Thoughts on an RPC low voltage issue

I was doing some RPC testing today for a friend. I am kind of thinking we are getting a little outside the operating tolerance on this one.

For reference, leg 3 is the wild leg

His idle voltages were 235, 258, 258.
I did not like it all that much loaded so we did a little tuning

Best I came up with was 235, 240, 248 unloaded
load with 1.5HP load 235, 240, 240

Problem is the inrush (spindle start) on his equipment (VFD) would pull the 1-3 down from 240V to 175V, plus when braking would shoot up to 280V. I did not discover any internal or external resistor but all other voltages were inline including the 2-3 which had a 220V inrush and 240V braking.

The RPC is 10HP, inrush current on L3 was 32A, was 80A on L1 (no check L2). They settle in with good voltages but I have concerns. I tuned the 1-3 all over the place and could not get the inrush voltage to go above 175V, mostly around 160-172V. Other voltages stayed tolerable.

1. Could there be a high resistance situation on L3 that could cause this? All connections look very good but still have to wonder.

2. This is a 7.5/10hp cnc spindle running on a 10hp converter, is this just asking too much?

3. brake voltage, this is the one that is puzzling me the most. All other voltages are fine but 1-3 spiking to 280-290V is concerning. This is a Fanuc drive. I am not sure if the RPC is unable to sink that level of power due to design or size and thus, the voltage is going high?

RPC have limited ability to sink returned current on manufactured leg.

RPC have somewhat limited ability to SOURCE current on the manufactured leg.

RPC have resistance and inductance in series with the manufactured leg that is not in series with the other two..... this fact is responsible for the largest amount of the imbalances.

If it isn't good enough, you have three choices..... 1) increase idler size to reduce impedance..... 2) change to some other form of 3 phase converter......3 get 3 phase service. Your choice.

Would a fanuc system not shut it self down with an over voltage condition? I would think we are getting close here. If this is an undersized converter, why is 2-3 voltage stay so well balanced? That one is confusing me... There is more than 2x the capacitance on 1-3 as 2-3. I am real surprised to see it go so imbalanced with load.

Viper, I just sent you an email on this topic you gotta read quick and let me know if you are interested.....

Mike, i don;t have a PM from you. I did send you an email as well. Feel free to ignore me at will...lol Once my brain goes into overload, random questions start popping up...

JST stated:

"RPC have limited ability to sink returned current on manufactured leg.

"RPC have somewhat limited ability to SOURCE current on the manufactured leg.

"RPC have resistance and inductance in series with the manufactured leg that is not in series with the other two..... this fact is responsible for the largest amount of the imbalances."

Thank you!

I have been stating the same for YEARS, but, alas, I had not stated those "first principles" in a SINGLE POST in ONLY three related sentences.

THANK YOU, JST!

Now, I will restate the above, but in more emphatic terms:

1) an RPC has NO ability to "sink" power on its manufactured phase, thereby rendering RPC three-phase essentially useless on certain three-phase regenerative braking drives (the high resistance of the manufactured phase, relative to the "apparent resistance" of the "apparent phases" is the issue, here; most regenerative drives will see a fault condition on account of high B Phase voltage, although the A and C phase voltages will be within specification),

2) an RPC has a limited ability to "source" power on its manufactured phase, for somewhat similar reasons,

3) the "equivalent circuits" for the three phases, the "apparent A Phase", the "apparent B Phase" and the "apparent C Phase" are quite different (sure, all three have an "idealized" voltage source, but the series resistance, the series inductance and the shunt capacitance are all different ... it is usually, but NOT ALWAYS, the same on the "apparent A Phase and the "apparent C Phase", and it is ALWAYS different on the "apparent B Phase"), and

4) the best transient performance ... the performance under varying loads ... is usually obtained with an unbalanced converter, usually 60/40 unbalanced, where the starting capacitor is temporarily placed across the 60 percent side (it is yet another of the RPC "first principles" that a "substantially unbalanced" condition is required in order to start an RPC in the first place, thereafter the choices are a perfectly balanced converter or an intentionally unbalanced converter, or somewhere in between, and my experience is a 60/40 unbalanced converter gives the best results ... YMMV, of course; an intentionally unbalanced converter was first popularized by Fitch Williams; previous authors apparently did not know the reasons for the imbalance, or why an unbalanced condition could be operationally superior to a perfectly balanced condition).

not pm; email to yahoo - ah, see spell checker stopped it going for a bit.....

Mike, got ya covered

Peter, per my other post, the question is, does a bigger converter offer more ability to sink power? IE, run a 10HP regen drive with a 50HP RPC? Obviously that big girl has quite some head room to keep voltages VERY stable during inrush and full load events. However, does it also offer improved sink ability to regen on B phase?

"Peter, per my other post, the question is, does a bigger converter offer more ability to sink power? IE, run a 10HP regen drive with a 50HP RPC? Obviously that big girl has quite some head room to keep voltages VERY stable during inrush and full load events. However, does it also offer improved sink ability to regen on B phase? "

A bigger converter provides a larger SOURCE of power, but not necessarily an ability to SINK power.

I have not seen a case where a much larger converter could actually SINK sufficient power to actually provide utility three-phase-like power to a regenerative braking load.

Now, I suppose in the so-called "last extremity", where the converter was so over-specified that it ... essentially ... provided what was an "infinite bus" that such would be possible.

THAT may be as much, if not more than TEN times the nominal load.

The problem, again, is that the equivalent source voltages are unequal, although they may be quite similar, although the source impedances are WILDLY unequal, leading to the obvious results.

