Abstract:
A method and apparatus for controlling flux in an induction motor. During initial start of an induction motor drive the motor flux has to be charged up. The method provides time optimal flux forcing under limited inverter ampere capability. Command stator flux and current are generated and coordinated through a motor flux model. Charging of the motor flux and flux current are accelerated up to nominal operating values in coordinated manner.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method and apparatus for controlling flux in an alternating current (AC) motor and, specifically, to a method and apparatus for accelerating flux to a nominal operating value in a coordinated manner without the need for flux feedback from the motor. 
     Control of an asynchronous AC motor is often achieved by directly or indirectly controlling motor flux. Conventionally, flux control requires determining the value of flux by measuring, for example, stator voltage and current and then deducing the motor flux from the measured voltage and current in combination with characteristics of the motor. For example, U.S. Pat. No. 5,172,041 to Bavard et al. discloses a method for asynchronous motor control using a closed-loop variation of magnetic induction flux as a function of a desired value. The motor flux is determined by detecting the motor voltage and the motor current and determining flux value by using the measured values in a motor operating equation. 
     U.S. Pat. No. 5,194,797 to Kahkipuro discloses a method for regulation of the stator flux and torque of an asynchronous motor using vector control. The absolute value of the stator flux is regulated by the real part of the stator voltage vector and the torque is regulated by the stator frequency. By adjusting the imaginary part of the stator voltage vector, the calculation coordinates are held in a position which allows the rotor flux to remain close to the real axis. The value of the torque and the absolute value of the stator flux are obtained by using measured parameters in a motor equation that is characteristic of the operation of the motor. 
     BRIEF SUMMARY OF THE INVENTION 
     A first aspect of the invention is a preflux forcing controller for an asynchronous, alternating current (AC) induction motor drive system comprising means for generating a flux reference value, means for generating a flux boosting value, means for generating a preflux current value, means for processing the flux reference value, the flux boosting value, and the preflux current value to determine an output signal, and means for providing at least one command signal to the motor based on the output signal. 
     A second aspect of the invention is a method of forcing flux in an asynchronous, alternating current (AC) induction motor drive system comprising the steps of generating a flux reference value, generating a flux boosting value, generating a preflux current value, processing the flux reference value, the flux boosting value, and the preflux current value to determine an output signal, and providing at least one command signal to the motor based on the output signal. 
     A third aspect of the invention is a preflux forcing controller for an asynchronous, alternating current (AC) induction motor drive system comprising a command flux generator configured to generate a flux reference value, a flux boost generator configured to generate a flux boosting value, a preflux current generator configured to generate a preflux current value, processing circuitry configured to process the flux reference value, the flux boosting value, and the preflux current value to determine an output signal, and a current regulator and voltage feedforward model of a motor drive configured to provide voltage commands based on the output signal. 
     A fourth aspect of the invention is a motor drive system for an asynchronous, alternating current (AC) induction motor comprising a command flux generator configured to generate a flux reference value, a flux boost generator configured to generate a flux boosting value, a preflux current generator configured to generate a preflux current value, processing circuitry configured to process the flux reference value, the flux boosting value, and the preflux current value to determine an output signal, a current regulator and voltage feedforward model of a motor drive configured to provide voltage commands based on the output signal, a DC bus, a voltage regulation and DC bus compensation module coupled to an output of said flux current model module and configured to generate a voltage signal and a phase angle signal, a vector rotate and pulse width modulation module coupled to an output of said voltage regulation and DC bus compensation module and configured to generate switching signals based on the voltage signal and the phase angle signal an inverter having bridges disposed across said DC bus and being coupled to said vector rotate and pulse width modulation module to generate a voltage signal intended to induce a desired current in an AC induction motor based on the switching signals, and a motor coupled to an output of said inverter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described through a preferred embodiment and the attached drawing in which: 
     FIG. 1 is a graph of motor flux current and command stator flux versus time; 
     FIG. 2 is a block diagram of a motor drive system of the preferred embodiment of the invention; and 
     FIG. 3 is a logic diagram of the optimal preflux forcing controller of the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the preferred embodiment, FIG. 1, is a graph of a command stator flux signal and the flux current of an AC induction motor drive system in accordance with a flux forcing procedure of the preferred embodiment of the invention. The flux current value (curve A is shown to remain substantially constant between 12:11.960 and 12:12.280 at a level of approximately 47 amps, as indicated by the axis on the right of FIG.  1 . During the same time period the command stator flux value expressed as volts/hz on the left axis (curve B), input into the motor model as described in detail below, continually increases until the command stator flux value reaches a value of the flux reference value plus a flux boosting value (determined in the manner set forth below) at time 12:12.280. At this time, the command stator flux value is reduced by the flux boosting value. Of course, the magnitudes of these values are merely examples of possible values which are determined based on characteristics of the motor and the desired operation thereof. 
