Motor torque control method and apparatus

What is claimed is:

1. A method of controlling the torque of an operating multiple phase induction motor having rotor means, stator means and an air gap therebetween comprising the steps of:

a) determining the third harmonic component of stator voltage and calculating the third harmonic component of air gap flux from said third harmonic component of stator voltage;

b) measuring the amplitude of a representative stator current;

c) measuring the phase angle between the fundamental component of said stator current and said third harmonic component of air gap flux;

d) determining the fundamental air gap flux and its magnitude from said third harmonic component of air gap flux;

e) calculating the rotor flux position and amplitude parameters from said fundamental air gap flux;

f) calculating a slip gain error from said rotor flux parameters;

g) calculating a correct slip gain based on said slip gain error; and

h) controlling the input slip frequency to said motor based on said correct slip gain using motor power voltage and input power frequency.

2. A method for controlling the rotor field orientation in an operating multiphase induction machine having stator means, rotor means and an air gap therebetween, said method comprising the steps of:

a) determining the third harmonic component of air gap flux from the third harmonic component of stator phase voltage;

b) determining the maximum value of the fundamental component of said air gap flux from said third harmonic air gap flux component;

c) measuring the phase angle between the maximum value of the stator current and the maximum value of said fundamental air gap flux component;

d) passing said phase angle and also the maximum value of said third harmonic air gap flux component through a quadrature oscillator means which produces direct and quadrature components of said fundamental air gap flux component;

e) computing the rotor flux from said direct and said quadrature components;

f) comparing said computed rotor flux to a reference motor flux, thereby generating a phase angle value which represents the difference between the fully oriented rotor field orientation and the actual rotor field orientation;

g) inputing said phase angle signal into a regulator means along with reference signals for said direct and said quadrature values of current being input into said machine, thereby generating current values for each phase of said machine; and

h) charging said current values into a current regulator from which power is fed to said machine, thereby regulating input power to control said rotor field orientation.

3. An apparatus for controlling rotor field orientation in an operating multiphase induction machine having stator means, rotor means and an air gap therebetween, said apparatus comprising in combination:

a) means for determining the third harmonic component of air gap flux from the third harmonic component of stator phase voltage;

b) means for determining the maximum value of the fundamental component of said air gap flux from said third harmonic air gap flux component;

c) means for measuring the phase angle between the maximum value of the stator current and said maximum value of said fundamental air gap flux component;

d) means for passing said phase angle and the maximum value of said third harmonic air gap flux component through a quadrature oscillator means which produces direct and quadrature components of said fundamental air gap flux component;

e) means for computing the rotor flux from values for said direct and said quadrature components of said fundamental air gap flux component and for identifying rotor field orientation;

f) means for comparing said rotor flux to a desired rotor field orientation and for generating an output signal representation of such comparison; and

g) means for regulating input current and frequency thereof to said machine, thereby to control rotor field orientation.

4. A method for correcting slip gain error in an operating multi-phase induction motor whose stator winding is connected to a power-providing inverter, said method comprising the steps of:

a) determining the third harmonic component of stator voltage by summing the stator voltage components and integrating said third harmonic component of stator voltage to calculate the third harmonic component of stator flux;

b) estimating the amplitude of the fundamental component of the air gap flux from (i) the characteristic relationship existing in said motor between the amplitude of said third harmonic component of said stator flux and said amplitude fundamental of said air gap flux, and (ii) the relative position with respect to the stator current as measured by the phase displacement between a first point which is along the waveform of said third harmonic component of said stator flux and which corresponds with the maximum value of said fundamental component of said air gap flux and a second point which is along the waveform of said stator current which corresponds with the maximum value of said stator current;

c) calculating each of the q-axis and the d-axis components of the rotor flux using q-axis and d-axis components of each of (i) the amplitude of said amplitude fundamental component of said air gap flux and (ii) said stator current;

d) comparing said q-axis and said d-axis components of said rotor flux with respective model reference values of said q-axis and said d-axis components of said rotor flux and calculating from the resulting difference values a slip gain error, said model reference values being selected from the rotor position;

e) calculating the reference slip gain from the reference values for each of (i) the q-axis value of said stator current and (ii) said d-axis value of said rotor flux;

f) comparing said slip gain error with the reference slip gain to produce the actual slip gain;

g) charging said actual slip gain, the rotor position, and each of said q-axis and said d-axis values of said stator current to an indirect field oriented controller and producing phased current outputs which are corrected for said slip gain error;

h) charging said corrected phase current outputs to a current regulated pulse width modulated inverter and generating currents and voltages which are corrected for said slip gain error; and

i) powering said motor with said corrected currents and voltages.

