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Newsgroups: rec.crafts.metalworking
Subject: Re: Rotary phase converters
From: John De Armond
Date: Sun, 23 Jan 94 08:21:32 GMT  (Jay Stryker) writes:

>In article <> (Bill W.
>Martin) writes:

>>>        If anyone has info on using a large 3-phase motor as a phase 
>>>converter, ie 1-phase power to 3-phase power, I have a couple of questions.
>>>First, how much power is consumed at idle load condition, relative to the 
>>>size of the motor being run?
>I have seen ordinary three phase motors used in this can wrap a
>rope around the shaft, give a strong pull, and then switch on 1 phase power.
>Once running, it will draw a few amps, but as long as there is no load on
>the shaft, it "idles".  When you place three phase motors on the output
>(recall that you have to use a three phase contactor) the current will rise.

Yes, this works well. and if you connect a capacitor in series with 
another winding hooked in parallel with the first, the motor will start
itself and will draw much less idling current.  For 3P motors in the
5 hp range, I'd start out with about 20 uF at the appropriate voltage.
If you're interested in best efficiency, tune the capacitor by varying
it to get the least idling current.

>How much...hmmm...maybe twice or more?  Anyone know?   I have a commercial
>single to three phase converter....I have measured the three phase outputs
>for balance but never measured the input current change.   

Amazing how power factor works, eh :-)  

>>>Second, can a dual-voltage 3-phase motor be configured as a kind of "
>>>rotating transformer" to step up 220V to 440V ?
>This sounds feasible if the windings are electrically separate (eg. the
>220 is not a mid-tap on the 440).  Heating, arc-over and insulation limits
>may be involved here.  

This works fine as long as the FLA limitation of the converter is
respected.  For dual voltage motors, simply connect the windings in 
series for the higher voltage and then apply the incoming power to
between one end and the junction of the series windings.  Take the output
power from the ends of each phase.

Another little known fact is that an induction motor driven by another
prime mover (engine, etc) is a fine generator.  It generates lots of
leading VARs so it counteracts the low powerfactor of the motors and the
"converter."  Simply connect the 3P converter motor to the prime mover,
bring the rotor speed up to nameplate (1800, 3600, etc) and apply
excitation as before.  Then crank the throttle open on the prime mover
and apply the 3P load.  Within the current rating of the converter, the
rotor will remain locked roughly in sync with the line and no govenor is
needed.  Watch the phase current to keep it within the FLA rating of the
converter.  Particularly watch the phase connected to the line; "excess"
power is fed back out the power line.


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