static phase converter

I did a bit of surfing for static phase converter design...
All I found was recommendations not to use them. I assume they are
just run caps between L1-L2 and L2-L3. How big is my question. I won't need a start cap as this is for a trial of using single phase and caps into a three phase transformer and then DC power supply on a plasma cutter.
Karl
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On 07/08/2011 04:37 PM, Karl Townsend wrote:

This won't work. You need the motor to provide the 3rd phase. the capacitors just balance winding current a bit, but don't really provide the 3rd phase. So, you need to add a large motor to this setup, and then size the caps for the motor.
Jon
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While I admire your electrical knowledge, I believe that a run cap places between L1 and L3 may induce third phase in a delta transformer.
i
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On Fri, 08 Jul 2011 17:13:45 -0500, Ignoramus3477

The electrical prints on a similar plasma cutter show just this. Mine's not quite the same. I'd like to know the science behind it.
Karl
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wrote:

Wish granted... http://www.play-hookey.com/ac_theory/ac_capacitors.html http://www.play-hookey.com/ac_theory/ac_inductors.html http://www.play-hookey.com/dc_theory/lc_circuits.html http://www.play-hookey.com/ac_theory/ac_lc_series.html http://www.play-hookey.com/ac_theory/ac_lc_parallel.html
This briefly explains the real + imaginary (j) components: http://www.play-hookey.com/ac_theory/ac_rc_series.html
I had to learn this to become a lowly radio lab tech at MITRE.
jsw
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"Voltage and Current" in this one shows how an inductor (motor or transformer winding) can delay or phase-shift an AC signal: http://www.play-hookey.com/ac_theory/ac_inductors.html . Capacitors help adjust the amount of electrical phase shift to match their mechanical 120 degree separation in a motor.
jsw
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On Fri, 8 Jul 2011 16:03:25 -0700 (PDT), Jim Wilkins

I should be careful what i ask for. I can see from these pages that caps should only go from L3 to L2. If I understood more, I could size the caps. Let's assume 20 amp load, how much caps to balance the L2 as close as possible?
Karl
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wrote:

Many moons ago when Fitch and Jim Hanrahan were discussing optimal balancing capacitance, I think they said the easy way was to measure the phase currents with a scope.
Clamp-on current probes are one of the goodies we don't pass up in surplus stores.
jsw
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You need to know the complete load profile, including the power factor of the leg using the capacitor to calculate the size needed. I am not familiar with that equipment but some experimentation, using phase angle meters may be in order to get it right.
------------
"Karl Townsend" wrote in message
I should be careful what i ask for. I can see from these pages that caps should only go from L3 to L2. If I understood more, I could size the caps. Let's assume 20 amp load, how much caps to balance the L2 as close as possible?
Karl
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On Fri, 08 Jul 2011 19:08:38 -0500, Ignoramus3477

Thanks iggy. I'm doing the same thing. This one used 70MFD, the prints I found used 280 MFD. Looks like I'm in the market for four 70 MFD run caps. Fleabay, here we come.
BTW, I was tossed off PM for having an avatar with an apple in it. Your avatar is no better.
Karl
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Do you want me to tell you a story of how I bought four 94 uF 480v run caps for $4?

I am curious what was exactly the 'reasoning' (I am using the term loosely).
i
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I meant 5 for $5

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On Fri, 08 Jul 2011 21:00:52 -0500, Ignoramus3477

I only need four. Give you $8, double your money. VBG
Actually, I did about as good when I built the 40 hp. converter. I had all winter in FL to buy the parts and sniped for a month to get everything.

If you're there at all, you know the owner is an anal control freak. When he first allowed avatars it was only with his approval and they had to be "on topic" My business Logo (see CNCzone) avatar didn't meet his approval. I let him toss me rather than remove it. I haven't been back.
Karl
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Not bad!

I am very aware that he is a control freak, yea.
i
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Maybe 8 or 9 years ago one was posted in the drop box and a number of Home shop guys build them.
It was designed and refined by a super and senior member in the group at the time. Now semi-retired in Penn.
Martin - I have a rotary.
On 7/8/2011 5:17 PM, Karl Townsend wrote:

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wrote:

Your actual phase lag will not be stable unless the load is constant.
For what it's worth, unless you get into a heavy cut, the thing most probably will operate just fine with single phase wired to L1 and L2 and nothing connected to L3
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On Fri, 8 Jul 2011 19:19:24 -0700, "PrecisionmachinisT"

Static phase conversion works with a 3 phase motor because the L1 to L2 capacitor feeding the motor magnetising inductance tries to generate approximately 60 deg phase shift (supply to L1 and L3). This results in an open vee 3 phase system. 60 deg is the vee included angle at L3. See Electric Motors 2nd edition for a fuller explanation.
It only works because the inherent iron airgap in an induction motor results in comparatively low value of magnetising inductance. The magnetising VA is now a significant or large fraction of the full load current (Low Power Factor). This enables the phase shift to be fairly constant over a usable load range.
With suitable choice of capacitor this inductive component also makes it possible for the phantom phase voltage to be close to the correct voltage.
Because standard 3 phase transformers have no inherent air gap the magnetising VA is a small fraction of the full load current. This means that the primary impedance appears predominantly resistive and directly proportional to load.. The best that a capacitor can now do is provide some phase shift that is critically dependent on load and necessarily lower in voltage than the L1-L2 voltage.
It's a bit better but a very long way from the desired balanced 3 phase. Adding also an L2-L3 capacitor can only make things worse - the two capacitors now act as a voltage divider so, for the same phase shift, the output voltage is even lower.
Since only a a limited improvement is possible it's well worth trying PrecisionmachinisT recommendation of L1-L2 without any capacitors.
If this isn't good enough then the simple solution is to use a 3 phase motor as a rotary converter as suggested by Lloyd Sponenburgh and Don Nichols.
Jim
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On Jul 9, 10:42am, snipped-for-privacy@yahoo.com wrote:

A Kill-A-Watt shows this clearly for a single-phase 120V motor, such as on a washing machine where the load varies substantially during the cycles.
Since the KAW is cheaper and easier to understand than a scope and current probe, do you know a way to balance the currents in a 240V 3 phase rotary converter with them?
jsw
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On Sat, 9 Jul 2011 08:34:35 -0700 (PDT), Jim Wilkins

An accurate true power meter is a good way of setting up a 3 phase converter because minimum copper loss occurs when the three winding currents are equal. It's a pretty flat optimum but that's not important - if you're within a few watts of optimum who cares!
It doesn't tell you in which direction to change the capacitor value(s) so you have to experiment. It's best to use it is a method for setting up at or near full motor load. Results on light load or no load will be inaccurate and not carry through to heavier loads.
You may find my reply to Bill Noble helpful.
Jim
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