Hi all,

Just noticed Mark Main's thread about using a variable capacitor in a
phase convertor. I thought about this a while back and discussed it with
Bob Swinney (Bob, sorry I haven't replied to your e-mail yet - I will,
I've just been busy with other stuff for a while). I meant to do some
more experimentation on the subject before I posted my thoughts here,
but so far haven't got round to it. The following is I think is an
interesting idea, although probably not one of any commercial value.

Traditional air-spaced capacitors aren't going to work, as people have
mentioned. I think I worked out that a small phase convertor would need
an air-spaced variable capacitor with an area of about 6000 m2. But then
I thought, what about a fake variable capacitor? In the form of a variac
and a fixed capacitor?

I tried it. For my experiment I used one 3/4 hp three phase motor
connected to a single phase supply. I didn't feel a second motor was
necessary for the initial experiment. I tried connecting capacitors
through an 8 amp variac between the supply and the third leg of the
motor. It worked. With a really low variac setting the motor won't
start. A bit higher and it starts very slowly. At the highest setting
the motor starts quickly but makes a whine when up to speed. In the
middle there is a "sweet spot" where the motor runs nice and quietly.

So it seems it is possible to make a form of variable capacitor using a
variac and fixed capacitor. I took a few measurements and reached the
following conclusions. (i) It's possible to adjust the voltage between
the third leg and one of the supply legs until it is equal to the supply
voltage (I didn't attempt to measure any phase differences). (ii) The
motor is noticeably quieter when these voltages are equal. (iii) The
voltage across the motor is definitely not linear with variac position.
(iv) Attepting to achieve equal voltages using various capacitor values
it seemed that the square of the capacitor voltage times the capacitance
was a constant under constant motor load.

Then I had another idea. I thought it would be nice if the system
regulated itself, so that a drawing a higher current from the convertor
would increase the variac setting. I figured that it might be possible
by rewinding a three phase motor with a few turns of thick wire and
connecting it in series with the load, then connecting the motor shaft
to the variac and also to a torsion spring. Bigger load - more current
drawn - more torque developed in rewound motor - rotor moves a little
further against torsion spring - variac setting increases. Obviously
this would be far too expensive to build commercially, and hard to
"tune", but fascinating if it could be made it work.

Here's a little diagram of the idea:
http://www.mythic-beasts.com/~cdt22/..._regulator.jpg

Enjoy. My head hurts and I've been puzzling this out for a while!

Best wishes,

Chris

In article , Christopher Tidy says...

IIn the form of a variac
and a fixed capacitor?

That's a really cool idea.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

Christopher Tidy writes:

position. (iv) Attepting to achieve equal voltages using various
capacitor values it seemed that the square of the capacitor voltage
times the capacitance was a constant under constant motor load.

If the relationship follows

Q = (C * E^2)/2

then it sounds like that relationship should be expected. Assuming
thats true and turning it around, the equation suggests the voltage
follows the square root of the power, so shouldn't vary widely with a
varying motor load. Did you make any measurements which might speak to
how the cap voltage varies with load? Or perhaps of more interest,
measurements of the generated leg voltage?

I think the feedback in your model might be working against the wrong
variables though, I think the variac perhaps ought to be adjusted
against the phase current difference between the generated leg and one
of the other two- so the system always tends towards balance. Its an
interesting idea though.

Greg

Christopher Tidy wrote:

I thought, what about a fake variable capacitor? In the form of a variac
and a fixed capacitor?

I tried it.

Heck Chris, based on what I have seen lately you should file for a
patent on that, especially with the closed-loop tuning.

Myself, I'm considering an application on "Method and Apparatus for
Wiping One's Own A** using the Non-Dominant Extremity". It would be just
as valid as some the extortionist .. sorry, I mean 'Intellectual
Property' ... firms are waving around.

--
Fred R
"It doesn't really take all kinds; there just *are* all kinds".
Drop TROU to email.

Chris, this is clever. Thanks for sharing it.

