Control device for AC motor

A control device for an AC motor receiving a voltage controlled by an inverter, comprising:

• a current sensing unit for sensing a current flowing between said inverter and said AC motor;
• and
• an overmodulation PWM control unit for controlling a pulse width modulation voltage provided from said inverter to said AC motor based on a voltage comparison between a phase voltage command having an amplitude exceeding a voltage amplitude of a carrier and said carrier, wherein
• said overmodulation PWM control unit includes:
• a voltage command producing unit for producing d- and q-axis voltage commands according to a deviation between a current command (Idcom, Iqcom) of said AC motor and an actual current based on a sensed value of said current sensing unit to eliminate said deviation,
• a voltage amplitude calculating unit for calculating an original voltage command amplitude based on original d- and q-axis voltage commands produced by said voltage command producing unit,
• a voltage amplitude correcting unit for correcting said d- and q-axis voltage commands such that a fundamental amplitude of said pulse width modulation voltage matches said original voltage command amplitude, and
• a voltage command converting unit for converting the d- and q-axis voltage commands corrected by said voltage amplitude correcting unit to said phase voltage command;
• said voltage amplitude correcting unit calculates a required correction rate with respect to said original voltage command amplitude according to preset voltage amplitude characteristics representing a relationship between said fundamental amplitude and a voltage command amplitude required for achieving said fundamental amplitude, and calculates the corrected d- and q-axis voltage commands based on the corrected voltage command amplitude produced by correcting said original voltage command amplitude according to the required correction rate;
• said voltage amplitude characteristics include first and second amplitude characteristics being set independently of each other according to presence and absence of superimposition of a three-order harmonic component on said phase voltage command converted by said voltage command converting unit;
• and
• said overmodulation PWM control unit further includes a harmonic determining unit for determining the presence and absence of the superimposition of the three-order harmonic component on said phase voltage command converted by said voltage command converting unit, based on said first and second amplitude characteristics and said original voltage command amplitude calculated by said voltage amplitude calculating unit.