Flux controller for induction motor

Disclosed is a flux controller for maintaining reliable flux estimation performance in a low velocity region, the controller including a velocity controller, a torque current controller for outputting a torque voltage command, a flux controller for outputting a flux current command, a flux current controller for receiving the flux current command to output a flux voltage command, a three-phase converter for converting the torque voltage command and the flux voltage command into a three-phase voltage command applied to the induction motor to output the three-phase voltage command, a flux estimator for outputting a rotating angle of a rotor of the induction motor, an estimated flux value of the rotor and an estimated velocity of the rotor, and a flux regulator for receiving the torque voltage command and the estimated velocity to output a gain value that regulates a magnitude of the flux command.

CROSS-REFERENCE TO RELATED APPLICATION

Pursuant to 35 U.S.C. §119(a), this application claims the benefit of earlier filing date and right of priority to Korean Patent Application No. 10-2010-0120433, filed Nov. 30, 2010, the contents of which are hereby incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a flux controller for an induction motor and, more particularly, to a flux control apparatus capable of minimizing a decline in flux estimation performance in a low velocity region in a drive inverter system of an induction motor using a sensorless vector control method.

2. Description of the Related Art

Generally, methods for driving an induction motor at variable velocity without using a velocity sensor or a position sensor includes a constant voltage/frequency (VF) control method that is an open loop control method, and a sensorless vector control method that estimates a flux position of a motor rotor from measurements of a voltage, a current, and motor parameters. Since the latter method is excellent in terms of performance for controlling velocity and load variations, the sensorless vector control method has been widely used.

However, the sensorless vector control method is problematic in that voltage required to drive a motor in the low velocity region is reduced, so that voltage (inverter output voltage) inputted into the motor may be erroneously estimated due to offset or dead time, whereby flux estimation performance is deteriorated. Consequently, the sensorless vector control method fails to exhibit sufficient performance in the low velocity region in comparison with a high velocity region.

FIG. 1is a block diagram illustrating an induction motor system driven at variable velocity by the sensorless vector control method without a velocity (position) sensor.

An inverter101outputs voltage to a sensorless induction motor102to operate the sensorless induction motor102according to a velocity command W*minputted by a user.

A subtractor103calculates a difference between a velocity command W*minputted from an exterior and an estimated velocity Wmthat is one of outputs of a sensorless flux estimator115, thus detecting a velocity error.

A velocity controller104functions to output a current command. Kp—smeans a proportional gain, Ki—smeans an integral gain, and s means a Laplace operator.

A subtractor105calculates a difference between a torque current command i*qand a torque current idto detect a torque current error.

In a torque current controller106, Kp—qmeans a proportional gain and Ki—qmeans an integral gain.

A subtractor107calculates a difference between a flux command λ*drand an estimated flux λdrthat is the output of the sensorless flux estimator115to detect a flux error.

In a flux controller108, Kp—fmeans a proportional gain and Ki—fmeans an integral gain.

A subtractor109calculates a difference between a flux current command i*dand a flux current idto detect a flux current error.

In a flux current controller110, Kp—dmeans a proportional gain and Ki—dmeans an integral gain.

A three-phase converter111receives an electric rotating angle θeof a rotor flux of the induction motor102from the sensorless flux estimator115, and converts a torque voltage command V*qthat is output of the current controllers106and110and a flux voltage command V*dinto a three-phase voltage command, V*a, V*band V*c.

A voltage controller112includes a power semiconductor device (IGBT), and receives the three-phase voltage command, namely, V*a, V*band V*c, thus applying three-phase output voltage controlled by the voltage command V*a, V*band V*cto the induction motor102through pulse width modulation (PWM).

Respective current sensors113a,113band113care coupled to a three-phase output cable of the voltage controller112to detect a three-phase current, ia, iband icflowing through the induction motor102.

A two-phase converter114receives a flux angle θeof the rotor of the induction motor102and converts a three-phase current of the motor, ia, iband icinto a torque current iqand a flux current id.

The sensorless flux estimator115receives the three-phase current of the induction motor102from the current sensor113a,113band113c, and receives a voltage command from the torque current controller106and the flux current controller110, thus outputting a rotating angle θeof the rotor flux, a magnitude λdrof the rotor flux and a velocity Wmof the motor rotor.

