Induction motor drive apparatus

Claim 1

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 N to the induction motor; actual speed detecting means for detecting the actual speed V 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 N and the actual speed V n of the induction motor; primary voltage signal generating means for generating a primary voltage command signal V 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 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 F and of an amplitude conforming to the primary voltage command signal V 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 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 F generating means comprises: constant speed signal generating means for generating a constant speed signal V 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 F by adding the respective constant speed signal ±V so to the actual speed V n of the induction motor and for outputting the absolute value of this sum; and the primary voltage command signal V E generating means comprising: means for forming the absolute value |V s | 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 |V s | and for outputting the selectively limited value |V s '| in response to the torque limit signal; and a multiplier for generating the primary voltage command signal V E by multiplying the selectively limited output |V s '| of the amplitude control means by said primary frequency command signal V F for determining said amplitude of said rectangular wave signals.

Claim 13

An apparatus for operating an inverter driving an AC induction motor selectively in normal- and low-torque modes of operation by selectively setting in said normal-torque mode of operation the approximate torque in said normal mode of operation by controlling approximately proportionally the amplitude and frequency of rectangular envelopes of three-phase waveforms applied to the inverter for driving the induction motor from a rectified power source, said apparatus comprising command speed means for setting a command speed for said motor, amplitude-adjusting means for constituting said waveform envelope of closely-spaced pulses and for modulating same in width and height to affect said envelope amplitude, and frequency-setting means for setting said frequency of said envelopes proportionally with the actual speed of said induction motor, said proportionality including a preselected minimum constant frequency at zero motor speed, said amplitude-adjusting means including means for generating and multiplying the absolute value of a speed deviation signal which is proportional to the difference between the actual and commanded speeds by a signal corresponding to said frequency of said envelopes, and amplitude control means for limiting said absolute value of the speed deviation signal during said low-torque mode of operation.

Claim 14

An apparatus for operating an inverter driving an AC induction motor selectively in normal- and low-torque modes of operation by selectively setting in said normal-torque mode of operation the approximate torque in said normal mode of operation by controlling approximately proportionally the amplitude and frequency of rectangular envelopes of three-phase waveforms applied to the inverter for driving the induction motor from a rectified power source, said apparatus comprising command speed means for setting a command speed for said motor, amplitude-adjusting means for constituting said waveform envelope of closely-spaced pulses and for modulating same in width and height to affect said envelope amplitude, and frequency-setting means for setting said frequency of said envelopes proportionally with the actual speed of said induction motor, said proportionality including a preselected minimum constant frequency at zero motor speed, said amplitude-adjusting means including means for generating and multiplying the absolute value of a speed deviation signal which is proportional to the difference between the actual and commanded speeds by a signal corresponding to said frequency of said envelopes, and amplitude control means for limiting said absolute value of the speed deviation signal during said low-torque mode of operation.