Author Topic: 3-phase motor drawing high current  (Read 15771 times)

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Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #25 on: August 15, 2016, 08:17:43 am »
Also, the phases are probably not hooked up wrong, as the pump is obviously functioning. If the rotation were reversed it wouldn't be working. The failing contactor is a good lead also - that can definitely cause a high load as well. Check all three legs for a voltage drop across the contactor to determine if it's OK.
« Last Edit: August 16, 2016, 01:25:33 am by eKretz »
 

Offline Halvmand

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Re: 3-phase motor drawing high current
« Reply #26 on: August 15, 2016, 09:51:25 am »
All good suggestions so far.

After you check the electricals I suggest you check the whole hydraulic system for obstructions? A filter may be plugged. And that would have a huge impact on the load of the motor.
If there is a filter in the pressure line, definitely check that.
Check for debris, sludge or foreign objects in the sump of the hydraulic tank.
Finally, if possible, separate the motor and pump and disconnect the output and se if it turns by hand easily. This is a little hit and miss since you don't know how much resistance the pump ideally should have.

 

Offline Seekonk

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Re: 3-phase motor drawing high current
« Reply #27 on: August 15, 2016, 11:42:43 am »
I don't think there is actually anything wrong.  This machine was likely set up for 50Hz.  At 60Hz it is pumping 6/5 more fluid even if the pressures remain the same.  It has likely been drawing more than its rated current for years, but still under the rating of the protection relay.  Now that your voltage is lower it is drawing more current.  Raise the voltage with some buck transformers and you will be fine.  Dropping the pressure a little after raising the voltage might also bring the currents more in line.  Consider yourself lucky that it has lasted this long without a problem.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #28 on: August 16, 2016, 04:50:34 pm »
It's doubtful to me that it's a winding issue. I would get that boost XFMR on first things first, then do the diagnostics the other guys recommended to check for extra load on the pump. You really shouldn't be running the motor under nameplate voltage if you want to keep it as healthy as possible for as long as possible. The motor won't draw rated amperage with no load BTW. Rated amperage is with a specified load - usually at rated torque.

So testing unloaded motor tells me... what? Nothing useful?

I can test it unloaded or loaded with the current pump load. One reading gives me much less than rated amps, the other much more than rated amps. What useful info is this in determining cause of overload?  :-//

Of course adding trannys are next the step.
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #29 on: August 16, 2016, 05:41:59 pm »
motors designed for 208 mains are usually dual voltage 200/400v motors.

either get three 32v transformers to boost your volts back up to 240, or replace the motor with a 1.5-2 hp 240v motor.

note that all the other motors in your shop are probably 240v nominal (230v nameplate) and draw 15% more current (producing 32% more heat) under nameplate "rated load" when run at 208 volts.
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #30 on: August 16, 2016, 08:44:07 pm »
Yes testing unloaded is not really useful IMO. Test for voltage drop across the contractor on all three legs first, if that's good, get the boost XFMR on there and see if that does the trick.
 

Offline Seekonk

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Re: 3-phase motor drawing high current
« Reply #31 on: August 16, 2016, 11:09:49 pm »
It can be done with just two transformers (and a lot safer) when trying to get the phase right.  Just did a 220 to 440 on 3/4HP motor.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #32 on: August 17, 2016, 04:46:52 am »
It can be done with just two transformers (and a lot safer) when trying to get the phase right.  Just did a 220 to 440 on 3/4HP motor.

I'm working with 120/208-Y mains. I thought on Y you must have 3 trannys. Only delta can use 2?  :-//
 

Offline Circlotron

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Re: 3-phase motor drawing high current
« Reply #33 on: August 17, 2016, 05:11:55 am »
Low voltage on 3-phase will definitely cause increased amperage draw, especially under heavier loads. For the same task (i.e. motor torque/horsepower draw) and lower volts, higher amperage is drawn. The same amount of work in watts is being done, so the energy has to come from somewhere.
Or more precisely, at full load if the voltage goes down the motor speed decreases and so the frequency induced into the rotor increases and this makes the rotor reactance (same inductance and higher frequency) exceed the rotor resistance. Therefore the power factor of the induced rotor current goes very low and as a consequence the stator winding power factor also goes very low. This means it has to pull lots more amps to maintain more or less the same input power and the same mechanical output wattage.
 

Offline qno

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Re: 3-phase motor drawing high current
« Reply #34 on: August 17, 2016, 08:01:19 pm »
Have you recently changed the oil?

You may use the wrong oil.
Why spend money I don't have on things I don't need to impress people I don't like?
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #35 on: August 18, 2016, 02:06:41 am »
It can be done with just two transformers (and a lot safer) when trying to get the phase right.  Just did a 220 to 440 on 3/4HP motor.