Steve

Christopher Tidy wrote:

Hi all,

Just noticed Mark Main's thread about using a variable capacitor in a
phase convertor. I thought about this a while back and discussed it
with Bob Swinney (Bob, sorry I haven't replied to your e-mail yet - I
will, I've just been busy with other stuff for a while). I meant to do
some more experimentation on the subject before I posted my thoughts
here, but so far haven't got round to it. The following is I think is
an interesting idea, although probably not one of any commercial value.

Traditional air-spaced capacitors aren't going to work, as people have
mentioned. I think I worked out that a small phase convertor would
need an air-spaced variable capacitor with an area of about 6000 m2.
But then I thought, what about a fake variable capacitor? In the form
of a variac and a fixed capacitor?

I tried it. For my experiment I used one 3/4 hp three phase motor
connected to a single phase supply. I didn't feel a second motor was
necessary for the initial experiment. I tried connecting capacitors
through an 8 amp variac between the supply and the third leg of the
motor. It worked. With a really low variac setting the motor won't
start. A bit higher and it starts very slowly. At the highest setting
the motor starts quickly but makes a whine when up to speed. In the
middle there is a "sweet spot" where the motor runs nice and quietly.

So it seems it is possible to make a form of variable capacitor using
a variac and fixed capacitor. I took a few measurements and reached
the following conclusions. (i) It's possible to adjust the voltage
between the third leg and one of the supply legs until it is equal to
the supply voltage (I didn't attempt to measure any phase
differences). (ii) The motor is noticeably quieter when these voltages
are equal. (iii) The voltage across the motor is definitely not linear
with variac position. (iv) Attepting to achieve equal voltages using
various capacitor values it seemed that the square of the capacitor
voltage times the capacitance was a constant under constant motor load.

Then I had another idea. I thought it would be nice if the system
regulated itself, so that a drawing a higher current from the
convertor would increase the variac setting. I figured that it might
be possible by rewinding a three phase motor with a few turns of thick
wire and connecting it in series with the load, then connecting the
motor shaft to the variac and also to a torsion spring. Bigger load -
more current drawn - more torque developed in rewound motor - rotor
moves a little further against torsion spring - variac setting
increases. Obviously this would be far too expensive to build
commercially, and hard to "tune", but fascinating if it could be made
it work.

Here's a little diagram of the idea:
http://www.mythic-beasts.com/~cdt22/..._regulator.jpg

Enjoy. My head hurts and I've been puzzling this out for a while!

Best wishes,

Chris

Fred R wrote:

Christopher Tidy wrote:

I thought, what about a fake variable capacitor? In the form of a
variac and a fixed capacitor?

I tried it.


Heck Chris, based on what I have seen lately you should file for a
patent on that, especially with the closed-loop tuning.

I thought about it seriously, which was why I didn't post the idea here
immediately. So far I haven't uncovered evidence that it has been done
before. But my financial resources are very limited, and my gut reaction
is that this isn't "the one". In the 1920s with less widespread
availability of three phase, cheaper labour and more skilled craftsmen,
it might have stood a chance. But now I think that at the most it will
become a curiousity, not a big thing. If I have time I may build a
prototype and do a write-up on a website, but I don't think this will
make money. You have to pick your battles. One day I'll find "the one".

Chris

patent or not, it was a very creative and profound idea.

i

On Thu, 18 May 2006 00:22:31 +0000, Christopher Tidy wrote:
Fred R wrote:

Christopher Tidy wrote:

I thought, what about a fake variable capacitor? In the form of a
variac and a fixed capacitor?

I tried it.


Heck Chris, based on what I have seen lately you should file for a
patent on that, especially with the closed-loop tuning.

I thought about it seriously, which was why I didn't post the idea here
immediately. So far I haven't uncovered evidence that it has been done
before. But my financial resources are very limited, and my gut reaction
is that this isn't "the one". In the 1920s with less widespread
availability of three phase, cheaper labour and more skilled craftsmen,
it might have stood a chance. But now I think that at the most it will
become a curiousity, not a big thing. If I have time I may build a
prototype and do a write-up on a website, but I don't think this will
make money. You have to pick your battles. One day I'll find "the one".