     In other words, the motor flux and flux current are forced, i.e. accelerated, up to nominal operating values in a coordinated manner. This is accomplished by injecting the current pulse of curve A into the motor stator winding during a flux forcing procedure. To accomplish this, a command stator flux and command stator flux current are determined in the manner described below and are input into the motor model as illustrated in FIG. 2 which is a block diagram of a motor drive system in accordance with the preferred embodiment. 
     As illustrated in FIG. 2, a command torque signal is input into torque compensation module  12  from an output of a speed regulator (not illustrated) or other device. Torque compensation module  12  determines the torque component of the stator current for the torque value of the command torque signal. The torque component stator current value is input into torque regulator  14  which maintains the torque stator current value at a constant value using a feedback loop. The constant torque stator current value for the desired torque is input into summing junction  16 . 
     In response to a motor start signal, command flux generator  18  calculates a flux reference value based on a “volts per hertz” model in which the ratio of stator voltage to the electrical frequency is a constant below the rated speed of motor  34 . The flux reference value is input into optimal preflux forcing controller  20  which calculates the command flux current and the command stator flux to control the flux forcing procedure as illustrated in FIG.  1 . The internal operation of optimal preflux forcing controller  20  is described in greater detail below. 
     Optimal preflux forcing controller  20  dynamically outputs the command flux current (which indicates the flux component of the desired stator current) and command stator flux (which indicates the desired operating flux). These signals are inputted into voltage feedforward model  22  along with the torque component stator current value output from torque compensation module  12 . Voltage feedforward model  22  includes the voltage equations of field-oriented motor based on stator flux command. Voltage feedforward model  22  outputs command voltage signals for the torque channel (x) command (which is input into summing junction  16 ) and the flux channel (y) command (which is input into summing junction  26 ). The torque channel signal is summed with the output of the torque channel regulator  14  and the flux channel voltage command is summed with the output of flux current regulator  24 . 
     The output of summing junction  16  is the torque channel voltage command and the output of summing junction  26  is the flux channel voltage command. These vector commands are input into voltage regulation and DC bus compensation module  28  which generates a voltage signal and a phase angle signal in a known manner. The voltage and phase angle signals are transformed into switching signals by PWM module  30  and the switching signals are used to gate bridges in inverter module  32  disposed across a DC bus in a known manner. The switching signals cause PWM module  30  to generate voltage pulses that cause a sinusoidal current, or a reasonable facsimile thereof, to be generated in the stator winding of motor  34 . With the exception of inverter module  32  and motor  34 , all of the elements of FIG. 2 are implemented as preprogrammed firmware or software  50 . 
     FIG. 3 illustrates optimal preflux forcing controller  20  of the preferred embodiment in detail. A motor start signal can be generated either locally at motor  34  or remotely on a control panel. Upon receipt of the start signal command, flux generator  18  will generate a flux reference value in the manner described above (see FIG.  1 ). Preflux current generator  62  calculates the maximum preflux current level and generates a preflux current value that represents the magnitude of the current pulse to be injected into the stator windings of motor  34  as illustrated in curve A of FIG.  1 . Preflux current is defaulted to the rated motor nameplate value but can be configured differently to achieve faster preflux forcing with a higher current level Also, flux boost generator  64  generates a flux boosting value described in greater detail below. The value of the flux reference, preflux current, and the flux boosting value are each predetermined based on motor data of the motor being started. The, preflux current value is representative of a value, i.e. magnitude, of preflux current to be injected into the stator winding of motor  34  to be started. The preflux current injected into motor  34  will increase the stator flux and flux current of the motor to a predetermined value. Once the stator flux and flux current have reached a predetermined value, injection of the preflux current will be terminated. The preflux current is maintained substantially constant during injection by a command flux current value input into summing junction  66  as described below. 
