What is the minimum number of poles in single phase motor

[srinaath]

three phase induction motor can have minimum two poles, so synchronous speed of the machine can be 3000rpm. While single phase motors have two windings - auxiliary and main winding, so minimum number of poles in single phase winding is 4??!...am i right?

 

[Andrew Mason]

three phase induction motor can have minimum two poles, so synchronous speed of the machine can be 3000rpm. While single phase motors have two windings - auxiliary and main winding, so minimum number of poles in single phase winding is 4??!...am i right?

It is a matter of getting the naming convention straight, which is a bit confusing.

The number of poles is the number of poles for each phase in a complete rotation of the rotor.

A two-pole, three phase motor actually has 6 poles or 3 sets of poles at 120 degrees apart. This creates a rotating field at the same frequency as the AC power. So if it is 60Hz, the field rotates around the motor 60 times per second. The synchronous speed (no slip, so no current induced in rotor so no torque) would be 3600 rpm. A 4 pole 3 phase motor has 6 sets of stator poles placed 60 degrees apart. Its synchronous speed would be half that of the 2 pole 3 phase motor.

A single phase motor has a minimum of 2 poles. A 2 pole single phase motor has 2 poles and a synchronous speed of 3600 rpm. A 4 pole single phase motor has 4 poles and a synchronous speed of 1800 rpm (assuming 60 Hz). But in order to create initial torque to start the motor, the single phase motor must have another set of windings with lower inductance creating poles in between the stator windings that are slightly out of phase so as to create a rotating field at startup. These auxiliary starter windings are not counted.

AM