An operation of the above components will be described below in detail. If a user inputs a velocity command W*mfor a rotation of the induction motor102, the subtractor103measures a difference between the velocity command W*mand an estimated velocity Wmoutputted from the sensorless flux estimator115to calculate a velocity error, and inputs the calculated velocity error into the velocity controller104. The velocity controller104calculates a torque current command i*qfor rotating the induction motor102according to the velocity command W*m, based on the inputted velocity error.

The subtractor105measures a difference between the torque current command i*qthat is the output of the velocity controller104and the torque current iqthat is the output of the two-phase converter114to calculate a torque current error, and inputs the calculated error into the torque current controller106.

The torque current controller106calculates a torque voltage command V*qfor enabling a torque current iqcorresponding to the command i*qto flow in the induction motor102, based on the inputted torque current error.

A subtractor107measures a difference between the flux command λ*drand the flux estimation value λdrinputted from the sensorless flux estimator115to calculate a flux error. A value of the flux command λ*dris previously calculated, and thereafter is stored in a memory unit (not shown) installed in the inverter101. In equation (1), Vrateand Freqrateare rated voltage and rated frequency of the motor, respectively.

The flux controller108calculates the flux current command i*dfor establishing internal flux of the induction motor102according to λ*dr, based on the flux error. The subtractor109calculates a flux current error, that is, a difference between the flux current command i*dand the flux current idthat is the output of the two-phase converter114, and transmits the flux current error to the flux current controller110.

The flux current controller110calculates a flux voltage command V*dfor enabling a flux current corresponding to the command i*dto flow in the induction motor102, based on the flux current error. The outputs V*qand V*dof the current controllers106and110are converted through the three-phase converter111into the three-phase voltage command V*a, V*band V*cand then are inputted into the voltage controller112.

The three-phase converter111receives the torque/flux voltage commands V*qand V*dand the rotating angle θeof the rotor flux of the induction motor102, and converts the two-phase voltage command of the torque/flux into the three-phase voltage command using equations (2) and (3). In the equations (2) and (3), SIN and COS mean sine and cosine trigonometric functions, respectively.
V*ds=−SIN(θe)·V*q+COS(θe)·V*d
V*qs=COS(θe)·V*q+SIN(θe)·V*d(2)
V*a=V*ds
V*b=−0.5·(V*ds−√{square root over (3)}·V*qs)
V*c=−0.5·(V*ds+√{square root over (3)}·V*qs)  (3)

The voltage controller112controls output voltage through pulse width modulation to apply voltage V*a, V*b, and V*cfrom the three-phase converter111to the induction motor102. The three-phase current ia, iband icof the induction motor102is detected by three current detectors113a,113band113c, and is converted into the torque current iqand the flux current idvia the two-phase converter114. The two-phase converter114converts the three-phase current ia, iband icof the motor into the torque current iqthat is in proportion to the output torque of the motor and the current idthat forms 90 degrees with the torque current and is in proportion to the flux of the motor, using equations (4) and (5). Here, idand iqmean flux/torque current in a synchronous coordinate system, and idssand iqssmeans flux/torque current in a stationary coordinate system.

The sensorless flux estimator115receives the three-phase current ia, iband icof the induction motor102, an output V*qof the torque current controller106, an output V*dof the flux current controller110, and outputs the rotating angle θeof the rotor flux of the induction motor102, the magnitude λdrof the rotor flux and the velocity Wmof the motor rotor. As shown inFIG. 2, the sensorless flux estimator115includes two units therein, that is, a flux estimation unit202and a rotating angle and velocity estimation unit203.

When using a stator circuit equation of the induction motor, d-axis (flux axis) and q-axis (torque axis) stator flux estimation values can be obtained as seen from the following equations (8) and (9). In the equations (8) and (9), rsmeans a stator resistance of the induction motor. A symbolabove the letter used in the equations does not mean an actual measurement value but means an estimation value. Generally, voltage values used in the equations (8) and (9) are not actual measurement values but are command values calculated from the equation (6).
{circumflex over (λ)}dss=∫(V*ds−rsidss)ds(8)
{circumflex over (λ)}qss=∫(V*qs−rsiqss)ds(9)

By using a relation between the stator and the rotor of the induction motor based on the stator flux obtained from the equations (8) and (9), rotor flux can be calculated as seen from equations (10) and (11). In the equations (10) and (11), σLsis stator leakage inductance and is given as represented in the equation (12). In the equation (12), Lsis stator inductance of the induction motor, Lris rotor inductance, and Lmis mutual inductance.

