Sync Motor Control

I bought a motor speed controller at work for a pallet stretch wrap machine.
The unit costs $600.00.  The unit is solid state, weights about 2 lbs,
measures about 3"x6"x8" and is rated 480 VAC 5 HP.  The way it works is by
changing the frequency of the 60 HZ to 0 HZ then it slowely comes up from 0
Hz to 60 HZ, this will slowly ramp up the speed of the motor.  Motor speed
can be ramped down again too. At 10 hz the motor turns very slow and at 20
hz the motor turns twice as fast as it did at 10 hz.  Every time the
frequency goes up the motor RPM goes up too.  This is logical, ac motors
sync themselves with the frequency of the applied voltage.  The unit has a
digital display and I can change the read out to several types of displays.
I can make the display read out only frequency and tell the unit to run at
30 hz and the motor runs at half it rated RPM.

This is an interesting idea for a rotary spark gap.  I don't have any
experience with rotary spark gaps but many of the guys on the TC list do and
will probably know if this is a good idea for a rotary spark gap.  AC motors
slip a little, it might be possible to build a unit adjustable from 55.01 hz
to 65.99 hz.  Fine tune the unit to sync an AC motor with the 60 hz line freq.

Someone on the list might have a circuit for building such a device or know
how to build one.

Just and idea. 

Gary Weaver