Driver for an induction motor

A driver for an induction motor comprising:

• speed sensing means for sensing a rotational speed of the induction motor;
• an invertor circuit for supplying power to the induction motor by an alternating current voltage based on an output voltage command signal and an output frequency command signal;
• output voltage command means for giving the output voltage command signal to said invertor circuit;
• output frequency command means for applying, as the output frequency command signal to said invertor circuit, a sum of a predetermined slip frequency and a frequency signal depending on the rotational speed of the induction motor and output from said speed sensing means, the driver being driven under slip frequency control,
• voltage limit level setting means for setting a upper limit value of an output voltage from said invertor circuit in accordance with an output signal from said output frequency command means or an output signal from said speed sensing means;
• voltage limiter means for limiting the output signal from said output voltage command means in accordance with a set value set by said voltage limit level setting means to limit the upper limit of the output voltage from said invertor circuit;
• lowest-frequency limiter means for limiting the output signal from said output frequency command means such that the lower limit of the output frequency from said invertor circuit is not less than a preset lowest frequency;
• and
• wherein a set level for the upper limit of the output voltage from said invertor circuit is changed in correspondence to a change in the output frequency from said invertor circuit, and the lowest frequency becomes the output frequency from said invertor circuit when the output frequency from said invertor circuit decreases to the lowest frequency of said lowest frequency limiter means.

A driver for an induction motor comprising:

• speed sensing means for sensing a rotational speed of the induction motor;
• an invertor circuit for supplying power to the induction motor by an alternating current voltage based on an output voltage command signal and an output frequency command signal;
• output voltage command means for giving the output voltage command signal to said invertor circuit;
• output frequency command means for applying, as the output frequency command signal to said invertor circuit, a sum of a predetermined slip frequency and a frequency signal depending on the rotational speed of the induction motor and output from said speed sensing means, the driver being driven under slip frequency control,
• voltage limit level setting means for setting a upper limit value of an output voltage from said invertor circuit in accordance with an output signal from said output frequency command means or an output signal from said speed sensing means;
• voltage limiter means for limiting the output signal from said output voltage command means in accordance with a set value set by said voltage limit level setting means to limit the upper limit of the output voltage from said invertor circuit;
• lowest-frequency limiter means for limiting the output signal from said output frequency command means such that the lower limit of the output frequency from said invertor circuit is not less than a preset lowest frequency;
• forward rotation limiter means for sensing whether the output frequency from said invertor circuit has reached a preset maximum frequency limit level and if so, for outputting a first power supply interruption signal;
• and
• reverse rotation limiter means for detecting whether the reverse rotation speed of the induction motor has reached a preset reverse rotation limit level and if so, for outputting a second power supply interruption signal;
• wherein a set level for the upper limit of the output voltage from said invertor circuit is changed in correspondence to a change in the output frequency from said invertor circuit, and the lowest frequency becomes the output frequency from said invertor circuit when the output frequency from said invertor circuit decreases to the lowest frequency of said lowest frequency limiter means, and power supply to the induction motor is interrupted when either said forward rotation limiter means outputs the first power supply interruption signal or said reverse rotation limiter means outputs the second power supply interruption signal.

A driver for an induction motor comprising:

• speed sensing means for sensing a rotational speed of the induction motor;
• an invertor circuit for supplying power to the induction motor by an alternating current voltage based on an output voltage command signal and an output frequency command signal;
• output voltage command means for giving the output voltage command signal to said invertor circuit;
• output frequency command means for applying, as the output frequency command signal to said invertor circuit, a sum of a predetermined slip frequency and a frequency signal depending on the rotational speed of the induction motor and output from said speed sensing means, the driver being driven under slip frequency control,
• voltage limit level setting means for setting a upper limit value of an output voltage from said invertor circuit in accordance with an output signal from said output frequency command means or an output signal from said speed sensing means;
• voltage limiter means for limiting the output signal from said output voltage command means in accordance with a set value set by said voltage limit level setting means to limit the upper limit of the output voltage from said invertor circuit;
• lowest-frequency limiter means for limiting the output signal from said output frequency command means such that the lower limit of the output frequency from said invertor circuit is not less than a preset lowest frequency;
• forward rotation limiter means for sensing whether the output frequency from said invertor circuit has reached a preset maximum frequency limit level and if so, for outputting a first power supply interruption signal;
• and
• reverse rotation limiter means for detecting whether the reverse rotation speed of the induction motor has reached a preset reverse rotation limit level and if so, for outputting a second power supply interruption signal;
• low-speed abnormality detection means for detecting whether the rotational speed of the induction motor is below a preset low-speed abnormality detection level and if so, for outputting a low-speed abnormality signal;
• and
• low-speed overload detection means including said low-speed abnormality detection means for latching and outputting a third power supply interruption signal when a state in which said low-speed abnormality detection means outputs the low-speed abnormality signal and said reverse rotation limiter means outputs no second power supply interruption signal continues for a predetermined time duration;
• wherein a set level for the upper limit of the output voltage from said invertor circuit is changed in correspondence to a change in the output frequency from said invertor circuit, and wherein the lowest frequency becomes the output frequency from said invertor circuit when the output frequency from said invertor circuit decreases to the lowest frequency of said lowest frequency limiter means, and wherein power supply to the induction motor is interrupted when said forward rotation limiter means outputs the first power supply interruption signal or said reverse rotation limiter means outputs the second power supply interruption signal, or when said low-speed overload detection means outputs the third power supply interruption signal.