5. The method of claim 4 wherein, in step (c), said calculating is carried out using the following relationships: ##EQU13## where: .lambda. indicates flux,

L.sub.r indicates rotor self inductance,

L.sub.m indicates magnetizing inductance,

L.sub.lr indicates rotor leakage inductance,

i indicates current,

superscript e indicates synchronous reference frame,

subscript s indicates stator,

subscript q indicates q-axis,

subscript d indicates d-axis, and

subscript r indicates rotor.

6. The method of claim 4 wherein, in step (d), said slip gain error is so calculated using the following relationship:

.DELTA.K.sub.s =.DELTA.m.times..lambda..sub.dr.sup.e*-1

where

.DELTA. indicates a variation in a quantity,

K.sub.s indicates slip gain,

m indicates machine parameters,

.lambda. indicates flux,

subscript d indicates d-axis,

subscript r indicates rotor,

superscript e indicates synchronous reference frame, and

* indicates the reference value of an indicated variable.

7. An apparatus for on-line, feed forward correcting of the slip gain error in a continuously operating multi-phase induction motor whose stator winding is connected to a power-providing inverter, said apparatus comprising in combination:

a) means for determining the third harmonic component of stator voltage;

b) means for estimating the amplitude of the fundamental component of air gap flux including a reference table interrelating for said motor the amplitude of the third harmonic component of the stator flux with said amplitude of the fundamental component of air gap flux and further including means for measuring the phase displacement between the point along the waveform of said third harmonic component of stator flux which corresponds with the maximum value of said fundamental component of air gap flux and the point along the stator current waveform which corresponds with the value thereof;

c) means for calculating each of the q-axis and the d-axis components of each of the stator current and the fundamental air gap flux amplitude, and also for calculating from such components each of the q-axis and the d-axis components of the rotor flux;

d) means for determining model reference values of said q-axis and said d-axis components from the rotor position and for comparing said q-axis and said d-axis components of said rotor flux with said respective model reference values;

e) means for calculating the reference slip gain from said reference values for each of said q-axis value of the stator current and said d-axis value of the rotor flux;

f) means for comparing the slip gain error with the reference slip gain to produce the actual slip gain;

g) indirect field oriented controller means responsive to said actual slip gain, the rotor position, and each of said q-axis and said d-axis values of said stator current and productive of phased current outputs which are corrected for said slip gain error; and

h) current regulated pulse width modulated inverter means responsive to said corrected phased current outputs and productive of current and voltage outputs which are corrected for said slip gain error.

8. A method for controlling torque by correcting slip gain error in an operating three phase, wye connected induction motor having rotor means, stator means, and an air gap therebetween, said motor being powered through a current-regulated inverter means, said method comprising the steps of:

a) determining the fundamental component of the air gap magnetic flux by:

(1) summing the stator phase voltages to produce resultantly the third harmonic component of stator voltage;

(2) integrating said third harmonic component of stator voltage to produce the third harmonic component of said air gap magnetic flux; and

(3) identifying the fundamental component of said air gap magnetic flux from said third harmonic component of said air gap magnetic flux;

b) estimating information for the rotor flux position and amplitude;

c) resolving said fundamental component of said air gap magnetic flux into its d-axis and q-axis components;

d) comparing said d-axis and q-axis components to a field oriented reference model and calculating slip gain error;

e) comparing said so calculated slip gain error to the calculated slip gain to produce an output signal;

f) regulating an indirect field oriented controller with said output signal and with said rotor flux position information to produce a resultant signal;

g) regulating phase voltages in said inverter by said resultant signal; and

h) operating said motor from the voltage and the current outputs of said inverter.

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FIELD OF THE INVENTION

The invention relates to methods and apparatus for using a flux determination to control the torque of a motor.

BACKGROUND OF THE INVENTION

Certain rotor flux field measurement techniques have been used in an attempt to provide motor control which is similar to that of the dc machine by controlling the stator current and frequency at the same time. However, problems still exist in practical implementation of the control algorithm. Rotor flux field orientation requires knowledge of the rotor flux position. This position can be computed by measuring or estimating the rotor flux, but it is not generally practical to modify the machine to install the flux sensors.

