Re: How to modify a motor for sync, was Design Problem

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

Original poster: "Brent Turner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bturner-at-apc-dot-net>

The trick is to create enough of a salient-pole configuration to provide
full-synchronous operation at the lowest voltage. (Lock-in voltage.)
Remove too much material and it will take much higher voltage to lock
in, and as you say, pull unecessary extra current and get hot. In fact,
if you remove too much material, you don't have sufficient salient-poles
to produce any sort of power out of the motor to overcome wind drag.
(Induced by the electrodes whirling around.)

Smaller motors don't work so well for salient-pole operation because of
this. A 1/4hp 1750RPM motor can reduce down to 1/8th or even 1/10th hp
running full-synchronous. I had a gap made from an old KSR-33 Teletype
motor (shows you how old *I* am... grin.) that was 1800RPM, full-sync,
and only rated at 1/40th hp... Obviously too small to reliably spin a
12" rotor with 1/2" tungsten electrodes...


- brent


Tesla list wrote:
> 
> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Kidd6488-at-aol-dot-com>
> 
> Hi John, CJ, all
> 
> I was thinking about the syncmot.zip file on hot-streamer and how it said
> don't make the flat to shallow, or too deep. I think I have a way to find
> perfect depth. Find the length of the dead-pole and trace it on to the rotor
> so it looks like two parallel lines. Then, instead of filing length wise,
> paralell to the rotor, file it perpendicular, until the flat is wide enough.
> then you should have exactly enough left. Unless I'm wrong, the flats should
> be that: "flat" right? Not like a canyon, with "walls". You guys know what I
> mean right?!?!
> 
> Jonathon Reinhart
> 
> In a message dated 7/8/01 01:16:00 PM Eastern Daylight Time, tesla-at-pupman-dot-com
> writes:
> 
> >
> > CJ,
> >
> > You can easily modify your motor with a large simple hand file, to
> > make it synchronous.
> > For your motor, you need to file 4 flats equally spaced around
> > the squirrel cage rotor in the motor.  Open the motor, remove the
> > shaft with the rotor, and carefully measure the rotor diameter.
> > It's probably about 3.3" diameter.  Make each flat about 1/4 or
> > 1/3 the total diameter.  For a 3.3" dia rotor, you could make
> > the flats 3/4" or 7/8" wide for instance.  Wider flats will make
> > the motor run hotter and draw more current, so there's no
> > point in making the flats wider than needed.   There is a dead
> > pole concept for modifying these motors, but many small
> > motors don't have dead poles, so the concept should be ignored
> > in those cases.  I don't think it helps anyway.  Measure
> > carefully so the 4 flats are all teh same width and and equally
> > spaced around the rotor.  You can put the rotor into a vise to
> > hold it while you're measuring and filing it.
> >
> > After the motor is modified, you can test it by observing a line
> > drawn onto a sheet of cardboard and taped to the shaft.  Use
> > fluorescent light to observe the line.  The line will form an
> > unmoving pattern when the motor is locked.  Raise the voltage
> > gradually via a variac, and you'll first see the pattern spinning,
> > then slow, and lock as the voltage is increased.  The sound
> > will change too and become very steady.   Just before locking
> > the motor will make an unsteady hunting sound.
> >
> > John Freau