Chris

According to Steve Smith :
Chris, this is clever. Thanks for sharing it.

I agree. It would be interesting to see the phase
relationships measured. And it would be particularly interesting to see
tests of the self-adjustor.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On Wed, 17 May 2006 22:44:07 GMT, Fred R "spam
wrote:


Christopher Tidy wrote:

I thought, what about a fake variable capacitor? In the form of a variac
and a fixed capacitor?

I tried it.

Heck Chris, based on what I have seen lately you should file for a
patent on that, especially with the closed-loop tuning.

Patents aren't nearly as easy to get in the UK and Europe as they are
in the U.S. Examiners there still actually examine. In the U.S.
they do a cursory examination for obvious infringement or prior art,
then take the attitude of "let 'em sort it out in court".

On Wed, 17 May 2006 21:07:28 +0000, Christopher Tidy
wrote:

Hi all,

Just noticed Mark Main's thread about using a variable capacitor in a
phase convertor. I thought about this a while back and discussed it with
Bob Swinney (Bob, sorry I haven't replied to your e-mail yet - I will,
I've just been busy with other stuff for a while). I meant to do some
more experimentation on the subject before I posted my thoughts here,
but so far haven't got round to it. The following is I think is an
interesting idea, although probably not one of any commercial value.

Traditional air-spaced capacitors aren't going to work, as people have
mentioned. I think I worked out that a small phase convertor would need
an air-spaced variable capacitor with an area of about 6000 m2. But then
I thought, what about a fake variable capacitor? In the form of a variac
and a fixed capacitor?

I tried it. For my experiment I used one 3/4 hp three phase motor
connected to a single phase supply. I didn't feel a second motor was
necessary for the initial experiment. I tried connecting capacitors
through an 8 amp variac between the supply and the third leg of the
motor. It worked. With a really low variac setting the motor won't
start. A bit higher and it starts very slowly. At the highest setting
the motor starts quickly but makes a whine when up to speed. In the
middle there is a "sweet spot" where the motor runs nice and quietly.

So it seems it is possible to make a form of variable capacitor using a
variac and fixed capacitor. I took a few measurements and reached the
following conclusions. (i) It's possible to adjust the voltage between
the third leg and one of the supply legs until it is equal to the supply
voltage (I didn't attempt to measure any phase differences). (ii) The
motor is noticeably quieter when these voltages are equal. (iii) The
voltage across the motor is definitely not linear with variac position.
(iv) Attepting to achieve equal voltages using various capacitor values
it seemed that the square of the capacitor voltage times the capacitance
was a constant under constant motor load.

Then I had another idea. I thought it would be nice if the system
regulated itself, so that a drawing a higher current from the convertor
would increase the variac setting. I figured that it might be possible
by rewinding a three phase motor with a few turns of thick wire and
connecting it in series with the load, then connecting the motor shaft
to the variac and also to a torsion spring. Bigger load - more current
drawn - more torque developed in rewound motor - rotor moves a little
further against torsion spring - variac setting increases. Obviously
this would be far too expensive to build commercially, and hard to
"tune", but fascinating if it could be made it work.

Here's a little diagram of the idea:
http://www.mythic-beasts.com/~cdt22/..._regulator.jpg

Enjoy. My head hurts and I've been puzzling this out for a while!

Best wishes,

Chris

Neat idea, Chris. The closed-loop control dynamics could get
interesting, but nothing that couldn't be sorted out.

On Wed, 17 May 2006 21:07:28 +0000, Christopher Tidy
wrote:

Hi all,

Just noticed Mark Main's thread about using a variable capacitor in a
phase convertor. I thought about this a while back and discussed it with
Bob Swinney (Bob, sorry I haven't replied to your e-mail yet - I will,
I've just been busy with other stuff for a while). I meant to do some
more experimentation on the subject before I posted my thoughts here,
but so far haven't got round to it. The following is I think is an
interesting idea, although probably not one of any commercial value.

