There are two ways to define motor speed. First is synchronous speed. The synchronous speed of an AC motor is the speed of the stator's magnetic field rotation. This is the motor's theoretical speed since the rotor will always turn at a slightly slower rate.
The other way motor speed is measured is called actual speed. This is the speed at which the shaft rotates. The nameplate of most AC motors lists the actual motor speed rather than the synchronous speed.
Standard AC induction motors depend on the rotor trying, but never quite succeeding, to catch up with the stator's magnetic field. The difference in the speed of the rotor and the synchronous speed of the stator's rotating magnetic fields is called the slip. Different motor designs will produce different amounts of slip.
AC motors are designed with various numbers of magnetic poles. Standard motors have two, four, six, or eight poles. These poles play an important role in determining the synchronized speed of an AC motor.
A motor's synchronous speed can be computed using this formula: synchronous speed equals 120 times the operating frequency, divided by the number of poles.
For example: A six-pole motor's synchronous speed is 120 x 60 = 7200 divided by 6, or 1200 RPM.
A four-pole motor's synchronous speed will be 1800 RPM. Use this formula to determine other speed/pole relationships.
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