Re: Sync motor mod, 4 flats ?

To: tesla-at-pupman-dot-com
Subject: Re: Sync motor mod, 4 flats ?
From: "Barton B. Anderson" <mopar-at-uswest-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Sun, 09 May 1999 14:05:42 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Hi Richie,

Sounds like the 2 flats are locking to the line frequency quite
well. I'm no expert at this, but yes, I have ground flats on an
induction motor for sync operation. I ground 4 flats, and it
spins precisely at 1800 rpm (60Hz) as read on a digital tach. If
I load it too much, it will fall out of sync and continually try
to come back in. It's a 1/3 hp motor. Since you have 2 flats
ground, I would think it is locking half the time, but it sounds
like that's enough based on your comments. Because your motor is
1/8 hp, I wouldn't load it too much (regardless of 2 or 4 flats
ground).

Bart

> Original Poster: "R.E.Burnett" <R.E.Burnett-at-newcastle.ac.uk>
>
> Hi once again list members,
> I have a question for all you induction motor experts,
>
> I have a 240V, 50Hz, 1/8th HP, 1420 RPM, capacitor run
> induction motor from a clothes drier machine.  I figured that
> since this is a 4 pole motor,  it requires 4 flats to be
> ground on the rotor.  (This procedure introduces saliency
> into the rotor and causes it to lock into the rotating field
> of the stator.  The motor will then run at synchronous speed,
> in this case 1500 RPM.)
>
> My problem is that I removed the rotor and initially ground
> only 2 flats 180 degrees apart,  because it was hard work.
> I tried powering up the motor,  and found that it instantly
> locked to the line frequency when the shaft was viewed with
> a stroboscope.  I even took a rag and tried loading the
> motor by hand,  and could not cause a pole slip to occur !
> I could only manage to cause a phase lag of maybe 10 degrees
> at most.
>
> I measured the line current at 600mA, and left the motor
> running for a few minutes without any problems.  From start-
> up it was found to lock into one of four shaft orientations
> (90 degrees apart) as expected.  So my question is should I
> remove the rotor again and grind the other 2 slots or should
> I leave it with only 2 slots  ?
>
> It seems to work OK,  gives plenty of torque,  and does not
> overheat,  so I am tempted to leave it with 2 slots,  but
> I know the theory says there should be 4.  I don't want to
> take off too much metal,  and end up with a motor which
> draws too much current.  I am worried that I am overlooking
> something because I cannot see how it can properly lock into
> the 4 pole stator field with only 2 slots.
>
> If you have any experience in this field, (no pun intended,)
> your advice would be very much appreciated.
>
> PS.  For those interested in this modification procedure it
> is explained in many many posts in the Tesla List Archives,
> but I could not find an answer to this puzzler.
>
>                                 - Richie
>
>                                 - Grinding iron laminations
>                                    in sunny Newcastle.