[Milsurplus] multi phase AC power

[Mike Hanz]

It's a common workaround for small shops that cannot get three phase 
power delivered to the doorstep.  I made one of the idler motor 
converters for my shop and used it about a year before switching to a 
VFD system.  Bill Schmidt's approach is essentially one of overpowering 
the tuning effects with an order of magnitude sized idler.  It works 
fairly well, though there are some drawbacks.  The size of capacitors to 
tune the "wild leg" with a 30hp motor gets /really/ expensive unless you 
find some that are surplus, and the waveform on the output is anything 
but a sine wave.  That's not much of an issue with another motor.  With 
smaller idler motors, you will notice considerable growling noises from 
the frame because of harmonic currents, and it doesn't adapt well to 
significantly varying loads - the rule of thumb is an idler motor with a 
minimum of  150% the load capacity of the driven load, and you have to 
derate the driven motor capacity by a third.  The noise is what bothered 
me the most, not to mention that a VFD has the ability to enormously 
pamper the motor with its CPU based design (not a trivial consideration 
when my Hardinge drive motor fetches $5K) and the ability to adjust 
frequency between 30 and 400Hz gives you a whole new dimension of 
usefulness.

Just another data point.  You will find religious wars between the 
rotary converter folks and VFD folks on the machining newsgroups.  :-)

Best 73,
Mike

Kenneth G. Gordon wrote:

>On 13 Mar 2006 at 16:26, Richard Brunner wrote:
>
>
>>Dr. William J. Schmidt, II wrote:
>>   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.  .... 
>>>
>>Do you mean you excite one phase single-phase, using a capacitor to the 
>>third terminal for capacitive starting?  This is an interesting concept. 
>>  Normally when a three-phase motor is single-phased it quickly 
>>overheats and burns up.  Of course you have no mechanical load on it, 
>>but I have seen three-phase motors overheat and fail with little load on 
>>them.  Because the excited winding heats up eight times faster than 
>>normal, normal overload protection is ineffective.
>>
>
>I don't know for sure, but I am pretty certain that you and Doc. Schmidt 
>are talking about two different things.
>
> I have several articles here on making three phase power from single 
>phase, and the one that provides almost commercial quality three-
>phase power from a single-phase uses an "idler" motor to do the 
>conversion. 
>
>I think this is what Dr. Schmidt is talking about.
>
>No power is drawn from the motor using this method. It just sits there 
>and runs. You have to size the motor to the load, and as I remember it, 
>the motor has to be at least twice the size the anticipated load would 
>indicate.
>
>I'll try to get some URLs together on it. I was fascinated when I read it. I 
>had never heard of this method before about a year ago, but apparently 
>it is being used successfully in many small machine shops around the 
>U.S.
>
>Ken Gordon W7EKB
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