Traditional air-spaced capacitors aren't going to work, as people have
mentioned. I think I worked out that a small phase convertor would need
an air-spaced variable capacitor with an area of about 6000 m2. But then
I thought, what about a fake variable capacitor? In the form of a variac
and a fixed capacitor?

I tried it. For my experiment I used one 3/4 hp three phase motor
connected to a single phase supply. I didn't feel a second motor was
necessary for the initial experiment. I tried connecting capacitors
through an 8 amp variac between the supply and the third leg of the
motor. It worked. With a really low variac setting the motor won't
start. A bit higher and it starts very slowly. At the highest setting
the motor starts quickly but makes a whine when up to speed. In the
middle there is a "sweet spot" where the motor runs nice and quietly.

So it seems it is possible to make a form of variable capacitor using a
variac and fixed capacitor. I took a few measurements and reached the
following conclusions. (i) It's possible to adjust the voltage between
the third leg and one of the supply legs until it is equal to the supply
voltage (I didn't attempt to measure any phase differences). (ii) The
motor is noticeably quieter when these voltages are equal. (iii) The
voltage across the motor is definitely not linear with variac position.
(iv) Attepting to achieve equal voltages using various capacitor values
it seemed that the square of the capacitor voltage times the capacitance
was a constant under constant motor load.

Then I had another idea. I thought it would be nice if the system
regulated itself, so that a drawing a higher current from the convertor
would increase the variac setting. I figured that it might be possible
by rewinding a three phase motor with a few turns of thick wire and
connecting it in series with the load, then connecting the motor shaft
to the variac and also to a torsion spring. Bigger load - more current
drawn - more torque developed in rewound motor - rotor moves a little
further against torsion spring - variac setting increases. Obviously
this would be far too expensive to build commercially, and hard to
"tune", but fascinating if it could be made it work.

Here's a little diagram of the idea:
http://www.mythic-beasts.com/~cdt22/..._regulator.jpg

Enjoy. My head hurts and I've been puzzling this out for a while!

Best wishes,

Chris

A slightly different way of looking at the problem.

The apparent value of a fixed capacitor can be varied by
feeding it through a transformer. For a 1:2 step up transformer
the voltage on the capacitor is doubled so the energy stored in
the capacitor is multiplied by four (CxVsquared).

With a reasonably efficient transformer, measured at the
primary, this appears at the primary as the primary inductance
shunted by a pure capacitor which stores the same amount of
energy. Because this is at half the secondary voltage the
apparent value is four times the value of the secondary
capacitance.

The capacitance value is multiplied by the square of the
turns ratio.

This can be turned into a true variable C by the use of a
Variac to feed the fixed capacitor. The impedance of the
magnetising current taken by the Variac needs to be much higher
than the impedance of the intended capacitor. This is easily
checked by comparing the Variac magnetising current with the
current drawn by the capacitor at a similar voltage. Because
Variacs are usually step down devices the apparent capacitance
value will be reduced by the step down ratio squared.

With a variable capacitor a fixed transformer ratio can be
used but, if 100uF is needed from a 1000pF variable, the values
are pretty extreme - more than 300:1 stepup ratio and, with 200V
in, more than 50Kv across the capacitor!


Jim








*




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In article ,
wrote:



A slightly different way of looking at the problem.

The apparent value of a fixed capacitor can be varied by
feeding it through a transformer. For a 1:2 step up transformer
the voltage on the capacitor is doubled so the energy stored in
the capacitor is multiplied by four (CxVsquared).

With a reasonably efficient transformer, measured at the
primary, this appears at the primary as the primary inductance
shunted by a pure capacitor which stores the same amount of
energy. Because this is at half the secondary voltage the
apparent value is four times the value of the secondary
capacitance.

The capacitance value is multiplied by the square of the
turns ratio.

