GB2135078A

GB2135078A - Speed control of synchronous motors

Info

Publication number: GB2135078A
Application number: GB08230761A
Authority: GB
Inventors: Masayuki Nashiki
Original assignee: Okuma Tekkosho KK; Okuma Machinery Works Ltd
Current assignee: Okuma Corp
Priority date: 1982-10-28
Filing date: 1982-10-28
Publication date: 1984-08-22
Also published as: GB2135078B

Abstract

Conventional control apparatus is additionally provided with an induced voltage signal generating device which produces signals representative of the voltages induced in the motor windings. The outputs from the signal generating device are added to either current error signals or to the current error signal which has been processed by controlling of PID compensation before being supplied to the motor windings. Thanks to such arrangement, the apparatus is capable of controlling the current at a higher precision by removing the influence of the induced voltage of the synchronous motor.

Description

SPECIFICATION Control apparatus for synchronous motors BACKGROUND OF THE INVENTION This invention relates to a control apparatus for synchronous motors which is capable of accurately controlling the intensity of the electric current which flows through the winding of a synchronous motor at a predetermined value by removing effect of induced voltage generated therein.

The control apparatus for synchronous motors in prior art is detrimental in that the current flowing through respective phases is influenced and varied by induced voltage in the winding of the motor to prevent precise control operation. Fig. 1 shows an embodiment of such conventional apparatus for a three-phase synchronous motor wherein the synchronous motor 1 7 is coupled with a tachometer generator 1 8 for detecting rotatipnal speed of the motor and a rotor position detector 19, the output PF from the position detector 1 9 is transmitted into a subtractor 1 together with a rotor position command signal RC, the output therefrom or a speed command signal SC is supplied into a subtractor 2 with a speed feedback signal SF from a tachometer generator 18, and the output SCA therefrom is fed into multipliers 25, 26 and 27 respectively as speed error signals SES via a PID (Proportional plus Integral plus Derivation) compensator 3. The position feedback signal PF is also fed into a three-phase (U, V, W) sine-wave generating circuit 24, the outputs SWu, SWv and SWw therefrom are fed into the multipliers 25, 26 and 27 converted to the sine-wave signals having phases deviated from each other by 7r, the outputs therefrom or three-phase AC current command signals CCu, CCv and CCw are made to pass into subtractors 5, 6 and 7 respectively together with current feedback signals CFu, CFv and CFw transmitted from current detectors 14, 1 5 and 1 6 for respective phases.Further, the outputs ECu, ECv apd ECw from the subtractors 5, 6 and 7 are inputted to PID compensators 8, 9 and 10, respectively, and the outputs CEu, CEv arld CEw therefrom are amplified by power amplifiers 11, 1 2 and 1 3 to be supplied to respective phases of the synchronous motor 1 7 as driving currents DRCu, DRCv and DRCw.

In such conventional control apparatus as mentioned above, when the synchronous motor 1 7 is rotated at a given speed under a given load, for instance at the current and the voltage of U-phase, the output signal SWu of the three-phase sine-wave generating circuit 24 is fed into the multiplier 25 together with the speed error signal SES to compose a current command signal CCu as shown in Fig.

2A. As a U-phase voltage Vu as shown in Fig.

2B by solid line is outputted from the power amplifier 11, the voltage Vru induced in the U-phase motor windings shows the form indicated in Fig. 2B by broken line and a phase current as shown by oblique lines flows in the winding of the motor. As the waveform is strongly affected by the induced voltage, however, it becomes to assume a waveform different from the one directed by the current command signal CCu shown in Fig. 2A, presenting difficulties in precise control of the current.

SUMMARY OF THE INVENTION Accordingly, an object of this invention is to provide a control apparatus for synchronous motors which is capable of preventing current command signals for respective phases from being influenced by the induced voltage of the phase current when a synchronous motor rotates at a given speed under a given load.

