Adjustable Speed Control

of A-C Motors L. R. FOOTE

ASSOCIATE M E M B E R A I E E

Description of α drive that uses saturable re­actors and a wound rotor motor to provide ad­justable constant speed performance comparable to d-c variable voltage drives. A line contactor is used to connect the system to the power line. All regulating components are static. Motor cur­rents are controlled in all three phases for motor

heating.

THE DRIVE described in this article was developed specifically to meet the requirements of a hoist drive. Use will not be limited to hoists. A-c motor

speed control has been achieved with essentially bal­anced primary voltage control. Control is effected through three saturable reactors, one of which has two power windings and is, therefore, a saturable trans­former. Reactor excitation is controlled by magnetic amplifiers. Static regulating action provides forward or reverse phase sequence for driving or retarding torque as required to hold any desired speed. The rotor circuit consists of two sets of resistors and a closed iron nonreg-ulated reactor. This provides a relatively uniform maxi­mum torque at all speeds. Performance has been proved on a test tower as well as on a dynamometer.

This is believed to be an improvement on previous a-c motor adjustable speed drives. Only three primary reactors provide essentially balanced, static, reversing control of primary voltage. Novel use of a saturating transformer makes this possible. The transformer also compensates in part for reactor drop. The simple three-element control provides low cost and simple installa­tion. Continuous speed adjustment through zero is pro­vided to set down hoist loads with minimum shock.

E L E C T R I C A L DRIVE REQUIREMENTS FOR HOISTS

O B V I O U S L Y , the basic drive requirement is to raise and lower a hook. Speed control is required to position ac­curately and to set a load down gently. Usually, the weight of load lifted should not vary the speed apprecia­bly. The load should be smoothly accelerated from a suspended position so that a smooth hoisting action results when the holding brake releases. Drive must change speed smoothly without shock or speed over­shoot. Automatic torque reversal may be required to break static friction and to accelerate an empty hook down.

L. R . Foote is with the General Electric Co., Roanoke, Va!

T h e author wishes to express appreciation of the assistance provided by his associates, P. L . Alger and P. A. Vance, in consulting, advising, and providing parts from which the test equipment was assembled.

In addition to these performance requirements, the type of construction is important. The control must be as static as possible to reduce maintenance. The number of wires between controller and trolley is limited by collector shoes. Equipment must resist vibration. Stand­ard motors should be used.

SYSTEM SELECTION

A STANDARD a-c wound rotor motor drive is desirable for several reasons. Most important is that the commu­tator has been eliminated. An a-c motor costs less than a d-c motor. A d-c power supply is not required.

Saturable reactors provide static, stepless control of motor current. The reactors may be used to control motor primary or secondary current. Primary current control was selected so that phase reversal could be used. Phase reversal is required to obtain reverse torque below synchronous speed. The reactors control both the phase sequence of the applied voltage and the magnitude of the resulting current. This eliminates reversing con­tactors. The unique arrangement of saturable reactors provides better current balance than 2^phase control, ; full reversing torque, and maintains the simplicity and economy of only three saturable reactors.

The motor rotor external resistance must be changed with speed to obtain high torque at all speeds. T o elimi­nate rotor circuit contactors a 3-phase reactor using a closed iron circuit and no control winding was used with two sets of resistors. The reactor inherently causes a low resistance circuit to predominate at high speed and the high resistance circuit to predominate at lower and plugging speeds.

The motor supplies both hoisting and braking torque so that the only accessory is a holding brake.

OPERATION

D E V I C E NAMES in the following description correspond with the Fig. 1 schematic diagram.

The rotor circuit is designed to produce necessary torque at all speeds with full stator voltage. The re­sistors, Rl, are selected to produce maximum torque around stall. Resistors R2 have about 40% the ohms in Rl. R2 alone in the rotor circuit would provide good torque at high speeds but not about stall and at nega­tive speeds. The closed iron reactor impedance makes R2 predominant at high speeds and Rl predominant at all other speeds. The motor plus rotor circuit thus requires only control of primary voltage phase rotation and magnitude for speed regulation.

As stated previously, three saturable reactors provide control of phase rotation and magnitude of the applied

840 Foote—Adjustable Speed Control of A-G Motors E L E C T R I C A L ENGINEERING

P O W E R CIRCUIT

| L 2 1 L 3

P A - P R E - A M P L I F I E R H H - H O I S T T O R Q U E A M P L I S T A T L A - L O W E R T O R Q U E A M P L I S T A T

M - L I N E C O N T A C T O R MX-UUE R E L A Y B - B R A K E C O N T A C T O R

F i g . 1. P a r t i a l s c h e m a t i c .

motor voltage. These reactors are labeled SX-1, SX-2 and SXT. In Fig. 1, SXT has two gate windings, SXT-P and SXT-S. This arrangement of saturable reactors is most clearly explained by building on more simple ar­rangements. Therefore, consider three separate satur­able reactors as in Fig. 2 A . It is obvious that if the direct-current control fields are not excited the only current in the motor will be the insignificant reactor magnetiz­ing current. Fully exciting the saturable reactors will apply nearly full voltage to the motor. Intermediate values of d-c excitation permit adjustment of motor torque to any intermediate value.

