Re: A more accurate approach to machine induction motor rotors to become synchronous

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

Original poster: "Rob Judd by way of Terry Fritz <teslalist-at-qwest-dot-net>" <canska-at-a5-dot-com>

Here's my success story for you: I just recently made my first SRSG. The
motor was an ancient 3/4HP 3450RPM cap start, made by Franklin Electric in
1972. Its a two pole, so I ground two flats.

I simply took the motor apart, removed the rotor, put it in a vice, and did
all the grinding with a bastard file. I reassembled the motor and tested it
every time the flat got another mm wider. I didn't measure it exactly, but
the flat ended up about 1" wide. It very reliably locks into sync at 110V
and slips out of sync at 95V. The motor doesn't seem any hotter (even after
long runs) than it did before modification. If the variac is set for 110V
when power is switched on, it reaches full speed and syncs in less than a
second. I've tried loading the motor down by grabbing the shaft with a
welding glove as hard as I can and it still maintains sync (though the glove
gets real hot really fast), so it didn't loose much torque. I can get more
precise measurements tomorrow if you're interested.

You can see tons of pictures of the entire process at
http://homepage.a5-dot-com/~canska/tesla/view.cgi?folder=Main/RobsCoil/RSG

Rob Judd - canska-at-a5-dot-com



 > Original poster: "Kurt Schraner by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <k.schraner-at-datacomm.ch>
 >
 > Hi RMC, all
 >
 > for my 40Watt 2-pole benchgrinder motor(=3000RPM), with a rotor diameter
 > of 54mm the 40% surface rule would lead to a 31.7mm-secant length of
 > the  flats. My realized 25.5mm flats are slightly over optimum of ~24.5mm.
 > This optimum corresponds to 30% surface, rather than 40%. No experience
 > from my side with 4-ploe motors, but I rely on Finn Hammer's posting.
 >
 > BTW: for coiler purpose, where only 2- or 4-pole motors are used, the
 > "%surface rules" can be converted to more directly applyable
 > "secant/diameter" rules:
 >
 > 2-pole motors: 30%surface ---> secant=0.454*diameter of motor-rotor
 > 4-pole motors: 40%surface ---> secant=0.309*diameter of motor-rotor
 >
 > It would be of interest, what other coilers have recorded as successful or
 > not so succesful flat sizes, in relation to their motors. Then, a more
 > reliable base for a "flat-formula" could be gained. I'm ready to collect
 > e-mailed data and try converting them to a formula and posting the
results,
 > if it's desired. In case, you are interested, please report (off-list, to
 > k.schraner-at-datacomm.ch) at least:
 >
 > - number of poles (2 or 4)
 > - rotor diameter
 > - flat length (=secant)
 > - successful sync?
 >
 > and, if possible, some or all of the following:
 >
 > - length of rotor
 > - name-plate power and -voltage
 > - run capacitor value
 > - voltage needed for falling in sync
 > - voltage for falling again out of sync
 > - size (dia.) and weight of rotary-disk + electrodes
 >    (ideally: moment of inertia)
 > - startup time, till sync
 >
 > I'm not quite shure, if just a geometric relation between flat secant
 > length and diameter is all we need for practice. Corrections welcome!
 >
 >
 > Cheers, Kurt