Abstract:
An apparatus for preventing malfunctions in a phase control mechanism of the type in which the firing angle of thyristors in a thyristor bridge is controlled to regulate the rotational speed of a DC motor comprising the load of the thyristor bridge. Thyristor firing pulses are produced each time a rising analog speed command signal generated in response to the phases of a N-phase power supply intersects N-number of sawtooth signals. Immediately after the generation of each firing pulse, the thyristors are not supplied with firing pulses for a time interval at least equivalent to the thyristor commutation overlap time.

Description:
FIELD OF THE INVENTION 
     This invention relates to a circuit for preventing a malfunction in a phase control mechanism for controlling the firing angle of thyristors in an apparatus of the type that drives a DC motor by means of thyristors. 
     A motor control system which is well-known in the art is adapted to drive a DC motor by using a thyristor bridge to rectify an AC voltage. A motor control system of this type is arranged to vary the firing angle of the thyristors that construct the thyristor bridge, thereby changing the mean value of the voltage applied to the armature of the DC motor so that the speed of the motor can be regulated. The described control system therefore is used in speed control devices such as those applied to regulating the speed of a spindle motor in a machine tool where there is a need to vary the rotational speed. 
     FIG. 1 is a block diagram showing the principal portion of unidirectional-type motor control apparatus for rectifying a three-phase AC power supply by a thyristor bridge in order to drive a DC motor. In FIG. 1, numeral 1 denotes a three-phase power supply whose output is connected, through a reactor 14, to a thyristor bridge 2 comprising six thyristors TH 1 , TH 1  &#39;, TH 2 , TH 2  &#39;, TH 3 , TH 3  &#39;. The rectified output of the thyristor bridge 2 is applied to a DC motor 3, the current I a  flowing through the motor being sensed by a direct current sensor 4. The output of the sensor 4 is fed to a summing point 5 whose other input is a firing control signal FS which will be described below. The output of the summing point is connected to an amplifier 6, whose output is in turn connected to a limiter 7. The output of the limiter 7 is applied to a sawtooth generator 8, and to a firing phase control circuit 9, the latter including U-, V- and W-phase firing circuits 9u, 9v and 9w for controlling the U-, V- and W-phase firing angles, respectively. A speed feedback loop, comprising a main loop, is deleted from FIG. 1 in order to simplify the description. 
     The sawtooth generator 8 generates three sawtooth signals 8u, 8v, 9w which are displaced in phase from one another by 60 degrees, as shown in FIG. 2A. The sawtooth signals 8u, 8v, 8w are applied as inputs to the U-, V- and W-phase firing circuits 9u, 9v, 9w, respectively of the the firing phase control circuit. Each of the firing circuits 9u, 9v, 9w includes a comparator (not shown) for comparing the respective sawtooth signal with the phase control input signal FS from the limiter 7, and a circuit (not shown) for forming a thyristor firing signal on the basis of the comparator output. More specifically, and again with reference to FIG. 2, the firing circuits 9u, 9w, 9v compare the sawtooth signals and the firing control input signal FS, as shown in FIG. 2(A), form three-phase rectangular signals based upon the results of the comparison operation, as shown in FIG. 2(B), and then produce thyristor firing pulses th 1 , th 1  &#39;, th 2  , th 2  &#39;, th 3 , th 3  &#39; on the basis of the rectangular signals, as shown in FIG. 2(C). 
     The apparatus of the aforesaid type has long been available in the art, and is described and illustrated in U.S. Pat. No. 4,288,782 and U.S. Pat. No. 4,322,671, by way of example. A detailed description of the apparatus will therefore be deleted. 
     When there is to be an abrupt increase in the r.p.m. of DC motor 3 in the foregoing apparatus, there is sharp rise in the level of an analog speed command signal provided by a numerical control device or the like. This is accompanied by a sharp rise in the firing control input signal FS, which is subjected to a current feedback signal. Consequently, as shown in FIG. 2(A), there are many instances where the signal FS crosses the sawtooth signal 8v at point A and then immediately crosses the sawtooth signal 8w at point B within the thyristor commutation overlap time period u. The reason for this is as follows, when the current rises without a time delay at point in time A, the rise in the signal FS is suppressed by the current feedback signal. In the above case, however, the rise of the current is delayed by the commutation overlap time, so the current feedback signal cannot suppress the signal FS. When the signal FS crosses the two sawtooth signals in such a short time interval, the thyristors are misfired and an excessive current of a possibly damaging magnitude flows into the thyristor bridge. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an apparatus for preventing a malfunction in a phase control mechanism that controls the firing angle, of the thyristors in a thyristor bridge, to regulate the rotational speed of a DC motor comprising the load of the thyristor bridge, the apparatus prevents a malfunction even when the level of an analog speed control signal is changed in an abrupt manner. 
     Another object of the present invention is to provide an apparatus for preventing a malfunction in a phase control mechanism that controls the firing angle, of the thyristors in a thyristor bridge, to regulate the rotational speed of a DC motor comprising the load of the thyristor bridge, the apparatus prevents irratic firing of thyristors even when the level of an analog speed control signal is changed in an abrupt manner. 
