Re: Help needed on understanding synchronous motors
In a message dated 99-01-21 01:01:48 EST, you write:
<< But how are higher powered motors (e.g. 1/2 hp.) built?
> A while ago I read a post on turning non sync motors into sync´d ones, by
> milling flats on the armature. How (by what principal) does this work ??
> Coiler greets from germany,
> Reinhard >>
When flats are milled on the armature of an induction motor, this
creates a salient pole sync-motor. The "bumps" that are created
by the milling operation, are *grabbed* by the rotating magnetic
field which pulls the bumps around with the field, so the armature
locks to the field. Since there's no continual slippage, the torque
capability is less than in the induction motor in which there is a
difference in speed (slippage) between the stator and rotor (armature).
When the sync-motor is loaded, the relative position of the bumps
on the armature, and the rotating field will shift somewhat. A constant
load will give a constant amount of shift, so it will still be locked.
There is shift, but not constant slippage. If the sync-motor is loaded
too heavily, it will lose lock altogether.
A 3450 rpm induction motor has 2 poles, so it needs 2 milled flats
to make it synchronous at 3600 rpm. A 1750 (1725) rpm motor has
4 poles, so it needs four flats to make it into a sync motor that runs
at 1800 rpm. (At 50Hz, the speeds will be 3000 rpm, and 1500 rpm).
The milling procedure can be used on small (1/20th HP) motors, and
on up to larger 2 HP motors, and higher.
And if you don't have a milling machine, the job can be done using
a drill press and a large hand file, and some care to keep it balanced.
But it does need quite a lot of filing!