The rotor flux calculated using the equations (11) and (12) is supplied to the rotating angle and velocity estimation unit203, and a rotating angle θeof the rotor flux is calculated as represented in equation (13). In the equation (13), tan−1is an inverse tangent trigonometric function. According to the kind of a sensorless controller, values of the equation (13) may be applied without correction, or calculation of a phase locked loop (PLL) may be added to the values of the equation (13).

A magnitude λdrof the rotor flux that is one of outputs of the flux estimation unit201is calculated as seen from the following equation (14), using the rotor flux of the stationary coordinate system obtained from the equations (11) and (12) and the rotating angle of the equation (13).
λdr=COS(θe){circumflex over (λ)}qrs−SIN(θe){circumflex over (λ)}drs(14)

In the rotating angle and velocity estimation unit203, the equation (13) is calculated to estimate the rotating angle and the velocity. An electric rotating angle of the flux obtained from the equation (13) is converted into a mechanical rotating angle with a motor pole through the following equation (15). In the equation (15), P denotes an induction motor pole. Further, the current electric rotating angle of the rotor is calculated using the equation (16). Also, the rotor velocity can be more precisely calculated through the phase locked loop. In the equation (16), s denotes a Laplace operator.

In order to calculate a result value of the flux and rotating angle estimation unit203of the equations (13) to (15), the stator flux of the equations (8) and (9) must be calculated. In such a process, an integral operation is required. However, if the voltage and current values used in the equations (8) and (9) include offset, an integrator leads to a divergence, thus making it difficult to use in practical. Generally, in order to solve such a divergence, a high pass filter is applied to eliminate an influence of a DC (frequency 0) component after the integral operation has been performed as represented in the following equations (17) and (18). In the equations (16) and (17), T denotes a time constant of the high pass filter.

FIG. 3shows a configuration of the rotating angle and velocity estimation unit203. This is composed of a rotating angle calculation unit302and a velocity calculation unit303. The rotating angle calculation unit302receives the rotor flux of the stationary coordinate system calculated by the equations (17) and (18) from the flux estimation unit202, and calculates the rotating angle θeof the flux through the inverse trigonometric function of the equation (13).

The velocity calculation unit303converts the rotating angle θeof the flux into a mechanical rotating angle and performs a differential, thus obtaining a mechanical rotating velocity Wmof the rotor. In the velocity calculation unit303, P means a motor pole and is stored in an additional memory unit (not shown).

In the case of driving the induction motor by the sensorless vector control method, a method of obtaining motor flux by performing integral for a back electromotive force of the motor (stator voltage−resistance×current) as used in the equations (8) and (9) has been widely used. Voltage outputted from the inverter to the induction motor is in proportion to a rotating velocity of the motor. Thus, high voltage is required in the case of a high velocity rotation, while low voltage is required in the case of a low velocity rotation. Various analog devices such as the current sensors113a,113band113c(seeFIG. 1) are used to control the inverter. In this process, offset is generally included in the calculated/measured voltage and current.

Since a magnitude of the inverter output voltage is sufficiently greater than that of the offset in a high velocity region having a large back electromotive force, it is possible to perform the integral operation of the equations (8) and (9) without any difficulty. However, in a low velocity region where a back electromotive force is small, so that the magnitude of the inverter output voltage is not sufficiently greater than that of the offset voltage, the operation of the equations (8) and (9) is affected by the offset, thus resulting in the divergence. Hence, it is impossible to use the integral operation of the equations (8) and (9) in practical, and the high pass filter is frequently used together with the integral operation as in the equations (17) and (18) so as to minimize an influence of the offset (frequency 0). In the case of using the high pass filter, the offset of the voltage and current can be overcome. However, the calculation of the equations (17) and (18) is greatly affected by the time constant of the high pass filter. In order to acquire a required flux estimation performance, the time constant of the filter must be sufficiently larger than the rotating velocity of the motor. Therefore, the time constant of the high pass filter must be appropriately selected in consideration of a main operating region of the motor and phase error characteristics of the filter. However, such a method is problematic in that the time constant of the filter must be reduced in a region having a very low frequency, so that it is difficult to expect good flux estimation performance because of the characteristics of the high pass filter such as a phase delay resulting from the reduced time constant.