Another object of this invention is to provide a control apparatus for synchronous motors which can accurately control the current by removing adverse effect of the voltage induced by the flow of phase currents through the winding of the motor.

The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Figure 1 is a schematic diagram to explain a conventional control apparatus for synchronous motors; Figures 2A and 2B are time charts to explain the operation thereof, respectively; Figure 2C is a time chart to show, in comparison with prior art, the operation of control apparatus according to this invention; Figure 3 is a schematic diagram to show an embodiment according to this invention; Figure 4 shows an equivalent circuit of the phase winding in a synchronous motor; Figures 5 and 6 are schematic diagrams of another embodiment according to this invention, respectively; and Figures 7A through 71 are time charts to explain the operation of the control apparatus indicated in Fig. 6, respectively.

DETAILED DESCRIPTION OF THE INVEN TION In Fig. 3 which shows an embodiment according to the present invention, subtractors 1, 2, 5 through 7, PID compensators 3, 8 through 10, power amplifiers 11 through 13, current detectors 14 through 16, a synchronous motor 17, a tachometer generator 18, a position detector 19, a three-phase sine-wave generating circuit 24 and multipliers 25 through 27 are identical to the above mentioned conventional elements, respectively.

The apparatus is so constructed that multipli ers 28, 29 and 30 multiply the outputs SWu, SWv and SWw from the three-phase sinewave generating circuit 24 and a rotor speed signal SF respectively to compose signals which are proportionate to the three-phase induced voltage, and the outputs TWu, TWv and TWw therefrom are fed into adders 20, 21 and 22 together with current error signals CEu, CEv and CEw from the PID compensators 8, 9 and 10. Further, the adding results AEu, AEv and AEw from the adders 20, 21 and 22 are in turn inputted to power amplifiers 11, 1 2 and 13, respectively.

The operation of the apparatus constructed as above will now be described mainly for a U-phase.

The characteristics of respective circuits comprising the U-phase are so determined that the U-phase current command signal CCu is 10 volts, the detection capacity of the Uphase current detector 14 is 1 volt/ampere, the amplification factor of the PID compensator 8 is 1 and the amplification factor of the power amplifier 11 is 20. The U-phase winding, if substituted by the equivalent circuit shown in Fig. 4, is such that the internal resistance R is 1 ohm and the induction voltage Vru is 100 volts/ 1000 revolutions per minute. The current control by the control device under the above assumed conditions will become as indicated in (A) to (C) below.

(A) When the revolution rate N of the motor 1 7 is 0 revolution per minute, as the induced voltage Vru becomes 0 volt and the correction signal for the induced voltage becomes 0 volt, too. The formula below holds: [(10-lu)X1 +0]X20-Vru= luX1 (1) According to the formula, the U-phase current lu is calculated to be 9.524 amperes and Vu 9.524 volts. As the U-phase current command value is 10 amperes, the control precision of about 95% can be obtained.

(B) When the revolution rate N of the motor 1 7 is 1000 revolutions per minute, as the induced voltage Vru becomes 100 volts, the formula below holds when the induced voltage is not corrected, [(10-lu)Xl + 0] X 20Vru = lu X 1 (2) According to the formula, lu = 4.762 amperes and Vu = Vru + lu R = 104.762 volts.

The U-phase current lu will take a value less than 50% of the commanded value, resulting in an inferior control precision. Fig. 2B shows with solid and broken lines the output voltage Vu and the U-phase induced voltage Vru of the above control apparatus under the above conditions.

(C) When the induced voltage is corrected in a case where the revolution rate N of the motor 1 7 is 1000 revolutions per minute and the induced voltage Vru is 100 volts, if it is assumed that the corrected volume AV = Vru X 1 /(amplification factor of the power amplifier) (3) the formula holds as belows: (10lu) x 1 + I x 20 - Vru = lu x 1 (4) According to the above formula, the U-phase current lu becomes 9.524 amperes and the U-phase voltage Vu becomes 109.524 volts.

