# Synchronous Motor speed

Discussion in 'General Electronics Chat' started by mnada, May 9, 2012.

Apr 5, 2012
26
0
Hi Dear Friends,
The speed of the synchronous Motor is given by the equation:
s=120 x f/n where

s = speed in rpm
f = frequency
n = no of pole (here starts the confusion)

My confusion starts with the n (no of poles) whether it refers to the no of poles of the stator windings or no of poles of the rotor winding, and if it refers to the stator no of poles, does that meant that the no of poles of the rotor has no effect on the speed.
If so what is the effect of the increasing the no of poles of the rotor ( and what is the restriction of the no of poles rotor)

2. ### StayatHomeElectronics Well-Known Member

Sep 25, 2008
864
40
From wikipedia entry on synchronous motors:

The speed of the motor is a result of a combination of both stator and rotor. The stator field just sets the mark for the synchronous rotation speed.

Apr 5, 2012
26
0
Thanks for your reply. Does that mean that n in the above equation refer to the number of poles of the stator?

What is the effect of number of poles of the rotor?

4. ### StayatHomeElectronics Well-Known Member

Sep 25, 2008
864
40
Yes, n will be the number of poles of the stator. A synchronous motor is synchronized when the rotor is rotating at the same speed as the magnetic field in the stator. When the rotor is rotating slower than the magnetic field in the stator, i.e. slip, torque is created.

Here is a small blurb from wikipedia again about the motor poles:

Just a really simple example.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Overly trusting in Wikipedia can be potentially misleading.

Here's another quote from Wikipedia

from

http://en.wikipedia.org/wiki/Synchronous_motor

In any case when one has a two pole (per phase) synchronous motor then the rotor poles and stator poles must be the same.

6. ### StayatHomeElectronics Well-Known Member

Sep 25, 2008
864
40
@t_n_k, I agree.

Apr 5, 2012
26
0
I quote the following from the lesson on synchronous motors:

"The three phases of stator excitation add vectorially to produce a single resultant magnetic field which rotates f/2n times per second, where f is the power line frequency, 50 or 60 Hz for industrial power line operated motors. The number of poles is n. For rotor speed in rpm, multiply by 60."

Regarding the rotating magnetic field speed is the red lined text correct?? I think it should be 2f/n, same synchronous speed?

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Yes you are correct - well spotted. You can report the error to the Feedback & Suggestions Forum.