Re: can this motor be made sync?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>


> No amount of flats is going to fix that. I don't know where that 
> idea came from, i've seen it before on some website. I just chuckled.

I have a synchronous motor made just this way, with 4-pole windings 
and four flats machined on an ordinary induction motor rotor. It came 
from a big old-fashioned tape deck. And it does actually work! It 
spins at 1500rpm happily even with a substantial load. (50Hz here)

The technical term for the "rotor with flats motor" is a reluctance 
synchronous motor. The magnetic flux much prefers being in iron over 
being in air, so it drags the iron along with it as it rotates, to 
try and minimise the airgap it has to pass through (iow, minimise the 
reluctance of the magnetic circuit, hence the name)

Another kind is the hysteresis synchronous motor that has an armature 
made out of hard magnetic material rather than transformer iron. As 
it pulls towards sync speed, and the slip frequency falls towards 
zero, the armature retains a bit of permanent magnetism and makes it 
lock in. This is the one that doesn't always sync in the same place, 
so can cause trouble with rotary gaps.

If you're grinding a motor yourself, as far as I know, 4-pole (that 
is 14xx/17xx rpm) capacitor run motors work best.

Steve Conner
http://www.scopeboy.com/