     These and other objects and features of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the conventional apparatus; 
     FIGS. 2(A) through 2(C) are waveform diagrams of signals associated with various portions of the apparatus shown in FIG. 1; 
     FIG. 3 is a block diagram illustrating an embodiment of the apparatus according to the present invention; and 
     FIGS. 4(A) through 4(D) are waveform diagrams of signals associated with various portions of the apparatus shown in FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 3, numeral 10 denotes an OR circuit having three signal inputs which are the firing pulses th 1 , th 2 , th 3  for the respective thyristors TH 1 , TH 2 , TH 3 , the firing pulses being obtained from the firing phase control circuit 9. The output of OR circuit 10 is connected to a wave-shaping circuit 11 which shapes the arriving firing pulses into short rectangular pulses, as shown in FIG. 4(B). These pulses are applied to a firing circuit input reset circuit 12. The latter circuit, which comprises a monostable multivibrator, which produces a rectangular pulse having a pulse width t, as shown in FIG. 4(C), each time a pulse is delivered by the OR gate circuit 10. The pulse width t is set equal to or slightly larger than, the thyristor commutation overlap time. The pulsed output of circuit 12 is connected to an input signal control circuit 13 comprising a transistor. When a rectangular pulse from circuit 12 enters the input signal control circuit 13, the output of amplifier 6 is shorted to ground for the interval t. Further, numeral 15 denotes a tachometer generator, 16 an amplifier, and 17 a summing point. Portions corresponding to those shown in FIG. 1 are denoted by like reference characters and are not be described again here. 
     The apparatus according to the present invention operates in the following manner. A speed command signal VCMD is applied to the apparatus input terminal IN. The signal is initially at a comparatively low level, so that the DC motor 3 is rotated at low speed. The speed command signal VCMD is subjected to negative feedback at the summing point 17 due to a signal from the tachometer generator 15. An error signal, derived at the summing point 17, from the signal VCMD and the feedback signal, is amplified by amplifier 16 and then applied to summing point 5 where it is subjected to negative feedback by the signal from the current sensor 4. The resulting error signal is amplified by amplifier 6 to produce the firing control input signal FS, which is in turn applied to the U-, V- and W-phase firing circuits 9u, 9v, 9w, whose other inputs are the sawtooth signals 8u, 8v, 8w, respectively. With the passage of time, the magnitude of signal FS equals the magnitude of the incline of sawtooth signal 8u, to the U-phase firing circuit 9 at time t a . This event is sensed by U-phase firing circuit 9u which responds by generating a signal initiating the firing phase control circuit 9 produce the firing pulses th 1 , th 3  &#39;. These pulses th 1 , th 3  &#39; are applied to the respective thyristors TH 1 , TH 3  &#39; which are fired and conduct. The firing pulse th 1 , after passing through the OR circuit 10, is shaped by wave-shaping circuit 11 into the pulse depicted in FIG. 4(B), and is then applied to the firing circuit input reset circuit 12. Circuit 12 responds to the received pulse by producing the signal pulse, shown in FIG. 4(C), that is applied to the input signal control circuit 13. Due to the width t of this signal, the input signal control circuit 13 is turned ON, conducts, for the time interval t, that is, at least for a period of time equivalent to the thyristor commutation overlap time. Accordingly, the firing control input signal FS is shorted to ground, temporarily dropping to zero for the interval t before rising again, as illustrated in FIG. 4(A). 
     The foregoing operation continues in the manner described until, at time t b , the firing control input signal FS experiences a sharp rise. The magnitude of the signal FS equals the magnitude of the incline of sawtooth signal 8v 2  applied the V-phase firing circuit 9v, which event is sensed by circuit 9v which responds by producing a signal. The firing phase control circuit 9 produces the firing pulses th 3 , th 2  &#39; of the signal, as shown in FIG. 4(D). The firing pulses th 3 , th 2  &#39; are applied to the respective thyristors TH 3 , TH 2  &#39; to render them conductive. Firing pulse th 3 , after passing through the OR gate 10, is shaped by wave-shaping circuit 11 into the pulse depicted in FIG. 4(B), and is then applied to the firing circuit input reset circuit 12. As described above, circuit 12 responds to the received pulse to produce the signal pulse shown in FIG. 4(C). This pulse is applied to the input signal control circuit 13 which therefore conducts for the time interval t, that is, at least for a period of time equivalent to the thyristor commutation overlap time. As before, the firing control input signal FS temporarily drops to zero for the interval t and then rises again. Thus, the signal FS, which attempts to rise sharply after intersecting the sawtooth signal 8v 2  at time t b , as shown by the dotted line in FIG. 4(A), is prevented from doing so by being shorted to ground for the interval of time that the input signal control circuit 13 is conducting. At the instant that the input control circuit 13 stops conducting, signal FS is already at a much lower level. In accordance with the invention, therefore, signal FS cannot cross the next sawtooth signal 8w 2  within the time interval t. This means that there is no disturbance in the timing of the firing pulses that fire the thyristors, thus assuring that the DC motor will be rotated in a smooth manner. 
     In the foregoing embodiment, the firing control input signal FS is shorted to ground, for an interval equal to at least the thyristor commutation overlap time, after the generation of each thyristor firing pulse. In an alternative arrangement, however, the level of the signal delivered by the firing circuit input reset circuit 12 can be adjusted in such a manner that the transistor comprising the input signal control circuit 13 is operated in the nonsaturated region, so that the firing control input signal is held at a constant level which is higher than zero. 
     The present invention is also applicable to an arrangement in which a DC motor is controlled by an open loop configuration, without the signal indicating the motor speed being fed back to the input side of the phase control mechanism. 
     According to the present invention, as described in detail above, when a firing control input signal changes abruptly during thyristor commutation, in a thyristor bridge, an input signal control circuit holds said signal at a level low enough to avoid misfiring during thyristor commutation. This makes it possible to prevent irratic firing control of the thyristors. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiment thereof except as defined in the appended claims.