To solve this problem, an approach was developed to estimate the magnitude of the rotor flux using terminal voltages and currents. However, this approach had difficulties at low speed and special attention was required due to the nature of the voltage signals. One example of this is U.S. Pat. No. 4,445,080 to Curtiss. The Curtiss device makes a measurement of the amplitude or magnitude of the flux and then this is used to regulate the flux. However, this does not give direct torque control but, in fact, is an attempt to regulate the average torque of the motor. Because the Curtiss device only measures the amplitude of the flux, it does not provide for instantaneous torque control but, in effect, operates as a low performance drive control.

Thus, as can be seen, while the Curtiss device does provide for some control of torque, it does not provide for the instantaneous control of torque which is necessary for a high performance drive system. This is because the Curtiss device operates by simple flux regulation.

Another scheme to estimate the rotor flux position uses a measurement of the rotor position. The sum of a calculated slip position and the measured rotor position yields the relative rotor flux position. The required position measurement is easier to obtain and is often already available. This scheme is referred to as indirect field orientation or feedforward field orientation.

While there are certain advantages, the machine parameter dependence in the computation of slip position (velocity) affects the performance of torque response and the efficiency of the drive system. Therefore, an enormous amount of work has been done over the past twenty years to solve this machine parameter dependence problem in indirect field orientation. It was not previously known that the air gap magnetic flux (and, in particular, the peak amplitude of the fundamental component of this flux and its angular position in the air gap as the flux rotates) could be accurately measured on an instantaneous basis by using some other machine characteristic, such as the third harmonic component of the stator phase voltage. Also, it was not previously known that such air gap flux could be used to accomplish slip gain correction.

No simple and reliable technique or apparatus was previously known which, when used in combination with an operating alternating current machine, would reliably and automatically determine the flux peak amplitude and relative position using only a sensed third harmonic component of the stator phase voltage.

Such instantaneously existing information about the peak amplitude and location of a flux such as the air gap flux, would be very useful in control devices and methods for regulating alternating current motor variables. Moreover, such control devices would themselves also be new and very useful, as would be the methods associated with their operation and use.

Electric motors consume much of the electric power produced in the United States. For example, motors consume about two-thirds of the total U.S. electrical power consumption of about 1.7 trillion kilowatt-hours. Over 50 million motors are estimated to be in use in U.S. industry and commerce with over one million being greater than 5 horsepower (hp). Over 7500 classifications for induction motors exist in the size range of 5 to 500-hp.

Although the efficiency of electrical machinery is improving, the efficiency of the typical squirrel cage induction motor ranges from about 78 to 95 percent for sizes of 1 to 100-hp. Thus, substantial energy savings can still be achieved. Energy can be saved in conventional constant speed applications when load conditions change considerably. Induction motor operation at normal operating conditions can result in high efficiencies by use of a favorable balance between copper and iron losses. Iron losses dominate at light loads. Thus, energy is saved by reducing motor magnetic flux at the expense of increasing copper losses so that an overall loss minimum can be maintained. However, the cost of the controller needed to adjust the motor flux is substantial.

In contrast to constant speed motor systems, variable speed induction motor systems characteristically involve variable torque loads over a range of speeds. Typical applications include compressors, pumps, fans and blowers of the type used in air conditioners, heat pumps, and the like. In these applications, improvement in operating efficiency is possible more economically because a controller for developing the optimum flux condition is derivable from the same converter that is used to vary the speed of the drive.

SUMMARY OF THE INVENTION

The present invention uses a solution which can minimize the dependence of machine parameters on the indirect field oriented drive system so that a robust high performance variable-speed ac drive can be realized.

The present invention is a method and apparatus for regulating the torque of an inductance motor. This is done by first determining the amplitude and position of a given flux of the motor. This can be stator flux, the magnetizing flux, the rotor flux, or any combination of these fluxes. For example, any combination of these fluxes can be arithmetically combined and utilized for the present invention.

A reference amplitude and position for the flux is also provided. A comparison can be made between the determined flux and the reference flux to obtain a correction in slip gain of the motor. The slip gain is the inverse of the rotor time constant. By using this correction in slip gain, the operation of the motor is then adjusted to the desired torque.

Because both the amplitude and position of the flux are measured, a vector quantity is used. This is quite different from the prior art, such as the device taught by Curtiss, which operates using a scaler quantity. Because a vector quantity is used, it is possible to calculate the correction in the slip gain and hence make the appropriate adjustment in the motor in a single step or in few steps. This also provides a robust motor control. Prior art devices, such as taught by Curtiss, require a series of adjustments which eventually attempt to reach the proper torque control.