The values obtained coincide with those calculated in the case (A), indicating the fact that the influence of the induced voltage has been removed. The U-phase voltage Vu and the induced voltage Vru are shown by solid and broken lines in Fig. 2C.

In Fig. 5 which corresponds with Fig. 3, a three-phase synchronous generator 31 is coupled with a synchronous motor 1 7 together with the tachorneter generator 18 and the position detector 19, and the output voltage CGu, CGv and CGw therefrom are transmitted into adders 20, 21 and 22 together with current error signals CEu, CEv and CEw, respectively. As the output voltages CGu, CGv and CGw from the synchronous generator 31 are in proportion with the induced voltages generated in the windings of the synchronous motor 17, respectively, the current can be controlled without receiving the effect from the induced voltage by feeding the output voltages CGu to CGw into the adders 20 to 22 in a manner similar to the control device indicated in Fig. 3.

Further, in Fig. 6 which shows another embodiment of the present invention, the output ES from an exciting circuit 32 is supplied to a resoiver 40 coupled with the synchronous motor 1 7 as well as into sampling pulse generating circuits 34, 35 and 36, the output RS of the resolver 40 is fed to an resolver signal detecting circuit 33, the output DS therefrom is inputted to sampled-hold circuits 37, 38 and 39 respectively together with the outputs SPu, SPv and SPw from the sampling pulse generating circuits 34, 35 and 36, and the output therefrom or sine-wave signals HGu, HGv and HGw are fed into the multipliers 25, 28; 26, 29 and 27, 30.

In the structure mentioned above, when an exciting signal ES is transmitted from the exciting circuit 32 into the resolver 40 as shown in Fig. 7A, a signal RS having a phase deviated by the rotor position B of the motor 1 7 will be supplied from the resolver 40 as shown in Fig. 7F. The exciting signals ES on the other hand are fed into the sampling pulse generating circuits 34, 35 and 36 wherein the signals are waveform-shaped and then the rise waveforms thereof are differenciated to produce sampling pulses SPu, SPv and SPw having phases deviated by 3rr from each other as indicated in Figs. 7C, 7D and 7E.

The signal DS from the resolver 40 is sustained with the sampling pulses SPu, SPv and SPw at the sampled-hold circuits 37, 38 and 39, respectively. If the sine-wave signals HGu, HGv and HGw or the outputs from the circuits 37, 38 and 39 are plotted with the rotor position of the motor 1 7 on the abscissa, they will show the waveforms indicated in Figs. 7G, 7H and 71 which are identical to those of the outputs SWu, SWv and SWw from the three-phase sine-wave generating circuit 24 as shown in Fig. 3, thereby achieving the current control similar to the one shown in Fig. 3. The position detector 1 9 may be omitted from the control apparatus shown in Fig. 6 because the position detection can be carried out by the resolver 40.

It is also possible to connect the exciting signal ES of the resolver 40 and the output RS of the resolver signal detecting circuit 33 in the opposite order for producing the sinewave signals HGu, HGv and HGw.

As explained in the foregoing, the control apparatus according to this invention is capable of controlling the current of a synchronous motor at a higher precision by removing the adverse effect of the induced voltage thereof, thereby achieving precise and effective operation of the synchronous motor.

Claims

1. In a control apparatus for synchronous motors of the type where a current setting signal is composed by subtracting a speed feedback signal from a speed setting of a synchronous motor, a current error signal is composed by subtracting a current feedback signal from the current setting signal and simultaneously the current error signal is fed into said synchronous motor through power amplifiers, which is characterised by comprising an induced voltage detecting device to detect voltages induced in windings of said synchronous motor, thereby to feed the outputs from said induced voltage detecting devices into said power amplifiers by adding said outputs to said current error signal or the current error signal which has been processed by controlling of PID compensation.

2. The control apparatus for synchronous motors as claimed in Claim 1, wherein said induced voltage detecting device comprises multipliers which receive as inputs speed feedback signals of said synchronous motor and rotor position detection signals of said synchronous motor.

