Operation Control Apparatus for AC Motors

This application is related to our copending application Ser. No. 176,337, filed Aug. 8, 1980 entitled OPERATION CONTROL APPARATUS FOR AC MOTORS, now U.S. Pat. No. 4,319,177.

This invention relates to an apparatus for controlling the operation of an AC motor, and more particularly to an AC motor operation control apparatus having improved circuitry for regenerative braking.

AC motors, and induction motors in particular, are employed in a variety of industrial fields. The kinds of loads that can be driven by these motors are equally diverse. Some induction motors, for example, undergo rapid and frequent increases and decreases in speed, while in others there are positive and negative changes in load torque so as to permit the raising and lowering of an object in the manner of a hoist. Thus, there is a requirement that an AC motor functioning as a driving source be operated so as to generate a positive torque as well as a braking torque.

A method of controlling the operation of AC motors, which has recently come into fairly widespread use, employs a variable-voltage variable-frequency inverter. While no major projects are encountered in this method when operating the motor in the driving mode, one difficulty which does arise is the difficulty in dealing with the rotational energy possessed by the rotor of the motor when it is to be operated in the braking mode at the time of a reduction in speed. Two exemplary methods of dealing with this rotational energy have been adopted in the prior art. In one method the flow of current to the AC motor is interrupted at braking time to permit the motor to slow down naturally due to mechanical loss attributed to the load. In the other method the slip which arises at the time of the speed reduction is suitably controlled and is allowed to dissipate within the motor. The first method, however, requires too much time to achieve the speed reduction and has a very poor control response, while the second method causes the motor to overheat to such an extent that it cannot endure frequent increases and decreases in speed. Another method which can be mentioned is one in which the rotational energy of the rotor is dissipated by allowing a smoothing capacitor, inserted in the inverter circuitry mentioned above, to charge until the charged voltage exceeds a specified value, whereupon the capacitor is discharged through a braking resistor connected in parallel with the inverter circuit, thereby to dissipating the energy. However, this method is disadvantageous in that it may lead to destruction of the apparatus if the smoothing capacitor is allowed to charge to an excessively high value. This method is also expensive since the braking resistor increases in size and cost in accordance with the size of the machine to be driven by the motor. Moreover, the method is undesirable in terms of enhancing efficiency because of the fact that the braking energy is wasted in the form of thermal loss.

A regenerative braking method, as illustrated by the circuit diagram in FIG. 1, has been proposed in an effort to improve upon the foregoing arrangements.

The apparatus includes an AC motor 1 such as a three-phase induction motor; a rectifier 2 comprising diodes D.sub.1 through D.sub.6 for rectifying the U, V and W phases of the AC input power; a regenerative thyristor bridge circuit 3 comprising thyristors S.sub.1 through S.sub.6 ; a smoothing circuit 4 having a capacitor C.sub.1 ; a variable-voltage variable frequency inverter 5 comprising transistors TA.sub.1 through TA.sub.6 ; a rectifier 6 comprising diodes D.sub.1 ' through D.sub.6 '; and a step-up transformer 7 for boosting the power source voltage. To control the induction motor 1 with this conventional arrangement, for example, to reduce the motor speed, the command speed is lowered to control the voltage and frequency of the variable-voltage variable-frequency inverter 5. Thus, the synchronous speed, in conformance with the newly set frequency, becomes smaller than the motor speed, giving rise to a negative slip condition. Accordingly, the motor begins to run in the regenerative braking region, with the result that the voltage induced in the motor is rectified by the rectifier 6, thereby raising the voltage on the DC line side. The smoothing capacitor C.sub.1, in order for it to exhibit the smoothing function, is charged to a voltage which is 1.3 to 1.4 times the AC power source voltage even when the motor is operating in the normal driving mode. Nevertheless, when the induction motor is operated in the regenerative region, the smoothing capacitor C.sub.1 is charged to, and held at, an even higher voltage. For example, if the AC power source voltage is 200 volts, the voltage to which the capacitor C.sub.1 is charged is raised to approximately 290 volts. Under such a condition, commutation cannot take place and regenerative operation is impossible because the AC power source voltage is lower than the voltage on the side of the DC line even if the firing of the regenerative thyristor bridge circuit 3 comprising thyristors S.sub.1 through S.sub.6 is controlled. To avoid this inconvenience the step-up transformer 7 is inserted between the thyristor bridge 3 and the AC power source, and the circuitry is arranged in such a manner that there will be intervals in which the AC power source voltage is always higher than the voltage on the DC line side. This enables operation in the regenerative braking region while assuring commutation of the thyristors S.sub.1 through S.sub.6. However, the apparatus that employs this system is large and expensive due to the need for the large capacity step-up transformer 7.

