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Sync motor mod & Remote phase cntrl


Hi Mark,

Regarding machining your sync motor:

I machined a 1/8th horse 1425RPM rotor down a while ago to run a 1500RPM
synchronous rotary.  I only machined TWO flats,  (4 is really the correct
number you should use for a 4 pole machine though.)  This motor has plenty
of power to turn my 7" phenolic disk with electrodes.  Even with only 2
flats I can only just induce a pole slip by pressing on the disk edge with
a finger.  At its rated supply voltage it slips just before the skin
starts to burn ;-)

Seriously though,  that's how it should be.  You really don't want the
phase angle to change whilst it is running for any reason.  How large is
your rotary disk ???

I think the disk radius is really important.  I did some tests and found
that the required power varies proportional to the radius cubed.  I'm sure
any Aerodynamics guru could have told us that anyway.  The result is that
a 7" disk may lock-up with a seemingly large margin,  but a 8" disk may be
too close to pole slipping for reliable use.

I am currently modifying a 1/2HP 2950RPM motor for 3000RPM use,  and have
the same problem as you.  This motor can barely hold a bare 7" disk in
lock at 3000RPM,  and I have reached the point where removing more metal
will cause a noticeable decrease in staring torque.  The only solution I
see in these situations is to use a smaller disk or over-volt the machine
slightly to coax a little more locking torque.

BTW. For those wanting to control sync RSG phase remotely, complex phase
shifting of the line voltage is not necessary if shifts of less than 90deg
are sufficient.  If the line voltage is merely decreased with a Variac,
the phase of the motor shaft moves through 90deg before falling out of
sync. 

Therefore a simple variable voltage source (Variac) can provide some phase
adjustment provided your motor locks-in over a reasonable voltage range.
Maybe the motor could be set to roughly the desired phase mechanically,
and a Variac could be used to get +/- 20 degrees or so. Beware trying to
delay the firing too far,  as it WILL hunt eventually !!!

Hope this helps,					Cheers,

							-Richie,


> Original Poster: "Mark Broker" <broker-at-uwplatt.edu>
>
> Hey,
>
> I finished machining 4 flats into the armature of a 1725 RPM 1/4 HP (or
> something close) motor recently.  Today, I attached my G-LE rotor (no
> electrodes yet :-(  )  to the motor and fired it up.  Ran pretty smooth
> after balancing.  I grabbed an old stroboscope and clocked it at 1800RPM!
> I was happy, despite there being no way with this scope to test for rotor
> position vs. the AC cycle.
>
> Then, I applied a little load to the rotor by gently pressing on it with my
> hand.  The rotor position rotated about 10 degrees, then a full 90 degrees
> (to the next pole).  It took surprisingly little pressure to cause this.  I
> was wondering if I need to grind a little more off the armature?  I
> machined 4 flats .675" across, which is the distance between the two "dead"
> poles in the winding.  That is according to the directions I found on
> Terry's site.  Is it possible that the motor is crap?  It was replaced in a
> furnace.
>
> Thanks
>
> Mark B.