CROSS COUPLED CURRENT REGULATOR

Abstract: A current regulator for a variable frequency power supply includes a proportional (51 q ; 51 d ) plus integral circuit (52 q , 53 q ; 52 d ; 53 d ) that produces a composite control output signal (5; 6) for each phase. To improve the performance of the current regulator at higher frequencies, the composite control output signal for each phase also includes a cross coupled component (55 q , 56 q ; 55 d , 56 d ) which is produced by multiplying a d.c. signal (10) proportional to frequency times an integrator (53 q ; 53 d ) output signal from another phase. The composite control signals (5; 6) are applied to a voltage source inverter which produces the output currents to a load.

Description: The field of the invention is variable speed drives for a.c. motors, and particularly, alternating current sources which provide polyphase, variable frequency, and variable amplitude currents to motor stator windings. Prior alternating current sources employ solid state switches to produce pulse waveforms that approximate sinusoidal waveforms suitable for application to the motor's stator windings. These are generally divided into two classes: those which employ current source inverters; and those which employ voltage source inverters. A current source inverter receives a controlled d.c. "link" current and switches it between the motor stator windings in such a manner as to approximate the application of polyphase sinusoidal currents of the proper frequency and amplitude. Such an inverter is disclosed, for example, in U.S. Patent No. 4,400,655. A voltage source inverter on the other hand, receives a d.c. voltage and chops it into a series of voltage pulses which are applied to the motor stator windings. The widths of the pulses are modulated such that the resulting currents produced in the motor stator windings have a sinusoidal shape of the desired frequency and amplitude. Such an inverter is shown, for example, in U.S. Patent No. 4,469,997. Both the current source and the voltage source inverters have their respective advantages and disadvantages which are well-known in the art. As a result, both technologies are employed in commercially available motor drive products, with the choice being determined primarily by performance and price consideration. The present invention solves a problem which is inherent in prior voltage source inverters that employ current regulators. It has long been recognized that at high motor speeds such voltage source inverters do not accurately control sinusoidal motor currents. This has been attributed by some to a loss in gain in voltage source inverters at high speeds, while others relate the problem to the back e.m.f. of the motor. Numerous schemes for solving this problem have been proposed, some of which involve compensation circuitry that is either very complex or that requires detailed knowledge about the particular motor being driven. The inability of such voltage source inverters to accurately produce current waveforms of commanded amplitude and phase is particularly troublesome when such inverters are used in a motor drive that relies on controlling the magnetic flux vector. Such vector control strategies require that the amplitude and phase of the sinusoidal current waveform applied to each stator winding be precisely controlled at all times. Only then will the total magnetic flux have the proper magnitude and direction to produce the desired motor torque and speed. EP-A-119 583 discloses an AC current control system for controlling multi-phase AC currents supplied from a power converter to a multi-phase load. Current detectors detect the AC currents supplied to the load and produce signals indicative of the detected multi-phase current values. A first coordinate transformer transforms the produced AC signals into two DC or quasi-DC current feedback signals which represent orthogonal two-axes current values in a rotating reference frame rotating at an angular frequency equal or close to the angular frequency of the AC currents. Each current control phase comprises a first summing means producing a DC or quasi-DC error signal which is indicative of the difference between a DC or quasi-DC current reference signal and the DC or quasi-DC current feedback signal. The error signal is supplied to a PI (proportional plus integral) control amplifier and to a first input of a multiplier. A signal proportional to the angular frequency equal or close to the angular frequency of the AC currents is supplied to a second input of the multiplier. The output signal of the multiplier is supplied to an I (integral) control amplifier. A second summing means receives the output signal of the PI control amplifier and a signal cross-coupled in a feed forward arrangement from the output of the multiplier from the other current control phase. A second coordinate transformer transforms the two-axes DC or quasi-DC output signals of the second summing means of both current control phases into multi-phase AC voltage reference signals which are used to determine the multi-phase output voltages of the power converter. While the AC current control system known from EP-A-119 583 attempts to correct deficiencies in the classical or stationary PI control system, it is load dependent, frequency dependent and gain dependent. The present invention relates to a current regulator for a variable frequency power