Now consider that any two of the three saturable reactors in Fig. 2 A are replaced with a single saturable reactor with two duplicate a-c power windings wound together. As before, the a-c power applied to the motor may be varied from almost zero to a maximum by the d-c current in the saturable reactors. Since the saturable reactor with the two windings is a transformer with zero d-c exciting power, the induced voltages across the two windings must be equal in magnitude and in phase op­position. Fig. 2 B indicates the two winding reactor. Fig. 2 C vectorially represents reactor voltages for connection shown in 2 A . Fig. 2 D shows how the vectors change when windings A and Β are placed on a common core. Saturating the reactors with d-c excitation changes the vectors as shown in Figs. 2 E and 2 F .

If vectors Β and C in Fig. 2 D can be increased in length, the phase rotation at the motor will be reversed. Fig. 2 G shows the motor side of reactor C connected to line L2. The phase reversal is shown in Fig. 2 H . Vector C is full line voltage and so is vector B. The motor will

run reverse. Refer now to the drive schematic, Fig. 1. Applying full d-c excitation to SX-1 causes the a-c wind­ings to have low impedance. This is similar to Fig. 2 G . Saturating SX-1 and SX-2 produces voltage vectors per Fig. 2 J . Thus, the voltage at the motor changes from Fig. 2 H to Fig. 2J—from full-phase forward, to off, to full-phase reverse. Reactor exciting currents correspond­ingly change from SX-1 and SX-2 saturated for full for­ward, to no exciting current for full off to SX-1 and SXT saturated for full reverse. Thus, full-forward to full-reverse torque control is obtained by regulating re­actor exciting currents.

SX-1 is not connected quite on the end of SXT-P in the schematic diagram. By connecting to a tap rather than the end of SXT-P, autotransformer action com­pensates for the small saturated drop in SX-1 and raises maximum torque.

Although the circuit is not symmetrical, there is re­actor control of current in each of the three phases. The currents through each of the two power windings on SXT, and through SX-2 are equal for balanced motor currents with SXT and SX-2 saturated. With SX-1 and SX-2 saturated, SX-1 current will be three times motor line current times the autotransformer ratio for bal­anced motor currents. Thus, SX-1 current and S X T current are in fixed ratio to SX-2 current. These rela­tions inherently provide balanced phase currents, if the saturable reactors are proportionately excited. The reactors are designed to work together so that the same exciting current in SX-2 and SXT or SX-1 maintains essentially balanced load currents.

With the aforementioned basic concepts established,

T H R E E S A T U R A B L E R E A C T O R S

T W O R E A C T O R S W I N D I N G S - B t C ON

C O M M O N C O R E LI Q o L 2 ο L 3

T W O R E A C T O R S R E V E R S E P H A S E

P R O M " Β " ) L I 0 L 2

V E C T O R V O L T A G E S F O R F I G . A

N O D C E X C I T I N G -C U R R E N T

L 2

V E C T O R V O L T A G E S F O R F I & . A

F U L L D C

V E C T O R V O L T A G E S F O R F I G . Β

N O D C E X C I T I N G C U R R E N T

C « L 3 M O T O R

A

F e J

V E C T O R V O L T A G E S F O R F I G . Β

F U L L D C

V E C T O R V O L T A G E S F O R F I G . G

B f c C R E A C T O R N O D C " A " R E A C T O R S A T U R A T E D

L 2 1 Β

\ T 3 T2/

' A

» L l

L2ft«

I

V E C T O R V O L T A G E S F O R F I G . t t l .

N O D C

B j M O T O R

F i g . 2 . V e c t o r e x p l a n a t i o n o f r e v e r s i n g a c t i o n . T h e s e s k e t c h e s a r e o n l y

e x p l a n a t o r y a n d a r e n o t a c c u r a t e in m a g n i t u d e , d i r e c t i o n , o r b a l a n c e .

AUGUST 1 9 5 9 Foote—Adjustable Speed Control of A-C Motors 841

LI LZ L 3 L I L X

S X - 2

F i g . 3 . T h e " w h y " a n d " h o w " o f stat ic r e v e r s i n g c o n t r o l . P i c t o r i a l l e n g t h o f r e a c t o r w i n d i n g is p r o p o r t i o n a l to i m p e d a n c e o f w i n d i n g .

F i g . 4 . P h o t o e l e c t r i c r e c o r d i n g o f s p e e d r e s p o n s e o n test t o w e r .