3. The control apparatus for synchronous motors as claimed in Claim 1, wherein said induced voltage detecting device comprises a synchronous generator coupled with said synchronous motor.

4. The control apparatus for synchronous motors as claimed in Claim 1, wherein said induced voltage detecting device comprises a resolver coupled with said synchronous motor, sampled-hold circuits to store the output signals or the input signals of said resolver, and multipliers which receive as input the output signals from said sampled-hold circuits and the speed feedback signals of said synchronous motor.

5. In a control apparatus for synchronous motors of the type where a position feedback signal transmitted from a position detector coupled with a synchronous motor is subtracted from a position command signal of said synchronous motor in order to compose a speed command signal, a speed feedback signal transmitted from a tachometer generator coupled with said synchronous motor is subtracted from said speed command signal and processed by means of a PID compensator in order to obtain a current command signal, the position feedback signal is inputted to a three-phase sine-wave generating circuit to obtain three-phase sine-wave signals having phases deviated from each other by 57r, the respective three-phase sine-wave signals are multiplied with the current command signal by multipliers to obtain three-phase AC current command signal and at the same time current feedback signals for respective phases are subtracted from the AC current command signal to compose current error signals, and the current error signals are supplied to said synchronous motor through power amplifiers, which is characterised by comprising multipliers to multiply said speed feedback signals and said current command signals, and adders which add the outputs from said multipliers and said current error signals in order to supply the sum thereof to said power amplifiers, thereby preventing the phase currents of said synchronous motor from influencing the current command signal.

6. In a control apparatus for synchronous motors of the type where a position feedback signal transmitted from a position detector coupled with a synchronous motor is subtracted from a position command signal of said synchronous motor in order to compose a speed command signal, a speed feedback signal transmitted from a tachometer generator coupled with said synchronous motor is subtracted from said speed command signal and processed by means of a PID compensator in order to obtain a current command signal, the position feedback signal is inputted to a three-phase sine-wave generating circuit to obtain three-phase sine-wave signals having phases deviated from each other by 51r, the respective three-phase sine-wave signals are multiplied with the current command signal by multipliers to obtain three-phase AC current command signal and at the same time current feedback signals for respective phases are subtracted from the AC current command signal to compose current error signals, and the current error signals are supplied to said synchronous motor through power amplifiers, which is characterised by comprising a synchronous generator coupled with said synchronous motor, and adders which add the output voltages from said synchronous generator and said current error signals in order to supply the sum thereof to said power amplifiers, thereby preventing the phase currents of said synchronous motor from influencing the current command signal.

7. In a control apparatus for synchronous motors of the type where a position feedback signal transmitted from a position detector coupled with a synchronous motor is subtracted from a position command signal of said synchronous motor in order to compose a speed command signal, a speed feedback signal transmitted from a tachometer generator coupled with said synchronous motor is subtracted from said speed command signal and processed by means of a PID compensator in order to obtain a current command signal, the position feedback signal is inputted to a three-phase sine-wave generating circuit to obtain three-phase sine-wave signals having phases deviated from each other by 57r, the respective three-phase sine-wave signals are multiplied with the current command signal by multipliers to obtain three-phase AC current command signal and at the same time current feedback signals for respective phases are subtracted from the AC current command signal to compose a current error signals, and the current error signals are supplied to said synchronous motor through power amplifiers, which is characterised by comprising a resolver coupled with said synchronous motor, an exciting circuit to excite said resolver, sampling pulse generating circuits which receive as input the exciting signals from said exciting circuit and outputs sampling pulses having phases deviating from each other by v, sampled-hold circuits which hold the outputs from said resolver in correspondence with the sampling pulses and produce three-phase sine-wave signals, multipliers which multiply said speed feedback signals with the threephase sine-wave signals, and adders which add the outputs from said multipliers and the current error signals, thereby preventing the phase currents of said synchronous motor from influencing the current command signals.