High Current Draw from RPC

I have an American Rotary AD20 that is powering a single cnc mill. When the mill is turned on but idling the current reading in each of the 3 supply wires is reading about 40 amps. I’m measuring this with a clamp style meter. The below voltages are taken with the machine on but not cutting or moving any axes.

L1-L2: 224v (machine off: 224v)
L2-L3: 210v (machine off: 244v)
L1-L3: 240v (machine off: 244v)
L3 in this case is the wild leg.

Is it normal to see a 40amp draw with my machine turned on but not under heavy load? Am I measuring the amp draw correctly?

Side question- The cnc is a fanuc robodrill. Does anyone successfully run a Robodrill off an RPC? The fanuc reps say it normally doesn’t work or work well since the mill will dump current if it sees a high voltage. Funny thing is I don’t get any alarms, just that concerning high amp draw.

What's the horsepower on the idler and on the machine? Which one are you reading 40 amps on? Most induction motors tend to draw near FLA when running unloaded until you start getting into really big stuff. It's mostly reactive power - the motor behaves like an inductor when unloaded, storing and releasing energy from and to the grid with each AC cycle. Your power company will silently hate you for it because it's not actually doing any useful work and isn't billable on a residential service - merely heating up their lines and equipment and dropping the local power factor without doing any good.

EDIT:

For a 20HP, 240V three phase motor I read a table FLA from NEC 430.250 of 54A. 40A should be well within the motor's nameplate FLA. Perfectly normal value for an unloaded motor that size. If you're bothered by it you can slap a few power factor capacitors in parallel with the idler motor.

Just a Sparky said:

What's the horsepower on the idler and on the machine? Which one are you reading 40 amps on? Most induction motors tend to draw near FLA when running unloaded until you start getting into really big stuff. It's mostly reactive power - the motor behaves like an inductor when unloaded, storing and releasing energy from and to the grid with each AC cycle. Your power company will silently hate you for it because it's not actually doing any useful work and isn't billable on a residential service - merely heating up their lines and equipment and dropping the local power factor without doing any good.

EDIT:

For a 20HP, 240V three phase motor I read a table FLA from NEC 430.250 of 54A. 40A should be well within the motor's nameplate FLA. Perfectly normal value for an unloaded motor that size. If you're bothered by it you can slap a few power factor capacitors in parallel with the idler motor.

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Interesting, thanks for the reply. When the cnc is turned off and the rotary phase converter is turned on the amp reading is zero. Is it normal for it to jump up from zero to 40a just from the cnc machine turning on without any motion? Is that current actually being pulled into the cnc and dumping somewhere? Or are you saying it’s flowing back to the city power?

Another interesting thing is that I’ve moved the 3 input wires around between the machine terminals and in some configurations I’ll read 20 amps with the cnc on but not moving.

The current reading before the cnc machine is turned on is about 6 amps. This is with the phase converter running.

Thanks!

If the draw before the CNC is turned on is low, then THAT is what the net draw for the RPC alone is. I would assume the RPC includes some power factor capacitors to get it that low.

Most smaller motors will draw around 40% of FLA when idling. I'd be a little concerned if I saw a motor pulling FLA at idle, and would want to know why.

So, the CNC is definitely pulling the remainder of the 40A, one way or another.

The questions include:

What is the CNC rated to pull? If there is both a full load and an idle rating, give both.

Where are you measuring? The RPC has one wire that it powers, and two that are "passed through" from the power line. If you measure the "pass thru" line that goes to the actual idler motor (but not onto the CNC) , you will read a different current than if you measure one of the incoming power lines so that you get both the current to the RPC and the current to the CNC.

What size motor is actually in the RPC? AD20 at 20 HP rating likely has a 30 HP motor.

Hmm. wonder what's happening here. 40 X 240 X 1.7 = 16 KW.

Probably something in the machine is getting *really* hot there. Is it water-cooled?

JST said:

Most smaller motors will draw around 40% of FLA when idling. I'd be a little concerned if I saw a motor pulling FLA at idle, and would want to know why.

