Re: Is this motor synchronous?

From: 	Gary Lau  27-Dec-1997 2012[SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: 	Saturday, December 27, 1997 6:27 PM
To: 	tesla-at-pupman-dot-com
Cc: 	lau-at-hdecad.ENET.dec-dot-com; wysock-at-courier8.aero-dot-org
Subject: 	Re: Is this motor synchronous?

To Bill Wysock:

Thank you very much for your detailed reply to my post.

It's now clear that the 1800 RPM motor described cannot be used for a
sync RSG due to the variable phase relationship.  What's got me puzzled
though is why the phase can change.  I opened the unit up and examined
the rotor.  Rather than being made of the usual soft iron laminations, it
is a cylindrical magnet mounted on an aluminum hub, the poles of the
magnet being 90 degrees apart.  I would have believed that since the
poles are in a fixed position wrt the shaft, this would rigidly define
the phase relationship.  BTW, I erred in my description, the unit in
question is .65 Amps, not .85.   It also employs a start (run?)

I did attempt to modify a 1725 RPM unit.  I found a .90 Amp 1725 RPM
unit in a squirrel cage blower and filed the four flats as had been
described.  I made the width of the flats the same as the distance
between adjascent flats, and the motor ran smoothly.  When I tested it
with just a magnet for my pickup coil as a load, it was indeed
synchronous (although I didn't check that the phase was constant from
start-up to start-up).  When I added a load disk, more often than not it
would not sync up and get hot quickly, although if I quickly rotated the
motor housing in the direction that would increase it's relative RPM, the
unit would sync up and run cooler.  This was just a bit too unreliable
though.  I wasn't sure if I had removed too much or too little, but from
your reply, it's clear I removed too much.  Guess I'll try again.

>Be sure that you specify a capacitor start/induction run type of motor,
>and make sure it is a ball bearing type.  Other types such as induction
>start/ induction run or capacitor start/capacitor run are not suitable
>for this modification/application.  

My 1725 RPM motor didn't have a cap, although I'm suspecting I might have
been successful had I removed less material.  How does one determine _by_
_observation_ if a motor is a capacitor start/induction run type of motor
vs an induction start/ induction run or capacitor start/capacitor run
type, short of having a spec sheet?

Thanks again for the help.

Gary Lau
Waltham, MA USA