In its preferred embodiment, the amplitude and position of the rotor flux is measured and it is used to regulate the torque of the motor. As described in more detail below, the amplitude and position of the rotor flux can be determined by measuring the third harmonic voltage and calculating the air gap flux of the motor.

In accordance with this invention, a slip gain error model is provided based on the conventional dq induction machine model equations in a synchronous reference frame. According to the principles of field orientation, it is known that q-axis rotor flux is zero when field orientation is achieved, hence any parameter change will cause a non-zero q-axis rotor flux to exist. If the model includes the nonlinearities of the ac machine, the slip gain corrector should be relatively independent of operating conditions. Such a model can correctly compute the slip gain error within a few correction periods. This is true even when the rotor resistance is suddenly changed to twice its original value. This slip gain error model can also function at start-up as well as at any other operation condition.

However, developing a parameter insensitive scheme to estimate the rotor flux is as important as developing an accurate slip gain error model. Among many possibilities, two systems have been implemented. In the first system, terminal voltages and currents are measured and used to estimate the stator flux, then rotor flux can be estimated based on the stator flux. To further decrease the dependence of machine parameter, a second system was also implemented.

In this second system, the fact that in most induction machines the air gap flux is saturated under normal operation causes a third harmonic voltage to exist in the phase voltage. This third harmonic voltage is measured and the air gap flux calculated and rotor flux reliably estimated. As a result, this system is sensitive to rotor leakage inductance only, hence it allows this system to successfully and accurately operate at zero speed as well as any other speed including slow speeds. Such a system has not been previously provided.

The present invention also provides a method and apparatus for correcting the slip gain error in an operating induction machine by an on-line, feedforward, field-oriented drive. This invention provides a solution to the problem of machine parameter dependence in indirect field orientation by minimizing such dependence so that a robust high performance variable-speed alternating current drive can be realized. The inventive method and apparatus provide advantages of direct and indirect field orientation in one system when used with an indirect field oriented controller. The inventive method and apparatus can be practiced in both the steady state and the transient dynamic state. Because of the predictive nature of the operating point of the inverse model in the correction method and apparatus, convergence time is nearly independent of the operating point and convergence generally occurs in a few correction periods.

In the practice of the present invention, the rotor flux is preferably determined. This can be estimated from measurements of the terminal currents and voltages. However, in the more preferred method, the rotor flux is determined from the air gap flux. The three phase voltages of a stator of a three phase induction motor are summed together. The fundamental voltage components cancel each other out and the resultant wave contains mainly the third harmonic stator voltage components and high frequency components due to interaction between stator and rotor slots in the operating motor. The phase position and amplitude of the third harmonic component of stator voltage is representative of the phase position and amplitude of the air gap flux.

The third harmonic of the air gap flux maintains a constant position with respect to the third harmonic component of the stator voltage and also to the fundamental of air gap flux. The third harmonic component can thus be used to determine the waveform and amplitude of the fundamental flux component or air gap flux.

By comparing the air gap flux with a field oriented reference model, the slip gain error is calculated. Then, by comparing calculated slip gain with calculated slip gain error, an output signal is produced that is employed for regulation of an indirect field oriented controller. Output from the indirect field oriented controller is fed to a current regulated inverter from which regulated phase voltages are fed to a multiphase induction motor or the like. Rotor position information is concurrently also fed to the indirect field oriented controller.

As used, the slip gain K.sub.r refers to the inverse of the rotor time constant T.sub.r. As described in more detail below, an estimated value T.sub.r * of the rotor time can be obtained using the present invention. The present invention uses techniques which are robust and do not require direct measurement of the total rotor inductance and rotor resistance to determine the rotor time constant. Using the techniques of the present invention, the slip gain continues to be set at the proper value even though the true rotor time constant is unknown or changes during the course of motor operation due to factors such as temperature changes or magnetic saturation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which comprise a portion of this disclosure:

FIG. 1 is a graph showing waveforms of both the fundamental air gap flux voltage V.sub.g1 and the third harmonic component V.sub.g3 as modulated by the rotor slot harmonics obtained after the summation of the stator phase voltages;

FIG. 2 is a graph showing fundamental components for the phase current i.sub.as, air gap flux of one phase .lambda..sub.m and third harmonic V.sub.s3 and also showing the angle of displacement .gamma..sub.im between the current and the third harmonic flux component .lambda..sub.3 ;

FIG. 3 is a plot showing the interrelationship between stator current air gap flux and third harmonic stator voltage vectors (v.sub.3) for one field orientation condition in the synchronous reference frame relative to the d-axis as ordinates and the q-axis as abscissae;

