Big leg drop during load, should I balance?

I have been running a Phase-Craft converter with a 10HP Baldor motor for a few months now.

Just recently, I have been able to really put a little bit of load on some aluminum on a Fadal VMC20 with 5hp spindle motor. We aren't running full out yet due to lack of coolant, but plan on pushing the machine harder, soon. Now, technically the machine is showing 25-30% load when cutting during an operation with a 3/4 3FL carbide cutter. I am noticing that one leg in particular is dropping quite a bit. I'm metering it close to 215V while the other two sit just right under 240V. With the machine running with zero load, I'm seeing 247, 236, and 241. At the spindle inverter side though the transformer, I am seeing 225, 215, and 238 while loaded at 30%. Should I be worried about this at all? If so, what would be the next step? Run caps? Thanks!

Hi Rob!

Hmmm...

Before you go to capacitors, try this... add another motor to your load... like... start up a second machine tool's spindle. Don't load it, just let it spin... and then put the Fadal to work, and see how that affects the sag. One wouldn't THINK that the Baldor motor would need more support, but it might. Might be that the Fadal has other loads on that leg that're asking a bit more.

Also- shut it down after working it, and carefully check the wiring and terminals in the converter box, and see if any are warm... if you've got any insufficiency in wire or connector, beef it up.

I would suspect that you don't have the correct number of, or arrangement of capacitors. Correctly sized, you should have much better balance, normally, I would expect 2-3% unbalance.

The leg that is dropping is your manufactured leg.

Tom

DaveKamp said:

Hi Rob!

Hmmm...

Before you go to capacitors, try this... add another motor to your load... like... start up a second machine tool's spindle. Don't load it, just let it spin... and then put the Fadal to work, and see how that affects the sag. One wouldn't THINK that the Baldor motor would need more support, but it might. Might be that the Fadal has other loads on that leg that're asking a bit more.

Also- shut it down after working it, and carefully check the wiring and terminals in the converter box, and see if any are warm... if you've got any insufficiency in wire or connector, beef it up.

Click to expand...

Dave, currently the Fadal is the only machine on the RPC. The manual lathe and mill are on their own single phase VFD drives that came with them.

The thing that I don't get is the spindle drive on the Fadal. It came with an upgraded 10k RPM spindle and 10HP spindle drive (Baldor H2 VS1SP210-1B) to match, even though they left the original 5HP motor on there (or swapped it back before selling it to me). I think during the course of the install the settings were lost on the control, so setting it all back was a pain, but I think I am good (setup for 5HP on the drive itself and only 6500RPM which is what the original motor ran at, not sure if 10k would be a good idea on such a motor). It doesn't help that there is zero information about the VMC20 these days as well. I have the transformer tapped for 240 on the two legs and 250 for the high leg to get them all under 240. I have no clue if the load meter is accurate as of now, even though it reads 25-30% in some of these cuts. Basically I need to figure out if it's the RPC that is struggling or the machine setup.

Thanks for the help!

Here's the meter ratings from a part being ran just a second ago:

Do you know what is being measured? Which meter corresponds to which phase? And which two are between teh manufactured leg and the supply legs?

Are the amp measurements in the delta legs or the Y wires?

Obviously the measurement is delta, but I see that one delta voltage changes quite a bit, while the other two change very little.

It appears that you culd raise the voltage with the so-called "balance" capacitors. You seem to have voltage available, your voltage on that phase is not too high.

You want to watch out, because balance caps will raise the unloaded voltage even higher than the loaded. So there is definitely a value beyond which you won't want to go.

Rob, there's some really nice workmanship that guys do building RPCs, but I'll have to admit that I've seen few 'commercially bought' RPCs, and no RPC 'KITS' that I had full confidence in. There seems to be a new guy every day who's selling 'RPC KITS- Add your own motor' from a five-minute website, or Ebay, or whatever, with no documentation support 'up front' to identify the contents, wiring, etc... I've seen some things that were downright hideous inside. Mix that with a motor that the 'panel manufacturer' couldn't accurately predict, and you have an unpredictable result. Might work fine, might need balancing caps, might be way, way wrong. So... until the covers are off, and it's clear just how it's all built and configured... there's really too many possibilities to venture a deep guess.

Shut it down, take off the covers, and take some pictures of the internals, and if the gave you a diagram, post that too.

To begin with, you do have sag. Is this affecting operation at all right now? Do you see an effect other than on the meters?

It's likely to be bad long term, especially at higher power, because it is similar to running the internal inverter on single phase, which it may not be at all rated for.

Dave is correct, there are unknowns, and a wiring diagram might help. However, an RPC is generally able to cope with unknowns pretty well, it is not a complex device, and the few factors affecting voltage are reasonably simple as well. There are not very many ways to set one up, and once it is working with reasonable output voltages, it is fairly certain that it is set up basically correctly.

