Author Topic: 3-phase motor drawing high current  (Read 15770 times)

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Offline iXod

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3-phase motor drawing high current
« on: August 12, 2016, 04:12:24 am »
0.75kw motor drives the hydraulic pump in a horizontal band saw. Label FLA (nominal 220v 60hz) is 2.9 A. Measured is 4.17. Motor overload trips after 30 seconds or so.

When the motor is running the saw operates normally... for 30 seconds.

What causes a motor to draw more current? Pressures are unchanged so I am thinking that the pump isn't loading the motor more than usual.

Bearings? No unusual noises are heard and the motor doesn't seem unusually warm.

The mains voltage is 205, not yet 10 percent off the rated 220, so that wouldn't cause this amount of current draw, would it?

I have read that an unbalanced phase (or 2?) of more than 5 percent can cause higher current draw. Is this true? I think the phases are 205, 205, 210.

Being a 35-year-old machine I'm not confident that a new motor/pump can be had, and if so, would probably make the repair not economical, so re-winding/re-bearing is the only option I think.

The electric utility changed the transformer supplying this building last weekend and I think this is when the trouble started. (There are no electronic components in this machine.) I don't know what the voltage was prior to the transformer change-out so I can't say if there is now a change in the mains voltage.

I think I've covered all the bases I have considered. Which are most likely and what haven't I thought of?   

Since I'm the one who will be making the call of whether or not to get the motor rebuilt, I'd like to be right.  ;D

Thanks.
« Last Edit: August 12, 2016, 04:26:37 am by iXod »
 

Offline noidea

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Re: 3-phase motor drawing high current
« Reply #1 on: August 12, 2016, 04:52:20 am »
From the photo the rated voltage on the motor is 230VAC not 220, at 220V it would draw more than the 2.9A rated and if you are down around 205 then even more.
If the utility has just replaced the transformer and you are only getting 205-210VAC on each phase then you should get them to adjust the taps to at least give you your nominal supply voltage.
Have you checked the winding resistance between phases? what is the percent difference between them as I doubt the actual resistance specifications would be easily available. Also what is the insulation resistance to earth using a insulation resistance tester (megger)
When you say the saw runs for 30 seconds is that under load cutting or just free running? Is there any way to run the motor without any load?
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #2 on: August 12, 2016, 06:03:34 am »
Oops... 230.

Will measure winding resistances. Have no megger (meaning to get one...).

The pump provides hydraulic pressure for clamping & maneuvering work on the saw. The blade is run by second motor. While the cut is happening the pump is providing a static pressure which keeps the work clamped in the vise.

The only way I know to run the motor without load is A) disconnect the hoses; B) unbolt the pump from the motor. Both not desirable... )c; Will this give a meaningful measurement, with no load at all? It should drop to FLA value if the problem is the pump? Is that what I should presume?

The voltage measured at the LV outlets is 115-117. Wouldn't changing taps push this over the max? Because of this I'm not sure the utility would consider changing taps, but haven't asked them.

Thanks.
« Last Edit: August 12, 2016, 02:07:46 pm by iXod »
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #3 on: August 13, 2016, 02:20:48 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.
 

Online SeanB

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Re: 3-phase motor drawing high current
« Reply #4 on: August 13, 2016, 03:25:17 pm »
Stupid check but have you checked all 3 phases, and that the current is the same in all 3 of them. Have you checked there is voltage on all 3 phases, and all are the same. The 4A current and overload trip with pretty much all seeming to be the same pressure and operation wise indicates that you likely have lost a phase somewhere.

To test the motor under no load without too much mess, just run the pressure output from the pump back to the oil reservoir, which will drop the pressure as low as it can possibly be, with maximum flow and almost zero head.
 

Offline johansen

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Re: 3-phase motor drawing high current
« Reply #5 on: August 13, 2016, 04:21:15 pm »
My guess is your motor was overloaded to begin with but only slightly, and when the the voltage was dropped from 240vac to 208 vac, that's when the current climbed from about 3.5 amps to the 4.2 amps you measured..


however, if your service was 240vac before, then you would expect to see about a 15% increase in motor amps when dropping the volts to 208. this helps explain how a slightly overloaded motor didn't trip before, because the 10% increase in motor current when dropping the volts from 230 to 208, is not enough to trip the thermal breaker.. usually.


if the voltage was 208 before then something is wrong with the motor or you are simply overloading it.
shorted turns are almost always catastrophic failures.


if you lost a phase the motor should not start.
« Last Edit: August 13, 2016, 04:35:46 pm by johansen »
 

Offline Seekonk

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Re: 3-phase motor drawing high current
« Reply #6 on: August 13, 2016, 05:03:04 pm »
Is the motor really 60Hz?  The pump load would greatly increase with a switch from 50.  It just might handle that, but not with a drop in voltage.  Still, I'm thinking shorted turn in the winding.  A check of each phase current would confirm that.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #7 on: August 14, 2016, 05:32:09 am »
Stupid check but have you checked all 3 phases, and that the current is the same in all 3 of them. Have you checked there is voltage on all 3 phases, and all are the same. The 4A current and overload trip with pretty much all seeming to be the same pressure and operation wise indicates that you likely have lost a phase somewhere.

