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A method and apparatus for controlling an induction motor, in which an AC power source is connected to the induction motor through a switch and a frequency converter, and the motor speed is controlled by controlling the output frequency and output voltage of the frequency converter. In the starting

A method and apparatus for controlling an induction motor, in which an AC power source is connected to the induction motor through a switch and a frequency converter, and the motor speed is controlled by controlling the output frequency and output voltage of the frequency converter. In the starting operation, the induction motor in a free-running state is supplied with an initial voltage in a given frequency which is expected to make the absolute value of the motor current minimum or near minimum, and then the supply voltage is increased gradually from the initial voltage with its frequency being retained at the given voltage. When the output frequency of the frequency converter reaches a predetermined value, the output frequency and voltage thereof are increased gradually with the ratio therebetween being maintained at a given value.

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1. In 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 controlling the output frequency and the output voltage of said frequency converting means, a

1. In 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 controlling the output frequency and the output voltage of said frequency converting means, a method for controlling said induction motor which is running by the inertia in case of reenergizing said motor comprising: a first step of 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; a second step of increasing said output voltage gradually from said initial voltage with said initial frequency maintained; and a third 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, wherein said first step includes: a fourth 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 fifth step of detecting the motor current while said output frequency is varied for a variation of said output frequency by a pregiven value; and a sixth step of determining, as said initial frequency, an output frequency which corresponds to the minimum value of the motor current detected in said fifth step. 2. A method for controlling an induction motor according to claim 1, wherein said fourth step decreases said output frequency and said search voltage gradually with the ratio therebetween maintained at the predetermined value, and said sixth step compares the present motor current value corresponding to the present output frequency with the previous motor current value corresponding to the previous output frequency which is larger than the present frequency value by the pregiven value, and determine, as said initial frequency, a previous output frequency the previous motor current corresponding to which increases for the first time when compared with the present motor current corresponding to the present output frequency. 3. A method for controlling an induction motor according to claim 1, wherein said fourth step increases said output frequency and said search voltage gradually with the ratio therebetween maintained at the predetermined value, and said sixth step compares the present motor current value corresponding to the present output frequency with the previous motor current value corresponding to the previous output frequency which is less than the present frequency value by the pregiven value, and determine, as said initial frequency, a previous output frequency the previous motor current corresponding to which increases for the first time when compared with the present motor current corresponding to the present output frequency. 4. A method for controlling an induction motor according to claim 1, wherein a time period required for performing said second step is not shorter than a second time constant of said induction motor. 5. A method for controlling an induction motor according to claim 1, wherein said third step increases said output frequency and voltage until said output frequency and voltage reach pregiven frequency and voltage respectively, and thereafter maintains said output frequency and voltage at said pregiven frequency and voltage respectively. 6. 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. 7. 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. 8. 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. 9. A method for restarting an AC motor according to claim 8, wherein said fifth step varies said output frequency and voltage until said output frequency and voltage reach a pregiven frequency and a pregiven voltage respectively, and thereafter maintains said output frequency and voltage at said pregiven frequency and pregiven voltage respectively. 10. A method for restarting an AC motor according to claim 8, wherein said first step decreases said output frequency and said search voltage gradually with the ratio therebetween maintained at a predetermined value; and said third step compares the present motor current value corresponding to the present output frequency with the previous motor current value corresponding to the previous output frequency which is larger than the present frequency value by a pregiven value, and determines, as said initial frequency, a previous output frequency at which the previous motor current increases for the first time when compared with the present motor current corresponding to the present output frequency. 11. A method for restarting an AC motor according to claim 8, wherein said fourth step first increases said output frequency and said search voltage gradually with the ratio therebetween maintained at the predetermined value; and said third step compares the present motor current value corresponding to the present output frequency with the previous motor current value corresponding to the previous output frequency which is less than the present frequency value by a pregiven value, a previous output frequency at which the previous motor current increases for the first time when compared with the present motor current corresponding to the present output frequency. 12. 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.