978, “AC feedback control in Japan: Part II”, Elevator World, May. 1978. [8] Fukuda, T., 1979, “AC feedback control in Japan: Part IV”, Elevator World, Jan. 1979. [9] Fukuda, T., 1979, “AC feedback control in Japan: Part V”, Elevator World, Feb. 1979. [10] Fukuda, T., 1979, “AC feedback control in Japan: Part VI(a)”, Elevator World, Mar. 1979. [11] Fukuda, T., 1979, “AC feedback control in Japan: Part VI(b)”, Elevator World, Jun. 1979. [12] Fukuda, T., 1979, “AC feedback control in Japan: Part VI(c)”, Elevator World, Aug. 1979. [13] Fukuda, T., 1980, “AC feedback control in Japan: The modification of AC feedback elevators in Japan”, Elevator World, Jun. 1980. [14] Al-Sharif, L., 1996, “Lift & Escalator Energy Consumption”, CIBSE/ASHRAE Joint National Conference, 1996, Harrogate, United Kingdom. [15] Tibbitts, J., 1995, “Linear motor drive systems”, Lift Report, Issue 4, July/August 1995. [16] -, 1996, “Breakthrough “Monospace” elevator concept”, in Elevator World, May 1996. [17] -, 1996, “An elevator go round”, in Elevator World, January 1996. [18] KONE, 1996, “Wheel of fortune”, in News & Views, In-House Magazine, Issue 1/96. [19] European Patent Office, 1995, “European patent application, Publication number 0 688 735 A3, Application number 95109094.3, Elevator machinery and its installation”, Date of publication of application 27/12/1995. [20] Sturgeon, W., 1996, “Odyssey: The introduction”, in Elevator World, November 1996.

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