Click to expand...

From 14 smaller motors I got an average of 86% FLA at no load. Several were above FLA. Most of it was reactive power.

Pardon my chicken scratch.

JST said:

What is the CNC rated to pull? If there is both a full load and an idle rating, give both.

Where are you measuring? The RPC has one wire that it powers, and two that are "passed through" from the power line. If you measure the "pass thru" line that goes to the actual idler motor (but not onto the CNC) , you will read a different current than if you measure one of the incoming power lines so that you get both the current to the RPC and the current to the CNC.

What size motor is actually in the RPC? AD20 at 20 HP rating likely has a 30 HP motor.

Click to expand...

The CNC is rated at a max load of 10kVa and my main supply voltage is 222 volts. The mill's 3 axes motors are 5.5amps max and the spindle motor is 17amp continuous or 25amp for 15mins.

I'm measuring each line individially. The 40amp draw I'm reporting is roughly what most of the 3 measurements were... look lower in my response here for the exact measurements on all three lines.

The idler motor has a 20hp badge on it.

jim rozen said:

Hmm. wonder what's happening here. 40 X 240 X 1.7 = 16 KW.

Probably something in the machine is getting *really* hot there. Is it water-cooled?

Click to expand...

I ran the cnc machine at idle for 5mins with the 40 amp draw. I checked the temp by hand of all 4 motors and nothing is getting hot. I also held the power cables for the 4 drives in my hand and none were hot.

New stuff I learned/measured,
-I moved the input wires around on the terminals and have new voltages across legs and the high amp draw is still present
T1-T2: 222v
T1-T3: 205v
T2-T3: 239v

-The high amp draw is seen on the supply lines to the cnc’s built in disconnect, on the wires on the other side of the disconnect and in the wires going into the controller box. The controller box does not get hot.
-The power lines from the controller to the drives are also not hot and measure zero amps with the machine at idle.
-The amp readings in the 3 supply lines vary line to line. I’ve been reporting the 40amps as a high average. The actual readings are,
T1: 51a
T2: 42a
T3: 34a (this is the wild leg)

-Is it strange that these are all different? Does it possibly indicate anything off/broken in the phase converter panel or the idler itself?

-The main supply wires are getting warm, but just barely noticeable. They are 8awg.

-The ground line shows no current

-When the machine e-stop is engaged the draw on every line drops to less than 1amp.

-The Fanuc tech I talked to said that some of these generation machines (this one is from 2007) would "dump" voltage if it sees something out of spec. I wonder if the initial high voltage of 239v is putting the machine into a mode that starts to "dump" voltage. I'm not familiar with this... is there a device that can even do this without current being picked up on the ground wire?

Any other ideas out there? The responses and help so far is much appreciated!

Just a Sparky said:

... Most of it was reactive power.
....

Click to expand...

What's sort of magic is that?? His meter reads 40 amps, that's got to be right yes? 16 killowatts is going someplace!

Remove the run caps from your rpc and the no load volts should drop to 240/216/210 volts. Then measure current again as you have been.

jim rozen said:

What's sort of magic is that?? His meter reads 40 amps, that's got to be right yes? 16 killowatts is going someplace!

Click to expand...

Kilovolt-amps. It would be rather obvious if it were 16 kilowatts. If it were dissipating that much power as heat you'd be able to feel it from the other side of the room unless it were being dumped into a big thermal mass like a motor or transformer. That's the same amount of heat a small central forced-air furnace puts out.

My understanding was that the RPC was consuming 40 amps at idle - but your latest post suggests otherwise. A 10kVA machine at 240V three phase equates to a nameplate full-load current of 24 amps. With the machine idle and the motors off it should not be drawing 40 amps.

You've got 40 amps going into a control panel and nothing coming out. Open the control panel. Figure out what particular component inside is passing that current. Check for a capacitor bank or a mis-applied filter choke on the line side of the drive. Chances are you don't have a 16kW space heater inside of there (if you did, it probably wouldn't have survived for 5 minutes) so I can only imagine there's some kind of reactor to blame. A motor, a capacitor bank, a choke, something funky with the drive itself...