Currents equal (4.45A). Voltages: 205, 205, 210 (if I remember...)

A missing phase would mean no motor movement (it's rotating full speed) and nearly-immediate OL pop (the OL takes up to 30 seconds to pop).
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #8 on: August 14, 2016, 05:37:23 am »
Is the motor really 60Hz?  The pump load would greatly increase with a switch from 50.  It just might handle that, but not with a drop in voltage.  Still, I'm thinking shorted turn in the winding.  A check of each phase current would confirm that.

Currents equal (4.45A). Resistances equal (10.9 ohm -- sorry, no greater resolution available). Will check insulation resistance (megg) and report.

The motor nameplate says motor is rated at several voltages and both 50 and 60 hz.

OL started tripping months ago (saw is 30 years old), so something has changed... Motor and pump are original part numbers.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #9 on: August 14, 2016, 05:50:56 am »
My guess is your motor was overloaded to begin with but only slightly, and when the the voltage was dropped from 240vac to 208 vac, that's when the current climbed from about 3.5 amps to the 4.2 amps you measured..

however, if your service was 240vac before, then you would expect to see about a 15% increase in motor amps when dropping the volts to 208. this helps explain how a slightly overloaded motor didn't trip before, because the 10% increase in motor current when dropping the volts from 230 to 208, is not enough to trip the thermal breaker.. usually.

if the voltage was 208 before then something is wrong with the motor or you are simply overloading it.
shorted turns are almost always catastrophic failures.

Report of 240v mains was in error. Mains was and is now 208 (common in industrial buildings in USA).

So pump has always run on 208v. Pump OL has been tripping for almost a year (operators thought it was a safety time-out thing and just reset it and kept sawing). The recent utility's upgrade of transformer in the neighborhood seems to have nothing to do with this failure. ("Red herring".)

Note that the pump motor is rated not at 208 but 230v/60hz (2.9A). Recalculating for measured 205 gives 3.2A, not near the measured 4.45A.  :-//
 

Offline pbendel

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Re: 3-phase motor drawing high current
« Reply #10 on: August 14, 2016, 05:53:06 am »
Did you current test the ground wire?  Possible motor leakage?
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #11 on: August 14, 2016, 06:30:34 am »
Did you current test the ground wire?  Possible motor leakage?

No, I didn't. Good idea. I'll do that next (after megger).

Thanks!
 

Online SeanB

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Re: 3-phase motor drawing high current
« Reply #12 on: August 14, 2016, 12:43:36 pm »
Stupid check but have you checked all 3 phases, and that the current is the same in all 3 of them. Have you checked there is voltage on all 3 phases, and all are the same. The 4A current and overload trip with pretty much all seeming to be the same pressure and operation wise indicates that you likely have lost a phase somewhere.

Currents equal (4.45A). Voltages: 205, 205, 210 (if I remember...)

A missing phase would mean no motor movement (it's rotating full speed) and nearly-immediate OL pop (the OL takes up to 30 seconds to pop).


Not always, if there is a failing contactor, that drops the phase after it initially makes, because the inertia is enough to get the contact past the sticking point, then it will start, especially with a pump load that is low starting torque. Was fun dropping that 130kg motor down from it's mounting where the shaft had galled to the gearbox, but as a bonus the replacement was only 83 kg to winch up with the chain block. Then I put in all new contactors and a soft starter, in a new IP44 box.
 

Offline Curtis

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Re: 3-phase motor drawing high current
« Reply #13 on: August 14, 2016, 02:10:15 pm »
Check the voltage drop from the mains to the load. If the pump was recently moved to a new location wire size plays a huge issue.
 

Offline EPTech

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Re: 3-phase motor drawing high current
« Reply #14 on: August 14, 2016, 02:10:23 pm »
Hi there,

I have had a case like Sean describes a few times, where one phase was (partially) interrupted at the fuse, contactor and motor OL trip. In such a case, the easiest way to spot the problem is to measure phase voltages under load, from the motor terminals upward, towards the supply.

