In recent years, a control device for a PM motor that performs speed control by vector control has been developed.
A conventional control device for a PM motor that performs speed control by vector control will be described below with reference to FIG. 10.
FIG. 10 is a block diagram showing a configuration of a conventional motor control device for a PM motor.
As shown in FIG. 10, the motor control device for a PM motor includes at least PM motor 101, speed calculating unit 103, speed control unit 104, current detector 105, current coordinate conversion unit 106, d-axis current control unit 107, q-axis current control unit 108, voltage coordinate conversion unit 109, and inverter 110.
The motor control device for a PM motor shown in FIG. 10 drives PM motor 101 by the operation described below.
More specifically, position detector 102 is attached to PM motor 101 to detect a rotational position of PM motor 101. Speed calculating unit 103 calculates speed ωm of PM motor 101 based on position information detected by position detector 102. Speed control unit 104 calculates q-axis current command Iq* by using a deviation between speed ωm and speed command ω* as an input such that speed ωm of PM motor 101 calculated by speed calculating unit 103 follows given speed command ω*. At this time, speed control unit 104 is operated by, for example, proportional-integral control.
Current coordinate conversion unit 106 performs coordinate conversion to a detection value of a motor current detected by current detector 105 to calculate and output d-axis current Id serving as a component having the same direction as that of an axis of magnetic poles of PM motor 101 and q-axis current Iq serving as a component having a direction perpendicular to the d-axis. Then, d-axis current control unit 107 calculates and outputs d-axis voltage command Vd* such that d-axis current Id follows d-axis current command Id* given in advance. Moreover, q-axis current control unit 108 calculates and outputs q-axis voltage command Vq* such that q-axis current Iq follows q-axis current command Iq*. At this time, d-axis current control unit 107 and q-axis current control unit 108 are operated by, for example, proportional-integral control.
Voltage coordinate conversion unit 109 forms a three-phase voltage command from d-axis voltage command Vd* and q-axis voltage command Vq*. Inverter 110 drives PM motor 101 based on the voltage command formed by voltage coordinate conversion unit 109.
At this time, in order to stably drive the PM motor by a conventional drive device for a PM motor, current control gains of d-axis current control unit 107 and q-axis current control unit 108 need to be properly set to realize stable current control. In order to obtain high speed-controllability of the PM motor, a current control gain of a current control system serving as a minor loop of a speed control system is desired to be high as much as possible.
Thus, in the conventional control device for a PM motor, some techniques that adjust current control gains are disclosed (for example, refer to PTL 1). The technique disclosed in PTL 1 targets an induction motor and uniquely determines a current control gain based on an arithmetic expression by using a resistance, an inductance, and a control delay time serving as a circuit constant (motor constant) of an equivalent circuit including a load.
Another technique that calculates motor constants such as a resistance and an inductance is disclosed in, for example, PTL 2. According to PTL 2, a resistance is calculated based on an input voltage and an input current obtained when a DC current is caused to flow in PM motor 101. Fundamental wave components of an input voltage and an input current obtained when an AC current is caused to flow in PM motor 101 are extracted, and an inductance is calculated based on the magnitudes of the input voltage and the input current and a phase difference between the input voltage and the input current. A current control gain is calculated based on the calculated resistance and the calculated inductance.
However, in PTL 1, a resistance and an inductance serving as equivalent circuit constants need to be examined in advance. Since a cut-off frequency that determines a response of current control is calculated by a fixed expression, a current response may not be always maximized.
On the other hand, in PTL 2, measurement of a motor constant that is a problem in PTL 1 is possible. However, in order to calculate a wire wound resistor, a time until a DC current caused to flow in PM motor 101 is set in a steady state is required. Since the resistance and the inductance are measured by using different test signals, respectively, long times are disadvantageously required for the measurement.    PLT 1: Unexamined Japanese Patent Publication No. 9-84378    PLT 2: Unexamined Japanese Patent Publication No. 2000-312498
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