×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Current imbalance persisting.

Current imbalance persisting.

Current imbalance persisting.

(OP)
Remember the Hydrualic Press...

The 2HP cushion pump is still giving me fits.  Today I went back and ran the cushion measuring everything.

A-B 492.5V
B-C 493V
C-A 494V

The electronic overload was tripping in about a minute.
Current:
A 1.4A
B 1.2A
C 1.8A

This represents a nasty 33% mismatch.

I rolled the three phases.
Current:
A 1.2A
B 1.5A
C 1.3A
20% mismatch

I rolled the three phases again.
Current:
A 1.5A
B 1.3A
C 1.15A
24% mismatch

I went back to the first roll.  It ran for 15 minutes and tripped.

This is a 2.8A motor barely loaded.

I checked the voltage drop from in front of the fuses to the three motor leads leaving the panel.  They all had a 1.0V drop.  Fuse contact, fuse, fuse contact, contactor, into terminal block, out of terminal block. (1.0V across the board)

In all cases the high current motor lead remained the high current leg.

I think I am down to the motor and the motor connections.  I was told the motor's nine leads were hooked up correctly and all the wire-nuts(groan) were tight.   I told them lets try re-terminating the motor connections with compression connectors.  If that doesn't improve things we need to yank the motor.

Do you guys concur?

If this motor is taken to a motor shop, (it is still under warranty being about 2 months old and never having run for long), can a typical motor shop check for current balance?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current imbalance persisting.

I don't know the answer.

One thing to note, a motor draws a lot more current unbalance at low load than at high load.  It holds true both when the source of the current unbalance is voltage unbalance or when the source of the current unbalance is within the motor (as it appears to be in your case).  I have heard of a motor with a coil  cutout (temporary repair) showing 20% unbalanced currents at no load and less than 5% unbalance at full load.

I kind of doubt your motor is anywhere near thermal limits at these low loads, even with that unbalance.  So I question whether a trip is really appropriate in this situaion.

At any rate, might be worth to see if you can load it up and check unbalance at higher load.

You also might want to do a winding resistance test with your multimeter (not particularly sensitive, but you never know).

The shop likely could repeat the winding resistance test, do a megger test, also try a  surge test (sometimes shows different pattern among phases if there is a miswiring). And sometimes they can easily load small motors for testing.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Current imbalance persisting.

Also thermography of the connections could be revealing.  Shop probably has that equipment readily available.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Current imbalance persisting.

I see that the high current goes with the supply lead. Since it does not stay with the motor terminal, I don't think the motor is the problem.

As pete says, the stated currents are much less than the rated. May be you need to change / bypass that trip unit.

RE: Current imbalance persisting.

(OP)
Hi edison123.

Quote:

In all cases the high current motor lead remained the high current leg.

Depends on how you look at the phases.  To keep my sanity I kept the readings to the color/phase of the source wires but those wires were the ones being rolled.  Sorry for any confusion.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current imbalance persisting.

itsmoked, I don't know the details of why you have nine connections and so on, but I would suggest perhaps there is a difference in reactance on each phase of the motor. Effective reactance will obviously depend on winding details, iron core but also the speed of the motor at different angles of rotation (pulsing load?). Since at low load, the power factor is very normally poor (I assume induction motor), the current in each phase is mostly controlled by reactance.
Anyway, I made many assumptions, but perhaps something I said might trigger a train of thought.

RE: Current imbalance persisting.

Is there any way you can access the air gap to check for a slightly off center rotor?
You can do a crude balance test by measuring the mid point voltages on a delta motor. Unfortunately about 99.99% of new motors are now star wound. Still it may be interesting to compare the voltages at the mid point splices. T4-T7, T5-T8, T6-T9.
Respectfully

RE: Current imbalance persisting.

(OP)
thread237-196453: Fuses blowing in the wind...  Is the motor.

Hi DaveScott;
It's a standard hydraulic pump so no pulsing load.

Bill; Too much work for a 2HP motor under warranty. You can well imagine the time this thing has already chocked up.  It is also 30 feet in the air making messin a little undesirable.

I wonder if we should change motor brands?

If a replacement has the same imbalance problem with light load, what other options do I have?  Any brands known to not have low load imbalances?  Use a choke?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current imbalance persisting.

I would consider changing the relay. Again, it's not clear why you need to trip at this low current, even with the unbalance

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Current imbalance persisting.

Good suggestion electricpete.
Respectfully

RE: Current imbalance persisting.

Which electronic overload are you using?

RE: Current imbalance persisting.

(OP)
This one!  The OOC version.

Note: It takes a while to load. ~30seconds.
http://ecatalog.squared.com/pubs/Motor%20Control/Overload%20Relays,%20NEMA%20Style/Solid-State%20Overload%20Relays/Motor%20Logic%20SSOLR,%20Type%209065S/9065HO9601R901.pdf

The load will vary all over because the press application requires the pressure to be adjusted from 100psi to 500psi(2HP) generally 100-200psi.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current imbalance persisting.

