PURPOSE: To detect the plugging operation precisely by providing the current detecting circuit which detects the difference between the current flowing via a field winding and the current flowing in a plugging diode and employing the output of this circuit.
CONSTITUTION: When the output value (the potential at a point C) of a differential amplifier 12 which amplifies the output signal of the current detecting circuit 11 becomes the value lower than the plugging detecting level, a comparator 25 detects the plugging brake. A capacitor 24 is discharged through a resistor 31, and the increase in the difference between the current IF flowing in a field winding 2 and the current IP flowing in a plugging diode 7 can be prevented. When the potential at the point C reaches the set value (overcurrent limiting value) in the current limiting circuit 32, a chopper 1 is cut off by a chopper control circuit 10. Furthermore, when the plugging diode 7 is shorted and failed, the failure is detected by a shorting failure detecting circuit 33, a transistor 40 is cut off, and the main circuit is stopped by the operation of an electromagnetic contactor 5.
TORII TAKASHI
NIIMI MASAYOSHI
