GB2175157A - Induction motor drive circuits - Google Patents

Abstract

A drive circuit for an induction motor with a pair of antiphase windings (3,4; 5,6; 7,8) associated with each phase of the motor has a d.c. power supply, and a switching arrangement (T3-8, DF3-8) supplying each winding in only one direction, controlled by a frequency control circuit. The switching arrangement comprises a bridge circuit having a first transistor (T3; T5; T7) connected between one supply rail and a first end of one winding (3; 5; 7) of the winding pair, and a second transistor (T4; T6; T8) connected between the other supply rail and a first end of the other winding (4; 6; 8) of the winding pair of the same polarity as the first end of the one winding. Thus only two transistors rated at the supply voltage are required for each phase of the motor. <IMAGE>

Description

SPECIFICATION Induction motor drive circuits This invention relates to drive circuits for induction motors, such as squirrel cage induction motors (SCIM) orwound rotor induction motors.

Reference is made to the Applicant's U.K. Patent Application No. 2,1 50,772A which describes an asymmetrical halfbridgearrangementforcontrolling a bifilar(orpartial bifilar) wound squirrel cage induction motor.

Reference is also made to U.K. PatentSpecifications Nos. 1,048,682, 1,167,320 and 1,385,205 which disclose inverter-motor configurations substantially of the type shown in Figure 1 of the accompanying drawings. In this arrangement there is associated with each phase ofthe three-phase motor a respective pair ofbifilarwindings 1, 2 connected in antiphase. A respective thyristor Sa, Sb, Sc, Sd, Se or Sf is connected in series with each winding 1 or 2, and thesethyristors are switched by a control circuit (not shown) so asto causethewindings of each winding pairto conduct current alternately and in opposite directions at a frequency which may be varied so as to varythe speed of the motor.In orderto prevent the occurrence of large voltage transients on switching of the windings, a respective diode Da, Db, Dc, Dd, De or Df is connected across each ofthethyristors so asto provide a path for the current induced in one winding of a pair by rapid turning off of the current in the other winding ofthe pair.

However, the arrangement of Figure 1 suffers from two serious disadvantages. Firstly leakage inductance between bifilarwindingscan cause high overvoltages atthyristor switch-off, and secondly, when one winding of a pair is conducting, a voltage will be induced in the other winding such that the associated switching device off voltage is twice the supply voltage. This means that switching devices rated at twice the supply voltage are required in such a configuration, resulting in poor device utilisation.

These disadvantages are overcome by the asymmetrical half bridge arrangement of Application No.

2,1 50,772A in which there is associated with each of the sixwindings a half bridge, as shown in Figure 2 of the accompanying drawings, comprising two semiconductor switches T1 and T2 connected to opposite ends of the winding 3, and two freewheeling diodes Df1 and Df2 also connected to opposite ends ofthe winding 3. Supply of currentto the winding 3 is obtained by simultaneous switching ofthe switches T1 and T2. The diodes limit all device voltages to the supply rail voltage, and winding leakage energy at turn-offisfed by the diodes into the supply. No overvoltage spikes occurwith such an arrangement.

However, the arrangement of Application No.

2,1 50,772A has one main limitation. It uses a total of twelve semiconductor switches (powertransistors or thyristors) whereas the configuration of Figure 1 uses only six switching devices.

It is an object ofthe invention to provide a novel form of induction motor drive circu it which is based on this arrangement butwhich can be configured to avoid the above-mentioned limitation.

According to the present invention there is provided a drive circu it for an induction motor having a pair of windings associated with each phase ofthe motor, with thewindings of each pair being connected in anti-phase,the circuit comprising supply means for connection to a d.c. supply, respective switching means associated with each winding pairfor supplying currentfrom the supply to each winding in only one direction, and frequency control means for switching the switching means so as to cause the windingsofeachwinding pairto conduct current alternately and in opposite directions at a frequency which may be varied to varythe speed ofthe motor, wherein the switching means associated with each winding pair comprises a bridge circuit having a first switching element for connection between one supply rail of the supply means and a first end of one winding ofthewinding pair, and a second switching element for connection between the other supply rail of the supply means and a first end ofthe otherwinding of thewinding pairofthesame polarity as the first end of said one winding, so that only two switching elements are associated with each phase ofthe motor.