That said, the load you have is not like a motor. All the Fadal info I found shows an inverter between the motor and the supply line (typical for CNC), so it is a rectified load, different from a motor load. There are other loads as well.

What works for a motor load may not work as well for an inverter load, because a rectified load is hard to drive. The rectified load has pulses of current considerably higher than the average, with a time between pulses, due to the rectification. The pulses can be from 2 to 5 (or more) times higher than the average or rms current, depending on source impedance and other factors in the supply itself.

That can cause an effective voltage drop for any supply phase with a higher impedance.

If the unit has a power factor corrector (PFC) on the input, which it may, then the load will be considerably "nicer", less hard on the supply. I don't know if that unit has a PFC input. The PFC will eliminate the high current pulses, instead drawing current much the way a plain resistor does. It "shaves off" the high peaks of current characteristic of a plain rectifier input.

Your meters are clearly measuring delta voltage, not "wye". Two remain pretty constant, while one sags quite a bit. That's not necessarily typical, as the voltages are all related. It's more typical to find one steady, and two that sag, but phase shifting due to the load can change that.

It isn't clear ho the current is being measured, but the easiest way is just in the supply lines, so that "wye" current is measured.

I would assume that the top voltage is the input, measured between the single phase wires, and the lower two are from input wires to the manufactured phase wire. This is because the top voltage stays steady under load, which a low impedance source would do.

It may be that a 'balance" capacitor change can help, and it's cheap, so worth a try.

It may also be the case that the load is obnoxious enough that the 10HP idler has too much impedance. With RPCs, it is often the case that a bigger idler cures all problems, because the problems are due to idler impedance, and a bigger one has less impedance. A bigger RPC acts more like a power company 3 phase supply

For a bigger idler, your medium load might be just a very light load, and the heaviest load might be less of an issue than the medium load is now.

Right now, you are getting quite a bit of sag (about 10%) on what is really only a couple HP load. I don't know exactly what would happen with a load much closer to 5 HP, but I very much doubt it would be any better.

Just throwing numbers around, assuming that the inverter load draws current pulses about 3 to 4 x what a motor would draw, one would come up with the idea of using an RPC of about 3 to 4 x the nominal draw. In this case a 20 HP idler.

I would NOT run out and replace what you have just yet, but keep that in mind as you try the balance caps.

Here's the inside guys, tell me what you think:



As for upgrading to a 15HP or 20HP idler, I worry about starting. I am limited on my service (only 100A) going into the shop right now off of the house's main 200A service. I've got a 7.5HP air compressor on single phase 230V in the main garage that takes up quite a bit of juice as well. Right now I am running the idler off of a 50A breaker and all seems well.

Plus, I have debated on upgrading anyway so I can move that 10HP idler I have into the machine so I can get more torque along with adding an encoder, which means rigid tapping.

Also, the gauges go in the following order:

Top = RED/BLACK (1-3)
Middle = RED/WHITE (2-3)
Bottom = BLACK/WHITE (1-2)

RED is the manufactured leg, which goes to T3 on the machine.
BLACK goes to T1 on the machine.
White goes to T2 on the machine.

So,

Top = B
Middle = C
Bottom = A

And with that said, I'm going to try and rearrange them in order to make more sense of things.

So the leg that is falling is the RED/WHITE, (2-3), (C on the CNC) leg. I need to figure out the wiring to see which legs are dealing with the control side of things (the 120V and +5 +12 DC side) and make sure those are stable first and foremost.

The idler is a CM3714T Baldor 10HP 208-230/460V, 27.5-25.6/12.8A, 1760RPM, 215TC frame, 60hz, 3phase, if that helps.

Initial comment would be that the use of white wire as an energized wire (not as the "groundED conductor", i.e. neutral) is frowned upon most strongly.

At least mark the ends at the connecting points with another color so it is shown to be a "hot wire". Even black. White and green are the colors which should never be used for a wire which does not connect to earth.

The white wire is the one that kills people, when they believe it is earthed/low voltage (as one would expect in USA), and instead, due to faults or miswiring it is actually hot.

Also, since this is a "wild leg" system, it is conventional to use orange for the generated leg, which has the high voltage to earth.

JST said:

Initial comment would be that the use of white wire as an energized wire (not as the "groundED conductor", i.e. neutral) is frowned upon most strongly.

At least mark the ends at the connecting points with another color so it is shown to be a "hot wire". Even black. White and green are the colors which should never be used for a wire which does not connect to earth.

The white wire is the one that kills people, when they believe it is earthed/low voltage (as one would expect in USA), and instead, due to faults or miswiring it is actually hot.

Also, since this is a "wild leg" system, it is conventional to use orange for the generated leg, which has the high voltage to earth.

Click to expand...

Outside of me throwing some colored heat-shrink on the white wires that are hot (Lowes did not have much for choice of color for the size of wire when we installed it), have any other advice towards my voltage sag?

How fast does your idler take to get up and running? What is the voltage and uf rating on the black can?

