PURPOSE:Stator winding is consisted of more than two sorts of coil having different cross-sectional areas of conductors and is able to start load of big inertia controlling temperature rise by changing to high speed side winding in turn after starting from minimum speed winding. CONSTITUTION:When pole of squirrel-cage induction motor is changed and operated in variable speed, two sorts of coil 4 for high speed use and coil 5 for low speed use are equipped in stator slot and have different cross-sectional aereas of conductors, then started from low speed winding, after operating in about synchronous speed of its winding during a certain period and temperature of rotor falls due to decrease of secondary loss and cooling air, they are changed to high speed winding with the same operation, whereby operation of excellent temperature rise characteristics can be attained without increasing cross-sectional area of slot, due to difference of conductor cross-sectional aerea taking load factor of low speed winding into consideration.
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