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The MG sets we were using were 3 phase 480 volt systems. We often used
the
light bulb or voltmeter system to put them in parallel. When the bulbs
went
out or the volt metes went to 0, then the switch was flipped. They then
drove a bank of about 16 motors that were around 3 or 5 HP at 480 volt, 3
phase.

OK, so basically mechanical phase converters?
The input was single phase AC? How did you vary the speed of the prime
mover?Mechanical variable speed drive?

I did not get into the prime mover that much, but I think it was a DC
electronic drive that powered the motor part. The main power fed to the
varitable speed part was 60 Hz 480 volt 3 phase. I think it was converted
to DC to drive the motor part that controls the speed and then to the
generator part that puts out lower frequency ( probably lower voltage also)
3 phase AC. I know it had a knob on it so we could change the speed of the
motor driving the AC generator. When I needed to put one on line, I hooked
a box up to some plugs on the speed control drive and adjusted the speed so
the bulbs went out and threw a switch. I was not too concerned about the
actual speed control part as others repaired that. My part of that job was
just to get the MG sets on and off line without causing prblems. My main
job was instrumentation, but had to know a little about most everything so I
could fill in for the electrician that worked on each shift when they went
on vacation.

We also had a newer part of the plant that did the same thing except it was
with all electronic controls and no MG sets. We had a box with meters on it
that we used to sync the inverters to put the spare on and off line when we
worked on the main speed control inverter.

This setup was at a plant that made polyester. The 'small' motors powered
gear pumps that forced the melted plastic out of a die. This speed had to
be constant so the plastic comming out of the die would be the same
thickness. There were 16 small motors being driven at the same time. We
could shut off one or two of the small motors and not cause much of a
problem,but if the whole thing shut off very long (much over 2 minuits) it
would cause a major problem and loss of production. That is why we had a
spare MG set for about 20 of the other MG sets.