Re: Modification to induction motor for Synchronos operation

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Charles,

At 01:45 PM 4/3/2001 +0100, you wrote: 
>
> Hello all
>  
> I just acquired a 0.37kW (~1/2 HP) induction motor labelled: at 1370 RPM and
> input voltage of 240V  at 50Hz. I have read numerous posts on modifying such
> motors for synchronous operation and am about to grind flats on the rotor for
> this purpose. However, some of the posts weren't clear as they referred to
> other posts on the subject which I don't have. 
>  
> 1. The motor is evidently 4 pole as the rpm is 1370, just under the 1500 rpm
> synchronous speed on 50Hz. Does that mean that 4 flats 90 degrees apart on
> the rotor are required?


Yes, 4 flats at 90 degrees.

>
>  
> 2. The motor has 2 sets of windings with one of them in series with a
> capacitor. I recalled that motors in the old days had a centrifugal switches
> which disconnected the winding (starting winding) with the capacitor in its
> circuit. This one doesn't have such a switch. I don't know if that is of any
> significance in my situation. 


Some motors have a high current winding that is switched and others have a
second winding that goes through a cap to skew the waveform for starting.  Both
work perfectly fine.


>
>  
> 3. I have a copy of the text of a post containing a method for determining
> flat width dimensions to be ground, but sadly I don't have the originator's
> details. In essence, the information given was a formula for the flat width
> with rotor radius as the variable. The information source was U. N. O. who
> ever that was. There were two options: one with  a 38 degree arc for a motor
> with non dead pole windings and one with a 40 degree arc for motors with dead
> poles. My width calculations are 8.84mm and 9.78mm respectively. Are these
> figures reasonable? Also, how does one recognise a non dead pole winding from
> a dead pole motor what ever dead poles are? 


Maybe the following:

http://hot-streamer-dot-com/TeslaCoils/Misc/sync_motor.txt
http://hot-streamer-dot-com/TeslaCoils/Misc/syncmot.zip


>
>  
> 4. I understand that European motors such as mine tend to have rotors with
> smaller radii (mine has a radius of 28.6mm (1.125") than U. S. motors of
> equivalent HP. I was wondering whether the U. N. O. formula will still be
> applicable as I don't understand why flats on the rotors make induction
> motors synchronous. 


Hopefully the second document about will explain the dead pole thing.  When in
doubt grind less ;-)  Many people have ground all kinds of flats (including two
flats instead of four for a four pole motor and it seems to still work fine. 
The real danger is to grind so much metal away that the motor draws very high
current and overheats.  Even small flats will probably lock up well considering
that a properly ground rotor has much torque to spare. 

>
>  
> 5. Are the flats ground along the entire length of the rotor (my rotor is
> ~125mm long)?


Yes.  Although it may still lock it you partially ground them (don't know why
it wouldn't) but that would be experimenting.

Cheers,

        Terry

>
>  
> Any help in the above would be most helpful and appreciated.
>  
> Chuck/England