supply employing a voltage source inverter, and is intended to improve the current regulating capability of such inverters, particularly, at high frequencies. The current regulator of the present invention eliminates errors which are introduced into the current regulator by the conventional proportional plus integral (PI) compensation circuit that is uniformly employed. It has been discovered that such conventional PI compensation circuits are not accurate when regulating a.c. currents. This inaccuracy increases as a function of the a.c. frequency of the currents being regulated, and this accounts for the difficulties which are encountered when current regulated voltage source inverters are employed to drive electric motors. Unlike prior circuits, the present invention provides accurate current regulation under steady-state conditions at any operating frequency and, hence, accurately regulates the a.c. current produced by a voltage source inverter. An advantage offered by the invention is to provide current regulation for a voltage source inverter which is economical to build and operate. The improved current regulator includes only a few passive components and integrated circuits in addition to those normally employed in a conventional PI current regulator. Furthermore, the present invention is adapted to improve the operation of polyphase motor drive systems which employ vector control strategies to control motor speed, torque or magnetic flux. The present invention may be employed to control the magnitude of n separate a.c. phase currents in response to two orthogonal a.c. current command signals. The magnitude and phase of the resulting motor currents accurately reflect the commanded currents at all operating frequencies. One way of carrying out the invention is claimed in claim 1 and is described in detail below with reference to the accompanying drawings which illustrate a preferred embodiment of the invention. In the drawings: FIG.1 is a block diagram of a current regulated voltage source inverter driving a two-phase load; FIG.2 is a block diagram of a current regulated voltage source inverter driving a three-phase load; FIG.3 is a functional diagram of a current regulator which employs the present invention; FIG.4 is an electrical schematic diagram of the current regulation of FIG. 3; and FIG.5 are graphic representations of currents and voltages which appear at various points in the system of FIG. 1. Referring particularly to FIG. 1, a load 1 is supplied with two-phase a.c. power by a voltage source inverter 2. The load 1 may be any one of a number of devices such as a synchronous motor, induction motor, electrostatic precipitator, induction heating unit or corona treater. The voltage source inverter 2 is a well-known circuit which produces pulse width modulated voltage pulses on the lines 3 and 4 in response to control signals on lines 5 and 6. As shown in FIG. 5, these voltage pulses have a constant amplitude (V), but their widths are modulated such that the currents, i q and i d flowing in the respective lines 3 and 4 are substantially sinusoidal in shape. The amplitude, frequency and phase of these output currents i q and i d are determined by the amplitude, frequency and phase of the control signals on lines 5 and 6. Voltage source inverters such as those described in U.S. Patent Nos. 4,469,997; 3, 830,003 and 3, 700,987 may be employed for this purpose. The output currents i q and i d are precisely controlled by a current regulator 7 which produces the control signals on the inverter input lines 5 and 6. The current regulator 7 receives two sinusoidal current command signals, and which are compared with sinusoidal current feedback signals i q and i d that are received from current sensors through respective lines 8 and 9. It is the function of the current regulator 7 to produce sinusoidal control signals on the lines 5 and 6 which will drive the voltage source inverter 2 in such a manner as to cause the respective feedback signals i q and i d to equal the current command signals i * / q and i * / d. There are numerous well-known current regulators which purport to perform this function, but in all the most complex circuits, these prior current regulators fail to perform accurately over a wide range of frequencies. As well be discussed in more detail below, as part of the solution to this problem the current regulator 7 of the present invention employs a d.c. input signal (ω) on line 10 which has a magnitude proportional to the frequency of the command currents i * / q and i * / d. Referring particularly to FIG. 2, a current regulated voltage source inverter system may also be employed to drive a three-phase load 20. A voltage source inverter 21 similar to that used in the two-phase system may be employed, however, it is driven by three sinusoidal control signals on lines 22-24 and it produces three output currents i a , i b , i c . The three-phase output currents (i a , i b , i c ) have the same magnitude and frequency, but they are displaced 120 degrees in phase. The same regulator 7 may be employed in this three-phase system, but phase conversions must be made. More specifically, the two-phase current regulator control signals which are output on lines 5 and 6 must be converted to the equivalent three-phase signals on lines 22-24. This conversion is performed by a 2-phase-to-3-phase converter circuit 25 of well-known