Almost all hydraulic pumps are constant flow rate pumps (positive displacement). If you pinch the output, ex. by operating a valve, the pump will try to maintain it's flow rate and as a result, pressure will increase. If the pump has to work harder, it will need more power and as a result current will increase. There are a few things you can check without having to decouple the pump  from the motor.
1. As suggested before, decouple the pressure side at the valve block and put the hose in the reservoir. This will effectively have the pump run at free load. Check motor current but also check whether the oil is clear of bubbles. If the oil is foamy, that meant the pump is cavitating, not good. If this is the case, check the filter in the reservoir. If the oil is milky white, there is water in the reservoir, very bad.
If the pump works normally and the oil is OK, the hydraulic system may be dirty or the valve does not open up correctly.
Reconnect the pressure hose to the valve block. If the machine is in the unclamped position, there should be almost a much oil coming out of the return hose, as coming from the disconnected pressure hose before.
2. Check whether the correct viscosity oil has been used. A lower viscosity means the pump has to work harder to push the oil through the system, especially at low temperatures.

Good luck.
Kind greetings,

Pascal.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #15 on: August 14, 2016, 05:16:06 pm »
Check the voltage drop from the mains to the load. If the pump was recently moved to a new location wire size plays a huge issue.

Not moved recently. Only recent external event is utility replacing transformer for the building/neighborhood. Doesn't seem to have had any direct effect as saw worked after that.

But will do test tomorrow.

Thanks.
 

Offline madires

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Re: 3-phase motor drawing high current
« Reply #16 on: August 14, 2016, 05:27:21 pm »
Do you got one those simple 3-phase testers showing the phase rotation or could borrow one?
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #17 on: August 14, 2016, 05:54:04 pm »
Do you got one those simple 3-phase testers showing the phase rotation or could borrow one?

No I don't. What do you think this will show?
 

Offline madires

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Re: 3-phase motor drawing high current
« Reply #18 on: August 14, 2016, 06:12:58 pm »
It would show if it's really 3-phase and the phase rotation. Some months ago I had an issue with a 3-phase motor running in the wrong direction, because someone swapped L1 and L3 in the IEC wall socket. If you think the problem is related to the change of the transformer, check if the phases (voltage & rotation) are correct. If those are fine, it's problem with your motor, which is triggered by accident after replacing the transformer. As always, check the power supply first ;)
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #19 on: August 14, 2016, 06:51:30 pm »
It would show if it's really 3-phase and the phase rotation. Some months ago I had an issue with a 3-phase motor running in the wrong direction, because someone swapped L1 and L3 in the IEC wall socket. If you think the problem is related to the change of the transformer, check if the phases (voltage & rotation) are correct. If those are fine, it's problem with your motor, which is triggered by accident after replacing the transformer. As always, check the power supply first ;)

Thanks for you reply.

Problem (pump OL tripping) preceded the transformer replacement. Operators thought it was a safety time-out thing and just reset it each time(!), not telling anyone. This has been for many months now.

So I think that either 1) short in a few windings (but all currents are the same, then unlikely shorts in all windings at the same time); 2) voltage reduced, either at mains source or higher resistance in path (v measured at motor is not low enough to cause increase of measured current (4.4A) over rated current (2.9A) -- v must drop to 140v to cause such an increase in A, if my calcs are correct); 3) pump load increased (maybe due to dirt or other restriction in hydraulic oil path)
 

Online Jr460

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Re: 3-phase motor drawing high current
« Reply #20 on: August 14, 2016, 06:52:20 pm »
My guess would be two things.

Something, age, bearings, pump is putting on more load to the motor than over a year ago, and you are running a motor not rated for 208 on 208.

Get the right motor on the unit, or the right voltage to the existing motor before getting into all kinds of other things.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #21 on: August 14, 2016, 06:53:07 pm »
Disconnecting pump hydraulic output should result in rated motor current (2.9A max), correct? If motor is healthy, that is.
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #22 on: August 14, 2016, 06:54:42 pm »
My guess would be two things.

Something, age, bearings, pump is putting on more load to the motor than over a year ago, and you are running a motor not rated for 208 on 208.

Get the right motor on the unit, or the right voltage to the existing motor before getting into all kinds of other things.

Autotransformer ("buck/boost") is next on the list. Options typically include boosting by 12v or 24v which would give 217/217/222 or 229/229/234, respectively, on the 3 phases. I'm opting for the higher voltages, I think.
« Last Edit: August 14, 2016, 06:58:57 pm by iXod »
 

Offline iXod

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Re: 3-phase motor drawing high current
« Reply #23 on: August 14, 2016, 09:07:48 pm »
I want to measure and compare resistance of each winding. I do not have a milliohm meter but do have ESR meter.

ESR meter should give accurate measurement of motor windings, yes? The 100KHz shouldn't make a difference?

If phase currents are identical, is there any reason to measure winding resistances?

Thanks.
« Last Edit: August 14, 2016, 11:52:44 pm by iXod »
 

Offline eKretz

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Re: 3-phase motor drawing high current
« Reply #24 on: August 15, 2016, 08:12:13 am »
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.
« Last Edit: August 15, 2016, 08:19:50 am by eKretz »
 


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