[Milsurplus] multi phase AC power

[Dr. William J. Schmidt, II]

Well, everyone has their own twist on this subject.  Just using caps to 
generate the 3rd phase is ok, but only works at one particular load and is 
not a perfect phase generation... may burn some equipment up (transformers 
in particular).  I've had nothing but problems with solid state 3-phase 
generators... they are sensitive to inductive loads (back emf).  I can't get 
3-phase power where I live, so made my own 3-phase generator to power a 20kW 
transmitter and various pieces of shop equipment.  For those in the know, 
you can get 3-phase equipment for a song simply because of the 3-phase 
requirement.  My generator is a 30hp 3-phase motor with cap start and caps 
used to correct the phase shift for the second and third legs of the 
(generated) phase.  It's much more tolerant of load differences.  The motor 
(30 hp, 3600 rpm, sealed) was brand new and was purchased for less than $100 
as surplus.  I suppose I have $250 in the whole thing and it will generate 
100 amps of clean 3-phase power.  I do have some phase correcting circuitry 
that adds capacitance to legs #2,3 to compensate for loads between 1 amp and 
100 amps.

Sincerely,

Dr. William J. Schmidt, II  K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC

Email: bill at wjschmidt.com
WebPage: www.wjschmidt.com

"It's not what you take with you... but what you leave behind that counts. 
Live each day as if it were your last."


----- Original Message ----- 
From: "J. Forster" <jfor at quik.com>
To: <gl4d21a at juno.com>
Cc: <milsurplus at mailman.qth.net>
Sent: Saturday, March 11, 2006 10:49 PM
Subject: Re: [Milsurplus] multi phase AC power


I'm not a real expert, but did look into this a while back There are a few 
possibles:

Capacitor phase converter. These are fairly cheap and have a cap phase 
shifter and Scott connected transformer. As I remember it did not make good 
3 phase, but it was OK for motors up to a few HP and likely OK for 3 phase 
power supplies where the input is transformer isolated and then rectified. 
As I remember, the lines are not symmetric about neutral, so an isolated 
load is a must. The efficiency is pretty good.

Rotary phase converters: These can either be belt driven or the Georator 
style. Big, HEAVY, and can be noisy. The phase relation of the output is 
stable and is symmetric about neutral. I have a roughly 1 KVA one as a 400 
Hz, 3 phase supply. It keeps the floor down well !!  IMO, the belt driven 
ones are less desirable.

Solid State Static:  These are nice and can sometimes be gotten cheap. 
Unfortunately, the bigger ones take 3 phase input power, so you are back 
where you started. They don't have much stored energy, so may have trouble 
starting loads. OTOH, they are often dial-a-frequency, which is nice. 
Caution is required though. I once applied 40 Hz to a 400 Hz transformer. It 
cooked pretty quickly. Again, big and heavy in larger sizes, but often cheap 
due to lack of demand.

The best choice really depends on your needs.

-John



gl4d21a at juno.com wrote:

> Group:
>
> All this discussion of single phase, 3 phase, Y, delta, etc., put me in 
> mind of a piece of information I found on one of the constant voltage 
> transformer manufacturer's web site.  The statement was that the output of 
> a CVT is shifted around 120° with respect to the input.  This caught my 
> attention, as it seems I often would like to generate some 3 phase power 
> from a single phase source.  So, now if I can find some device which will 
> produce a few hundred watts at -120°, I'll have something.  I thought of 
> using the input phase and this +120° as two legs of an open Y, but I 
> suspect that would be pretty inefficient.  I suppose I could connect a 
> second CVT to the output of the first and generate 240°, and I may try 
> that sometime when I have my collection of Solas out at one time.  Any 
> power experts out there who have gone through all this?
>
> 73,
> George
> W5VPQ



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