Protective Method and Apparatus for a Controlled Current Inverter and Motor Control System

The subject invention is related to U.S. Pat. No. 4,230,979 entitled, "Controlled Current Inverter and Motor Control System," issued Oct. 28, 1980 in the names of Paul M. Espelage, et al., which application is also assigned to the assignee of this invention and is specifically incorporated hereinto by reference.

The present invention relates generally to controlled current inverter drive systems for powering an AC motor load, and more particularly to means for preventing unacceptably high levels of motor current from occurring as a result of system response to abnormal operating conditions.

In the past, DC (direct current) motors have been used for operation over a wide speed range as desired. More recently, AC motors have been finding greater application in variable speed drive applications. This is due in a large measure to the fact that an AC induction motor for example is inherently rugged, thereby reducing maintenance problems due to the lack of brushes, which makes AC motors particularly desirable for certain applications. There are, however, certain problems associated with the use of AC motors, particularly when the motor is supplied by power from a variable frequency thyristor controlled inverter (DC to AC) which in turn is fed power from a thyristor controlled converter (AC to DC).

In the above cross-referenced related U.S. Pat. No. 4,230,979 and in a publication entitled, "A High Performance Controlled Current Inverter Drive," authored by Loren H. Walker, et al., a IEEE Conference paper and appearing in the IAS Annular Conference Record data Oct., 1979 at pages 927-936, both of which are specifically incorporated herein by reference, there is disclosed a controlled current inverter system comprised of a thyristor bridge configuraton for supplying an AC load such as an induction motor with an AC current of variable magnitude and variable frequency. The system described therein employs a variable DC power source which is connected to a variable frequency inverter preferably by way of a DC link including an inductor. Means are included to develop signals representing the instantaneous electrical torque of the AC motor and the instantaneous angle .theta. (defined as the angle between the air gap flux and the stator current) which is representative of the air gap power factor. The electrical torque signal and the angle signal are utilized to control the DC current in the link and the gating angle of the inverter thyristors with respect to the motor flux such that the air gap power factor or angle .theta. is controlled in a predetermined manner over its prescribed operating range.

It is therefore an object of the present invention to provide an improved power conversion system, particularly as applied to the operation of an AC motor load.

It is another object of the present invention to provide a method and apparatus directed to a protective arrangement for an AC motor control and drive system.

Yet another object of the present invention is to provide an improved method and apparatus for an AC motor control and drive system which can quickly shift the operational mode of the motor in response to abnormal operating conditions.

It is a further object of the present invention to provide an improved method and apparatus for a motor control and drive system which is operative to prevent excessive circulating currents from occurring in the event of a system abnormality.

The foregoing and other objects are satisfied in accordance with the present invention through the provision of a thyristor controlled current inverter system for supplying an AC load such as an induction motor with an AC current of variable magnitude and frequency. The system employs a thyristor controlled DC power source in the form of a converter which is connected to a thyristor controlled inverter preferably by way of a DC link including an inductor. The motor drive is controlled by a single torque command signal which may be provided by a larger overall system of which the subject control and drive system is a part. In response to the torque command signal, the current in the DC link and the frequency of the inverter are controlled to thus control the motor torque and flux. In the event that the converter develops a short circuit condition, either because of a converter failure or because of converter response to control stimuli, when the motor is operating in a regenerative mode, method and apparatus are set forth in the present invention for sensing converter condition and quickly advancing the gating angle of the inverter for a brief time interval to shift motor operation from a regenerative mode toward a motoring mode, the resulting effect being to quench any excessive build-up of circulating current between the inverter, motor and converter. In a preferred embodiment, a signal generated in response to a detection of predetermined converter condition; e.g., a short circuit across the DC link, triggers a single pulse generator which operates to inject a voltage pulse into a variable frequency oscillator which in turn advances the phase of the gating angle of the inverter to cause the motor operational mode to shift from regeneration toward motoring.

