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Electrical Induction Motors - Synchronous Speed

The speed at which an induction motor operates depends on the input power frequency and the number of electrical magnetic poles in the motor

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The synchronous speed for an electric induction motor is determined by

  • the power supply frequency, and
  • the number of poles in the motor winding. 

The synchronous speed can be calculated as:

n = f (2 / p) 60                      (1)

where

n = shaft rotation speed (rev/min, rpm)

f = frequency of electrical power supply (Hz, cycles/sec, 1/s)

p = number of poles

Note - an induction motor will never reach its synchronous speed. If it did - the rotor would appear to be stationary to the rotating stator field since it would rotate with the same speed. With no relative motion between stator and rotor field no voltage will be induced in the motor. The speed of an induction motor is therefore limited to a speed below synchronous speed and the difference between synchronous speed and actual speed is called slip.

Example - Synchronous Speed of Electric Motor with Two Poles

A motor with two poles is supplied with power with frequency 50 Hz (1/s). The rotation speed can be calculated as

n =  (50 1/s) (2 / 2) (60 s/min) 

   = 3000 rpm (1/min)

Synchronous rotation speed at different frequencies and number of poles

Shaft rotation speed - n - (rev/min, rpm)
Frequency
- f -
(Hz)
Number of poles - p -
24681012
10 600 300 200 150 120 100
20 1200 600 400 300 240 200
30 1800 900 600 450 360 300
40 2400 1200 800 600 480 400
501) 3000 1500 1000 750 600 500
602) 3600 1800 1200 900 720 600
70 4200 2100 1400 1050 840 700
80 4800 2400 1600 1200 960 800
90 5400 2700 1800 1350 1080 900
100 6000 3000 2000 1500 1200 1000
  1. Motors designed for 50 Hz are most common outside U.S
  2. Motors designed for 60 Hz are most common in U.S

Variable Frequency Drive

A variable frequency drive modulates the speed of an electrical motor by changing the frequency of the power supply.

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