Nice unit. It even says:

"Phase unbalance protection provides
additional degree of motor protection"

I don't think you need that "additional degree of protection". An antique Bi-metal protection would probably do the work, they have more of a "phase loss" protection than a phase unbalance one. Is your motor hot at all? - I doubt it.

There are many little motors running under conditions you wouldn't like to know about. And they either survive - or they die. In the latter case, the post mortem usually reveals the tru cause of death.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Current imbalance persisting.

Hello itsmoked

How are you working your imbalance formula? This is the way I understand it, Please correct me if wrong.

The electronic overload was tripping in about a minute.
Current:
A 1.4A
B 1.2A
C 1.8A

This represents a nasty 33% mismatch.

(1.4 + 1.2 + 1.8)/3=1.4666 or 1.47 rnd

(highest phase-average)/average x 100

(1.8-1.47)/1.47 x 100 = 22.4%

RE: Current imbalance persisting.

Sorry, I should have separated your quote from my formula!

RE: Current imbalance persisting.

itsmoked, looking at the relay, the looping is effectively multiplying the 'real' (not %) current imbalance. Wouldn't the OOB model be more suitable?

RE: Current imbalance persisting.

Does the unit give the reason for the fault (i.e. overload or phase imbalance)?  You need to contact Square D to see what their algorithm is for determining phase imbalance and trip time.  You might be having a very high transient imbalance (which you don't see on your amprobe) and the Square D unit is tripping on that.  Probably their method of imbalance calculation and your motor conditions don't get along well together.

RE: Current imbalance persisting.

(OP)
Hi flexoprinting.  Nope I went for the maximum imbalance not the average imbalance, figuring any relay would care more about that.

100% - (1.2/1.8 x 100) = 33%

DaveScott;  I see those loops as multiplying any  current running thru the wires.  So the OVLD relay is really seeing 2.4A, 2.8A, 3.6A.  Same percentages.  You are correct.

And You bet!!! The OOB would be MUCH better!  Just try to find one... banghead  Rare as hen's teeth.  Nobody stocks them as "You can just double or triple the loops mon".

Hi gepman; I'd love to talk to someone on  that.  I can well imagine that could take several days of nagging hard work to reach someone in the bowels of Schneider Electric who's bought SquareD and created their lame website.  I don't understand the transient imbalance theory since this thing runs more than a minute regularly before tripping.  The unit has only a plastic red button that appears on trip.  No useful info at all.

Six inches from this unit is an identical OVLD relay running a 3/4HP fan using three wire loops.  We've never had any problem with that one.  It has NEVER tripped.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current imbalance persisting.

I just wanted to mention that the definition that flexoprinting gave for unbalance is consistent with NEMA MG-1.  So it was probably a good clarification.  But since we have the raw numbers, the definition of balance is not so critical.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Current imbalance persisting.

fwiw, here are my my thoughts (right or wrong... my job is not to select unbalance protection setpoints, so I may be missing something).

We have less that 25% current unbalance using the NEMA definition.

The NEMA derating curve gives a derating by voltage unbalance.

Current unbalance is 5 – 10 times voltage unbalance.

The edge of the curve is 5% voltage unbalance which corresponds to 25 – 50% current unbalance.  That certainly envelopes our case.

The derating factor is about 75%.

Apply the derating factor to nameplate current 2.8 amps.  2.8*0.75=2.1

The highest current should not exceed 2.1A on a continuous basis.      I can see no reason to trip at 1.8 amps.

This would be a very conservative approach as far as I know (for example, if we assumed current unbalance 5x voltage unbalance, we could have selected 2.5% volt unbalance and 92% derating).    I think the unbalance is much less a concern for small motors than for large motors (where the rotor is the concern).    The fact that the cause of the current unbalance is something other than voltage unbalance doesn't change the thermal conclusions as far as I know.

Note again that as you increase the load, you can expect the current unbalance to decrease so if the trip wasn't so sensitive you may well be able to operate this motor all the way up full load.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Current imbalance persisting.

Thank you Gentlemen, itsmoked thanks for this post I am watching with interest for a conclusion.

RE: Current imbalance persisting.

Keith

"Six inches from this unit is an identical OVLD relay running a 3/4HP fan using three wire loops.  We've never had any problem with that one.  It has NEVER tripped."

Why don't you swap the damn relays and see whether the problem travels with the relay or stays with the motor ?

RE: Current imbalance persisting.

