25 hp Phase converter

Ive been using a 25 hp phase converter for several years now and it's worked
perfectly except with no load there is "ghost voltage" on the generated leg.
As soon as a load is applied, the voltage is quite even. The ghost voltage
is twenty or thirty volts higher on the generated leg and is no problem for
motors but cnc equipt is very sensitive. To be safe I would turn on a
bridgeport or band saw before applying power to a cnc. I just bought a new
30 hp cnc safe converter for around $1900. If anyone is interested, I feel
my old one should be worth $800. I'm in W. Mich and to lazy to arrange
shipping, so if anyone is interested and within driving distance, let me
know.
Dixon
Reply to
Dixon
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"Dixon" wrote: (clip) with no load there is "ghost voltage" on the generated leg.
^^^^^^^^^^^^^^^^^^^^^ Just a suggestion: might you put a little resistive load on--just enough to kill the ghost?
Reply to
Leo Lichtman
My guess is that the RPC has unequal capacitance between legs 1-3 and 2-3 respectively. That was the cause of this symptom on my old RPC.
i
Reply to
Ignoramus5390
Hey Dixon,
Glad you brought that up. I had real 3 phase in my last shop, but made a converter when I moved. It had really (still has really) flaky traces, although under load it is only a 20 volt spread. Anyway, it runs all the "straight" three phase stuff just fine, that is until I hooked it up to the CNC lathe. The only thing on the CNC lathe that RUNS 3 phase is the spindle motor which is controlled by a plain old motor-starter panel that is just switched on and off by the computer. But when they built the whole thong (OEM), they "balanced" the three phase across all the goodies in the box. so it ran OK for about a minute, and poooofffff....something went. It had never occurred to me to re-lug everything across just the "hot terminal", or simpler yet, start the mill or another lathe or the grinder!! That might have save me! Anilam wants $9,000 for an upgrade!!
Take care.
Brian Lawson, Bothwell, Ontario.
Reply to
Brian Lawson
There is no such thing as "ghost voltage" from a rotary phase converter. A RPC is an electrical network consisting of : 2 three-phase motors or 1 single-phase motor and 1 or more 3-phase motors running from a common source. That source may be 3-phase or single phase and 1 of the motors (the idler) can be single phase. The usual RPC consists of a single-phase motor and a three-phase motor (load) connected across a single-phase supply with appropriate "balancing" and starting capacitors. See
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for some good articles on RPCs.
Bob Swinney
Hey Dixon,
Glad you brought that up. I had real 3 phase in my last shop, but made a converter when I moved. It had really (still has really) flaky traces, although under load it is only a 20 volt spread. Anyway, it runs all the "straight" three phase stuff just fine, that is until I hooked it up to the CNC lathe. The only thing on the CNC lathe that RUNS 3 phase is the spindle motor which is controlled by a plain old motor-starter panel that is just switched on and off by the computer. But when they built the whole thong (OEM), they "balanced" the three phase across all the goodies in the box. so it ran OK for about a minute, and poooofffff....something went. It had never occurred to me to re-lug everything across just the "hot terminal", or simpler yet, start the mill or another lathe or the grinder!! That might have save me! Anilam wants $9,000 for an upgrade!!
Take care.
Brian Lawson, Bothwell, Ontario.
Reply to
Robert Swinney
Bob, over time I have read many of your posts and they all were well done and made a lot of sense---until now. WTF are you talking about! I certainly appreciate you trying to help me, but are you kidding? I've had close to thirty years experience with phase converters and diesel, real three phase power, and I've never heard such a convoluted explaination of a phase converter. I'm sorry, but your explaination sounds like a story one would come up with when coming home at 5 A.M. and the wife is unexpectedly awake when you try to sneak into bed! Dixon
Reply to
Dixon
"Robert Swinney" wrote: There is no such thing as "ghost voltage" from a rotary phase converter. (clip) ^^^^^^^^^^^^^^^^^^^^ I don't believe in ghosts, either, but are we just quibbling about words here? The OP described a voltage imbalance which evidently has a high output impedance, so that as soon as it encounters a load the value becomes negligible. Why do you object to calling it "ghost voltage?" Your description of how phase converters are configured doesn't answer the question for me. If you can clarify, I'd appreciate it. Thanks.
Reply to
Leo Lichtman
Hi Bob
I especially liked the referenced article titled "Induction Motors and Phase Converters. I have read it many times and am still learning things about 3 phase motors from the information included therein.
Jerry
Reply to
Jerry
Dixon, Leo, et al,
Sorry!. The term "wild leg" is sometimes used in the description of RPCs, but it is almost as innocuous and vague as "ghost voltage". I will admit to having fallen into that verbal trap myself, but truth be known, there is no simple explanation for the operation of RPCs. There have been discussions of RPCs on this news group (and others I'm sure) for a very long time. A RPC is not merely a single phase motor and some randomly selected capacitors. Nor is it simply 2 three-phase machines running on the same line. On the contrary, a RPC is a complex electrical network in which circulating currents emulate 3-phase "power". One of the paths in which current flows in a RPC is sometimes called a "wild leg", but that leg cannot be separated from the rest of the network. Do not view a RPC as a generator of 3-phase current. It is a network unto itself. Balancing caps effectively tune current paths within the network in a series resonance fashion. Comparatively large amounts of capacitance effectively "steer" single phase current in such a way that it appears to the load as 3 phase. The same thing happens even with no balancing caps, except the manufactured leg ( wild leg ?? ) voltage wiil sag down below the level of the of the single-phase line voltage.
Bob Swinney
"Robert Swinney" wrote: There is no such thing as "ghost voltage" from a rotary phase converter. (clip) ^^^^^^^^^^^^^^^^^^^^ I don't believe in ghosts, either, but are we just quibbling about words here? The OP described a voltage imbalance which evidently has a high output impedance, so that as soon as it encounters a load the value becomes negligible. Why do you object to calling it "ghost voltage?" Your description of how phase converters are configured doesn't answer the question for me. If you can clarify, I'd appreciate it. Thanks.
Reply to
Robert Swinney
Thanks, Jerry. There is a diagram and brief explanation of RPC current flow in my article.
Bob Swinney
Hi Bob
I especially liked the referenced article titled "Induction Motors and Phase Converters. I have read it many times and am still learning things about 3 phase motors from the information included therein.
Jerry
Reply to
Robert Swinney

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