Hi all,

Thanks for your compliments and feedback. So far I've only performed one
basic experiment, which shows that this arrangement can form a variable
phase convertor of sorts. I haven't attempted to measure the impedance
of the variac, or to take measurements with the motor running under load
(except to establish that it does run under load without apparent ill
effects).

Once I've created a working RPC I'm hoping to perform more experiments.
My progress has been slowed a little by the need to wind a 240 V - 415
V autotransformer, as a 415 V RPC is rather more useful in the UK than a
240 V one. Now that the weather is warm enough to work in the shed
comfortably I should have this finished soon.

Bob sent me an interesting article about how a specially-wound
autotransformer can be used to achieve good balance in a phase
convertor. The first terminal of the motor is connected to the first end
of the transformer winding, the second terminal to a centre tap, and the
third terminal through a capacitor to one of a series of taps between
the centre of the transformer winding and the second end. So it seems
someone has had a similar, if not identical, idea to mine. The advantage
of using a variac, it seems, is that it avoids the need to design and
build the special autotransformer.

I'm still keen to have a go at building the automatic balacing unit,
although it will be quite a challenge. I like the way that it is an
entirely mechanical solution to the problem. We'll have to see if I have
time. What I am very tempted to do anyway is to add a variac, capacitor
and handwheel to my RPC, so that it can be balanced by hand. My only
concern is that if I increase the variac setting while the RPC is under
load, then forget to reduce it when the load is removed, the over
voltage could damage the idler motor. In my experimental set-up it was
quite easy to adjust the variac to give over 300 V across a motor, so in
a 415 V RPC it might be possible to accidentally subject the motor to
550 V, say. 300 V didn't appear to damage the 3/4 hp motor I used for
the experiment, but then it can also be wired for 415 V, so I imagine
the insulation will be designed to cope with it. Whether a 415 V motor
would safely cope with 550 V for a short time though, I don't know. Any
thoughts?

I'll let you know how the project progresses. If anyone else does
experiments on adjustable phase convertors, do let me know. I'd be very
interested to discuss the results.

Best wishes,

Chris