construction. For example, a circuit such as that described in "Control and Simulation of a Current Fed Linear Inductor Machine" by B. K. Bose and Thomas Lipo published in IEEE-IAS Conference Record , pp. 876-883, 1987, may be employed for this purpose. Conversely, the three output currents i a , i b and i c which are fed back through lines 26-28 are converted to two-phase feedback signals i q and i d by a 3-phase-to-2-phase converter circuit 29. A circuit such as that described in "Control Methods for Good Dynamic Performance Induction Motor Drives Based on Current and Voltage as Measured Quantities", by Robert Joetten and Gerhard Maeder and published in IEEE-IAS Transactions , IA-19, No. 3, May/June 1983 , may be employed for this purpose. It should be apparent to those skilled in the art that the current regulator of the present invention may be employed in a wide variety of applications to control a.c. loads having any number of phases. Also, the current command signals i * / q and i * / d, as well as the speed signal ω, may be produced by any one of a number of well-known control circuits. The specific construction of the control circuit will depend on the nature of the load and on the particular control strategy which is being implemented. A number of different control circuits for a.c. motors are disclosed in U.S. Patent Nos. 4,506,321 and 4,266,176. The current regulator of the present invention provides a combination of proportional control action, integral control action, and cross coupled speed compensation action. The "q" phase and the "d" phase are handled in the same manner, and the corresponding functional blocks and circuit elements in each phase of the current regulator have been given the same reference number. Referring particularly to FIG. 3, the sinusoidal current feedback signal i q is subtracted from the sinusoidal current command signal at a summing point 50 q . The resulting error signal (e q ) produces a proportional control signal through a proportional block 51 q and an integral control signal through blocks 52 q and 53 q . These two control signals are added together at summing point 54 q to produce the composite control signal on line 5. This composite control signal also includes a cross-coupled component which increases in magnitude at higher frequencies (ω). It is produced by a multiplier block 55 q that receives a cross-coupled signal from the integrator block 53 d of the other phase. The amplitude of this cross-coupled signal is modulated by a second input to the multiplier 55 q which is proportional to frequency. This second input is the d.c. frequency signal (ω) on the line 10. The cross-coupled component which is output by the multiplier 55 q is added at a summing point 56 q which connects to the input of the integrator 53 q . Under steady state conditions the error signals e q and e d should be zero. The composite control signals on the lines 5 and 6, however, must be waveforms which cause the voltage source inverter 21 to produce output currents corresponding to the commanded currents i * / q and i * / d. Under most operaing conditions these composite control signals are substantially sinusoidal in shape as shown in FIG. 5. These steady state sinusoidal composite control signals are produced by the cross-coupling of the present invention. More specifically, under steady state conditions the cross connections between the two phases from an oscillator which operates at the commanded frequency. The amplitude of the cross-coupled component produced by this oscillator is proportional to frequency. If for any reason an error signal develops in either phase, the input to the affected integrator 53 q or 53 d will at that instant include both the cross-coupled component and an error component (K I e). The resulting composite control signals on the corresponding output line 5 or 6 will include the integral of these two components and a component proportional to the error signal (K p e). This composite control signal will force the error signals e q and e d to zero through a coordinated control between the phases to insure balanced control even at higher frequencies. Referring particularly to FIG. 3 and 4, the preferred embodiment of the invention is constructed using passive components and standard commercially available integrated circuits. The summing point 50 q , for example, is implemented with an operational amplifier 100 q which receives the current command signal i * / q and current feedback signal i q at its inverting input. The values of resistors R₂ are the same to provide unity gain at the summing point 50 q . Similarly, the summing point 54 q at the output is implemented with an operational amplifier 101 q . The values of resistors R p and R₄ provide the gian (K p ) for the proportional block 51 q , and the values of resistors R₃ and R₄ are the same to provide unity gain for the integral component. The integral block 53 q is implemented with an operational amplifier 102 q having a feedback capacitor C connected between its output and its inverting input. An input resistor R I connects to the same inverting input and its value relative to the value of capacitor C provides the integral gain (k I ). An inverter having unity gain is formed by an operational amplifier 103 q and associated resistors R₁. This inverter insures that the signal produced by the integral block 53 q has the same sign as the signal produced