I was kind of wondering if the minimal power sink ability on B phase was actually mostly being bled through the motor itself or the caps eventually getting back to the grid lines.

Is there any way to quantify the sink ability at all?

peterh5322 said:

1) an RPC has NO ability to "sink" power on its manufactured phase, thereby rendering RPC three-phase essentially useless on certain three-phase regenerative braking drives (the high resistance of the manufactured phase, relative to the "apparent resistance" of the "apparent phases" is the issue, here; most regenerative drives will see a fault condition on account of high B Phase voltage, although the A and C phase voltages will be within specification),

Click to expand...

"NO ability" is a bit extreme...... the voltage rise for decel will be *at least* as much of an overvoltage as what the "drop" is for accel, for equal powers transferred through.

The idler has no idea what a source and load is...... it is voltage, frequency, and current operated. It is essentially linear for small changes, and so cannot actually "block" return power. It might be "not so good" at passing it.

Not the same as "ZERO ABILITY" to return power, but might effectively be almost as bad, depending on the load which is acting as a source.

"Not the same as 'ZERO ABILITY' to return power, but might effectively be almost as bad, depending on the load which is acting as a source."

Might as well be ZERO as it is about as close to zero as is possible without being zero.

Bottom line is A and B can sink almost infinite power ... well, as much power as the load can source, up to the limit which is imposed by the feeder protection, anyway ... and B can sink almost vanishingly little power, perhaps 0.01 times that which A and C can sink.

A regeneration-type drive will see good sinking on A and C and almost no sinking on B and B will most likely have an over-voltage condition which will shut down the drive.

One possible solution is to inhibit regeneration on B. This can be effective on drives which are software-free and don't use firmware to monitor the regeneration function. This will give roughly 2/3 of the (regenerative) braking power. Such is the price one may have to pay for using an RPC as a three-phase power source.

A PP is designed with regeneration in mind and is an obvious solution for those with expensive regenerative drives (late 10EEs, for example) which are operated in single-phase premises.

Hmm. A rotary has to be able to sink *some* power.

The observables: I can easily plug-reverse the spindles on my machines, even when
running at considerable speed and with substantial work in the spindle.

The idler motor grunts but carries on.

The energy in the rotating parts goes somewhere.

1) heat in the load motor
2) heat in the wiring
3) some back to the idler or else why would it complain so
4) some back to the utility?

I was always of the thought that fitch advocated a balanced converter.

peterh5322 said:

"Not the same as 'ZERO ABILITY' to return power, but might effectively be almost as bad, depending on the load which is acting as a source."

Might as well be ZERO as it is about as close to zero as is possible without being zero.

Bottom line is A and B can sink almost infinite power ... well, as much power as the load can source, up to the limit which is imposed by the feeder protection, anyway ... and B can sink almost vanishingly little power, perhaps 0.01 times that which A and C can sink.

Click to expand...

Elsewhere you suggest that "if big enough" it would be closer..... So I assume you agree that there will be "some" power passed back, as suggested by your "1%" comment.

And I assume that you agree that the power passed back is limited by the impedance, which imposes a voltage drop, and requires the "source" of regen power to provide it at a higher voltage for that phase......

Since anything operating off a fixed DC rail will have a limited peak voltage, I will assume that is the origin of your "1%" suggestion.

I suppose we can agree that the limitation in this case is "regen source voltage" more than some sort of "one-way valve" action. There is bidirectionality of the idler motor.

And therefore, we can agree that a source of regen power that is not so strictly voltage limited could pass back significantly more power than a voltage limited source with a voltage limited near the peak of the AC line.

Just wanted to post some results of some tests yesterday. I added an additional idler in parallel with the main RPC motor to double the capacity to 20HP. Ran the same tests. L1-3 now pulls down to 185V and rises to 275V. So it is "better" but still nowhere near where it should be for max life of the drive. All other voltages stay very stable, even the L2-3 Which pulls down from 237V to 220V.

However, I did note that inrush was pulling 95A from L1 and L2, only 35A from L3. That is big girl current for sure. A 10HP across the line start pulls 100-130A so that is certainly not a 10HP motor inrush. I think we determined this spindle uses a Fanuc 5.5/7.5kw system which is actuall a 30HP motor.

All in all, short of running a test with a 50HP unit, it is safe to say that RPCs must be vastly oversized to handle inrush loads with good stability. I might find a 50HP motor just for a test and see what happens. However, you also have to figure in the 2500watts of power being dumped constantly while idling which heats the shop and burns your wallet. Could easily waste $100/mo for that RPC but I guess that is chump change if it works.

viper said:

... However, you also have to figure in the 2500watts of power being dumped constantly while idling which heats the shop and burns your wallet. Could easily waste $100/mo for that RPC but I guess that is chump change if it works.

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2500watts?? 10 hp motor for rpc idling pulls like 15 amps with clamp on meter but only 1-2 amps of that is REAL that you pay for, no? 240v*1.5a= 360watts?

Sorry Mike, that is 2.5kw for the 50HP. I see about 50w/hp over here Real power but the 20HP I just built seems to be doing better at 800w. Still, that is enough heat to want to move it outside in the summer.

viper said:

Sorry Mike, that is 2.5kw for the 50HP. I see about 50w/hp over here Real power but the 20HP I just built seems to be doing better at 800w. Still, that is enough heat to want to move it outside in the summer.

Click to expand...

OH.. thought u were talking about the 10hp...if it pulled 2amps that is ur 240*2=480w or 50w/hp, yep.... so 20hp of 800 is about right for 1.5ish amps friction. and so 50hp shud likelywise be around 45w/hp*50= 2200-ish watts. yep. sorry 'bout misreading post