Abstract: In a servo control apparatus constituted by a converter unit 3 for rectifying an AC power supply voltage to produce a DC power supply voltage, and a plurality of inverter units 5 for producing drive power supply voltages of motors from the DC power supply voltage, the servo control apparatus is provided with a power supply breaker 2 which is connected to the side of the AC power supply of the converter unit 3; and a ground fault detecting circuit 16 built in the converter unit 3. When a ground fault happens to occur, the servo control apparatus stops the operations of all of the inverter units 5 so as to cut off the breaker 2.

Abstract: In a servo control apparatus constituted by a converter unit 3 for rectifying an AC power supply voltage to produce a DC power supply voltage, and a plurality of inverter units 5 for producing drive power supply voltages of motors from the DC power supply voltage, the servo control apparatus is provided with a power supply breaker 2 which is connected to the side of the AC power supply of the converter unit 3; and a ground fault detecting circuit 16 built in the converter unit 3. When a ground fault happens to occur, the servo control apparatus stops the operations of all of the inverter units 5 so as to cut off the breaker 2.

Abstract: A bridge-type power converter has, in each phase, a inductor (L.sub.1) connected between a connection point of said two switching elements (Q.sub.1, Q.sub.2) and an output terminal; capacitors (C.sub.1, C.sub.2) connected between the output terminal and a positive or negative side of a d.c. bus for changing an output voltage along a curve determined by a phenomenon of resonance with the reactor (L.sub.1) when the semiconductors switching elements (Q.sub.1, Q.sub.2) operate, causing abrupt changes of a voltage at the connection point; diodes (D.sub.3, D.sub.4) for changing a current path at a time when the resonance goes approximately one quarter of period to thereby stop the resonance; and constant voltage diodes (ZD.sub.1, ZD.sub.2) for absorbing the energy stored in the inductor (L.sub.1) during the resonance.

Abstract: An angular position detector for generating trains of A- and B-phase pulses which are 90.degree. out of phase with each other and a C-phase pulse per rotation are mounted on a permanent-magnet synchronous motor such that a magnetic pole origin of permanent magnets and a position where the C-phase pulse is generated agree with each other. After a power supply voltage is applied and in an initial stage before the permanent-magnet synchronous motor is operated normally, a motor control apparatus operates a current control loop with an alternating current command having a constant amplitude and a low frequency to supply an alternating current having a low frequency to the permanent-magnet synchronous motor for thereby rotating the rotor of the permanent-magnet synchronous motor from a stop at a low speed.