by the proportional block 51 q . The inverting input to the operational amplifier 102 q also forms the summing point 56 q which receives the cross coupled signal from the multiplier 55 q . The multiplier 55 q employs a commercially available integrated circuit manufactured by Motorola, Inc. and sold as part number MC 1595L. Its output connects to the summing point 56 q through a resistor having a value R I /C. This provides a unity gain for the cross coupled component. The multiplier 55 q is described in more detail starting at page 6-83 in the book "Linear Integrated Circuits", published by Motorola, Inc. in 1979. One of its inputs connects to the line 10 to receive the frequency signal ω, and its other input connects to the output of the integrator in the other phase. The multiplier 55 d is connected in a similar manner, but it connects to receive the inversion of the integrator output from the operational amplifier 103 q . An oscillator is formed by these cross connections. More specifically, a loop is formed by the multiplier 55 q , integrator 102 q , inverter 103 q , multiplier 55 d and integrator 102 d . The phase shifts around this loop total 360 degrees which causes it to oscillate at the frequency ω. The amplitude of the cross coupled signals produced by this oscillator is determined by the frequency ω. The magnitude of the frequency signalω is set to produce maximum possible signals at the outputs of the multipliers 55 q and 55 d when the highest operating frequency is reached. While the preferred embodiment of the invention employs two phases (q and d) which are in quadrature with each other, current regulators having other numbers of phases may also be constructed. It is only necessary that the cross coupled signals have the proper phase and amplitude relationship. Referring to FIG. 3, for example, the q-phase composite control signal on the line 5 lags the d-phase composite control signal on line 6 by ninety degrees. The cross coupled signal from integrator block 53 q is shifted 180 degrees in phase by the inversion of the frequency signal ω into the multiplier 53 d , and it is delayed another 90 degrees by the integrator block 53 d to bring it into phase with the d-phase composite control signal on line 6. Conversely, the cross coupled signal from the output of integrator block 53 d is merely delayed 90 degrees by the integrator block 53 q to bring it into phase with the q-phase composite control signal on line 6. It should be apparent to those skilled in the art that with three or more phases cross coupled signals would be received from each of the other phases, and that the vector sum of these cross coupled signals should be in phase with the subject phase and have unity gain as described above.

Claims: A current regulator for a variable frequency power supply employing a voltage source inverter which in response to composite control signals provided by said current regulator produces sinusoidal output currents to a load, said current regulator having a plurality of phases for receiving a plurality of sinusoidal current command signals, each current regulator phase comprising: first summing means (50 q ; 50 d ) for receiving at one input one of said sinusoidal current command signals and receiving at a second input a sinusoidal current feedback signal indicative of sinusoidal output current supplied to said load (1; 20), the first summing means (50 q ; 50 d ) being operable to produce an error signal which is indicative of the difference between the sinusoidal current command signal and the sinusoidal current feedback signal; second summing means (56 q ; 56 d ) for receiving at one input a signal proportional to said error signal and for receiving at a second input a cross-coupled signal, the second summing means being operable to produce a summed signal which is proportional to the sum of the signals applied to its two inputs; an integrator (53 q ; 53 d ) having an input connected to receive the summed signal from the second summing means (56 q ; 56 d ) and being operable to produce an integrator output signal which is the integral of the summed signal applied to its input; third summing means (54 q ; 54 d ) for receiving at one input the integrator output signal and for receiving at a second input a signal proportional to said error signal, the third summing means (54 q ; 54 d ) being operable to produce one of the composite control signals which is proportional to the sum of the signals applied to its inputs; and means for producing said cross-coupled signal which includes: a) a multiplier (55 q ; 55 d ) having its output connected to the second summing means (56 q ; 56 d ), b) means for coupling to one input on the multiplier (55 q ; 55 d ) a signal which is proportional in magnitude to the frequency of said one sinusoidal current command, and c) means for coupling to another input on the multiplier (55 q ; 55 d ) a signal proportional to the integrator output signal produced by an integrator (53 d ; 53 q ;) in another of said current regulator phases. The current regulator as recited in claim 1 in which the current regulator has two phases and the two sinusoidal current command signals are phase displaced ninety degrees with respect to each other.

Inventors: KERKMAN, RUSSEL J., ROWAN, TIMOTHY M.
Applicants: ALLEN-BRADLEY COMPANY, INC.
Application year: 1986
Main classification: H02M7
IPC codes: H02P21, H02M7, H02P27