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Re: Induction motor conversion
Tesla List wrote:
>
> >From sgreiner-at-mail.wwnet-dot-comSun Oct 13 22:20:08 1996
> Date: Sun, 13 Oct 1996 09:35:58 -0700
> From: Skip Greiner <sgreiner-at-mail.wwnet-dot-com>
> To: tesla list <tesla-at-pupman-dot-com>
> Subject: Induction motor conversion
>
> Brent, Chip and All
> I am addressing this to the list rather than direct to Brent because it
> may be of interest to others considering conversion of an induction
> motor to synchronous operation.
>
> My 3450 rpm motor has 24 wire slots. 2 groups of 4 slots each have heavy
> wire in them. The other 16 slots are split into 2 groups of 8 slots and
> have finer wire in them. Per the diagrams which Brent has posted I would
> assume that the active poles in this motor are the 2 groups of 4 slots
> and the poles are therefore 15 degrees wide. There are no empty slots in
> my motor. I therefore must mill the rotor for 2 flats which are 15
> degrees wide.
>
> I would appreciate it if you concur in my diagnosis and thank you for
> the very definitive note and sketches on funet.
>
> Skip
--- To Skip personally:
This is a bit unusual, as the 3450 RPM motor I have has the main RUN
windings in all 24 slots: one side with 12, and the other side with the
other 12. The finer starting windings bridge the poles. (The motor starts
via a current phase shift, BTW.)
Start with a cut that skips one pole and 1/2 a pole on either side. See
the rest of this message below:
--- Skip (and others)
The info I supplied is more of a general guideline than anything --
you will inevitably see variations on the number of poles, etc. Standard
AC induction motors rely on the relationship of the number of stator
poles and the number of 'shorted' turns in the armature, plus a 'slip'
factor which is a sort of RPM drop factor under load.
I just got done modifying a 3450 motor this weekend. It is a brand
new GE 1/3HP, ball-bearing unit. We really needed to shave that armature
down! In this particular motor, there was 1 magnetically 'dead' pole.
(I must clarify a few things here -- and yes, will update the files
on funet...)
The flats on the armature 'spoil' the repulsive effects between the
stator poles and the shorted armature turns. What is required is enough
flux concentration through the armature to cause it to 'lock' into the
physical magnetic poles on the stator.
Most 1750 RPM motors have at least 1 'dead' pole between magnetic poles
(as defined by the stator windings: all poles incorporated by the main
run windings (heavy wire) are 'active' poles). Ergo, you will need to
cut a flat on the armature so that *any* dead pole(s) are magnetically
decoupled on the armature. You will need to decouple additional poles
depending upon the motor -- this will require a bit of experimentation.
All the (modified) 1750 RPM motors I have use flats that measure across
the 'dead' pole, plus one on either side - in other words, it skips
3 full poles. This amount seems to work OK for 4-pole (1750 RPM) motors.
3450 RPM motors use fewer stator poles, and higher winding current. In
the case of my new GE unit, we needed to skip the 'dead' pole between
the stator pole windings, one on either end (as per a 1750 RPM motor),
*and* 1/2 of the next poles on either end!
Regrettably, there isn't an exact formula (at least that I have found)
that will universally work, though the info I supplied to the funet
server will at least get you going.
What you need to do is this: cut the armature so that you skip the
'dead' pole(s) between the main run windings, plus 1 pole in either
direction from those poles. This is your starting point. Attach a larger (6
to 8 inch) disk or your actual gap rotor (minus electrodes if possible),
and apply 2 strips of contrasting tape radially (from center to outside)
on the disk surface facing out, opposite each other across the rotor's
center. For a 3650 RPM motor, use a single strip from the center out.
Assemble your motor, clamp it down, and fire it up using a variac. Try
setting the variac at 50%. The motor should start and spin up. If your
working area is lit by fluorescent lamps, you should see alternating
light and dark patches on the rotor, which most likely will be drifting
slowly. Bring the voltage on the variac up slowly. If the cuts on the
armature are sufficient, the motor will lock into salient-pole mode at
roughly 75 to 85% of full voltage. You can actually feel the motor
'hunt' for the poles as you bring the voltage up. When it locks in, you
can feel the motor 'strength' kick in a bit. The visual pattern should
stabilize at a particular angle.
(This 'hunting' effect I believe is the cross-over point in the motor
where it drops out of repulsion-mode and into flux-concentration salient-
pole mode.)
You can test for positive indexing by removing the power and letting
the motor come to a complete rest. Re-powering it up should result in
the visual pattern locking into the same angle.
If your motor doesn't lock up yet, you will need to extend the flats
on the armature a bit. A word to the wise - don't take off a lot at
one time - try extending things out by 1/2 pole in either direction.
Eventually you should reach a point where the motor starts, comes up
to speed and locks in. Test the lock in voltage range!!!! If your motor
locks in only at max voltage, you need to trim the armature a *small*
amount more...the idea is to have a voltage window to allow for minor
voltage fluctuation on the line. It's not a good idea to have your gap
suddenly shift out of sync if the line voltage hiccups!
One thing you need to note - when you cut a motor like this, you *will*
raise the starting surge current as the inductive properties have been
drastically altered. This is particularly true on 3450 RPM motors. My
new GE unit popped a 10-amp fuse on startup. (Really big motors might
need to have their voltage ramped up via a variac to limit the surge
curent.)
I hope this helps clarify things a bit. The best advice I can give is
to proceed slowly until you get it running right. You might also want
to get a bunch of cheap surplus motors to 'practice' on. This kind of
modification isn't particularly hard, but there is a bit of a 'knack'
to it.
- Brent