GB2114780A - Current control pulse width modulated inverter machine drive system - Google Patents

Abstract

A machine drive control system for providing stable motor operation in the presence of a cyclic load, having a current controlled inverter 24, a first control loop 30 for maintaining motor terminal voltage under varying load conditions, and a second control loop 58 for stabilizing motor frequency against hunting oscillations. A waveform generator 10 produces three sinusoidal reference signals in three-phase relationship. These signals are compared with feedback current signals from the motor phases, and PWM control signals are supplied to the inverter 24. The loop 30 responds to motor flux determined from phase voltage measurements and a flux command signal psi * to produce amplitude commands for waveform generator 10. Loop 58 responds to phase current supplied by a synchronous switch 62 and an amplifier 70 responsive to transient changes to determine the frequency commands for generator 10. <IMAGE>

Description

SPECIFICATION Current control pulse width modulated inverter machine drive system This invention relates generally to alternating current machine drive systems, and more specifically to alternating current machine drive systems having a current controlled inverter.

In alternating current machine applications where regulation of machine speed and torque is desired, inverter machine drive systems are employed. Typically, an inverter machine drive system comprises a DC-AC variable frequency inverter supplied by a DC source, delivering variable frequency power to an alternating machine, either a synchronous or induction type.

Such inverters are commonly configured of a plurality of pairs of switching devices with the switching devices of each pair coupled in seriesaiding fashion and each of the pairs of seriallycoupled switching devices coupled across a direct current source. Connected at the junction between each of the serially coupled switching devices is a respective machine phase. When the switching devices of each pair are alternately rendered conductive in sequence, alternating power is supplied by the inverter to the machine.

It is an object of the present invention to provide a simplified drive control system which can be used in applications requiring speed control and in which precision torque regulation is not needed.

It is a further object of the present invention a control system that will achieve more stable operation of the motor in the presence of a cyclic load than is typical of open loop operation with a voltage controlled inverter.

Generally stated, in one aspect, the present invention provides a motor control interposable between an electrical energy source and the stator of an alternating current motor. The motor controller has a current controlled inverter containing current regulators which compare sinusoidal reference waveforms produced by a waveform generator to a corresponding actual motor line current to develop error signals. The error signals are compared to a hysteresis band and pulse width modulated signals are sent to the inverter so that the error signals remain confined within the hysteresis band. A first control loop is also provided to maintain the voltage at the motor terminals under varying load conditions. Motor flux is determined in the first control loop by integrating the voltage sensed at the output of the inverter.The motor flux is rectified and compared with a reference flux which may be constant, if constant volts/Hertz operation is desired to produce an error signal. The error signal is amplified and used to control the inverter current input.

In another aspect of the invention a second control input can be added to stabilize motor frequency. The power producing component of the motor line current is determined and increases of the power producing component of the current result in brief reductions of the inverter frequency to achieve stabilization of the motor rotor against hunting oscillations.

While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, the objects and advantages of the invention can be more readily ascertained from the following description of a preferred embodiment when read in conjunction with the accompanying drawing in which: Fig. 1 is a part schematic and part block diagram representation of the present invention.

Fig. 2 is a phasor diagram showing the relationship between line to neutral voltages and air gap flux in the motor.

Fig. 3 are waveforms diagrams showing the input and output waveforms of the synchronous rectifier in Fig. 1.

Fig. 1 illustrates an alternating current motor control. A pulse width modulated current controlled inverter 8 comprises a waveform generator 10 supplying each of 3 current regulators 1 2, 14, and 1 6 with one of three sinusoidal reference signals, each of the reference signals being in a three phase relationship with one another. The amplitude and frequency of each of the three sinusoidal signals generated by the waveform generator, varying in accordance with.a frequency command and an amplitude command signal input to the waveform generator. The current regulators 1 2, 14, and 1 6 in addition to having inputs from the waveform generator each have an input from current sensors 1 8, 20 and 22 which are connected to the output of inverter 24.

The current regulators provide pulse width - modulated signals to inverter 24. Power is supplied to the inverter by the DC supply 26. The output of the inverter which consist of 3 lines A, B, and C is connected to the stator winding of motor 28.

A first control loop 30 has a different amplifier 32 comprising a resistor 34 coupled to the negative input terminal of an amplifier 36 and a resistor 38 coupled to the positive input of the amplifier. Connected to the resistor 38 is the line A voltage and connected to resistor 34 is the line B voltage. Resistor 40 is connected between the positive terminal of amplifier 36 and ground. A feedback resistor 41 is connected between the output of amplifier 36 and the negative input of the amplifier 36.

The output of the differential amplifier which is the difference between line voltage A and B is input to the-integrator 42. Integrator 42 comprises a resistor 44 to which the input is connected. The other side of resistor 44 is connected to amplifier 46. A feedback resistor 48 and a feedback capacitor 50 are connected between the input and output of the amplifier 46. The output of the integrator is passed through a rectifier and filter 52 and the output is connected to a negative input terminal ofsumming junction 54. Connected to the positive input of summing junction 54 is a flux command signal *, The output of the summing junction is connected to a flux regulator 56. The output of the flux regulator is connected to the amplitude control input of the waveform generator 10.

