RE: SRSG motor power

["David Dean" <deano-at-corridor-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

> Someone suggested using a variac to vary the phase angle of the SRSG while
> it is running.

Huh?!?  A variac can be used to vary the voltage or current feeding the
motor, but I don't see how it can vary the phase, unless the added
inductance of a variac in series would cause sufficient phase shift, but if
you do that, then you're also varying the available current to the motor,
which doesn't seem like a very good idea.

- Gomez (Bill Lemieux)

OK, think of it this way. In a normal motor, that is one that has not been
modified for sync operation, the stator _slips_ in the (apparently rotating)
magnetic field. This slip is responsible for most of the torque that is
delivered to the load. If the motor is unloaded, the stator speed will
approach sync. speed. If the motor is loaded to its design rating, the speed
will slow down to the rated speed. If the motor is overloaded, it will slow
down even more.

If one were to take a normal motor and lower the voltage to it, the magnetic
field will be weaker and allow more slip thus slowing the motor. If the
voltage is raised a little bit above the rated voltage, the motor will run a
little bit faster. Of course there are limits to how much you can vary the
speed of an induction motor because if the current gets too low the magnetic
field is too weak and not enough torque is left to keep the motor running.
If the current gets too high the motor will burn up. The lower limit will
depend on the torque requirements of the load. The upper limit will depend
on the saturation point of the armature core.

Once the motor has been modified for sync. operation it will loose a lot of
its torque. It will also _seem_ to be more heavily loaded and draw more
current. But if the right amount of material is removed and the motor can be
locked in sync. at a lower than rated voltage, and still not pull more than
rated FLA at a bit higher voltage, a range of operating voltages (and hence
currents) can be had. At the lower voltage the current will be lower and the
magnetic field weaker. The physical poles of the stator will not be aligned
with the position of the armature poles to the same degree as when rated
voltage is used. The stator will be _lagging_ or retarded in timing (to make
an analogy to a car engine). At the high voltage the stator poles will be
aligned more perfectly with the rotating magnetic field of the armature, and
thus _advance_ the timing to some degree.

I can't tell you how many degrees of latitude you can get by this method,
but when I modified my 1/3 HP 3450 RPM AO Smith motor I had the idea in mind
so I was very careful to remove just the right amount of metal from the
stator to get the maximum amount of range. I bought one of those Radio Shack
strobe lights and modified it per Terry's instructions, but alas, the local
Radio Shack store does not carry much in the way of parts so I was only able
to get one of the 1uf caps so was not able to use it. I got a cap the other
day, though, and when I get a chance I will find out.

Later
deano