SX-1 and SX-2 are proportionately excited lor forward torque. SXT and SX-2 are proportionately excited for reverse torque. Speed regulation can be accomplished with any of many well-known servomechanism tech­niques. Power amplistats are shown and were used for the tests. The hoist amplistat, HA, excites SX-1 and SX-2. Lower amplistat, LA, excites SXT and SX-2. The amplistats are excited by the difference between a master switch reference voltage and a tachometer feedback. It is interesting that the steady-state characteristic of the reactor drive produces an increasing torque with in­creasing speed at some fixed value of reactor excitation. This is similar to a rising speed regulation in a d-c motor generator loop which normally requires a very fast regu­lator. However, the increase in torque with speed in the a-c system is dependent on rotor frequency and therefore speed. The situation is, therefore, quite dis­similar to an over-compounded d-c system and presents no particular challenge to the regulator.

TEST PERFORMANCE

M A X I M U M TORQUES and load regulation are shown in Fig. 3. Maximum down torque is only about 7 5 % at zero speed because a reduced size lower amplistat was used to produce down torque. With a full size lower amplistat, maximum torque in the down direction is only slightly less than in the up direction. The differ­ence is due to the au to transformer compensating action.

Fig. 3A shows maximum torque-lowering. In this position, SXT and SX-2 are saturated and, therefore, have very low impedance. This allows maximum flow of the a-c current from LI to Tl, L2 to T2, and L3 to T3.

Fig. 3 Β shows zero torque. When SXT, SX-1, and SX-2 are unsaturated, they present maximum imped­ance to the current flow in all three lines. This means that for all practical purposes, there would then be zero voltage at the motor terminals and motor torque would be zero.

Fig. 3C shows the maximum torque-hoisting position. In this position, SX-1 and SX-2 are saturated and pre­sent a very low impedance. Since SX-1 and SX-2 are

Fig. 5 . S p e e d - t o r q u e c u r v e s f o r I C 7 4 0 6 c r a n e c o n t r o l .

842 Foote—Adjustable Speed Control

F i g . 6 . B r e a d b o a r d s e t u p a n d test m o t o r . T o w e r is v i s i b l e t h r o u g h d o o r .

of A-C Motors ELECTRICAL ENGINEERING

F i g . 7 . P r o d u c t i o n v e r s i o n o f h o i s t c o n t r o l . F i g . 9 . R o t o r c i rcui t r e a c t o r s h o w i n g t a p s f o r o p t i m u m a d j u s t m e n t .

effectively short circuits as a result of saturation, L2 is connected to Τ 3 and LI is connected to TL SXT-P, the transformer primary, has L2-L3 voltage. Since SXT-P and SXT-S have a one to one turns ratio, SXT-S also has voltage equal to L2-L3. Τ2 voltage becomes the same as L3. Notice that the voltage sequence L1-L2-L3 is now applied to T1-T3-T2, compared with T1-T2-T3 in the lowering position. This is a phase reversal and, therefore, gives a torque reversal.

Load regulation is better than required for hoist op­eration. The increase in load regulation for down torque is a function of the components used and is not basic. High gain is not necessary for down torque in normal hoist operation.

Safety requires that the minimum hoist speed be high enough so that 125% load will not lower the hook. Similarly, an empty hook must lower on the minimum lower speed. This creates a range of speeds between minimum hoist and minimum lower that has been tra­ditionally unobtainable. Continuous speed adjustment

between these points with a thumblatch on the master switch provides minimum landing speeds.

Response was rapid enough to permit smooth hoist­ing acceleration of loads suspended in air by a holding brake. Fig. 4 shows photoelectric tracings of speed vs time.

These tracings were taken on the test tower and show the response to step changes of the master switch. The test drive had more inertia than would be normally encountered because of safety brakes and oversize coup­lings on test gear. Zero speed was taken with a holding brake and suspended load. These data show that the load hoisted without dropping or hesitating from a suspended condition. It is also apparent that speed over­shoot has been damped so torque oscillations do not occur in the gears. The response is not as fast as is usually experienced in the d-c system. However, in practice, the response has been fast enough to give the operator the illusion of increased torque over time ac­celeration resistance control.

Communication via Rope A special technique developed by The Colorado Fuel

and Iron Corporation utilizes the wire hoisting rope to carry electrical conductors, embedded in the fiber cores for protection against damage. It makes possible zinc socketing of wire hoisting rope without damaging the enclosed electric conductors. These are then brought out through the basket of the socket and suitable term­inal connections made. At the other end of the wire rope, where it is attached to the hoisting drum, the con­ductors terminate in slip rings. The electric current is picked off the slip rings by carbon brushes and then

F i g . 8 . P r i m a r y r e a c t o r s sx-i, sx-2, S X T . conveyed to transmitting and receiving equipment.

AUGUST 1959 Foote—Adjustable Speed Control of A-C Motors 843