The present invention is directed to an inexpensive and compact operation control apparatus for an AC motor, wherein a step-up transformer is not employed for disposing of the braking energy generated by regenerative braking when the speed of the AC motor is reduced.

Accordingly, an object of the present invention is to provide an AC motor operation control apparatus equipped with a regenerative braking circuit which is cheaper and which is capable of being constructed in a much smaller size than known apparatus.

It is another object of the present invention to provide an AC motor operation control apparatus which is capable of regenerative braking without the use of a step-up transformer.

It is still another object of the present invention to provide an AC motor operation control apparatus in which stable regenerative braking can be achieved inexpensively without a large apparatus.

It is a further object of the present invention to provide an AC motor operation control apparatus equipped with a regenerative braking circuit in which transistors are turned off with ease, in which inexpensive thyristors are employed, and in which there is a high level of current efficiency.

Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings.

FIG. 1 is a circuit diagram of a regenerative type control apparatus for controlling the operation of an AC motor in accordance with the prior art;

FIG. 2 is a circuit diagram of an embodiment of an apparatus for controlling the operation of an AC motor in accordance with the present invention; and

FIGS. 3A and 3B are waveform diagrams which illustrate waveforms associated with the various portions of the circuit shown in FIG. 2.

Reference will now be had to FIG. 2 to describe the circuit of an embodiment of the AC motor operation control apparatus of the present invention. In FIG. 2, the sections denoted by the reference numerals 1, 2, 4, 5 and 6 are identical to those designated by the same reference numerals in FIG. 1. The section denoted by numeral 3" corresponds to the regenerative thyristor bridge circuit 3 of FIG. 2 but in this case it is a regenerative compound bridge circuit which is constructed by connecting the regenerative thyristor bridge circuit, comprising the bridge-connected thyristors S.sub.1 through S.sub.6, between the DC lines through switching transistors TR.sub.1 and TR.sub.2. Shown at numeral 8 is one of two elements which constitute a photocoupler. Elements such as light-emitting diodes are shown at P.sub.1 ' through P.sub.6 ', and diodes at D.sub.1 " through D.sub.6 ". The light-emitting diodes P.sub.1 ' through P.sub.6 ' and diodes D.sub.1 " through D.sub.6 " are connected to form a bridge circuit whose AC input side receives the three-phase AC power for the motor in a phase relationship which corresponds to that of the respective thyristors in the regenerative compound thyristor bridge circuit 3" that receive the three phases U, V and W. The diodes D.sub.1 " through D.sub.6 " serve also to protect the circuit against reverse voltage. The other element which constitutes the photocoupler is shown at reference numeral 8' and consists of, for example, phototransistors P.sub.1 " through P.sub.6 " . The arrangement is such that when light-emitting diode P.sub.1 ' emits light in response to an inflow of current, the element P.sub.1 ", serving as a light receiver, is rendered conductive by receiving the emitted light. The output terminal of each light-receiving element P.sub.1 " through P.sub.6 " is provided with an inverter 9 for inverting the output signal. The output signals from the inverters 9 are respectively applied to a corresponding differential circuit 9' which generates a pulse at the leading edge of the output signal from the inverter 9. The pulses from the differential circuits 9' are applied to an OR circuit 10 whose output is in turn coupled to a first monostable multivibrator circuit 11 which generates negative pulses of a fixed time width. A second monostable multivibrator circuit 12 responds to the output of the first monostable multivibrator circuit 11 also to produce pulses of a fixed time width. AND circuits 13 take the logical AND between the outputs of the respective inverters 9 and the output of the second monostable multivibrator 12, and correspond in number to the inverters 9. The outputs of the AND circuits 13 are second firing control signals SG.sub.1 through SG.sub.6, serving as gate signals, which are applied to the gates of corresponding ones of the thyristors S.sub.1 through S.sub.6 that form the regenerative compound thyristor bridge circuit 3". The output of the first monostable multivibrator circuit 11 is a first firing control signal, serving as a base signal, which is applied simultaneously as signals TRB.sub.1, TRB.sub.2 to the switching transistors TR.sub.1, TR.sub.2 connected in series with the thyristor group in the regenerative compound thyristor bridge circuit 3".

The elements 8, 8' that construct the photocoupler, the inverters 9, the differential circuits 9', the OR circuit 10, the monostable multivibrators 11 and 12, and the AND circuits 13 construct a firing control circuit for the regenerative compound thyristor bridge circuit 3". In addition, the photocoupler elements 8, 8' and the inverters 9 form a phase signal forming circuit; the differential circuit 9', the OR circuit 10 and the first monostable multivibrator circuit 11 construct a first firing control signal generating circuit for forming firing control signals for the transistors TR.sub.1, TR.sub.2 in the regenerative thyristor bridge circuit 3"; and the second monostable multivibrator circuit 12 and AND circuits 13 construct a second firing control signal generating circuit for forming firing control signals for the thyristors S.sub.1 through S.sub.6 of the regenerative thyristor bridge circuit 3".

