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Well, many moons ago I used a VFD for the gap. No it was not synchronous
but could hold a frequency relatively well. The VFD never experienced
any issues from the sparks, but the issues I did run into was turning
the 3ph 2HP motor under such minimal load as a spark gap. It's very easy
to go over with such a light load and cause the VFD to fault (adding
ramp up and down time solved that issue). When a VFD fault occurs, you
can usually set a VFD to coast at fault or to brake at fault. I chose
to let it coast (for obvious reasons). I also used a fault output from
the VFD into a PLC to control the contactor and remove power when a VFD
fault occured.
I mainly ran it to vary brake rates via a potentiometer at the control
panel (I'm was not a fan of synchronous gaps). I did note my coil gap
speed and spark output was best at about 360 bps, however it was more
fun to just throttle back and forth the gap. You do need to set the
phase adjustment as with any gap either mechanically or electrically. In
my case, I used a super low rpm gear motor to vary the gap angle on
movable arms.
It worked reasonably well. If I were to do another RSG coil (I won't,
I'm way too lazy), I would again go the VFD route. I just had a lot of
fun with that.
Take care,
Bart
On 4/23/2014 8:25 AM, Bert Hickman wrote:
A VFD makes most sense if you use a DC resonant charging system. A
properly designed DC resonant design will allow you to smoothly vary
output voltage and power without the need for any phase-syncing, since
the tank cap is always recharged between firings at any firing rate.
This approach works especially well for high power systems using
3-phase gap motors.
However, A VFD can be used to make an asynchronous variable speed gap.
Using a VFD in your existing system would work in a fashion similar to
driving a DC motor from a variable DC supply. You WILL lose the
capability of truly syncing and properly phasing your motor to the
line. Also, some speeds near multiples of line frequency have the
potential to overvolt your tank cap or HV transformer, so make sure
you have a safety gap bridging your rotary gap or a Terry filter.
I like Jim's idea of driving your HV transformer from the same VFD and
using a true synchronous motor would permit you to sync the gap to the
line. I'm not sure just how well the VFD, HV transformer, and motor
will all play together, and I don't recall any coilers using this
technique before. If you try this method, you'll probably need to use
a fixed mechanical phase adjustment on your gap since the Freau
controller's phase relationship will change with operating frequency.