     The output of summing junction  66  is multiplied by a design constant K in multiplier  68 . The design constant is determined based on the design parameters of motor  34 . The output of multiplier  68  is an injection signal which represents the voltage being developed within motor  34  for each ampere of current injected into the stator winding of motor  34 . Switching device  70  receives a control signal from comparator  72  to selectively couple the output of multiplier  68  to limiter  74  for reasons which will become apparent below. If the injection signal is not received at limiter  74  and thus summing junction  76 , there will no longer be a signal to inject current into the stator winding of motor  34 . Limiter  74  limits the maximum voltage for current injection to avoid current overshoot during current acceleration. 
     The flux reference value from command flux generator  18  is received by summing junction  78  and is added to the flux boosting value output by flux boost generator  64 . The flux boosting value is representative of the leakage flux of motor  34  and is used to compensate for flux losses during stator flux acceleration. The output of summing junction  78  is received by comparator  72  and is compared with a command stator flux signal generated by flux current model  90  described below. When the command stator flux signal is equal to the flux reference plus the flux boosting value, i.e. the output of summing junction  78 , comparator  72  will activate switching junctions  70  and  80  to prevent the injection signal from being received by summing junction  76  and to eliminate the flux boosting value from being received by summing junction  82 , i.e. to disable generation of the injection signal and the flux boosting value. The elimination of the flux boosting value at the proper time will establish the command stator flux value at the desired steady-state flux value of motor  34  while compensating for any leakage flux during acceleration of the stator flux. 
     There are two paths (one from limiter  74  and other from limiter  84 ) which contribute to the required flux forcing voltages (output of summing junction  76 ). The output of limiter  84  is the linear command stator flux ramping portion which controls the command stator flux to rise linearly with time. The output of limiter  74  is the acceleration command stator flux which accelerates the rate of change of command stator flux such that the current will rise to the desire injection level (as shown in FIG. 1 curve A) and be maintained until the command stator flux reaches Flx_Rated plus Flx_Bst. The output of summing junction  76  is input into flux current model  90  which represents a transfer function between the stator flux current and the stator flux of motor  34 . 
     In flux current model  90  integrator  92  integrates the output summing junction  76  over time to obtain a signal representative of the stator flux during a stator flux acceleration procedure. The output of integrator  92  is input into comparator  72  and voltage feedforward model  22  as the above-noted command stator flux signal. The output of the summing junction  76  is also received by transient current component module  94  of current model  90 . The output of the transient current component module  94  is a value in units of amperes and is received by current module summing junction  98 . In particular, transient current component module  94  divides the voltage value signal input thereto by R 2  which is the roter resistance or motor  34 . The output of transient current component module  94  is representative of the transient current component of motor  34  during the stator flux acceleration procedure. 
     The output of integrator  92  is also received by the saturation curve module  96  of current model  90 . The output of saturation curve module  96  is representative of the steady-state current component of motor  34  during the stator flux acceleration procedure. The output of saturation curve module  96  is also received by current model summing junction  98  which adds the transient current component value and the steady-state current component value and sends the result to low pass filter  99  which has a leakage time constant representing a flux model of motor  34 . The resulting command flux current signal is input to current regulator  24  and voltage feedforward model  22  as noted above. 
     It can be seen that the flux acceleration procedure of the preferred embodiment injects a preflux current having a substantially constant magnitude, to the motor stator during flux forcing, i.e. acceleration. When the stator flux reaches the rated flux plus a flux boost amount, the preflux current is eliminated to step the stator flux to its rated value. This permits the motor to be charged up quickly and allows the flux forcing current to reach a nominal value quickly without exceeding the inverter current capability. The invention permits coordination of command flux and flux current to maintain a field oriented de-coupled drive system. 
     The preferred embodiment illustrates functional blocks. However, the various functions of the invention can be accomplished through analog logic devices, preprogrammed solid state devices, or in any other manner. Known feedback loops can be used to control torque and other operating values of the motor. 
     While the foregoing description includes many details and specificities, it is to be understood that these have been included for purposes of explanation only, and are not to be interpreted as limitations of the present invention. Many modifications to the embodiments described above can be made without departing from the spirit and scope of the invention, as is intended to be encompassed by the following claims and their legal equivalents.