While the present invention is particularly defined in both method and apparatus claims annexed to and forming a part of this specification, a better understanding can be had from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an electrical block diagram illustrating the present invention in its preferred embodiment;

FIG. 2 is an electrical schematic diagram illustrative of the single pulse generator portion of the diagram shown in FIG. 1;

FIG. 3 is a set of voltage waveforms helpful in understanding the operation of the subject invention; and

FIG. 4 is a set of voltage waveforms also helpful in understanding the operation of the subject invention.

Referring now to the drawings and more particularly to FIG. 1, which shows the present invention in its basic configuration, the invention centers around a controlled current inverter system including a source of variable DC current 10 which is implemented, for example, by means of a thyristor controlled AC to DC converter under the control of suitable control means 12. The means 12 comprise a thyristor gating circuit capable of generating and applying suitable gating signals to a three phase thyristor bridge circuit 10 coupled to a source of three phase (3.phi.) electrical power supplied via lines L.sub.1, L.sub.2 and L.sub.3. A current I.sub.DC is supplied from the converter by way of a DC link circuit, including a suitable filter for smoothing DC current from the converter 10 such as an inductor 14, to a thyristor controlled inverter 16. The inverter 16 is preferably comprised of a six thyristor bridge under the control of a variable frequency oscillator, such as a voltage controlled oscillator 18 and ring counter 20. In this well known type of control, the magnitude of the input signal to the voltage controlled oscillator controls the gating frequency of the thyristors of the thyristor bridge and accordingly the output frequency of the current fed to the load 22 which preferably comprises an AC induction motor.

As is known, the instantaneous air gap power factor or phase angle .theta. of the motor can be varied by changing the inverter output frequency, since any difference between the frequency of motor back EMF (flux) and the frequency of inverter current will appear as a rate of change of phase angle of current with respect to flux. Accordingly, the controllable parameters are the DC link current I.sub.DC and the inverter frequency which are used to control torque and flux as a function of the torque command. As shown in FIG. 1, a current control loop 24 and a frequency control loop 26 are coupled to the output of a torque command block 28. The current control loop 24 has for its purpose controlling the current I.sub.DC by means of the gating circuit 12 coupled to the converter 10 while the frequency control loop 26 has for its purpose providing a slip command signal which is applied to a signal summing junction 30 whose output is coupled to the voltage controlled oscillator 18.

The system up to this point comprises a simplified version of the system shown and described in the aforementioned U.S. Pat. No. 4,230,979 and can be referred to should a more detailed description be desired.

The present invention has for its primary objective the prevention of undesirable current build up should the converter 10 develop or be commanded to provide a short circuit condition at a time when the motor acts as a regenerative load causing power to be fed back to the converter through the inverter and the DC link circuit. In this type of power converting motor control, power may flow either from the AC source to the motor (motoring) or from the motor to the AC source (regeneration). The sense or direction of the direct current I.sub.DC is, as shown by the arrow in FIG. 1, independent of the direction of power flow. The sense of DC voltage appearing across terminals 29 and 31, however, reverses as the direction of power flow reverses. The polarity of voltage across terminals 29 and 31, indicated in FIG. 1, is for power flow toward the motor 22.