Accordingly,where such an arrangement is usedto drive athree phase motor, only six switching elements are required, and furthermore these switching ele mentsneedto be rated on Iy at the su pply voltage.

In a preferred form ofthe invention each bridge circuit further incorporates a first freewheeling diode coupled between said other supply rail and the connection point of the first switching element to said one winding, and a second freewheeling diode coupled between said one supply rail and the connec tion pointofthe second switching elementto said other winding, whereby each each freewheeling diode provides a path forfreewheeling motor current when the associated switching element is switched off. This prevents the occurrence of voltage spikes due to leakage.

In orderthatthe invention may be more fully understood, reference will now be made to the remaining figures ofthe accompanying drawings, of which: Figure 3a is a simplified circuit diagram of a drive circuit in accordancewiththeinventionfora starconnected induction motor; Figure 3b is a simplified circuit diagram of a drive circuit in accordance with the invention for a deltaconnected induction motor; Figure 4a represents a modification ofthe circuit of Figure 3a; Figure 4b represents a modification ofthe circuit of Figure3b; and Figure4crepresentsa modification of the previous -i rcu its fo r a star-delta configuration motor.

Referring to Figure 3a a SCIM motorcomprises three pairs ofbifilarwindings 3,4,5,6 and 7,8 connected in a star-configuration as shown. Each winding 3,4,5,6,7 and 8 is connected to a respective terminal A3, A4, A5, A6, A7 and A8 of a drive circuit at one end of a determined polarity, and the other ends ofthe windings ofthe opposite polarity are connected in common. Each terminal A3, A4, A5, A6, A7 and A8 is connected to the common connection point of a respective switching transistor T3, T4, T5, T6, T7 and T8 and a respective freewheeling diode Df3, Df4, Df5, Df6, Df7 or Df8, each switching transistor and the associated freewheeling diode being connected in series across the supply rails.

A control circuit (not shown) serves to switch the switching transistors on and offsuch that each pair of transistors T3, T4 o rT5, T6 orT7, T8 associated with each winding pair is switched sothatthetransistors are turned on and off alternately so as to cause the windings of each winding pairto conduct current alternately and in opposite directions at a frequency which may be varied by the control circuit so as to vary the speed ofthe motor.

In this arrangementthere is no possibility of a short circuit path occurring which does not incorprate one ofthe motorwindings, since a winding is disposed in series with each switching transistor, so thatthe rate of rise of fault current is finite and over-current detection circuitry can be used to turn offthe switching transistors orto operate a crowbar. In addition the freewheeling diodes provide an alternative current path by means of which leakage energy and winding coupled energy can be harmlessly conducted to the supply rails when each ofthe switching transistors is turned off.

Figure 3b shows an equivalent drive circuit for a delta-connected motor comprising three winding pairs3',4',5',6'and7',8'.

In the arrangements of Figures 3a and 3b high inter-bifilarwinding leakage inductance is a desirable feature since, underfault conditions, the higherthe leakage inductance Lthe lowerthe rate of rise offault current (v=L di/dt). By contrast, in use ofthe circuit of Figure 1 ,the leakage inductance is ideally zero (although this is impossible to achieve in practice) so asto avoid high voltage spikes atturn-off. However, in the absence of leakage inductance,fault current protection would not be available in viewofthe high rate of increase of the fault in the current of a short circuit.