In operation, when the AC motor 1, comprising the induction motor, is running in the ordinary driving mode, each phase of the AC power source is rectified into a direct current by the rectifier 2. The variable-voltage variable frequency inverter 5 converts the direct current into an alternating current of a prescribed frequency and voltage which is supplied to the induction motor 1 to bring the motor speed into agreement with the command speed. The output frequency of the inverter 5 can be varied by adjusting the repetition frequency of the firing pulses applied to the transistor elements TA.sub.1 through TA.sub.6 of the inverter, and the output voltage of the inverter 5 is capable of being varied by adjusting the conduction time of the transistor elements TA.sub.1 through TA.sub.6. When a speed reduction command signal is applied at such time that it is required to reduce the speed of the motor in accordance with the load, the output voltage and frequency of the variable-voltage variable-frequency inverter 5 are decreased, so that the rotational speed of the operating motor at this time is higher than the synchronous speed which conforms to the new operating frequency. As a result, the induction motor 1 operates in a region where the slip is negative, that is, in a regenerative braking region, so that the output of the motor raises the voltage on the DC line side upon being rectified by the rectifier 6. Thus, if the motor is operating at an AC power source voltage of 200 volts, the voltage at the terminals of the smoothing capacitor C.sub.1 will be raised to approximately 290 volts. If the regenerative bridge circuit were composed solely of the thyristors S.sub.1 through S.sub.6 as in the prior art, the thyristors would fail to commute even though the intent is for regenerative braking to occur by firing the thyristors which are connected to the two phases of the AC power source that exhibit the largest interphase voltage at that time. Such commutation failure short-circuits the power source and can destroy the equipment. In contrast, according to the present invention, the switching transistors TR.sub.1, TR.sub.2 are connected in series with the thyristor group of the regenerative bridge circuit and are rendered conductive during regenerative operation, and momentarily non-conductive at the same time when any of the thyristors S.sub.1 through S.sub.6 is to commute, thereby to interrupt the flow of current to the thyristors. The transistors TR.sub.1, TR.sub.2 are then fired again after taking into account the time during which all of the thyristors are to be turned off. In this way the regenerative current is always fed back to the power source without any of the thyristors experiencing commutation failure even though the voltage on the DC line side is higher than the AC voltage of the power source.

The formation of the control signals that make the foregoing operation possible will now be described. The input side of the bridge circuit 8, comprising one of the photocouplers composed of the elements P.sub.1 ' through P.sub.6 ' such as light-emitting diodes, is connected to the U, V and W-phases of the motor driving source. With such a connection the bridge circuit 8 and the regenerative thyristor bridge circuit 3" receive phase voltages of the same phase, so that it is possible to detect the phase corresponding to that at which firing must occur when the regenerative thyristor bridge circuit 3" is to operate in the regenerative mode. This will be described in connection with FIG. 3A. Sinusoidal phase voltages U-V, V-W, and W-U are impressed upon the corresponding phase input terminals of the bridge that forms the photocoupler. A current flows only into that light-emitting diode that receives the largest phase voltage of those that are applied, that particular diode responding by emitting light for the duration of the current flow. The light-emitting diodes P.sub.1 ' through P.sub.6 ' are actuated in succession in accordance with the time variation of the interphase voltages. The other photocoupler element 8' is arranged so as to intercept the light from the light-emitting diodes. Hence, when the diodes are actuated and caused to emit light, the light-receiving elements (such as phototransistors) that constitute the photocoupler 8' intercept the light from corresponding ones of the light-emitting diodes and are rendered conductive, their collector potentials going to zero volts and remaining there for as-long as they continue to receive light. The outputs of the phototransistors P.sub.1 ", P.sub.2 ", P.sub.3 ", P.sub.4 ", P.sub.5 " and P.sub.6 " are illustrated by the respective waveforms P.sub.1, P.sub.2, P.sub.3, P.sub.4, P.sub.5 and P.sub.6 of FIG. 3A. The signals P.sub.1 through P.sub.6 are inverted to produce the phase signals P.sub.1, P.sub.2, P.sub.3, P.sub.4, P.sub.5 and P.sub.6 by means of the inverters 9 connected to the output of each phototransistor. The inverted phase signals are applied to one input terminal of the corresponding AND circuits 13, and to the input side of the OR circuit 10. Accordingly, the OR circuit 10 successively delivers pulses to the monostable multivibrator circuit 11, a pulse being produced each time a pulse in the signals P.sub.1 through P.sub.6 arrives. The monostable multivibrator circuit 11 is triggered by the leading edge of each received pulse and is rendered conductive for a fixed period of time only, after which the output of the multivibrator returns to its original potential, this operation continuing in repetitive fashion to produce the pulse train M.sub.1 in FIG. 3A. The pulse train has a zero potential with approximately 0.5-millisecond intervals which correspond to the conductive intervals of the multivibrator. This zero-potential pulse train is applied as first firing control signals TRB.sub.1, TRB.sub.2 to the bases of switching transistors TR.sub.1, TR.sub.2 which are connected in series with the regenerative thyristor bridge circuit, and functions to bring the base potentials of the transistors to zero potential to render the transistors non-conductive. In addition, this pulse train is also applied to the second monostable multivibrator circuit 12 which responds by producing the pulse train M.sub.2 in FIG. 3A. The pulses are positive pulses having a pulse width of approximately 10 microseconds and they are timed by the leading edge of each input pulse as it returns to its original potential. The pulses M.sub.2 are applied to each of the AND circuits 13. The AND circuits 13 take the logical AND between the phase signals P.sub.1 through P.sub.6, obtained from the inverters 9, and the output pulses M.sub.2 from the second multivibrator circuit 12, thereby producing the second firing control signals SG.sub.1, SG.sub.2, SG.sub.3, SG.sub.4, SG.sub.5 and SG.sub.6 which are delivered to the gates of the thyristors S.sub.1 through S.sub.6 that construct the regenerative thyristor bridge circuit. The phase relationship among the signals SG.sub.1 through SG.sub.6, as shown in FIG. 3B, is of course regulated by the output signals P.sub.1 through P.sub.6 of the photocoupler.

