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
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
Karl Townsend fired this volley in news: snipped-for-privacy@4ax.com:
Karl, you can also do that experiment with a big 3-phase motor and a pull-starter.
Your phases won't be exactly balanced, but you'll still get 3-phase power off'n it.
Just feed two windings on single-phase, pull-start with a cord, then extract three-phase from all three windings.
LLoyd
My 18 year old friend did this to a welder that I sold him, works great. He placed caps on the INPUT to the transformer.
i
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
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
Do you know if he just did L2-L3? Or both L1-L2 and L2-L3? About how much MFD?
Karl
I will ask him. I sold him the welder, but gifted the caps.
i
Wish granted...
jsw
"Voltage and Current" in this one shows how an inductor (motor or transformer winding) can delay or phase-shift an AC signal:
jsw
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
I believe so, L1-L3. (L1 and L2 are symmetrical and the same). MFD depends on your desired current.
i
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
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
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
I meant 5 for $5
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
Not bad!
I am very aware that he is a control freak, yea.
iMaybe 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.
Mart> >
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