December 2012
Beginner
528 pages
17h 31m
English
Content preview from Introduction to Electrical Power and Power Electronics
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412 Introduction to Electrical Power and Power Electronics
Since the V/f ratio of the motor input is kept constant at reduced speed, its
air gap flux remains constant. To produce 1/4 torque at the same flux, the rotor
current must be 1/4 of the rated value. At constant flux, the rotor current linearly
varies with the rotor slip rpm, which will become 1/4 × 72 = 18 rpm. This is 2% of
the new synchronous speed, as we would have derived from Equation 15.13 also.
Therefore, at 30 Hz, exact rotor speed = 900 – 18 = 882 rpm.
The exact speed (882 rpm) is about 2% higher than the approximate speed
(864 rpm). This difference may matter in some application ...
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