I'm working with 120/208-Y mains. I thought on Y you must have 3 trannys. Only delta can use 2?  :-//

you can use two, but you will need a transformer that can accept 208 in and then add 32 volts (or 24, and that's good enough) to it. the connections for this may be rather confusing.

or you can use three transformers that will take 120v in and add 18 volts. (or 12 volts, and call it good enough)


the two transformer method leaves the third leg a little weak. this isn't a problem for smaller, less efficient motors but for large ones it is a problem.
a 5% voltage imbalance requires a 25% derating (meaning you can only load the motor to 75% of nameplate full load), otherwise you exceed "nameplate full load temperature rise"
« Last Edit: August 18, 2016, 02:11:14 am by johansen »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #36 on: August 18, 2016, 03:46:57 am »
you can use two, but you will need a transformer that can accept 208 in and then add 32 volts (or 24, and that's good enough) to it. the connections for this may be rather confusing.

Thanks for the thorough and informative reply. (This why I love BBV forum--nothing but the greatest input.)

No reason to not go with 3 transformers. Just curious to understand the "why".

Cheers.
« Last Edit: August 18, 2016, 05:47:13 am by iXod »
 

Offline k2teknik

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Re: 3-phase motor drawing high current
« Reply #37 on: August 18, 2016, 04:47:36 am »
What was the result when you measure resistance (between the 3 coils and ground) with the megger?
It can make a huge different from a normal resistance measurement when you use a megger.
The megger is used isolating measurement, many people had been in for a surprise when they discovered the different in measurement results between a normal resistance measurement and when you use a megger.
A megger is a must use instrument in a case like yours even if you think that it is unlikely that all 3 coils should have the same leakage to ground.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #38 on: August 18, 2016, 05:03:22 am »
What was the result when you measure resistance (between the 3 coils and ground) with the megger?

5.5M between isolated windings, about the same to ground, using the 500v test.

Quote
It can make a huge different from a normal resistance measurement when you use a megger.
The megger is used isolating measurement, many people had been in for a surprise when they discovered the different in measurement results between a normal resistance measurement and when you use a megger.

My favorite YT video re. insulation testing -- including the surprise, as you say. (c;  ::



Quote
A megger is a must use instrument in a case like yours even if you think that it is unlikely that all 3 coils should have the same leakage to ground.

Yes. Now that I own a Fluke 1587 I can see how useful it is to eliminate these unknowns from the troubleshooting equation.

Cheers.
« Last Edit: August 24, 2016, 06:20:09 am by iXod »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #39 on: August 23, 2016, 06:09:19 am »
Installed 3 boost transformers. Input approx 205, output approx 225. Current dropped to 4.05 amps, still 50 percent above rated.

Except for the tripping overload this machine works perfectly. The pump pressure is per spec. Oil viscosity is per spec. Filters not new but look good. I'm running out of ideas. I've emailed the motor manufacturer (Yaskawa Electric USA) and will see what they say.

Cheers.
 

Offline EPTech

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Re: 3-phase motor drawing high current
« Reply #40 on: August 23, 2016, 09:19:27 am »
Hi again,

there is one more thing I can think of. Maybe the oil seals between the pump and motor have started to fail and oil is getting in the motor. If that is the case and the cable passage between the stator and the terminal box of the motor is open, you will see oil in the terminal box. If the passage is potted, you will have to separate the motor from the pump. Yaskawa support is going to ask you the same question: to  have the motor run free and check currents.

5.5 Mohm is on the low side for an isolation test but not problematic. I have seen motors short out during operation but measure perfectly fine at stand-still. Best is to perform a megger test right after the overload condition to increase your chances that the fault is stil there.
Kind greetings,

Pascal.
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #41 on: August 23, 2016, 12:04:52 pm »
No voltage drop on the contactor? I missed it if you already checked. The only thing left after that is a high mechanical load. Perhaps the pump needs a rebuild. Is it a positive displacement pump?
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #42 on: August 24, 2016, 06:18:33 am »
Vdrop across contactor + overload is 0.2v on each phase.

I changed the hydraulic oil and replaced with factory-recommended type, and cleaned the pickup screen. No joy.

I'm coming to the conclusion that it's mechanical overload that is causing the problem. If I adjust the pump's pressure regulator down to near zero the current draw drops to about 2A. In my mind this means that there is no mechanical binding happening, just hydraulic load. Speaking to a pump rebuilder he said that this type of pump (trochoid or gerotor*) can lose its seal from input to output and "re-circulate" fluid which increases the load on the motor.

