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DIY RPC - voltage on case & large imbalance between legs

I had a manual 5 HP lathe running on a static converter and it was fine. It would even instant-reverse for tapping and whatever. People here say you can see the irregular pulses of the motor in the finish but, I never ran into that. It's possible, in theory, I guess. If it was a production machine, yeah, maybe running on 2/3 of the motor would put an undue load on it...I guess...in theory.

I just threw it out there as an alternative to spending more money on the RPC caps and balancing and whatever. Starting the RPC and starting the lathe are exactly the same process. The third leg isn't buying you anything that I can see.
thanks, is there an equation required to make the amps make sense? Some of the posts/write ups I’ve read really make a point about measuring amps, but give no indication how they utilize the info, other than “balancing legs”

Amps will not make much sense unless measured with the driven machine under load.

Current supplies power to the motor (if "in-phase"... we won't go there now). Ideally, the current on all three wires should be equal.

With an RPC that is not a huge overkill in power capability, that is not always the case. In fact it is rare. There are some reasons for that.

First, the amp draw on each leg depends on fairly small voltage differences. The idler has a "back EMF" or generated voltage, which is the voltage on the 3rd leg output, which depends on rpm.
The load motor is probably turning at about the same speed, so it has a very similar "back EMF" which is going to balance against the idler output, resulting in low current flow. The idea of balance capacitors is to boost the idler output voltage and balance out currents, but that is not very important at no load.

As you already know, the generated leg has a natural voltage lower than the voltage applied to the RPC. Balance capacitors help, but the help is less effective with increasing load, adding voltage drop. So as the load increases, the "boost" they provide becomes less.

But, the load motor slows under load, so it's back EMF becomes less. That means the idler, which is not loaded as much in proportion to it's rating, should hold rpm more steady, and the "back EMF" will not drop as much, increasing the current output.

Since the straight through wires have little voltage drop, the generated leg tends to get closer to equal current with the other wires. It is unlikely to even out, but the current balance should improve under load, which is where it needs to be better.

If you get the voltages right, the amps should sort themselves out well enough.

Some RPCs have apparently been made with special idler motors that have more turns of wire in the generated leg, to produce a higher voltage, more nearly equal to the incoming line voltage. That should improve performance. I have never seen one of those, I do not even know who made them, so I do not know how effective they were.
For Crusty:

My personal experience with measuring currents into load motors, from a rotary converter is this. When the load motor is running but with not much mechanical load on it, the two utility wires will show some reasonably large current value, approaching the FLA number on the load motor nameplate. If measured at the same time, the current flowing into the load motor on the manufactured leg will show a very very low current. Around 1/10 of the FLA number for the load motor.

The observation will be the load motor starts and runs fine, and can be used to cut metal fine.

My rough explaination for this apparent huge imbalance in currents is this: The amp clamp is measuring current with a very low power factor on the two utility lines. For some reason the amp clamp will measure only the in-phase current in the manufactured leg.

I have found that as the load motor takes up mechanical load, the power factor on the utility legs improves, but the amp clamp reading remains the same. When the mechanical load is applied in this way, the amp clamp reading on the manufactured leg will increase, until when the motor is fully loaded, all three of the amp clamp numbers will be much closer.

If you do have a current meter that measures power factor as well as current, this make the effect more apparent. In the meantime it sounds as if your converter is working just fine as is.
Nope. A meter that is an amp-clamp will ONLY read "total current". Not just in-phase, not just out-of-phase. It has no phase reference, it cannot determine phase.

If the relative proportion of in-phase and out-of phase changes, the meter might not see much change in the "total" current.

The idler provides power on the manufactured leg. To do that it draws power, which is in-phase current, on the input wires. Current there increases.

Because the "natural" idler output voltage is always lower than the line, at no-load, very little current is supplied on the manufactured leg ("ML"). The back EMF of the load motor generally matches that of the idler, so very little voltage difference is present to drive current.

When the load motor is doing work, it slows down,dropping the back EMF and allowing more voltage difference so there is more current flow.

Yes, there are both reactive and power current flows.

No, your "amp-clamp" meter cannot distinguish between them. It just reads whatever they total up to.
You guys have a grasp of this stuff well beyond my abilities. Thanks for the info. I’m going to see if I can borrow an amp meter from electrician to see first hand what you guys are discussing.
I have to pick up some supplies in town tomorrow, so will hopefully be back in to it the end of the week. Will report back.

" A meter that is an amp-clamp will ONLY read "total current"

Stop, you're preaching to the choir here. =)

The gist of my post to him was, amp clamp on the utility legs will show a suprisingly high current when the motor is unloaded. It will show a suprisingly low current on the manufactured leg. This is a natural result of your absolutely correct description of what an amp clamp meter reads.

My discussion of what's really happening was driven my measurements that took into account the power factor, and were not done with an amp clamp.

The exact reason why most of the current going into the manufactured leg to the load motor, being nearly all in phase under all conditions, has a simple explaination I suspect.