PURPOSE: To improve the readhesive performance of an electric motor vehicle at idling and sliding times by controlling the reduction in the slip frequency when at least one of deviation between the rotating speeds of a plurality of induction motors operating in parallel with each other and ageing change rate exceeds the preset value.

CONSTITUTION: When the deviation fr1-fr2 of the rotating frequencies fr1, fr2 of induction motors of which the output of variable voltage and variable frequency inverter is supplied and the ageing changes Δfr1/Δt, Δfr2/Δt exceed the preset value, idling and slide detection signals are outputted through an OR circuit 19 from a differential speed detector 16 and change detectors 17, 18 thereby synthesizing the throttled current limiting value ΔIc with the current limiting value Ic through a time constant circuit 21. When the signal is vanished, it is gradually returned to the original state by the circuit 21.