Control apparatus for an induction motor

A control apparatus for an induction motor, comprising;

• a dc power source;
• inverter means for converting dc power supplied from said dc power source into ac power of the variable voltage and variable frequency to feed the induction motor;
• and
• modulation means for producing signals for operating said inverter means in accordance with the pulse width modulation method on the basis of a predetermined ratio of voltage to frequency in an ac output voltage of said inverter means and a reference for frequency of the ac output voltage,
• characterized by
• current detecting means for detecting an effective value of a motor current of the induction motor;
• frequency detecting means for detecting an actual rotation frequency and an actual slip frequency of the induction motor;
• pattern generating means for generating a reference for a torque to be produced by the induction motor;
• comparing means for comparing the motor current detected by said current detecting means with the torque reference generated by said pattern generating means to obtain a deviation therebetween, in which at least one of the motor current and the torque reference is converted so that both are torque-related control variables of the same dimension;
• and
• operating frequency reference generating means for calculating a reference for a slip frequency in accordance with the deviation obtained by said comparing means and obtaining the frequency reference to be given to said modulation means on the basis of the calculated slip frequency reference and the actual rotation frequency detected by said frequency detecting means.