Electric Motor Phase Correction Question

I have a 7.5 HP TEFC Motor on a hot high pressure washer. My 3 phase power is generated from a rotary converter. This motor is not happy with my home made power and trips the heaters. Normal load is about 9 Amps under load but the one leg pulls 13-14A and trips the heaters. The contactor is a small GE unit with integral heaters and I have them set on max which is 14A IIRC.
The motor checks out just fine at the local motor shop. Coil resistances are all equal. There are no excessive mechanical loads on the motor. I have narrowed it to the current overload on one leg only.
I'll test with an added load on the RPC to see if that helps, but in the mean time; can I use a capacitor to adjust the current draw on that leg down? If so what MF values? I think that the caps in my surplus are all 600V rated.
Thanks,
Bob.

Bob wrote:
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It sounds like the voltages on the three legs aren't balanced, or perhaps the three legs are grounded in one configuration (delta or Y), in the phase converter and in the other in the motor.
I'd be surprised if it was a grounding issue, but the more screwed up systems of various kinds I repair, the less surprised I get when I find them.
If the problem is voltage balance, you could find it by measuring between pairs of the three legs with a volt meter. If it's a phase imbalance (which it would almost have to be) you'd either need a phase meter (which would be a power company or an industrial electrician thing) or an oscilloscope.
Did the motor shop actually run the motor, or just do static checks? You could have something perverse like a shorted winding in a motor coil that would only show up if you put AC on the thing.
Do you have any other three-phase motors running? If all are fine except for the one motor then it's more likely a problem with that motor, or a deficiency in both the motor and the system (like the colliding grounds that I posited earlier), than it is with your converter.

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And that, like a shorted turn in a transformer, will result in excessive current through that one winding. It doesn't have to be a "dead short"; just ONE turn shorted can do that sort of thing.
LLoyd

Lloyd E. Sponenburgh wrote:
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Yup. And one shorted winding won't show up if you do DC resistance checks.

Swap two pair to keep the same direction and move the 'wild' third wire to another winding. It might be that you need to have a smaller three phase motor to balance the rotary and then switch in the big one. Rotaries generate the third and the small starter motor might be just what you need - the mag field in it will supply some surge energy needed. It will also load the wild one and have it better under control when the contactor is snapped on.
I run a rotary when I use my grinders. I have it running a few minutes before I kick in the grinder. That might help. Martin
Bob wrote:
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You don't give any information on your home brew rotary converter but it it's the usual type without "run" capacitors, for 230v 60Hz and a 7.5 HP motor, your need to add about 200 to 250uF run capacitor to the phantom phase.
This should be enough to bring the current balance within acceptqble limits.. Some further improvement can be made by adding a second capacitor from phantom phase to the other side of the supply and messing about with the relative size of the two capacitors but the complication is rarely justified. Incidentally the"obviously symmetrical" arrangement of two equal capacitors is not useful - most or all of the capacitance needs to be in the top run capacitor.
You are absolutely right in aiming for current balance. Voltage balance only gives geneneral indication. It is current balance that results in minimum I squared R loss and minimum full load power drawn from the supply.
Jim

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A 7.5 HP motor is supposed to pull 3.4*7.5%A.
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What are the respective voltages?
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Yes, you can do so. But I would eliminate other possibilities first and measure voltages.
i

Bob wrote:
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9 A? What voltage? In the US, a 240 V 3-phase motor draws roughly 3 A per HP. So, I'd expect a 7.5 Hp 3-phase motor to draw roughly 22 A per line on 240 V. Ohhh, pressure washer -- must be some of those inflated HP readings. So, it probably is really a 3 Hp motor.
Jon

Gunner, what do you mean motor died?
i

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I've lost the origins of this thread. Is this a single-phase, capacitor start motor?
If so, it could run at full speed with the start winding still connected, and if the centrifugal switch didn't cut out, it would overheat and stop. (without there being a short in the windings)
LLoyd

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Deliberately disconnect the start winding from the centrifugal switch.
Wind a cord around the shaft, and "pull-start" it (very, very damned soon after you apply power).
If it runs without kicking the over-temp, it's the start switch that is failing to open, and that should be _really_ easy to fix, since it's purely mechanical.
The other alternative is just to open it up and fiddle with the switch. It could be corroded and jammed, or just welded shut from a bad high- current "make" at some time.
LLoyd

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I will buy this motor for $5 plus shipping.
i

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Free reading: http://www.archive.org/details/armaturewindingm00brayrich
Wes