Accordingly, let it be assumed that the U-V phases of the AC power source exhibit the largest interphase voltage suited for regeneration. When the zero-potential pulse signals TRB.sub.1, TRB.sub.2 of FIG. 3B are applied to the switching transistors TR.sub.1, TR.sub.2 of the regenerative thyristor bridge circuit 3", the transistors TR.sub.1, TR.sub.2 are rendered non-conductive simultaneously to temporarily cut off the flow of current to the thyristor bridge circuit 3", thereby turning off all of the thyristors S.sub.1 through S.sub.6. After this occurs the second firing control signals SG.sub.1 and SG.sub.4 applied to the gates of thyristors S.sub.1 and S.sub.4 which are fired as a result. Hence a regenerative current I flows from the U-phase of the power source to the V-phase to feed back the energy. After a prescribed time period the zero-potential pulses are re-applied to the switching transistors TR.sub.1, TR.sub.2 to render them non-conductive, thereby again turning off all of the thyristors in the regenerative thyristor bridge circuit 3". Then the switching transistors TR.sub.1, TR.sub.2 are again rendered conductive, and at the same time the second firing control signals SG.sub.1 and SG.sub.6 are applied to thyristors S.sub.1 and S.sub.6, so that the regenerative current can flow from the U-phase to the W-phase of the power source. Thereafter, the operation continues in similar fashion. Thus, all of the thyristors are turned off by rendering both switching transistors TR.sub.1, TR.sub.2 non-conductive by applying the zero-potential pulses, namely the first firing control signals TRB.sub.1, TRB.sub.2, to the switching transistors prior to the commutation of the thyristors. Then, in successive intervals during which the switching transistors are non-conductive, the second firing control signals SG.sub.3 and SG.sub.6 are applied to the thyristors S.sub.3 and S.sub.6, the second firing control signals SG.sub.3 and SG.sub.2 are applied to the thyristors S.sub.3 and S.sub.2, and the second firing control signals SG.sub.5 and SG.sub.2 are applied to the thyristors S.sub.5 and S.sub.2. As a result, a regenerative current can be supplied between the two phases of the AC power source that exhibit the largest interphase voltage. In addition, commutation can take place in a reliable manner since the thyristors are turned off temporarily prior to the arrival of the second firing control signals due to the fact that the current is interrupted without fail by the switching transistors TR.sub.1, TR.sub.2. The conduction of the thyristors S.sub.1 through S.sub.6 and the regenerative current waveforms based on the foregoing operation are illustrated by S.sub.1 through S.sub.6 and by I, respectively, in FIG. 3B.

The present invention, as described in detail above, allows regenerative braking to be applied to an AC motor such as an induction motor so that a highly efficient braking action can be achieved, and permits comparatively inexpensive thyristors to be employed as the elements that construct the regenerative bridge. In addition, only two switching transistors, which have a high cost per unit of current capacity, need be connected in series between the DC lines and the regenerative bridge to enable reliable commutation and stable operational control without regard to the size of the AC power source interphase voltage. Therefore it is not necessary to provide the step-up transformer for commutation which was an essential component in the prior art, nor is it required to construct the regenerative bridge entirely of switching elements. This is commercially advantageous since the size and cost of the apparatus can be reduced.

As many widely different embodiments of the present invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.