On the silver cans or run capacitors, what is the volt & uf rating on them? Are they using the same hotline or different hotlines jumping to manufactured leg?
Traditionally you want between 12 and 16 µF per hot leg to generated leg,per horsepower of Idler. But then you should shift the run cans so there is roughly 1/3 ,2/3. For example a 10 hp RPC should have 200 uf run capacitor from black to blue, and 100uf from red to blue. Now this is just a ballpark, some further tweaking usually needs to take place.

Here is a good example. This is a professionally built Unit 10 hp
150uf from Black to blue, 100uf from red to blue.

Bill is correct, any RPC can be balanced to 75% of Idler hp., but with heavy loads a 4hp machine motor at FLA can chew up the all of the VA of a 10hp. idler. Ideally a 15hp RPC would have been ideal. But properly tuned, your voltage will be high at light loads and a little low at heavy loads. Right now my guess is your tuned for 1 1/2 - 2 hp.

I have no idea whether the compressor was on or not when the test above was done... But the voltages during the test show a good stable input voltage. AND the problem still exists....

So the compressor can likely be totally forgotten about as a contributor here. It's a distraction, and not an issue.

Adding more idlers might be diagnostic, but isn't a great solution... they add more losses than a correctly sized RPC.

The whole issue of "balancing" is to some extent a matter of "deck chair arrangement".... it is NOT REQUIRED, and instead is a refinement.

What is seen from the voltage readings is TWO stable readings, and ONE that sags. Essentially, "the phase triangle is closing up".

OK, then that is typically from a LOAD ISSUE, where the load is unbalanced, or the load is unusual, not a motor or resistive load. It CAN be from a too-high motor load also, one beyond the capacity of the RPC.

A CNC machine has servos, inverters, and a power supply for it/them. They present a heavy capacitive load, with big currents to recharge the filter capacitors. That can easily overload the RPC, despite the fact that it nominally is of ample size. If that's the issue, balance caps will not help, they are swamped by the load.

So, put on an added idler, as big as you can get. See how that affects the voltages.

As I mentioned, you may as well TRY balance caps first, they are cheap, and it won't hurt. You may get lucky and we may be wrong. If no joy, put added idler(s) on and re-check.

Are the meters, particularly the current meters, true RMS. Average reading meters can be very misleading.

Tom

That's absolutely true. The only saving fact here is that they are always the same, and all 3 the same. So whatever goofiness they have is shared.

Since two show stability, the assumption is that the readings are not overly affected.

There is still a question about what is different with that last meter.

Maybe it has an smps as a load, and the EMI is fouling up the reading. Quite possible. May be a case where a reading with a plain old Simpson or equivalent would be a good idea.
Maybe the whole problem is a bad reading....did we get told of any actual operational problem? that's an interesting thought.

Let me see if I can answer some questions.

The compressor rarely kicks on and I have yet to see it affect the voltages on the three phase side of things. I can turn it off easily. I do remember it kicking on during one of my runs in aluminum before the coolant arrived (had to use the air nozzle during the milling) and you can see the lights dim for a second when it kicks on, but nothing changes load wise. The compressor is sitting on the main 200A panel in the garage while the shop has all 100A to itself.

The idler starts up extremely quickly without issue. I can't get the voltage and uf ratings from the cans due to the way they were hot glued together and into the enclosure (pretty inconvenient if you ask me).

I think I have an old 5HP? idler up in my attic that a friend left here, how would I go about wiring that in to my current setup to see if it helps at all?

The FLA of the VMC20 is rated at 25A on three phase. The load on the machine is only showing 30%, and the amperage on the meters make since being around 7-8A load.

I am going to get a hold of a true RMS meter and see whats up and if that helps at all.

I went back and looked at your unit. I do see the hot glue. It would be a shame if you ever had to replace a cap. I think it's difficult to help somebody to tune something unless you know where you're starting from. You can add a second idler 5 hp to your system, but to make it ideal you need to add run caps there as well. 100 µF black to blue and 50 µF from red to blue. Without doing this it uses up some of the main load. I have two 5 hp RPC run in parallel. The first one has start caps and run caps and the second one starts off the first one but just has run caps. Both idlers are properly balanced at 66% load. If you understand fully how a true RMS meter works, you will be out of the gate. I'll post a link, read it, fully understand it, and you'll find a cure for perfection.

Something else you might want to check and document (document everything)
1/ Single phase voltage/amp draw prior to RPC. At any and all loads
2/ Amp draw of all three legs going to/form Idler/s. At all loads
3/ Real voltage readings at each machine with amp read. At all loads
4/ At higher loads, real voltage drops + amp reads at all locations.

Good luck, I wish you the best, And work safe.
http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

See if you can get a plain OLD Simpson meter, and read the voltage with that also. What we are interested in is the amount of drop as load increases. Just to make sure that the drop is real. It's a huge drop, especially with the other phases not dropping. Makes a body suspicious.