Induction motor drive apparatus

An induction motor drive apparatus comprising:

• an inverter for converting DC voltage into three-phase AC primary voltage;
• an induction motor driven by the three-phase AC primary voltage;
• speed command means for providing a command speed V.sub.N to the induction motor;
• actual speed detecting means for detecting the actual speed V.sub.n of the induction motor;
• speed deviation signal generating means for generating a speed deviation signal based on the difference between the command speed V.sub.N and the actual speed V.sub.n of the induction motor;
• primary voltage signal generating means for generating a primary voltage command signal V.sub.E for controlling the amplitude of said three-phase AC primary voltage;
• primary frequency signal generating means for generating a primary frequency command signal V.sub.F corresponding to the frequency of said three-phase AC primary voltage;
• rectangular wave signal generating means for producing three-phase rectangular wave signals of a frequency which conforms to the primary frequency command signal V.sub.F and of an amplitude conforming to the primary voltage command signal V.sub.E ;
• pulse forming means for converting each said rectangular wave into a series of closely spaced pulses and for modulating in width each said pulse of each of the three-phase rectangular wave signals in accordance with the magnitude of the primary voltage command signal V.sub.E ;
• and
• means for applying said width-modulated closely spaced pulses selectively to said inverter to produce said three-phase AC primary voltage from said DC voltage for driving said induction motor;
• wherein the primary frequency command signal V.sub.F generating means comprises:
• constant speed signal generating means for generating a constant speed signal V.sub.so of a predetermined constant value and with a polarity determined by the polarity of the speed deviation signal;
• and
• adding means for generating the primary frequency command signal V.sub.F by adding the respective constant speed signal .+-.V.sub.so to the actual speed V.sub.n of the induction motor and for outputting the absolute value of this sum;
• and
• the primary voltage command signal V.sub.E generating means comprising:
• means for forming the absolute value .vertline.V.sub.s .vertline. of the speed deviation signal;
• torque limit signal generating means for selectively generating a torque limit signal for selectively operating said motor in a low-torque mode;
• amplitude control means for selectively limiting the absolute value of the speed deviation signal .vertline.V.sub.s .vertline. and for outputting the selectively limited value .vertline.V.sub.s '.vertline. in response to the torque limit signal;
• and
• a multiplier for generating the primary voltage command signal V.sub.E by multiplying the selectively limited output .vertline.V.sub.s '.vertline. of the amplitude control means by said primary frequency command signal V.sub.F for determining said amplitude of said rectangular wave signals.