Patent ID: 7315144

Claim:
An AC drive apparatus comprising: a bidirectional converter for converting DC power to variable frequency AC power for driving a motor, and converting an AC output from said motor to DC; a plurality of electric brakes each including a resistor connected to said bidirectional converter to consume an electromotive force regenerated by said motor, and a switch for connecting and disconnecting said resistor to and from said bidirectional converter; and a controller adapted to control duty in said switch, which duty is a time ratio of connection of said resistors to DC power in accordance with a comparison between voltage value and setting value of said DC power, wherein: when the voltage value of said DC is within a first voltage range, said duty is rendered a predetermined value, when the voltage value of said DC is within a second voltage range which is a larger voltage compared to said first voltage range, said duty, which is in accordance with the voltage value of said DC, becomes larger compared to said duty of said first voltage range at least in a part of said range, and when the voltage value of said DC is within a third voltage range which is a larger voltage compared to said second voltage range, said duty is set to a large predetermined value compared to at least the duty corresponding to said second voltage range.