PURPOSE: To prevent a long time from being taken for a rotor before it is stopped by applying a brake to an inverter circuit by the use of a regenerative braking circuit and a DC brake from a free-run condition to a complete stop after braking action is stopped or decreased, when a three-phase induction motor is stopped.

CONSTITUTION: In the case of a three-phase induction motor driving power unit using a pulse amplitude modulation(PAM) system, an output of a rectifier circuit 11 is controlled through a chopper circuit 12 to predetermined voltage supplied to an inverter circuit 13 on-off controlling switching elements Q1 to Q6 by a signal from a phase generator 17, and the voltage is converted into AC power of predetermined voltage and frequency supplied to a three-phase induction motor 22 driven. In deceleration operation, when the power unit is decreased in its output frequency, voltage between terminals a-b increases by generating action of the three-phase induction motor 22, so that power is consumed by a resistor R applying a generation brake. In low and intermediate speed operation, when generated power decreases, a DC current is allowed to flow between phases UV of the three-phase induction motor 22 applying an electromagnetic brake.