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Home The Polyphase Induction Motor Operation at other than Rated Voltage and Frequency  
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Operation at Other Than Rated Voltage and FrequencyAuthor: 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 60cycle frequency may usually be operated on 50cycle frequency at rated load with a temperature rise of not more than 55 deg C. A 60cycle motor connected to a 25cycle 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.


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