to the speed corrector 111 the corrector works to output a specified speed correction value obtained in processing the deviation voltage by proportion integration or differentiation the output is added to the speed instruction from the speed setter 107 in the adder 142 when a negative deviation voltage is input a correction quantity corresponding to the deviation voltage is subtracted from the speed instruction given by the speed setter 507 and the speed instruction for the oscillator 508 is decreased owing to this the motor 105 is made to be in a regenerative condition and the speed instruction is corrected in relation to the deviation voltage between the voltage of the dc circuit and the target voltage so that similar to the case of basic constitution the motor 105 is decelerated continuously and smoothly in the state where the voltage of the dc circuit is kept at the target voltage value e the operations after the occurrence of a service interruption are sustained by the control power supply 117 connected to the dc circuit in the present embodiment it is possible to adopt a constitution in which the setting of a target voltage is performed by a microcomputer cpu in other words it is possible to have a constitution in which the functions to be performed by the voltage detector 110 the target voltage setter 112 the mild acceleration deceleration circuit 533 the adder 142 the oscillator 508 and the pwm converter 509 are executed by the microcomputer cpu in the above.