Unless there's been another breakdown in communication.

I've got a number of small motors, but they are all older ones. I have never seen any of them pull anything like 80% of FLA. It's around 40%. Maybe 50%.

If these are newer motors, "value engineered" with welded stators, etc, then they may be made intentionally with minimum winding and iron, and may have any of several reasons they run that high in current.

I have not seen it.

And, I very much doubt if what is done in small appliance motors is related to what is done with a 20 HP motor.

About the first time I have seen a 20 HP idler rated at 20 HP load..... more usual would be 30 HP.... perhaps they use a tag indicating the Load HP.

The NEC requires a tag on the RPC indicating current draw and output load, etc. Otherwise you have no idea what to rate the overload protection at, nor the wire size, etc.

No telling if that RPC has a UL tag on it, so it may just be labeled any way they thought was good. Most of the small outfits do not get any "agency recognition" for their products, so technically they cannot be used "in a workplace". My Arco does have an agency tag.

Just a Sparky said:

Kilovolt-amps. It would be rather obvious if it were 16 kilowatts. ...

Click to expand...

But his meter *says* it's 40 AMPS, and that meter cannot be wrong, can it? The amp-clamp meter is just saying what is happening in the circuit.

Maybe, just possibly, the amp-clamp meter is somehow *misleading* him? Giving a reading that is *confusing* or *misleading* the operator? I wonder if this has ever happened before anywhere?

Jim's on it again, blaming a meter for a person perhaps not understanding electric power concepts.....

We better ban those ammeters.... and the voltmeters too. What if someone measures to GROUND instead of line-to-line? Perhaps that would *mislead* someone? They might be *confused* by the reading.

If you are going to use a measuring device, a mic, a meter, a thermometer, whatever, you really need to know how to understand the reading you get. Otherwise it is just a number that could mean anything in the wide world.

And that would be YOUR fault, not the meter's. Remember that "personal responsibility" thing?

Per the OP, the RPC does not pull 40A. The 40A comes when he turns on the CNC. As for where it goes, I do not know yet. But since he says it drops to 1A when he hits e-stop, I assume it is going to run some internal part, which is not actually drawing 40A.

There may be something about that 40A. Since the CNC has large DC supply for the servo drives, it will likely be pulling current in short pulses near the peak of the waveform.

That 40A may be due to the meter not reading a pulse current correctly. That can happen if the meter does not read true rms, or if the meter IS true rms, but has not got sufficient "crest factor" (ability to deal with peaks).

When dealing with rectified AC, it is often best to use a 'scope, because you are no longer dealing with a sine wave. A clamp-on probe with a 'scope could tell a lot of dirty secrets in THIS particular case.

What sort of meter is in use? Is it true RMS?

JST said:

...
What sort of meter is in use? ..

Click to expand...

He said: it's an amp-clamp.

Let's cut to the chase here, for his sake: the single most likely cause for the high reading is - your meter is lying to you. As with most inexpensive clamp on meters, yours reads the sum of real and imaginary currents. AKA in-phase, and out-of-phase currents. AKA low power factor currents and high power factor currents.

There are probably capacitors in the supplies in your machine that draw large imaginary currents (the votages and currents are nearly entirely out of phase so there is no sixteen KW dissipation!

1) absent any other obvious problems, disregard the erronious reading on your existing current.

2) if you want, buy a current meter that can determine the phase angle of waveforms.

3) a multi-input oscilloscope with a clamp-on probe so you can simultaneously display currents and voltages, with the correct phase information is most informative as mentioned.

4) unless your existing clamp on meter is set to "peak hold" (some meters have this) then a pulsed wave form will probably integrate correctly on an analog meter.
By all means, say if this feature was there and enabled, as it would probably explain the readings you saw.

