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# Operation at Other Than Rated Voltage and Frequency

Author: E.E. Kimberly

The torque of an induction motor is proportional to the square of the applied voltage. If the voltage applied is 10 per cent less than the rated value, the torque at any speed will be only 0.90x0.90 = 0.81 or 81 per cent of normal. If the starting requirements are particularly severe, a reduction of 10 per cent in voltage may cause failure to start.

Synchronous speed is directly dependent on frequency, and will consequently vary as the frequency is varied. Motors designed for 60-cycle frequency may usually be operated on 50-cycle frequency at rated load with a temperature rise of not more than 55 deg C. A 60-cycle motor connected to a 25-cycle line will sometimes overheat without load because of excessive exciting current. In general, a polyphase induction motor may be operated at rated voltage at rated frequency or as much as 20 per cent above rated frequency without injury, but with somewhat impaired characteristics.

 TABLE 18-1 OPERATION OF 60-CYCLE INDUCTION MOTOR UNDER VARIOUS LINE AND LOAD CONDITIONS CONDITIONS RESULTS* Load Time Voltage Frequency Current Speed Square of Torque Temperature Rise Normal Continuous Normal Normal Normal Normal Normal Normal Normal Continuous 10% High Normal Lower Normal 20% Higher Normal or Higher Normal Continuous 10% Low Normal Higher Normal 20% Lower Normal or Higher Normal Continuous Normal 5% High Small Change 5% Higher 7% Lower Small Change Normal Continuous Normal 5% Low Higher 5% Lower 7% Higher Higher Normal Continuous 10% Low 5% High Higher 5% High Too Low (i) Higher 15% High Continuous Normal Normal + 15% Small Change Normal 50° 35% High Intermittent Normal Normal + 35% Slower Normal 50° 35% High Continuous Normal Normal +35% Slower Normal Too High (70°) Normal Continuous Normal 50 Cycles Higher 17% Lower 35% Higher 50° 15% High Continuous Normal 50 Cycles Too High 17% Lower 35% Higher Too High (60°) 35% High Intermittent Normal 50 Cycles Too High 17% Lower 35% Higher Too High (80°) * All compairisons based on motor at normal load, voltage, and frequency.

Last Update: 2010-10-05