It might be said that when the power flow is toward the motor (as shown in FIG. 1) the converter voltage drives the DC current while the inverter voltage opposes the current flow and thus draws power from it. Conversely in regeneration, when polarity of the DC link voltage across terminals 29 and 31 is reversed, the inverter 16 is the source of voltage which drives the current and the converter voltage opposes the current and thus draws power from it. It is evident that any condition which causes the converter 10 to fail to exhibit an opposing voltage on the DC link, i.e. to short circuit the link, would place the entire voltage from the inverter 16 across the inductor 14. The sense of this voltage is such as to cause an ever increasing current in the inductor 14. It is readily apparent that unless immediate action is taken, the motor current will increase to unacceptable levels. It has been found that this increase in current can be controlled if the polarity of the back EMF of the motor appearing across terminals 29 and 31 of inverter 16 is quickly reversed or quenched by shifting from a regenerative mode toward a motoring mode. The system as shown in FIG. 1 is adapted to accomplish this through a momentary or transient advancement of the gating angle of the thyristors of inverter 16. This is illustrated by the waveforms of FIG. 3 in which A, B and C are motor terminal voltages, and 84 is the instantaneous voltage at the DC terminals 29, 31 of the inverter. In FIG. 3, reference numeral 32 designates the preexisting regenerative mode of operation, while reference numeral 34 designates a motoring mode of operation. It is seen that upon the advancement of the gating angle, for example by 60.degree., between phase B and C for one three phase interval, the required transition in operating mode takes place. Bearing in mind that the gating angle of the inverter thyristors is related to the operating frequency of the voltage controlled oscillator 18, the subject invention contemplates injecting a pulse signal of predetermined amplitude and time into the input of the voltage controlled oscillator 18 to speed up and trigger the ring counter 20 earlier than beforehand as shown.

The advancement of the gating angle is accomplished by means of a single pulse generator 36 (FIG. 1) which is triggered by an output signal from a condition detection circuit 38 along with a power flow sense signal provided from a power flow direction sensor 39. A typical example of a condition detection circuit is shown and described in U.S. Pat. No. 4,150,322, entitled, "Power Conversion System Having Inversion Fault Detecton and Correction Circuit," granted to Richard Miller, et al, on Apr. 17, 1979, which patent is also assigned to the present assignee. The teachings of this latter patent are herein incorporated by reference. The output of the condition detection circuit 38 is also coupled to and operates a normally closed electronic switch 40 coupled between the torque command block 28 and the control loops 24 and 26.

When the converter 10 causes a short circuit to be placed across the DC link 15, it is necessary to ultimately bring the motor 22 to a zero torque condition which corresponds to a zero power flow condition on the DC link. Opening switch 40 will cause the control loops 24 and 26 to bring the motor to a zero torque condition to satisfy this requirement. The response of the control loops, however, is not fast enough to prevent excessive current build up in the DC link. Accordingly the single pulse generator 36 acts in concert with switch 40 to satisfy the immediate and ultimate requirement of the system.

The power flow sensor 39 shown in FIG. 1 produces an output signal which is indicative of the direction of power flow; i.e. into or out of the inverter 16. The exact form of this sensor is not important to the present invention and the sensor may comprise merely a resistive attenuator, not shown, connected across terminals 29 and 31 of the inverter 16. The polarity of the signal from this attenuator; i.e. the output of sensor 39, is dependent upon the direction of power flow.

Having considered the present invention in terms of the block diagram shown in FIG. 1, reference is now made to FIG. 2, wherein there is disclosed the means by which the advance gating of the inverter 16 is implemented. The single pulse generator 36 referred to in FIG. 1 is shown in FIG. 2 comprised of, among other things, a well known one shot (monostable multivibrator) circuit 42 which is adapted to produce a single pulse of predetermined pulse width in response to a trigger signal applied thereto via signal lead 43. The trigger signal appearing on lead 43 is the output signal from the condition detection circuit 38 of FIG. 1. This single pulse is used to produce the waveform segment 44 shown in FIG. 4 in the following manner. The pulse is applied to one input of an AND gate 46 whose other input comprises an enabling signal from the output of a comparator circuit 48 which is responsive to two input signals, one being the power flow sense signal provided by the sensor 39 of FIG. 1 and a reference signal -V.sub.ref. The power flow sense signal from the sensor 39 comprises a negative DC voltage during a regenerative mode of motor operation. Accordingly, it is compared against the negative reference -V.sub.ref and if the sense signal is more negative than the -V.sub.ref voltage, the circuit 48 produces a positive going output signal which is applied to the AND gate 46 along with the positive going pulse waveform from the one shot circuit 42. Under such conditions, the AND gate 46 puts out a positive pulse of the same pulse width as that from the one shot 42 which is applied simultaneously to two other AND gates 50 and 52 whose other inputs are derived from a sense of rotation signal of the motor 22. This latter signal is the resultant output of a comparator 47 which has a first of its inputs connected to ground and a second input connected to and receiving the output from the tachometer 23 (FIG. 1). Rotation of the motor 22 in a first direction causes AND gate 50 to be enabled, while rotation in the opposite direction will cause AND gate 52 to be enabled due to the presence of the signal inverter circuit 54. The outputs of the AND gates 50 and 52 are respectively coupled to and control a pair of electrical switch devices 56 and 58 which are adapted to close for a time period corresponding to the pulse width of the pulse from the one shot circuit 42, i.e. so long as the respective AND gate 50 or 52 is enabled. Upon closure of either switch 56 or 58, a voltage having an amplitude of either +V.sub.pulse or -V.sub.pulse will be present at circuit junction 59 for a time equal to the pulse width generated by the one short circuit 42. The sense of motor rotation determines which polarity of V.sub.pulse is applied to summing junction 59. The signal at junction 59 is applied to junction 30 by means of summing resistor 60 along with the slip command signal from the frequency control loop 26 (FIG. 1) via summing resistor 62 and the positive feedback signal indicative of motor speed provided by the tachometer 23. The motor speed signal is applied to the summing junction 30 by means of a summing resistor 64. The composite of these three signals appears as waveform 65 in FIG. 4.