Further, in the general sensorless vector control, flux is estimated not by real voltage inputted into the motor but by a command voltage of the controller. In the low velocity region where the magnitude of voltage is reduced, an error in voltage caused by dead time affects the estimation of the flux, so that there occurs a difference between command voltage used in the calculation of flux and voltage inputted actually into the motor. The difference leads to an error in the flux estimation.

SUMMARY OF INVENTION

Accordingly, an object of the present invention is to provide a flux controller for a motor, which is mounted on an inverter-motor drive system for driving the induction motor having no velocity or position sensor at variable velocity, thus maintaining sensorless flux estimation performance even in a low velocity region where a magnitude of voltage is small.

In order to accomplish the object, there is provided a flux controller for an induction motor using a sensorless vector control method, the controller comprising a velocity controller for receiving a difference in velocity between a velocity command and an estimated velocity to output a torque current command, a torque current controller for receiving the torque current command to output a torque voltage command, a flux controller for receiving a difference in flux between a flux command and an estimated flux value to output a flux current command, a flux current controller for receiving the flux current command to output a flux voltage command, a three-phase converter for converting the torque voltage command and the flux voltage command into a three-phase voltage command applied to the induction motor to output the three-phase voltage command, a flux estimator for receiving a three-phase current of the induction motor, the torque voltage command and the flux voltage command to output a rotating angle of a rotor of the induction motor, an estimated flux value of the rotor and an estimated velocity of the rotor, and a flux regulator for receiving the torque voltage command and the estimated velocity to output a gain value that regulates a magnitude of the flux command, wherein the flux regulator increases the magnitude of the flux command when a rotating velocity of the rotor is low. Therefore, the flux controller is advantageous in that a magnitude of torque command voltage is not reduced below a predetermined value, so that flux estimation performance can maintain reliability even at low velocity.

The flux controller further includes a multiplier for receiving the gain value of the flux regulator to output a value produced by multiplying the gain value and the flux command.

Further, the flux regulator includes a flux regulation controller for receiving a difference between the torque voltage command and a preset minimum flux command voltage to control a gain, a comparator for determining whether the estimated velocity is a preset velocity or less, and a selector for outputting the gain value when it is determined by the comparator that the estimated velocity is the preset velocity or less.

The flux regulator further includes a limiter for limiting the gain from the flux regulation controller within a preset range to output the gain value.

Further, the preset range is 100% in a lower limit, and is 200% in an upper limit.

Further, the minimum flux command voltage is set to be 10% of rated voltage of the induction motor.

Further, the selector outputs a gain value that does not affect a value of the flux command when it is determined by the comparator that the estimated velocity is the preset velocity or more.

The present invention provides a flux controller for an inductor motor configured to generate a torque voltage command and a flux voltage command to control the induction motor by a velocity difference between a velocity command and an estimated velocity and a flux difference between a flux command and an estimated flux, the flux controller including a flux regulator for outputting a gain value that is increased by multiplying the estimated velocity and the flux command when the estimated velocity is a preset value or less.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a flux controller according to the present invention will be described in detail with reference to the accompanying drawings.

FIG. 4is a block diagram illustrating a configuration of an induction motor system that is equipped with a flux controller according to the present invention and is driven at variable velocity by a sensorless vector control method without a velocity and position sensor.

According to a concept of the present invention, a flux regulator416is disposed in an inverter401to maintain flux estimation performance in a low velocity region. The flux regulator416receives a present motor velocity Wmfrom a sensorless flux estimator415, and receives a torque voltage command V*qfrom a flux current controller406. Further, the flux regulator416outputs a gain to a multiplier417. Here, the gain regulates a magnitude of a flux command inputted into a subtractor407for calculating a flux error.

The multiplier417multiplies the gain inputted from the flux regulator416and a flux command λ*drto obtain a final flux command.

Since an operation from the subtractor403to the sensorless flux estimator415as shown inFIG. 4remains the same as the operation described in the prior art, a duplicated description thereof will be omitted. The operation of the flux regulator416will be chiefly described.

Since a configuration of the sensorless flux estimator415according to the present invention remains the same as an existing configuration, the flux estimator415has a limit on flux estimation performance in the low velocity region, which is the feature of an integral of a back electromotive force+a high pass filter method.

The flux regulator416according to the present invention is characterized in that it regulates the magnitude of the flux command in the low velocity region so as to minimize the limit on performance of the conventional sensorless flux estimator.

