Current imbalance persisting.

[itsmoked]

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

 

[electricpete]

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.

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[electricpete]

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

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[edison123]

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.

 

[itsmoked]

Hi edison123.

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

 

[DaveScott]

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.

 

[waross]

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

 

[itsmoked]

thread237-196453 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

 

[electricpete]

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

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[itsmoked]

A very good point electricpete.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Good suggestion electricpete.
Respectfully

 

[gepman]

Which electronic overload are you using?

 

[itsmoked]

This one! The OOC version.

Note: It takes a while to load. ~30seconds.

http://ecatalog.squared.com/pubs/Mo...or Logic SSOLR, Type 9065S/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

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[flexoprinting]

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%

 

[flexoprinting]

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

 

[DaveScott]

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

 

[gepman]

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.

 

[itsmoked]

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... 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

 

[electricpete]

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.

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