>> I use the Silicon Chip VFD

> Nice circuit, in some ways overkill but looks not too difficult to make.

Very quick kit to put together and I added foot controls (speed, on/off).
I've read a few reader comments about the diverse things it's been used
for, even though it was originally designed to save money when running
a pool pump/cleaner for long periods

> Wondering if there's a simpler method of doubling the frequency
> Thought came to mind of a full wave rectifier followed by a DC
> blocking cap

I had thoughts along that line too, when I was aiming at 100Hz. Given
the uncertainty of whether the motor will perform at double speed I'd
probably use some sort of PIC circuit, as the chances are frequency
will have to be lower.

Another option would be an amp driving a backwards transformer.

Initially I rejected that instinctively because I was thinking bulky and
expensive. However, it would be a good jig to make up for testing
the motor. And overdriving the secondary (maybe just a little) would
be a way to boost output voltage of the primary. It does mean the
circuit overall would be a little less "live" and probably safer

By sheer chance, Silicon Chip's May issue has almost exactly what I
would have built, a speed controller for 50/60Hz vinyl turntables. A
16F88-derived sinewave drives a push-pull FET H-bridge with a
0-9V transformer secondary load and variable frequency (+/- 12%)
230VAC 15W on the primary. Theirs is crystal based but for a
clock you'd of course sync the PIC to mains cycles and stretch the
range out to +50% or +100%

Joe


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