View Full Version : question about electric motors.

02-20-2013, 10:27 AM
What are the differences in the stators of synchrounous as to non synchronous motors?

I can see that the armatures are different, that's really visible. The coil windings, that's another story.


02-20-2013, 11:02 AM
If you put a permenet magnet rotor into an induction motor you now have a synchronous AC motor.

If the windings were redistributed for a trapezoidal waveform, you would have a Brushless DC motor.

The efficiency map and how the losses are divided between the motor and the drive determine which would be chosen.

02-20-2013, 01:00 PM
What are the differences in the stators of synchrounous as to non synchronous motors?

Going to have to define different as in technical theory or practice. What I'm getting at is if I recall correctly the starting torque of a shaded pole sync stator is so low you'll not see that in practice, although they theoretically exist. So pick up a motor and see shaded pole stator that means 99.99% odds its non-sync although someone could build a a sync shaded pole stator motor.

How does a shaded pole sync "work"? Well not very well, but basically the shaded pole rather optimistically spins it up until the sync can "lock" and the "locking" part of the sync torque has to be quite a bit stronger than the shaded part of the torque so they typically don't spin up too well. You could probably use it for a clock but nothing hooked up to a "real" load.

Then too you get into definition arguments like is a brushless electronically controlled motor technically a sync motor, well yeah, sorta, if you're gonna call a 3phase with a VFD a sync motor. So the windings for a low voltage brushless sync are going to look fat compared to a high(er) voltage plain ole non-sync induction motor. But thats an engineering implementation difference because of the voltage it runs at, not a theoretical science difference, although I suppose plain ole induction motors might exist "somewhere" that run off 12 volts AC (not DC)

02-20-2013, 01:30 PM
There was a synchronous motor that was popular decades ago for Very large HP applications, where the motor was ran up to maximum off load speed as an induction motor and the slip frequency was read off of a rotor winding through slip rings, when the slip was within a small % value, the rotor winding was injected with DC and the rotor came up to synchronism.
The way a shaded pole works is that it is effectively a shorted turn on a part of the stator, this causes a phase shift in this part of the winding, effectively creating a split phase motor.

Alan Douglas
02-20-2013, 03:01 PM
The stators may be the same. The difference is in the rotor. A synchronous rotor has "salient" poles, always in the same physical position, so there can be no slip. The rotor either runs at synchronous speed or it stops. It could still have a squirrel-cage winding to get it close to synchronous speed.

You could make a sync motor from a non-sync by milling flats on the rotor.

J Tiers
02-20-2013, 09:54 PM
A synchronous motor usually has a wound field on the rotor, although it could be PM. A few tiny non-self-starting ones are made with what looks like a gear that proceeds from pole-to-pole as the AC reverses. I have somewhere a clock like that.

An induction motor has magnets on the rotor also, but they are "induced" by the stator current.

Alan Douglas
02-20-2013, 10:23 PM
Not always tiny -- I have a 1930s phono turntable directly-driven by a 92-pole sync motor. Works out to 78.26 rpm.