This can be turned into a true variable C by the use of a
Variac to feed the fixed capacitor. The impedance of the
magnetising current taken by the Variac needs to be much higher
than the impedance of the intended capacitor. This is easily
checked by comparing the Variac magnetising current with the
current drawn by the capacitor at a similar voltage. Because
Variacs are usually step down devices the apparent capacitance
value will be reduced by the step down ratio squared.

With a variable capacitor a fixed transformer ratio can be
used but, if 100uF is needed from a 1000pF variable, the values
are pretty extreme - more than 300:1 stepup ratio and, with 200V
in, more than 50Kv across the capacitor!


Jim

I'm stretching long unused knowledge here, but hasn't the OP just
created a big tank circuit to tune the created leg?. The added
inductance of the transformer would tend to even out the current lead of
the caps and balance the created leg's current flow in relation to the
other two.

Or am I totally offbase? (which wouldn't be a first by any means)

--
Bring back, Oh bring back
Oh, bring back that old continuity.
Bring back, oh, bring back
Oh, bring back Clerk Maxwell to me.

In case you don't already know, there are motorized variac models
available for numerous types of applications. You're application might
require a simple/small circuit to operate a small gearmotor acting as a
servo motor.

I have an unsual variac device that regulates/adjusts the AC output
voltage. It's labeled as a Regulated AC Power Supply, and is switchable
for either 120 or 240VAC output with an input line voltage of 120VAC.
It's not line isolated, but a cool little unit.
The sense/motor drive circuit is only a few square inches, and the
motor is a small DC hobby-type motor. A geartrain allows the small
motor to adjust the variac.

Another variac I got from a surplus dealer has a small AC gearmotor to
adjust the variac, and it waits for a project to surface, and I'll need
to implement a sense/drive circuit for it when that time comes. I dunno
what type of equipment it was from originally.

WB
...............

Christopher Tidy wrote:
Hi all,

Just noticed Mark Main's thread about using a variable capacitor in a
phase convertor. I thought about this a while back and discussed it with
Bob Swinney (Bob, sorry I haven't replied to your e-mail yet - I will,
I've just been busy with other stuff for a while). I meant to do some
more experimentation on the subject before I posted my thoughts here,
but so far haven't got round to it. The following is I think is an
interesting idea, although probably not one of any commercial value.

Traditional air-spaced capacitors aren't going to work, as people have
mentioned. I think I worked out that a small phase convertor would need
an air-spaced variable capacitor with an area of about 6000 m2. But then
I thought, what about a fake variable capacitor? In the form of a variac
and a fixed capacitor?

I tried it. For my experiment I used one 3/4 hp three phase motor
connected to a single phase supply. I didn't feel a second motor was
necessary for the initial experiment. I tried connecting capacitors
through an 8 amp variac between the supply and the third leg of the
motor. It worked. With a really low variac setting the motor won't
start. A bit higher and it starts very slowly. At the highest setting
the motor starts quickly but makes a whine when up to speed. In the
middle there is a "sweet spot" where the motor runs nice and quietly.

So it seems it is possible to make a form of variable capacitor using a
variac and fixed capacitor. I took a few measurements and reached the
following conclusions. (i) It's possible to adjust the voltage between
the third leg and one of the supply legs until it is equal to the supply
voltage (I didn't attempt to measure any phase differences). (ii) The
motor is noticeably quieter when these voltages are equal. (iii) The
voltage across the motor is definitely not linear with variac position.
(iv) Attepting to achieve equal voltages using various capacitor values
it seemed that the square of the capacitor voltage times the capacitance
was a constant under constant motor load.

Then I had another idea. I thought it would be nice if the system
regulated itself, so that a drawing a higher current from the convertor
would increase the variac setting. I figured that it might be possible
by rewinding a three phase motor with a few turns of thick wire and
connecting it in series with the load, then connecting the motor shaft
to the variac and also to a torsion spring. Bigger load - more current
drawn - more torque developed in rewound motor - rotor moves a little
further against torsion spring - variac setting increases. Obviously
this would be far too expensive to build commercially, and hard to
"tune", but fascinating if it could be made it work.