A second control loop 58 receives the line voltage C at one input of a single pole double throw switch 62. The other input to the switch is inverted line voltage C from a signal inverter 64.

The output of integrator 42 is connected to the positive input of a comparator 66 and the negative input terminal is grounded. The output of comparator 66 controls the position switch 62.

The synchronously rectified output of switch 62 is passed through a low pass smoothing filter 68 which in turn is connected to a circuit 70 having a transfer function of eos/(s+c.)) which acts to remove the steady state value of the smoothing filter output and pass only the transient changes.

A unity gain amplifier has a resistor 72 having a value of R connected to the input of amplifier 73.

Connected between the input and output of amplifier 73 is a resistor 74 also having a value of R. Connected in series with the input to the unity gain amplifier is a capacitor 71. The output of the circuit 70 is connected to the negative input of summing junction 76. Another implementation of the transfer function could be a series capacitor connected to a resistor which goes to ground. The output being taken between the resistor and capacitor. The positive input of the summing junction 76 is connected to an operatorcommanded frequency F*. The error signal is applied to the frequency command input of the waveform generator 10.

The operation of Fig. 1 will now be described.

The waveform generator 10 supplies a sinusoidal reference signal to each of the current regulators 12, 14 and 16. The sinusoidal signals are in a three phase relationship with one another. The reference signal supplied to each current regulator is compared to a corresponding motor line current signal to produce an error signal. The error signal is then applied to a comparator (not shown) located in the current regulator, and when the error signal is outside a predetermined hysteresis band in one direction a logical "1" is produced and when the error signal exceeds the hysteresis band in the other direction a logical "-1" signal is produced. The train of logical "-1" and "1" pulses from each of the current regulators are the pulse width modulated switching signals for a respective phase of the inverter.The inverter switching signals are connected to a gating circuit which controls the pairs of switches in the inverter, each pair is associated with a phase of the motor. A logical "1" signal, for example, associated with a pair of switches in the inverter causes the upper switch to turn on and the lower switch to turn off and connects the DC supply across a corresponding phase of the motor. A logical "-1" from the same current regulator causes the lower switch of the pair to turn on and the upper switch to turn off, connecting the DC source in an opposite polarity across the motor phase. The repeated switching of three pairs of inverter switches result in three phase power being supplied to the motor, with the current supplied in each phase remaining within the corresponding predetermined hysteresis band.For more detailed information concerning the inverter operation, reference can be made to our U.S.

Patent No. (Serial No. 080,479) entitled, "Transistorized Current Controlled Pulse Width Modulated Inverter Machine Drive System", (U.K.

Patent Application No. 8031395).

The first control loop 30 monitors the motor voltage of lines A and B. Differential amplifier 32 obtains the difference of B and A and integrator 42 integrates the difference to obtain the flux of A-B. In addition, integrator 42 removes the chopping ripple and a lot of the noise present in the circuit. The varying flux signal A-B is rectified and filtered and then compared to a commanded flux level *. The resultant error signal is connected to a flux regulator which provides a minimum and maximum output amplitude level with gain between the two limits. The minimum level prevents operation at zero flux (if an induction motor is used) and the maximum iimit is necessary to avoid overcurrent in the inverter output.The output of the flux regulator is used to control the amplitude input of the waveform generator which in turn controls the inverter current. If the commanded flux * is constant, then to a first order constant volts/Hertz operation can be obtained. When the load on the motor increases and the motor is being supplied with a constant current, the motor voltage will drop. The decreased motor voltage will be detected by the first control loop which will increase motor current amplitude to compensate and return the voltage to its previous value. Also, as the frequency varies, the integrator in the control loop automatically compensates for frequency variations, since the voltage should linearly increase with speed.The operation of the inverter with the first control loop results in characteristics similar to an open loop voltage converter system while still maintaining the inherent features of a current controlled pulse width modulated system, namely, varying of the pulse width of the inverter switching pulses to keep the ripple current low and thereby keep the motor losses down. The drive circuit will operate on any three-phase motor by just attaching the output of the inverter to the motor stator terminals. The pulse width modulated current inverter with the first control loop can also be used as a three-phase constant voltage power supply.

When used as a power supply, the flux regulator would have unity gain and the integrator time constant of integrator 42 would be relatively longer than the time constant used in a drive circuit application.

The second control loop 58 operates to provide stabilization against rotor oscillations or hunting.

Rotor oscillations result from the harmonics present in the inverter output which cause oscillatory torque which are a problem especially at low speeds. In a 60 cycle motor, for example, rotor speed pulsations are most prevalent at 1 20 cycies of applied frequency. The second control loop 58 determines the real component of motor stator current by providing the total stator current of line C at one of the two terminals of a single pole double throw switch 62 and providing inverted current from line C to the other input terminal. The position of switch 62 is determined by the comparator 66 which monitors the zero crossings of the flux of B-A which is in phase with the line current.