The voltage controlled oscillator 18 shown in FIG. 1 comprises a bi-directional VCO and is of an integrated type in that it integrates the frequency command applied to summing junction 30 until the potential of the output of amplifier 66 at point 67 reaches a predetermined positive or negative value at which time the oscillator 18 produces an output pulse and resets the integrator. FIG. 2 includes an illustration of the integrator portion of the voltage controlled oscillator 18 and is shown including an operational amplifier 66 an integrator capacitor 68 and a reset switch 70. A typical example of this type of circuit is shown and described in the aforementioned Walker, et al. publication. The circuit is operative to produce a ramp voltage output as shown in FIG. 4 by the waveform 72. Output pulses 74 for triggering the ring counter 20 are produced whenever the amplitude of the ramp waveform 72 exceeds a predetermined DC threshold 76. It can be seen from FIG. 4 that in absence of the waveform 44 a regularly occurring ramp voltage 72 of constant slope will be produced by action of the integrating capacitor 68. However, upon the generation of the waveform 44 by the one shot circuit 42, the ramp voltage is caused to rise at a faster rate as indicated by waveform segment 78 due to the fact that the feedback capacitor 68 charges at a faster rate. Accordingly, the threshold value 76 will be reached at a time earlier than normal and an extra pulse 80 will be produced. This pulse 80 is produced but once with the succeeding pulses 74 appearing at their otherwise normal times.

While in the specific embodiment described a single extra pulse providing 60.degree. phase advancement in a six-pulse inverter is utilized, it is to be understood that the phase advance may be varied, as desired, according to the application desired and equipment used, within the range of from 0 to 180 electrical degrees. This variation may be accomplished by changing either or both the amplitude and duration of the pulse 44 (FG. 4) so that the required number of extra pulses such as shown at 80 are produced.

It can be seen now with reference back to FIG. 3 that for a regenerative mode of operation as further depicted by segment 32 of the power waveforms 80 that if the gating angle is advanced 60.degree. to point 82, for example, the operating mode of the motor will shift to a zero torque mode as depicted by segment 34 of waveforms 84, which will continue upon a regular gating sequence being thereafter applied. Thus by generating and applying a single pulse waveform 44 of predetermined width and amplitude to the voltage controlled oscillator 18 a single thyristor gating pulse will be produced earlier than usual and thus effect the 60.degree. advanced gating desired. This provides an automatic protection against motor current build up, which would otherwise have a deleterious effect on the system operation, should a sudden short circuit condition occur in the converter 10 during a regenerative mode of motor operation.

While there has been shown and described what is at present considered to be the preferred embodiment of the present invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described, but it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.