In the event anyone is trying to decipher my illogical parenthetical statement, I should correct it.  Should have been: "for example, if we assumed current unbalance 10x voltage unbalance, we could have selected 2.5% volt unbalance and 92% derating"

And just to elaborate the two cases, since there is a range current unbalance = 5-10x volt unbalance, if we pick 5x we convert 25% current unbalance into 5% volt unbalance (derate to 75%), while if we pick 10x, we convert 25% current unbalance into 2.5% volt unbalance (derate to 92%)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Current imbalance persisting.

(OP)
electricpete;  Yes I was trying desperately to decipher your, "illogical parenthetical statement". LOL.

But thru it all I was rapidly coming to the conclusion that the SS OVLD relay may be defective and that I should just swap the damn thing with the adjacent one, even though that is a royal pain in the butt.

Agreed edison123 a splendid idea!

flexoprinting; I'll stick it out and certainly make sure you guys hear the ultimate solution.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current imbalance persisting.

Well, If you want to follow MG-1 then the derating is given by voltage unbalance. A derating only comes into effect above about 0.8% voltage unbalance which you do not have.

I expect its just an average cheaply built small motor. Almost all motors show a fair amount of unbalanced currents at low loads so what you are reporting is not odd to me.

Another interpretation of MG-1 used is to take the largest difference in phase currents and divide by the rated current to get the imbalance.

Check the connections for fun but you will most likely have to change the motor to make that overload work. The simplest is to just change the overload to one that does work.

RE: Current imbalance persisting.

(OP)
Thanks guys!  This is one of those annoying ones.  I called the plant guy today because they were going to redo the connections this morning and then yank the motor if the current stayed greatly imbalanced.

Today's response: "Ah.. They said they needed the parts run so I told the operator to go try it.  He ran the full 4 hour job and it never tripped once."

So I will order a new SS Overlord ~$120 to have on hand for that panicky call I'll get in the near future.

Can you run a 2HP motor safely with just delay fuses?  Seems a bit big for that. Type http://www.bussmann.com/pdf/1001.pdf

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current imbalance persisting.

Hi Keith;
Back in the 70's the preferred protection in a lot of plants was the melting alloy type of overload relay. No differential protection at all. Many industries ran on these without differential protection and had years of dependable service.
In the event of a complete phase loss the current would usually be enough to trip off.
The calibration of the bi-metal type of overload was not very good and it was not uncommon for motors to fail even though the protection was nominally correct.
One of the fuse manufacturers was promoting dual element fuses for motor protection.
I used to have a sizing chart for dual element fuses for the protection of motors.
I believe you can still meet code with fuse protected motors.
Respectfully

RE: Current imbalance persisting.

Just to follow up on LionelHutz' comment.  Even though the NEMA derating curve applies for voltage unbalance vs current unbalance, I believe the NEMA curve has relevance to this discussion for two reasons:

1 - A smart current-sensing relay providing protection for unbalanced currents will presumably implement derating roughly equivalent to that given by NEMA MG-1.  It doesn't know the difference between current unbalance caused by the motor or the power system.  So bringing the NEMA curve into the discussion gives us a rough idea when we might expect the relay to trip (regardless of the source of the unbalance).
2 - If we were not concernred about the relay characteristics, but only protecting the motor, we would want to understand the thermal effects of the current unbalance.  I believe the thermal effects are in generall not too tremendously different whehter the current unbalance is caused by a power sytem voltage unbalance or may a magnetic circuit unbalacne.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Current imbalance persisting.

I looked at MG1 1998 section 14.36 and it covers derating due to voltage imbalance. I didn't see anything specific about current imbalance in MG1 but it's a big publication and I didn't spend too much time looking.

So, it seems to me that an overload using current imbalance as part of it's algorithm has had the current imbalance part derived from reading MG1 and converting the section about voltage imbalance to watch current imbalances instead (using the typical 6-10x current imbalance multiplier). This is mostly done because the overload is not built to read the phase voltages but also to provide some form of unbalance protection for the motor. However, this method is not spelled out in MG1 so it's the manufacturer that has decided how to do it.

If I missed something then let me know the section because I'd like to read it.

I guess what I'm saying is that if the voltages are balanced then you can theoretically just set the overload to work with no concern for current imbalances. The motor manufacturer could theoretically build their motors so they run at rated load with a 20% current imbalance but still meets MG1 operational specs. And, with the cheaply built small motors these days, they probably do. Watching current imbalance is probably a good idea for a big expensive motor but may not be for a cheap little 2hp motor.

In the end, the motor is running under rated load and all of the phase currents are below the rated current so it is unlikely the motor is going to be damaged by the current imbalance being read.

RE: Current imbalance persisting.

Lionel - I agree 100% with everything you said.  I also don't think it contradicts anything I said.  

Before I thought perhaps you were disagreeing with the way I had referenced MG-1 voltage unbalance derating with the 5-10x conversion but apparenlty you didn't.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources