Another electrical head-scratcher
At this point, the office is going to make the supplier provide a new power head, but what could possibly be wrong with this one??? Everything I've checked seems to be ok, & whatever I've compared to the working one agrees pretty well. I'm at a loss as to what could be wrong, and I really don't like not knowing what's going on.
Comments
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Is it "easy" to switch the motors to see if the issue follows?0
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What are the motor specs? What size wire?
Too much voltage drop will spike the amps."If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 -
IIRC ≈1½ HP, confused by the fact that it's metric, i.e. 1.1-1.9 kW tri-rated 50/60/85 Hz, & the winding taps change with voltage & frequency. Fed with #12 THHN, total resistance of the feeders, connections, & windings is 3.3 Ω AB, BC, CA. But it's tapped the same as the one that works & the winding resistance on the one that works is 3.4 Ω across each winding.
The only odd thing is that I couldn't get a stable resistance measurement with my milliohmmeter, but I though that might have more to do with trying to juggle Kelvin connections whilst holding the meter and not stripping the motor leads longer to allow both alligator clips more space. It uses 10+ mA to make it's measurements, maybe even 100 mA, so a lot more than the Fluke clamp meter; but I would have though that a weak spot in the insulation would have been obvious to the megger. And during operation the motor current is ±0.1A on all three legs.
WTH.
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A very long shot -- indeed, a WAG -- but I wonder if the longer feed is causing some kind of capacitance effects... which upset the VFD.Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
That was my initial thought too—I ended up installing a load reactor in an attempt to correct that. Made a few tenths of an amp difference, nowhere near enough. It's nearly certainly a motor issue, but for the life of me I can't think of a fault that would only present itself during operation. Shorted windings on all coils, similar enough to maintain ±0.05 Ω resistance? I don't think so!
Do you think an LCR meter would give me any insight into the state of the windings? I'm leading towards some kind of mfgr'ing booboo.
On a side note, any familiarity with Kelvin connections? The four leads on my milliohmmeter are too short for useful field work. I'd like to pick up a longer set, but it seems that the most common configuration is one wire in either jaw of the alligator clip, which seems to me isn't a proper Kelvin connection. But I have no clue if any subsequent error would be measurable, let alone meaningful.
[edited for spelling]0 -
@ratio,
The mfgr said X is the max length. You have one under that length that works, and one over that doesn't.
Resistance isn't the only issue. I'm pretty sure capacitance and inductance can also be problems.Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
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Is it in EMT or some other ferrous raceway? If so, that is causing reactance most likely. Can you temporarily wire one motor to the other drive? Preferably directly from the drive to the motor with romex or similar for testing purposes?
I'd try running the "bad" motor with the good drive and its wire run intact.Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
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I no longer have one over the max length, the drive has been relocated to within the mfgr's requirements. It did not solve the problem. FWIW, the drive itself (Mitsubishi D700 series) has a max feeder length of 350' before difficulties arise.
The feeders are 3 #12 THHN w/ #12 ground in a dedicated ¾" EMT. Before I relocated the control panel, I swapped motors from one drive to the other with jumpers—the fault followed the motor & the good motor operated fine with the other drive. The mfgr still had me replace the drive itself, that was, of course, ineffectual.
The feeders are IIRC ±90' unbroken #12's in a dedicated ¾" conduit, with c. 5' of #12/4 SOOW feeding the motor itself. I haven't checked impedance on it yet, only resistance, but there isn't anything there that should cause a problem.
My big fear is that the motor is tapped wrong and that is all that is causing the problem, but I checked it numerous times, against the other (correctly operating motor), the motor nameplate, the motor data sheet, and under the direction of tech support. I can't get back in to the job to recheck until the panic is over.
One test I haven't done is to tie the motor in direct & skip the drive, but this is a 16' fan and I'm a little reluctant to put that kind of stress on the system.
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I would suspect maybe the motor leads are marked wrong. I would swap the motor since you have already swapped drives and the length of run has been shortened. What voltage are you running?0
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The office ordered another power head for it. It's a large metric gearmotor, so 200/400 volts 50/60/87(!)Hz Y/Δ wiring. It's fed from a 208V drive. The part numbers are right, the factory checked them during a few of my calls. Curiously, the nameplate says to wire it Y at 400V 50Hz, Δ at 230V 50 Hz, Y at 460V 60 Hz, and Δ at 400V 87 Hz. Note: this is right per the factory and as compared to the working motor.
I guess mis-marked taps could be it, but I would have thought that I'd see that with my ohmmeter.
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Re-read his post, @ChrisJ . He said he changed everything to meet the manufacturer's suggestions and relocated the drive.ChrisJ said:@ratio,
The mfgr said X is the max length. You have one under that length that works, and one over that doesn't.
Resistance isn't the only issue. I'm pretty sure capacitance and inductance can also be problems.
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Have you tried moving the phases on the contractor?
Say just move all to the right one pole and the extreme one on the right put on the first one the left.
I have had some wacky situations on 3 phase where this fixed things.0 -
I hadn't thought of that. I'll give it a try next chance I get. Of course, that probably won't be until the new power head is here …0
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Is it a EuroDrive motor? This is a pic I took of one for reference if the cover or sticker is illegible.Serving Northern Maine HVAC & Controls. I burn wood, it smells good!0
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That is confusing that Lenze doesnt show 230volt at 60hz. And that at 87hz it shows 400Volts with a delta connection.
Does this motor have marked leads or the posts with shorting bars like the one I showed? I have worked on many Lenze servo motors but not induction motors like you have.Serving Northern Maine HVAC & Controls. I burn wood, it smells good!0 -
746 watts/hp means your 1.1kw motor is 1.5hp. My NEMA chart shows a 1.5 ho motor at 208volts to be 6.6full load amps.Serving Northern Maine HVAC & Controls. I burn wood, it smells good!0
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Yes, why doesn't it show 208 or 230 at 60HZ?0
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Posts with shorting bars. So I'm back to not doubting that the motor is tapped right.
IDK why the table is so odd, I guess it's just left-handed metric.
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I'd try it across the line and see what it draws. I know you said it's a big gear reduction fan, so I dont know if that is possible. Could be a mistake at the factory as rare as that seems and I hate to say it. Can you wire the other drive to this motor through your existing wireing and see if it overamps?Serving Northern Maine HVAC & Controls. I burn wood, it smells good!0
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Originally, the drives were side by side & I crisscrossed them, the working drive & bad motor faulted, the bad drive & working fan ran as expected.
I don't think I'm going to start the motor across the line, at least not without orders from tech support. It's just too dangerous-looking.
My possibilities are:
- operator (me) error: This is my biggest fear (& most likely), but gosh darn it I checked everything!
- factory/internal motor fault: Not very likely, but the most satisfying answer.
- feeder fault: I can't see how this could be, but I'm including it since I haven't proven it to not be possible.
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