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Speed of an Induction Motor

Author: E.E. Kimberly

It has just been shown that in two-thirds of one cycle the magnetic field rotated through two-thirds of one revolution. With a frequency of 60 cycles, the field would rotate 60 times per second and 3600 times per minute. In one cycle the field moves through 360 electrical degrees. Since there are 360 electrical degrees for every two poles around the winding, a four-pole motor contains 720 electrical degrees; so two cycles are required for every revolution of the field. Hence, a four-pole motor runs only one-half as fast as a two-pole motor. A few common synchronous speeds for 60-cycle motors are given in the following table.

Number of Poles Speed in rpm
2 3600
4 1800
6 1200
8 900
10 720
12 600

Rotating fields for motors with two, four, and six poles are shown in Fig. 18-8.

Fig. 18-8. Induction-Motor Rotating Fields

Last Update: 2010-10-05