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Re: SRSG motor anomoly
Tesla List wrote:
>
> Original Poster: "Finn Hammer" <f-hammer-at-post5.tele.dk>
>
> Mark!
>
> I think you are lucky, really, and have not yet taken enough off the
> armature.
> I have seen this behaviour too.
> Try to run the motor up with a variac, with a amprermeter in series. A
> analog amp. meter is needed for this.
> What you need to observe, is the hunting that takes place before the
> motor locks into sync. mode, and the voltage that this happens at.
> In your case, the locking probably happens too close to full line
> voltage, therefore there is not enough power to hold it in sync mode.
>
> Try to shave, say 4 mills more off each flat (this is thickness, not
> width of the flats) then reassemble the motor and try again. You will
> see that the motor now locks in at a slightly lower voltage, and draws a
> bit more current at sync speed.
>
> The tradeoff now is to stop shaving off material, before the motor draws
> too much power at full line voltage, but still is able to pull the rotor
> with the electrodes.
>
> Hope this helps!
Increasing the air gap will indeed increase the torque and reduce the
magnetizing inductance, resulting in more current being drawn from the
line for the same power. A suitable power-factor correction capacitor
in parallel with the motor will reduce the line current (about 22 ufd
per amp of line current on a 120 V circuit, if I didn't make a mistake)
but the current will still be flowing in the motor windings and, if it
is high enough, cause excessive heating. Of course, if you don't make
very long runs the heating won't be much of a problem and, in addition,
if the motor isn't too big you can probably stand the extra current
without problems.
Give it a try and let us know what happens. If you only take off 4
mils at a time it may take several iterations of disassembly, grinding,
and reassembly to get to the pull-in torque you need, but that is
clearly the safest way to go.
Ed