Patent ID: 7317292

Claim:
A method of controlling an alternating current motor and a power converter including: a converter unit for converting an alternating current voltage supplied from an alternating current power supply into a direct current voltage, and a smoothing capacitor for smoothing the converted direct current voltage, and an inverter unit for converting a direct current intermediate voltage into an alternating current voltage having a frequency corresponding to a torque instruction by a PWM control manner, by setting an electromotive torque limit value and a regenerative torque limit value in advance, and by restricting the torque instruction with the torque limit values to generate a PWM switching pattern to be output to the inverter unit, the method comprising the steps of: allowing a power failure detecting unit provided in the power converter to detect a power failure of the alternating current power source; outputting a deceleration start instruction to the inverter unit in response to a power failure detecting signal output from the power failure detecting unit; calculating a first reduction rate, on the basis of a detection value and a target value of the direct current intermediate voltage, such that the direct current intermediate voltage is made constant during the deceleration of the alternating current motor; calculating a second reduction rate on the basis of a variation in the direct current intermediate voltage; controlling a deceleration time by controlling a value obtained by multiplying the two reduction rates together in a PI control manner; calculating the torque instruction to allow the alternating current motor to be decelerated for the deceleration time; changing the electromotive torque limit value and the regenerative torque limit value on the basis of the value of the detected direct current intermediate voltage; stopping the deceleration when the direct current intermediate voltage is equal to a voltage before the power failure is detected or it rises during the deceleration; and when the alternating current motor is returned to a normal control mode, storing an output frequency before the power failure is detected.