Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2000-09-05
2001-05-08
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S041000, C318S432000, C318S802000
Reexamination Certificate
active
06229719
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling an inverter which drives an induction motor at a variable speed, and in particular, to an automatic torque boost control method and an automatic torque boost control apparatus.
2. Description of the Related Art
In a known method to control an inverter to drive an induction motor at a variable speed, an output voltage V
1
of the inverter is controlled in proportion to a primary frequency f
1
of the inverter, i.e., V/f is kept fixed. The exciting current of an induction machine can be kept almost fixed in this method. However, when a load imposed onto the motor is increased, the voltage drop due to primary resistance
1
of the motor becomes greater and an induction or induced voltage of the motor is minimized. Resultantly, the exciting current of the motor is reduced and torque of the motor is lowered. Since an actual speed is lowered in response to a speed instruction, the speed regulation is increased. To overcome these difficulties, there have been proposed many automatic torque boost control methods to increase the motor torque particularly in a range of low and intermediate frequencies.
In general, a two-phase motor current is detected, the current is transformed from a fixed coordinate system to a rotating coordinate system (uvw/dq conversion) using a magnetic flux phase to obtain a torque current It. A voltage drop of primary resistance r
1
·It corresponding to the torque current It is added to fixed voltage (induction voltage) V/f. As a result, a voltage instruction Vm is produced as represented by expression (1).
Vm=K·f
1
*+r
1
·It (1)
where, * indicates an instruction value and K is a proportional gain.
A slip frequency fs proportional to the torque current It is added to a reference frequency fn* to obtain an inverter output frequency instruction f
1
* as follows.
f
1
*=fn*+fs=fn*+Ks·It (2)
where, Ks is a proportional gain.
In a method to detect a torque current from the two-phase current of the motor, instantaneous motor currents of two phases are transformed by a uvw/dq conversion, namely, from a fixed coordinated system to a rotating coordinate system. Therefore, the current of the torque can be instantaneously and correctly detected. The torque boost is automatically controlled according to the torque current. Consequently, the automatic torque boosting operation can be controlled with high response and high precision. However, this method requires motor current sensors or detectors for two phases and accordingly current sensor circuits for two phases. This soars the price of the inverter system.
JP-A-10-225199 describes a method to estimate a current vector using a motor current of one phase. In this method, alternating currents of other phases are estimated according to a state equation expressed by an inverter output voltage instruction, a motor current, and a motor impedance. The alternating currents are then transformed by a uvw/dq conversion to estimate current vectors. However, the state equation includes parameters, which in the motor control operation include such motor constants as a primary resistance, a secondary resistance, and mutual inductance. For each type of the motor, it is necessary to set these constants. This invention is related to relatively small-sized motors, which are produced in many types. For each type of the motor, it is disadvantageously required to collect the motor constants. The motor constants vary depending on a temperature corresponding to a load change. For example, the resistance value varies in a range from about 20% to about 30% of the original value according to an operating state. Therefore, a large error appears with respect to a setting value and the estimated current value is decreased in precision.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a current vector estimation method of estimating, using a motor current of one phase, currents of other phases without using the motor constants as well as a low-priced inverter system applicable to general purposes.
Another object of the present invention is to provide an inverter control method and an inverter control apparatus in which a high-torque control operation is conducted particularly in low and middle speed zones with a minimized speed regulation by improving the precision of current estimation.
One aspect of the present invention resides in a closed loop system. In this closed loop system, a current vector including an actually detected motor current of one phase and a motor current of other phases finally estimated and supplied as a feedback item is converted, using a reference phase obtained by integrating an output frequency from an inverter, into a rotating coordinate system to obtain instantaneous values respectively of torque current Iq and exciting current Id. By calculating an average value of each of torque current Iq and exciting current Id and by inversely converting the coordinate system of each average value into the fixed coordinate system, there are estimated currents of other phases.
Consequently, there is provided according to the present invention an inverter control method of converting a dc power into an ac power having a variable frequency and a variable voltage according to an output frequency instruction (f
1
*) and output voltage instructions (Vd*, Vq*) of an inverter (
4
) to control an ac motor (
1
) at a variable speed, comprising the steps of inputting to an input point a current detection value (iu) of one phase of the ac motor (
1
) and current estimation values (iws, ivs) of other phases thereof, converting the current detection value (iu) and the current estimation values (iws, ivs) into a rotating coordinate system (uvw/dq) according to a reference phase obtained by integrating the output frequency instruction (f
1
*) of the inverter (
4
) to obtain instantaneous values respectively of a torque current (Iq) and an exciting current (Id), inputting average values of the instantaneous values, converting the values into a fixed coordinate system according to the reference phase to obtain the current estimation values of the other phases and feeding the values back to the input point, and controlling an output voltage or an output frequency of the inverter (
4
) according to the instantaneous value or the average value of the torque current (Iq).
According to the present invention, the inverter control method further includes the step of controlling an output voltage or an output frequency of the inverter (
4
) according to a primary current (I
1
) of the ac motor (
1
) attained according to the instantaneous value or the average values respectively of the torque current (Iq) and the exciting current (Id). The primary current (I
1
) is obtained as a square root of a sum of Iq
2
and Id
2
.
According to the present invention, the inverter control method further includes the steps of estimating the torque current (Iq) for each sampling of the current detection value of one phase (iu) and increasing, when a load is increased, the output voltage or frequency of the inverter (
4
) according thereto. As a result, even when the load is changed, the torque current can be estimated with high response, and hence the inverter output voltage and the inverter output frequency can be also compensated for with high response and with high precision.
To estimate the torque current according to the present invention, there are only required a reference phase instruction and a detected value of a current of one phase. Therefore, even if information of voltages, motor constants, and the like is not clearly determined, the torque current can be estimated. Consequently, this method can be applied to various general purposes. Since the parameters employed in this method are fixed regardless of variation in the load, the estimated values have high precision. Additionally, for the currents of other phases estimated according to t
Fujii Hiroshi
Okuyama Toshiaki
Sakai Keijiro
Antonelli Terry Stout & Kraus LLP
Han Jessica
Hitachi , Ltd.
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