Method and apparatus for controlling AC motor

A control apparatus for an induction motor, wherein an alternating current power source is connected to the motor through a switch and a frequency converting means and the motor speed is controlled by a controller through controlling the output frequency and the output voltage of said frequency converting means, said controller including:

• first means for causing said frequency converting means to produce at a time of starting of said motor an initial frequency which is expected to cause the absolute value of the current flowing in said free-running induction motor to become substantially minimum and an initial voltage which is insufficient to drive said motor;
• second means for increasing said output voltage gradually from said initial voltage with said initial frequency;
• and
• third means for increasing said output frequency and output voltage, with the ratio therebetween being maintianed almost at a predetermined value, after said output voltage has reached a predetermined value,
• wherein said first means includes:
• fourth means for causing said output voltage to be used as a search voltage which is insufficient to drive said motor, and varying said output frequency and said search voltage with the ratio therebetween being maintained at a predetermined value;
• fifth means for step of detecting the motor current while said output frequency is varied for a variation of said output frequency by a pregiven value;
• and
• sixth means for determining, as said initial frequency, an output frequency which corresponds to the minimum value of the motor current detected by said fifth means.

A method for restarting an AC motor which is running by inertia, in which upon restarting an a.c. voltage source is connected to the motor through a frequency converter and the motor speed is controlled by controlling the output frequency and the output voltage of said frequency converter and the motor rotates by the action of its moment of inertia, comprising:

• a first step of causing said output voltage to be used as a search voltage which is insufficient to drive said motor, and varying said output frequency and said search voltage with the ratio therebetween being maintained at a predetermined value;
• a second step of detecting a motor current while said output frequency is varied for a variation of said output frequency by a pregiven value;
• a third step of determining, as an initial frequency, an output frequency which corresponds to the minimum value of the motor current detected in said second step;
• a fourth step of increasing said output voltage gradually from an initial voltage which is insufficient to drive said motor with said initial frequency maintained;
• and
• a fifth step of increasing said output frequency and output voltage, with the ratio therebetween being maintained almost at a predetermined value, after said output voltage has reached a predetermined value.

A method for restarting an AC motor which is running by inertia, in which upon restarting an a.c. voltage source is connected to the motor through a frequency converter and the motor speed is controlled by controlling the output frequency and the output voltage of said frequency converter and the motor rotates by the action of its moment of inertia, comprising:

• a first step of causing said output voltage to be used as a search voltage which is insufficient to drive said motor, and varying said output frequency and said search voltage with the relation therebetween being maintained at a predetermined relation;
• a second step of detecting a motor current while said output frequency is varied for a variation of said output frequency;
• a third step of determining, as an initial frequency, an output frequency which corresponds to the minimum value of the motor current detected in said second step;
• a fourth step of increasing said output voltage gradually from an initial voltage which is insufficient to drive said motor;
• and
• a fifth step of varying said output frequency and output voltage, with the ratio therebetween being maintained almost at a predetermined value, after said output voltage has reached a predetermined value.

A control apparatus for an induction motor, wherein an alternating current power source is connected to the motor through a frequency converting means and the motor speed is controlled by a controller through controlling the output frequency and the output voltage of said frequency converting means, said controller comprising:

• first means for causing said output voltage to be used as a search voltage which is insufficient to drive said motor, and varying said output frequency and said search voltage with the relation therebetween being maintained at a predetermined relation;
• second means for detecting a motor current while said output frequency is varied for a variation of said output frequency;
• third means for determining, as an initial frequency, an output frequency which corresponds to the minimum value of the motor current detected by said second means;
• fourth means for increasing said output voltage gradually from an initial voltage which is insufficient to drive said motor;
• and
• fifth means for varying said output frequency and output voltage, with the ratio therebetween being maintained almost at a predetermined value, after said output voltage has reached a predetermined value.