In orderto obtain a high leakage inductance between the windings of a winding pair, whilst ensuring that both windings are tightly coupled to the magnetic circuit formed by the motor stator iron, it is advantageous if the bifilarwindings are replaced by winding pairs in which each coil is wound separately and the two coils are placed one upon the other in the machine stator slots. The physical separation of each coil gives a high leakage inductance ofthe one relative to the other, whilst each is coupled tightly (low leakage inductance) to the stator iron. The stator iron circuit cannot differentiate between the use of such coil pairs orthe use of bifilarwindings, so that both arrangements perform magnetically in the same way with respect to the motor rotor and the power output.

It is also advantageous if special arrangements are madetoreducethemotorstatorcopperlosses. In the conventional induction motor, current flows in both directions(bipolar)throughthewindingsasthea.c.

supply alternates. In the bifilar(orcoil)wound motors of Figures 1,2 and 3, however, currentflows through each coil in onlyone direction (unipolar),sothat,fora given motor stator slot size, the copper losses (i2R) due to copper wire resistance are doubled. Since these copper losses result inthe production of heat and motors are rated on the allowable motortemperature rise, a unipolarstatormotorwill require a reduced maximum currentto producetheallowabletemperature rise.Furthermore reduction ofthe maximum allowable current will resultin reduction of the maximum torque output, and thus a lesserpower outputwill be attainablefora given motorframe size and a given temperature rating by use of a unipolar motor, in viewofthe unipolar utilisation of the copper windings.

The copper losses in the windings can be reduced by having part of each winding bipolarand part unipolar. If this is doneforthe star configuration of Figure 3a, a winding arrangement as shown in Figure 4a is obtained. In this arrangementthe windings of each phase comprise a pairofwindings 3, 4or5, 6 or7, 8 (bifilarwound or coil pairs) and, in addition, a bipolar winding 9,10 or 11. In Figure 4athe current paths for one phase are shown bythe arrows 1 and i2.Itwill be seen thatthe winding 9 experiences bipolar current flow, whereas the windings 3 and 4 experience unipolarcurrentflow. If the winding 9 has turns and the windings 3 and 4 have respectively ki1 1 and $l2turns,theratio(2i1:i2)11 (12=11)canvaryfrom no11 or12turns(which istheconventional 3-phase induction motor) to no #1 turns which is the unipolarmotorconfiguration as shown in Figure 3.

Thefewerthe numberof12 and k)1 1 turns (implying more ml turns for a fixed total number ofturns) the lowerthe copper losses, but the lowerthe leakage inductance,which is undesirable forfault protection.

Thus the optimum numberofturnsforeachwinding will depend on a compromise based on the following relative analysis: (i) If there are no IZ11 1 turns, the copper losses are 2i2R (where i2R is the conventional bipolar motor loss) andthe leakage inductance isa maximum,say 1 per unit (p.u.).

(ii) If ratio ofturns 1/ki1 1=1 ($1/$12=1 ),the copper losses reduce to 1.5 i2R, but inter-winding leakage ((2)11 to 12) fallsto 0.25 p.u. (L octurns squared).

(iii) Ifthe ratio ofturns (2)11(2)11=2, the copper losses reduce to 1.33 i2R and inter-winding leakage drops to 0.111 p.u.

(iv) Generally, if (2)11(2)11 =n, the copper losses are (n+2)/(n+1) morethanthe normal bipolar motor losseswhilethe leakage inductance reduces two 1/(n+1) thatof a unipolar motor.

The practical compromise is between reducing copper losses and trying to maintain high leakagefor fault protection.

It is also advantageous to appropriately select the copperwirediameterofthe bipolar and unipolar portions ofthe windings so as to optimise copper area utilisation and ensure uniform heat dissipation throughoutthe stator slot cross-section. If the wire diameters of all the windings 3,4 and swerve the same, the winding 9 wouid be expected to get hotter since, forthe same cross-sectional area, it conducts a higher r.m.s. current (bipolar current as opposed to unipolar current in the case ofthe windings 3 and 4). Thus, in ordertorenderthelossesperunitstatorslotarea uniform,the wire diameter of each ofthe windings 3 and 4 is preferably 0.707 ofthe wire diameter ofthe winding 9, that is halfthe area and twice the resistance.

