Motor drive apparatus equipped with dynamic braking control unit

A motor drive apparatus which converts DC to AC to provide drive power for driving a synchronous motor, comprising:

• a full-bridge inverter which has an upper arm and a lower arm each provided with a plurality of semiconductor switching devices and free-wheeling diodes connected in reverse parallel with respective ones of said plurality of semiconductor switching devices, wherein said semiconductor switching devices provided in said upper arm and said lower arm are controlled on and off to convert DC to AC;
• a short-circuiting unit which includes a selector switch between motor phase windings of said synchronous motor, said selector switch being opened and closed under the control of a command;
• and
• a dynamic braking control unit which, upon reception of a dynamic braking start command, performs control so as to turn on all of said semiconductor switching devices provided in either one of said upper arm and said lower arm and to turn off all of said semiconductor switching devices provided in the other arm, and thereafter controls said short-circuiting unit so that said selector switch is closed, and further, after closing said selector switch, performs control so that all of said semiconductor switching devices turned on before closing said selector switch are turned off when a finite amount of time until the chattering between contacts of said selector switch settles down has elapsed, and thereafter said synchronous motor produces a decelerating torque.