For the proper operation of the synchronous inverter 62, it is necessary to have a signal that is in phase with the line voltage C of the motor.

Referring now to Fig. 2 which shows the phase relationships between the voltages and the airgap fluxes in the mOtor, it can be seen that the difference between the line to neutral voltage A and the line to neutral voltage B results in a line to line voltage of A-B. The voltage of B--A lags the voltage C by 900. The flux due to the voltage of A-B is in phase with the voltage of phase C and is in phase with the real or power producing component of the line current in phase C.

The flux of A-B is determined by the output of intergraior 42 in the first control loop. When the flux of A-B is positive, switch 62 passes line current, when flux is negative, switch 62 passes negative line current from signal inverter 64.

Referring to Fig. 3, the waveforms involved in the synchronous rectification are shown. Fig. 3a shows the flux signal from the comparator. Fig. 3b shows a current that is in phase with the voltage.

(With in induction motor the current would typically be 300 out of phase with the voltage at full load.) A current in phase with the voltage means that the entire current is real and power producing. Fig. 3c shows the output of the switch acting as a synchronous rectifier when the conditions of Fig. 3b prevail. Fig. 3d shows the current waveform for a zero load condition with the current 900 out of phase with the voltage. The current in Fig. 3d has no real component. Fig. 3e shows the output of the switch acting as a synchronous rectifier when the conditions of Fig.

3d prevail.

In actual operation, the current is not a pure sine wave but includes harmonics which reduce the accuracy of the real current determinations.

The real current component measured in this way, however, is sufficiently accurate for stabilization purposes. The output of the switch 62 is sent through a smoothing filter. The waveform in Fig.

3b results in a much larger signal at the output of smoothing filter than the waveform of Fig. 3e. The real current is then sent through a unity gain amplifier 70 that has a transfer function of c.)s/(s+es) where s is a complex variable of the La Place Transform, and a; is a function of the frequency of the waveform applied to the amplifier. The characteristics of the transfer function is such that the DC component is removed and increases in the real current input to the transfer function result in momentary abrupt changes in the output of the transfer function. The output of circuit 70 is connected to act as a negative feedback to the frequency command signal. As the current increases because the load increases due to the action of the first control loop, the second control loop will detect the increase of the real component of current and will output a brief pulse that will momentarily depress the frequency command to the waveform generator. Depressing the frequency input to the function generator causes the rotor to briefly slow down a little as the load increases, keeping the motor from picking up the torque immediately.

Briefly depressing the frequency during increasing load conditions acts as a negative feedback and stabilizes the rotor against hunting oscillations.

It is understood that the foregoing detailed description is given merely by way of illustration and that many modifications can be made therein without departing from the spirit or scope of the present invention.

Claims (5)

1. A motor control interposable between an electrical energy source and the stator of a multiphase altornating current motor for controlling motor speed, said control comprising: a current controlled inverter having waveform generator means for supplying multiphase reference signals which vary in frequency and amplitude in response to frequency and amplitude command signals, respectively, feedback means for providing signals proportional to each of the motor line currents, and current regulator means for comparing each of said waveform generator reference signals to a corresponding line current from said feedback means to produce an error signal and said current regulator means also comprising means for comparing each of said error signals to a hysteresis band to provide pulse width modulated signals to said inverter so that said error signals remain confined within said hysteresis band; and a first control loop for maintaining the voltage of the stator of said motor under varying loads having integrator means for providing a signal proportional to motor flux from the output voltage of said inverter, rectifier means for rectifying said signal, summing means for determining the difference between said output of said rectifier means and a predetermined flux command to produce a flux error signal; and flux regulator means for providing gain to said flux error signal, said error signal being connected to said amplitude control input of said waveform generator means.

2. The motor control system of claim 1 further comprising: a second control loop for stabilizing the rotor frequency having means for determining the real component of motor current, means for providing a brief pulse output in response to an increase in the real component of motor current; summing junction means for determining the difference between the output of said means for providing a brief pulse and an operator commanded frequency signal, said difference being connected to said frequency control input of said waveform generator.

3. The motor control circuit of claim 1 further comprising: a second control loop for stabilizing the rotor frequency comprising means for rectifying the inverter output current of a third output of said inverter in synchronism with the zero crossing to the flux due to the difference between the second and first output invertervoltage, and smoothing filter means for smoothing the output of said synchronous rectifier; and means for providing a brief pulse in response to an increase of the real component of the third output current of the inverter as a negative feedback signal to the frequency command.

4. The motor control of claims 2 and 3, wherein said means for providing a brief pulse comprises a unity gain operational amplifier having a capacitor in series with said amplifier input terminal.

5. A motor control circuit substantially as described herein with reference to the accompanying drawings.