Figure 4b shows a modification of the delta configuration of Figure 3b in which each phase comprises twowinding pairs3',4'3",4"or5',6',5",6"or7',8', 7",8" and an intermediate bipolarwinding 9', 10' and 11'. Finally Figure4cshows a modification ofthe arrangements of Figures 3a and 3b in which the windings are connected in a star-delta configuration with the delta section comprising three bipolar windings 12,13 and 14 and the outerstarsection comprising three unipolarwinding pairs 3,4,5,6 and 7,8.

Claims (9)

1. Adrivecircuitforan induction motor having a pair ofwindings associated with each phase ofthe motor, with the windings of each pair being connected in anti-phase, the circuit comprising supply meansfor connection to a d.c. supply, respective switching means associated with each winding pairfor supplying current from the supply to each winding in only one direction, and frequencycontrol meansfor switching the switching means so as to cause the windingsofeachwinding pairtoconductcurrent alternately and in opposite directions at a frequency which may be varied to varythe speed ofthe motor, wherein the switching means associated with each winding pair comprises a bridge circuit having a first switching elementfor connection between one supply rail ofthe supply means and a first end of one winding ofthewinding pair, and a second switching element forconnection between the othersupply rail ofthe supply means and a first end ofthe otherwinding of the winding pair of the same polarity as the first end of said one winding, so that only two switching elements are associated with each phase of the motor.

2. A drive circuit according to claim 1, for driving a three phase motor, wherein the switching means compriseonlysixswitching elements in all.

3. Adrivecircuitaccordingtoclaim 1 or2,wherein each bridge circuitfurther incorporates a firstfreewheeling diode coupled between said other supply rail and the connection point ofthefirst switching element to said one winding, and a second freewheeling diode coupled between said one supply rail and the connection point of the second switching element to said other winding, whereby each freewheeling diode provides a path forfreewheeling motor current when the associated switching element is switched off.

4. An induction motor including a drive circuit according to claim 1,2 or3, wherein the windings are connected in a star config u ration with thefirstend of one winding of each winding pair being connected to a respective first switching element, the first end of the otherwinding of each winding pair being connected to a respective second switching element, and the second ends ofthewindings of all thewinding pairs being coupled to a common connection point.

5. An induction motor according to claim 4, comprising, in additiontothewindingsforconducting current in on Iy one di rection, fu rther windings for conducting current in both directions, wherein a respective further winding is connected between said common connection point and the second ends of the windingsofeachwinding pair.

6. An induction motor including a drive circuit according to claim 1,2 or3,wherein thewindings are connected in a delta configuration with the first end of one winding of each winding pair being connected to a respective first switching element, the first end of the otherwinding of each winding pair being connected to a respective second switching element, and the second ends ofthewindings of each winding pair being coupled to the first ends ofthe windings of an adjacent pair.

7. An induction motor according to claim 6, comprising in addition to the windings for conducting current in only one direction, furtherwindings for conducting current in both directions, wherein first and second pairs of windings for conducting current in only one direction are associated with each phase of the motor, and the first and second winding pairs assocated with each phase are connected together in series by means of a respective fu rtherwinding.

8. An induction motor including a drive circuit according to claim 1,2 or 3, comprising, in addition to thewindingsforconducting current in only one direction, furtherwindings for conducting current in both directions, wherein the windings are connected in a stardelta configuration,with the first end of one winding of each winding pair being connected to a respective first switching element, the first end of the otherwinding of each winding pair being connected to a respective second switching element, and the second ends ofthewindings of each winding pair being connected together and to the second ends of thewindingsofanadjacentwinding pair by means of a respectivefurtherwinding.

9. An induction motor drive circuit substantially as herein before described with reference to any one of Figures 3a to 4c ofthe accompanying drawings.