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The Polyphase Induction Motor

Operation at other than Rated Voltage and Frequency

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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.

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The Polyphase Induction Motor

Operation at other than Rated Voltage and Frequency

Last Update: 2010-10-05