Simple clamp-on meters often give confusing (wrong, to a person unfamiliar with reactive currents) readings, and are an endless source of discussion on this sub-forum.

Alright, new update for you all. Again, thanks for all of your interest in troubleshooting.

The meter I am using is a Klein Tools CL390 from Home Depot. It has the words "true RMS" on it. I am not in a peak or max measurement mode, as I am aware of that function and have not been using it for these measurements. Although, I wouldn't put it pass me to miss something like that so thanks for asking.

I did a pretty long test where the machine was turned on, the high current draw was measured and then I proceeded to monitor temperature and take current readings on other wires and components. The high current comes into the machine, goes through a manual disconnect, through a reactor and then to the power drive unit of the controls. The high current is not noted on any wires exiting the power drive unit. The power drive unit itself is not warm itself nor is the cooling air coming out the exhaust fan. I worked backwards from there and checked the reactor for heat. It increased from 40F to 65F and then maintain the 65F temp for as long as the test ran (about 15mins). This seems normal. There is also no unexpected continuity from leg to leg or leg to ground on the reactor.

During this test I did notice that the power drive unit had an LED lit up near the power input cables that I know now means "voltage too high".

I've also learned that the RPC does not generate the third leg at a voltage relative to my incoming single phase voltage. Meaning, my house supply voltage of 245v bucked to 224 via a transformer has no impact on the voltage of the generated leg output of the RPC. That third leg is generated by a 240v motor, so it starts there and varies based on no load or load situations. This is why L1-L1 is 224v and the other leg measurements are very high (240 to 250v). The solution is to feed the power straight from my house to the RPC without the single phase transformer. Then, run a 3 phase buck transformer after the RPC (between the RPC and the cnc machine). This will produce output voltages from the RPC of 240-250 on all three legs. Then the 3 phase transformer will buck it down to 208v (the sweet spot in my 200-220v range that is accepted by my cnc machine). From that setpoint of 208, I can expect the 5% spec of the American Rotary converter to hold true.

So, at this point I still don't know why I'm getting that 40amp draw near the machine power drive unit but I can only assume its caused by large delta of voltage between the incoming power or by too-high of voltage itself. If there are any theories on this please let me know! In case this helps any theories - the motors on the axes are all 150-200v three phase AC servo motors.

My solution at this point to do the adjustment I described above and get lower, more balanced voltage into the cnc machine and hope that corrects the strange high amp draw. The lead time for the new transformer is TBD, but I'm guessing I can report back in about 2 weeks.

At low load, but still SOME load, a rectifier takes current in narrow pulses. A "true rms" meter can give you a high reading because pulses have a disproportionately high rms value.

There may be other things going on as well, but a lowish source impedance, and a high current rectifier can really give you a high but very short pulse current, and a higher than expected "rms" current. Added to the normal draw from things in the control, it might look crazy-high.

In THIS CASE, if you had an "amp clamp" 'scope probe and a 'scope, you would probably see that.

Since the unit is evidently not drawing excess current, since it is not getting hot, and no breakers or thermals are opening, no faults showing, I would not be concerned at this point. Run a part, and see what happens.

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BTW, "amp clamp" is not a meter type, and there is nothing about an "amp clamp" that is a problem. The same type (true rms) series ammeter would give the same answer as the clamp-on.

It is, however, necessary to know what you are measuring, and how it is being measured, plus to have some idea what you ought to expect, to know what to think about the "number" you get as a result.

In this case, an old clunky "Amprobe" averaging-type meter might well give a better answer, even though it is still an "amp clamp meter" (see....? There are many types). The "average" of that sort of pulses is going to be much lower than the "rms" value.

I think "true rms" meters should be able to read "average" by changing the setting..... very few if any do that, though, the "true rms" is generally regarded as the "better" reading.... That is not always true, and the comparison between the two is often very telling.

Fool-proof way to tell is with a trio of 24/25A dual-element fuses. Fuses always open for a reason.

That may cost as much as a decent meter..... depending on the fuses.