So that's the theory I'm working with. Presently.  ;)

* https://en.wikipedia.org/wiki/Gerotor
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #43 on: August 24, 2016, 06:28:20 am »
there was recently a thread on the practical machinist forum about those types of pumps. dammit, i might be thinking of arduino vs evil. or both, i can't recall.

not something you can just throw together, or repair. more like trade secret voodoo involving edm, polishing/lapping , running the pump for a certain number of hours, perhaps with very, very fine grit in the oil before you ship it.

anyhow, i do apologize for incorrectly expecting a lot more than the 5% current reduction you achieved by boosting the voltage 10%, but, were your measurements true rms ?  :popcorn:   |O

anyhow, as i mentioned earlier, shorted turns are almost always catastrophic failures. you can't just dump a half kilowatt of heat into a single shorted turn, to account for the half hp of missing shaft torque, and not expect the motor to warm up.
« Last Edit: August 24, 2016, 06:36:46 am by johansen »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #44 on: August 24, 2016, 06:31:26 am »
there is one more thing I can think of. Maybe the oil seals between the pump and motor have started to fail and oil is getting in the motor. If that is the case and the cable passage between the stator and the terminal box of the motor is open, you will see oil in the terminal box. If the passage is potted, you will have to separate the motor from the pump. Yaskawa support is going to ask you the same question: to  have the motor run free and check currents.

The motor is open type (not "potted" inside; can see the windings) so I rule out this possibility. Oil would be escaping in every crack of the motor housing if this were so. But a very creative deduction in any case. Thanks.

Yaskawa Electric motor tech support was not helpful. No new suggestions.

Quote
5.5 Mohm is on the low side for an isolation test but not problematic. I have seen motors short out during operation but measure perfectly fine at stand-still. Best is to perform a megger test right after the overload condition to increase your chances that the fault is stil there.

The "fault" isn't momentary or intermittent; it is constant high current. The overload takes time (it's thermal type) to go open-circuit. So I don't see any particular point in the cycle (power-on to OL trip) that would be advantageous to make insulation resistance measurement.

Cheers.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #45 on: August 24, 2016, 06:36:54 am »
there was recently a thread on the practical machinist forum about those types of pumps. not something you can just throw together, or repair. more like trade secret voodoo involving edm, polishing/lapping , running the pump for a certain number of hours, perhaps with very, very fine grit in the oil before you ship it.

Yeah, these are very close-tolerance devices. I'll look for a new pump first. Thanks.

Quote
anyhow, i do apologize for incorrectly expecting a lot more than the 5% current reduction you achieved by boosting the voltage 10%, but, were your measurements true rms ?  :popcorn:   |O

No Apologies! Theorizing is part of troubleshooting. All theories (and guesses) welcome. Thanks!
« Last Edit: August 24, 2016, 06:29:18 pm by iXod »
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #46 on: August 24, 2016, 06:41:33 am »
there was recently a thread on the practical machinist forum about those types of pumps. not something you can just throw together, or repair. more like trade secret voodoo involving edm, polishing/lapping , running the pump for a certain number of hours, perhaps with very, very fine grit in the oil before you ship it.

Yeah, these are very close-tolerance devices. I'll look for a new pump first. Thanks.

Quote
anyhow, i do apologize for incorrectly expecting a lot more than the 5% current reduction you achieved by boosting the voltage 10%, but, were your measurements true rms ?  :popcorn:   |O

No Apologies! Theorizing is part of troubleshooting. All theories (and guesses) welcome. Thanks!


well, at a minimum you need to disconnect the pump and measure the no load idle current of the motor. it should be around one third of the nameplate full load amps.


but i was a bit surprised that the amps didn't decrease as much as i expected. your motor may be more overloaded than i expected as well. who knows.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #47 on: August 24, 2016, 07:10:47 am »
...as i mentioned earlier, shorted turns are almost always catastrophic failures. you can't just dump a half kilowatt of heat into a single shorted turn, to account for the half hp of missing shaft torque, and not expect the motor to warm up.

Windings can short one turn at a time, decreasing resistance and increasing current fractionally. Usually from chemical or thermal breakdown of the insulation. That's what insulation resistance tests show. This is often a non-zero ohm result. Good motor maintenance uses these tests over time and can graph (and predict) the change in the breakdown of the insulation.

A winding doesn't usually melt into a 0-ohm resistor.

Cheers.
« Last Edit: August 24, 2016, 07:17:49 am by iXod »
 

Offline Jeroen3

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Re: 3-phase motor drawing high current
« Reply #48 on: August 24, 2016, 08:57:04 am »
What was the result when you measure resistance (between the 3 coils and ground) with the megger?

5.5M between isolated windings, about the same to ground, using the 500v test.
That's like only slightly above fail value... But also way below pass value (25M). Did you order a new one yet?
 

Offline k2teknik

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Re: 3-phase motor drawing high current
« Reply #49 on: August 24, 2016, 10:00:35 am »
Filters not new but look good. I'm running out of ideas.
Change that filter, it may be blocked and you may not be able to see that visually.
 


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