PURPOSE: To reduce driving frequency of mechanical system and to stabilize thrust and brake force by providing an exciting current detector, an operational circuit and a means for selecting between a calculated value and a detected value thereby performing speed control with selected exciting current.

CONSTITUTION: Signals I or IP from an exciting current detector and an exciting current operating circuit 22 are connected to the terminals 25a and 25b of a switch 25. Signals are fed from the common terminal 25c of the switch 25 through an adder 24 and a voltage pattern operating circuit 23 to form an output voltage pattern VM/fINV. When the speed is controlled with the exciting current selected through the switch 25, driving frequency of mechanical system can be reduced resulting in stabilization of thrust and brake force.