If Laplace transformation is carried out for the equation (17) without considering a resistance term of a stator, a relation between voltage and flux is obtained as represented in the following equation (19).

Generally, the induction motor controls flux to keep it constant, below a rated velocity. Thus, in order to keep the flux constant, it can be seen that high voltage is required in the case of a high velocity and low voltage is required in the case of a low velocity when substituting jw for s in the equation (19), where s is a Laplace operator and w is an angular frequency.

When estimating sensorless flux under a constant flux control, an error occurs in estimation in the low velocity region where the magnitude of voltage is small. If the magnitude of the flux is intentionally increased in the low velocity region, voltage inputted into the motor is also increased as seen by the equation (19), and voltage used in sensorless calculation is also increased. The larger the magnitude of voltage is, the less an influence such as offset or deadtime is. As a result, it is advantageous to estimate the sensorless flux.

FIG. 5is a block diagram illustrating a configuration of the flux regulator416.

As described above, an aim of the flux regulator416is to maintain the magnitude of a torque command voltage so that it does not fall below a predetermined value, so as to increase the flux in low velocity region and thereby keep the magnitude of voltage applied to the motor large.

The torque flux command voltage is used for the following reason: because the magnitude of the torque command voltage in a normal state may approximate the following equation (20) in the case of ignoring a resistance value of the stator, the magnitude of the torque flux command voltage may be used as a physical quantity that is in proportion to a rotating velocity regardless of a change in load (change in q-axis current) that may occur during an operation.
V*q≈WeLsidse(20)

The subtractor502calculates a difference between a minimum flux command voltage Vminand an output of a flux current controller410, and inputs the difference into a flux regulation controller503.

The minimum flux command voltage Vminis a minimum torque voltage that allows an integral of a back electromotive force for sensorless flux estimation to be smoothly performed without being affected by offset, is experimentally determined by taking 10% of rated voltage as an initial value, and then is stored in an additional storage unit (not shown).

When the torque command voltage V*qis smaller than the minimum flux command voltage Vminthat is a reference value, the flux regulation controller503increases a gain multiplied by the flux command λ*drvia the multiplier417, thus resulting in increasing a command flux level and thereby keeping torque command voltage constant. Kp—mis a proportional gain, Ki—mis an integral gain, and s is a Laplace operator.

Meanwhile, a limiter504limits the flux regulation controller503such that it is operated to merely increase flux, namely, voltage. The limiter504sets a lower limit to be 100%, and sets an upper limit to be less than 200%. Since the flux regulator of the present invention is intended to improve sensorless performance in the low velocity region, it is unnecessary to operate the flux regulator in the high velocity region.

A comparator506compares a present velocity with a predetermined velocity, and changes a state of a selector507depending on the compared result. In response to a value of the comparator506, an output of the selector507is connected to an input ‘A’ so that an output of the flux regulation controller503is multiplied by the flux command λ*drvia the multiplier417when the velocity is a predetermined value or less. Meanwhile, when the velocity is the predetermined value or more, the output of the selector507is connected to an input ‘B’ to ignore the operation of the flux regulation controller503.

When it is determined by the comparator506that the motor velocity is the predetermined value or more, a storage unit508previously stores a value (100%) which prevents the output of the flux regulation controller503from affecting the flux command λ*dr.

The flux regulator416according to the present invention enables the torque voltage command V*qof the synchronous coordinate system to maintain a predetermined value or more. It should be noted that sensorless flux calculation does not use a reference voltage of the synchronous coordinate system but uses a reference voltage of the stationary coordinate system.

As seen from the equation (6), when a value of V*qmaintains a predetermined value or more, the magnitude of the reference voltage of the stationary coordinate system used in the sensorless flux estimation of the equations (17) and (18) is also increased, thus compensating for incomplete flux estimation in the low velocity region where the voltage is low.

Therefore, the flux controller is advantageous in that the flux estimation performance can be maintained regardless of velocity.

The present invention is directed to a flux controller for a motor, which is mounted on an inverter-motor drive system for driving an induction motor having no velocity or position sensor at variable velocity to prevent torque command voltage from being reduced below a predetermined value, thus preventing a magnitude of voltage applied to the motor from being reduced below a predetermined value even in a low velocity region and thereby maintaining sensorless flux estimation performance even in the low velocity region where the magnitude of the voltage is small and an error occurs in sensorless flux estimation.