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Re: sync motor mod question

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 5/27/01 12:16:22 AM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes:

> I have modified a motor to run at 1800 RPM for my spark gap.  When I 
>  checked the RPM's on it, it came out at 1790 steady.  Did I do something 
>  wrong, or is this normal?  if not normal, what did I do wrong.  the motor 
>  is 1/6 HP, 1725  RPM and it is turning a 8" diameter x 1/2" thick phenolic 
>  disk with 4 electrodes.
>  Thanks for the help
>  Tom


One of the best ways to tell if a rotor is sync'ed is to observe it
under flourescent light, or with a 60Hz strobe.  If the "pattern"
seen on the rotor is rock steady (not rotating), then the motor is
locked.  Another thing I like to do is to start the motor using a 
variac.  As the variac is slowly turned up, you'll see the pattern
rotate in various ways.  Suddenly, at some voltage, the pattern
will lock steady, which means the motor is locked and is 
exactly synchronous.  At the point just before locking, the
pattern may actually rotate in little jumps.... as if it's trying to
lock.  These jumps can be heard by ear.  The motor will sound
somewhat unsteady...  a sort of hunting sound.  When it locks,
the sound will become very steady.  If your motor can't quite
lock up at 120 volts, it may be able to lock at 140 volts using a
step up variac.  If this is the case, then the use of the phase
shifter circuit may be able to lock the motor without the voltage
variac.  If the motor can't lock with that rotor... but if it's very 
close to locking, you may be able to get it to lock by using a
little larger run cap, if the motor uses a run cap.  Another trick
is to use thinner electrodes for less air resistance.  Still another
trick is to reduce the diameter of the rotor a little to reduce air
drag and weight.

Sync gap phase shifter circuit is at:  


If you use the phase shifter circuit to lock the motor (assuming
the motor can't lock at 120 volts), then select a phase shift
cap value that gives some reso- build up at some point along
the variac's range... just enough voltage to lock the motor at
that point, no more.  Once the motor is locked, it should remain
locked over the phase variac's range or most of the range.

John Freau