| 摘要 |
PROBLEM TO BE SOLVED: To protect a motor against burning even if an undue load is applied during a low speed operation while saving the operating energy by suppressing voltage ripples produced through PWM control of AC voltage being supplied to the motor upon elapsing a predetermined time under such a state as the speed detection value is lower than a specified level. SOLUTION: When a motor is operated under such a state as the line speed reference SPL is lower than a specified level and that state is sustained for a specified time or longer, a switch 21 outputs a voltage reference Vd2r selectively. The voltage reference Vd2r is set lower than a voltage reference Vd1r and the DC output voltage Vd from a converter is reduced to a level corresponding to the voltage reference Vd2r. Consequently, the DC supply voltage Vd to inverters 4, 4a is decreased and the modulation rate of PWM control is increased and thereby ripples of supply voltage to induction motors 5, 5a are suppressed. According to the arrangement, voltage loss in the induction motors 5, 5a can be reduced and the possibility of burning the induction motors 5, 5a can be reduced even if an undue load is applied during low speed operation. |