Here's a little diagram of the idea:
http://www.mythic-beasts.com/~cdt22/..._regulator.jpg

Enjoy. My head hurts and I've been puzzling this out for a while!

Best wishes,

Chris

On Thu, 18 May 2006 14:48:43 GMT, John Husvar
wrote:

In article ,
wrote:



A slightly different way of looking at the problem.

The apparent value of a fixed capacitor can be varied by
feeding it through a transformer. For a 1:2 step up transformer
the voltage on the capacitor is doubled so the energy stored in
the capacitor is multiplied by four (CxVsquared).

With a reasonably efficient transformer, measured at the
primary, this appears at the primary as the primary inductance
shunted by a pure capacitor which stores the same amount of
energy. Because this is at half the secondary voltage the
apparent value is four times the value of the secondary
capacitance.

The capacitance value is multiplied by the square of the
turns ratio.

This can be turned into a true variable C by the use of a
Variac to feed the fixed capacitor. The impedance of the
magnetising current taken by the Variac needs to be much higher
than the impedance of the intended capacitor. This is easily
checked by comparing the Variac magnetising current with the
current drawn by the capacitor at a similar voltage. Because
Variacs are usually step down devices the apparent capacitance
value will be reduced by the step down ratio squared.

With a variable capacitor a fixed transformer ratio can be
used but, if 100uF is needed from a 1000pF variable, the values
are pretty extreme - more than 300:1 stepup ratio and, with 200V
in, more than 50Kv across the capacitor!


Jim

I'm stretching long unused knowledge here, but hasn't the OP just
created a big tank circuit to tune the created leg?. The added
inductance of the transformer would tend to even out the current lead of
the caps and balance the created leg's current flow in relation to the
other two.

Or am I totally offbase? (which wouldn't be a first by any means)

This is the reason for specifying that the Variac impedance
be much higher than the capacitor impedance - both of course
measured at supply frequency. So far as the motor is concerned
the the apparent capacitative impedance is the reflected
capacitative impedance paralleled by the inductive impedance of
the variac primary. Since this is a leading power factor paralled
by a lagging power factor this inductance slightly increases the
nett capacitative impedance. This means that the effective
reflected capacitance will be slighly less than the expected
value.

It is of course true that a variac plus capacitor forms a
tuned circuit but, for the conditions specified, this is resonant
at frequency much lower than supply frequency. It is no more and
no less a resonant system than the tuned circuits normally formed
by the motor inductance and the normally added capacitors.

These comments apply to the use of the Variac to trim the
capacitance value - i.e. with the Variac set to the upper half of
its coverage.

If the Variac output is set to a very low value the
impedance of the reflected capacitance may no longer be much
smaller than the Variac primary impedance and this reduces the
value of the apparent reflected capacitance.

The limiting case is when the step down ratio is so large
that the reflected capacitative impedance is equal to the Variac
primary impedance. In this case it is resonant at supply
frequency and the nett reflected capacitance will be zero i.e.
unity power factor.


Jim





















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On Thu, 18 May 2006 14:53:30 +0100, wrote:



The apparent value of a fixed capacitor can be varied by
feeding it through a transformer. For a 1:2 step up transformer
the voltage on the capacitor is doubled so the energy stored in
the capacitor is multiplied by four (CxVsquared).

Actually, the impedance (and by implication the apparent capacitance) varies
as the square of the transformer ratio.

For any mechanical engineers looking on:-
If the transformer doubled the voltage that the capacitor saw it would show
double the current at the input side for the same voltage on the capacitor.
But since the voltage on the capacitor has been doubled, you get double the
doubled current on the input side for your original voltage. i.e. the apparent
capacitance varies as the square of the transformer ratio.

Of course, If you manage to get the leakage reactance of the variac
(thankfully quite low) and the capacitor in resonance at the mains frequency,
then all bets are off and you need to buy a new fuse.


Mark Rand
RTFM

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