FIG. 4 is a graph showing the relationship between the amplitude of the air gap flux component (plotted as ordinates) and the amplitude of the stator third harmonic voltage (plotted as abscissae in volts) for a 3-hp induction machine;

FIG. 5 illustrates in block diagrammatic form one embodiment of a slip gain corrector device of the present invention;

FIG. 6 illustrates one embodiment of a control algorithm suitable for use with a slip gain corrector such as shown in FIG. 5;

FIG. 7 is a block diagram similar to FIG. 5 but showing an alternative embodiment of a slip gain corrector device of the present invention;

FIG. 8 is a simplified diagram of a hard switched pulse width modulated (PWM) servo amplifier wherein gating signals are controlled in the digital regional processor (DSP) interface;

FIG. 9 is an oscilloscope trace for the speed waveform (R.sub.20) of a field oriented motor drive after tuning for a square wave torque command using a device of FIG. 7 operating at a rotor speed of 75 rpm/div;

FIG. 10 shows oscilloscope traces, illustrating transient responses occurring in the tuning process with a step change of rotor resistance between (R.sub.20) and (2R.sub.20) using a device of FIG. 7, the top trace being the response of the motor speed waveform and the bottom trace being the response of the slip gain in the slip gain corrector device at a rotor speed of 75 rpm/div and a slip gain of 2 volts/div;

FIG. 11 is an oscilloscope trace showing spectrum contents (uncorrected) for the summed phase voltages with frequency in Hz being shown as abscissae and root mean square voltage being shown as ordinates in a operating device of FIG. 5;

FIG. 12 shows oscilloscope traces illustrating the closed loop slip gain response of the device of FIG. 5 for an associated motor operating at rated torque and flux commands with a step change of rotor resistance between (R.sub.20) and (2R.sub.20) wherein trace I is the slip gain of the device (2 volts/div), trace II is the estimated q-axis rotor flux (1 volt/div), trace III is the estimated d-axis rotor flux (1 volt/div) and trace IV is the shaft speed (300 rpm/div);

FIG. 13 shows oscilloscope traces similar to FIG. 12 for the device of FIG. 5 similarly operating but in a locked rotor test wherein trace I is the slip gain of the device (2 volts/div); trace II is the estimated q-axis rotor flux (1 volt/div); trace III is the estimated d-axis rotor flux (1 volt/div) and trace IV is the shaft speed (150 rpm/div);

FIG. 14 is a block diagrammatic view of another embodiment of a controller for a three phase induction machine utilizing a slip frequency calculator assuming invariant magnetizing inductance;

FIG. 15 shows in block diagrammatic view of an embodiment of model reference adaptive controller for implementation of a rotor time constant correction scheme with magnetizing inductance based on the amplitude of the third harmonic voltage signal;

FIG. 16 shows a block diagrammatic view of a feedforward/predictive feedback controller using the rotor time constant with the magnetizing inductance estimate based on the amplitude of the third harmonic voltage signal;

FIG. 17 shows a block diagrammatic view of a direct rotor field orientation controller using a scheme of locating the absolute position of the rotor flux from the third harmonic voltage signal; and

FIG. 18 shows a block diagrammatic view of a direct rotor field orientation control implementation system for driving the rotor flux angle .gamma. to zero.

DETAILED DESCRIPTION

The present invention is applicable to all alternating current machines. However, for present disclosure purposes, three-phase induction motors are described. For a general discussion of the operation of motors and terms used in the art, see Electric Machinery by Fitzgerald et al. (5th ed.) McGraw-Hill, New York (1990) which is incorporated herein by reference. See also parent application Ser. No. 591,517 which is also incorporated herein by reference.

The present invention uses measurements of a motor flux to provide estimations of the rotor time constant T.sub.r or its inverse, the slip gain K.sub.r. The resulting slip gain or the difference in correct slip gain, the slip gain error, is then used to control the motor inputs to control torque.

The use of a motor flux provides a high performance control over the torque. By using the combination of both the amplitude and position of the motor flux, a vector analysis can be made and a rather fast control of the torque can be provided. Several motor fluxes can be used including the stator flux, the magnetizing flux and the rotor flux. Preferably, the flux chosen is one which does not change as a function of the temperature over the normal operating range of the motor. As discussed in more detail below, it is preferred to use the rotor flux. The preferred method of determining the rotor flux is by calculation of the air gap flux as discussed in more detail below.

In an induction mac