Re: SRSG motors

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

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Greg,

Most common is the run-cap (fixed-cap split-phase motor) and less common
(but still many around) is the
start-cap (centrifugally switched split-phase motor). The run-cap motor is
common because it is easily
configured for CW or CCW operation. Split-phase motors typically require
only one winding to actually run on
when operational speed is reached. Start-cap motors run on one winding
after the cetrifugal switch disconnects
the start winding and cap. The motor is free to run at the synchronous
rotating field speed. However, run-cap
motors are in the circuit all the time (there is some phase shift) and the
shaft speed will not match the
synchronous field speed. The term is "slip" and specified as the percent of
shaft speed slipping behind the
rotating field speed.

** slip rpm = sync field rpm - shaft rpm (1800 rpm - 1710 rpm = 90 rpm slip).
** percent slip = slip rpm / sync field rpm (90 / 1800 = 0.05 = 5%).

So with a run cap, it will never be synchronous. It will always be
"slipping" behind the synchronous field rpm.
However, because split-phase motors can usually run on one winding at it's
operational speed, a modification
"might" be possible by installing a manually operated external switch to
remove the cap and one winding after
operational speed is reached. This should remove the slip rpm and allow the
modification for synchronous
operation. But, you would need to do some testing to determine how the cap
is wired into the circuit (probably
parallel to the two windings and the windings joined together at the other
end). I'm not sure which winding
should be used (it may not matter with this motor type). Maybe some others
better experienced have some
thoughts on this?

Take care,
Bart

Tesla list wrote:

> Original poster: "Mr Gregory Peters by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>
>
> Hello all,
>
> I was just reading an old post by Bill Wysock regarding SRSG motors. He
> says that capacitor run motors can't be used for SRSGs, only induction
> run/capacitor start motors can be used. Is this true? This poses a
> problem to me as large induction run motors seem pretty rare nowdays.
> Someone I spoke to at the motor shop says he sees no reason why a cap
> run motor can't be used.
>
> Cheers,
>
> Greg Peters
> Department of Earth Sciences,
> University of Queensland, Australia
> Phone: 0402 841 677
> http://www.geocities-dot-com/gregjpeters