Motor imbalance

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Small pump motor, 15 HP operating on 240v, corner ground delta. Four wire to PP. Grounded conductor bonded to EG in POCO meter socket. EG bonded & Earthed via 1 rod at PP. Fifty (?) year old motor runs so smooth, you would never know there was a problem by listening to it. Long story why I was even there.

Customer:
Date:
% VOLTAGE IMBALANCE
PHASEVOLTAmp
A-B
252.00​
41.00​
4.67​
0.33​
B-C
248.00​
47.00​
0.67​
6.33​
A-C
242.00​
MAXMAX-AVG
34.00​
MAXMAX-AVG
5.33​
6.67​
AVG
247.33​
252​
4.67​
40.67​
47​
6.33​
5.33​
6.67​
STAND DEVIATION
4.11​
5.31​
TOTAL IMBALANCE
2.16%​
Investigate
16.39%​
Investigate

Usually rotating the leads helps. Not so this time. Current stayed the same which points to POCO or lateral. Lateral was fine and POCO with bucket truck found nothing at transformers after I left. They did not like the current readings they took after I left and are changing xfmr bank to a Delta with hi leg this morning.

Is this voltage:current imbalance typical? Is it predictive?
 

paulengr

Senior Member
Motors don’t care about grounding. A 2% voltage unbalance will cause a 12-16% current unbalance. Keep measuring voltages upstream until the voltage unbalance disappears and this points to your issue. Chasing current is a waste of time in this situation unless you can live with a large derating factor on the motor.
 

NewtonLaw

Senior Member
You commented on the transformer bank that the utility is using. Is it an Open-Delta bank, that is only two transformers used to supply you a three phase service? If so, are they both the same size? Generally speaking, the Utility will use one larger transformer to supply the single phase load the customer has and the second transformer suggests what the Utility company has determined is necessary to supply the three phase load. If this is so, changing out a transformer will have little impact, especially so if the voltage imbalance is on the Utility side. You could open the main service circuit breaker and take voltage reading, phase to phase and phase to ground and compare them to the same voltages you read during normal load at the customers main. If the open circuit voltages are still unbalanced, the Utility has a problem. If they are now balanced, it may still be a problem with the total load on an open Delta transformer bank. If the transformer bank has all three transformers, it is still likely one will be larger for the purposes supplying the customers single phase load. Performing the same voltage checks mentioned above will still help to determine if the Utility voltage is out of balance. Hope this helps.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
The only thing I can think of making a difference would be insulation degradation, lowering the voltage to ground would reduce the leakage current, causing less voltage drop, only one phase would see a higher voltage to ground instead of two? Not a very good theory, but a theory all the same.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
The motor and lateral did get a quick pass/fail with the megger. Also checked resistance of windings. A-B, B-C, A-C.

The original 16% was the result of one xfmr putting out 260 vs 240 of other two.

The PFC did have only two caps in it but is pulling 6 amps.
 

NewtonLaw

Senior Member
The motor and lateral did get a quick pass/fail with the megger. Also checked resistance of windings. A-B, B-C, A-C.

The original 16% was the result of one xfmr putting out 260 vs 240 of other two.

The PFC did have only two caps in it but is pulling 6 amps.
You noted the PFC had one phase out of service? If so this causes most 3 phase banks to appear as two single phase capacitors and the impedance doubles (due to current flow on the neutral that is not there with all three phases in service) and this will cause a voltage increase multiplier to as much as 4.5 times the increase caused by the three phase bank. This could be a problem that is now fixed.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
You noted the PFC had one phase out of service? If so this causes most 3 phase banks to appear as two single phase capacitors and the impedance doubles (due to current flow on the neutral that is not there with all three phases in service) and this will cause a voltage increase multiplier to as much as 4.5 times the increase caused by the three phase bank. This could be a problem that is now fixed.
Not out of service it only uses two. Think open delta.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Wait? In a previous post I though you indicated it was a closed Delta?

"Closed delta. Single motor only.
Voltage is very good now. All even.
Current imbalance is now slightly above 2%, but I’ve done all we can. "
The POCO supply is closed Delta.

PFC capacitor is connected open delta. Internal Factory connection, no Ag or Field Engineering involved.
 

paulengr

Senior Member
The POCO supply is closed Delta.

PFC capacitor is connected open delta. Internal Factory connection, no Ag or Field Engineering involved.
That would create voltage imbalance big time. You are better off disconnecting all the PFCs than to run with one out of service. I don’t really care if it’s “factory” or not. The engineer that did this needs to be reprimanded at best. If it came in factory like that, I would have the salesman sign off on a warranty for 10 years unlimited warranty on all three phase loads. Then make them either fix it right at their cost or refund everything when they won’t sign.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Not an EE by any means and maybe my description of how it is connected is wrong. Removing it made no changes to the voltage and appeared to make no change to the current draw while we had the bad POCO transformer involved. It drew the 6 amps on all three of the wires going to the PFC when we were done. It may have been a bit different prior, but not significant.
 

paulengr

Senior Member
Not an EE by any means and maybe my description of how it is connected is wrong. Removing it made no changes to the voltage and appeared to make no change to the current draw while we had the bad POCO transformer involved. It drew the 6 amps on all three of the wires going to the PFC when we were done. It may have been a bit different prior, but not significant.
NEMA defines % unbalance this way. Average all three values. Calculate the three differences between the original values and the average and throw away the signs. Then multiply by 100 and divide by the average. IEC does it differently and it requires a bunch of engineering math (ratio of the negative sequence to positive sequence). IEC is more accurate but hard to use,

If you have say two high currents and a low one, that missing current creates a negative torque, creating heat rather than output power. If one current is higher the effect is similar...the extra current just contributes to motor heating. Whether the current unbalance is caused externally (voltage unbalance) or internally (weak connections or turn to turn faults) does not matter...the effect is the same.

If you are looking at currents it’s almost linear so if you have say 10% current imbalance, derate the motor 10%. But if you have only voltages a 1% voltage unbalance will cause a 6-8% current unbalance, and NEMA derates accordingly.

So it’s OK if all voltages or all currents are a little high or a little low but if they are even a little bit different phase to phase that’s a huge issue.

This is where electricians are generally taught that anything “close” like say 440 to 500 V is “close enough” or that 1 or 2 A won’t make much of a difference. If all three values are the same, it doesn’t. But small differences have a huge impact.

Also if you are operating the motor at low loads a bad (or missing) capacitor won’t make much difference. Under load though the voltage unbalance will be much more obvious, especially at stall. At low loads the motor flux is most of what you see which is almost purely inductive (all VARs) and low load do voltage drop is small. At name plate current flux will be only 10-15% of the total (the rest is torque...resistive) and voltage unbalance will be much more obvious since most of that 10-15% flux current should be cancelled by the capacitors.

In my area in Dominion and Duke West areas power factor is a big deal and capacitors frequently get ignored and subsequently cause trouble. In Duke East there is almost no value so they aren’t typically used.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Ok. Thanks. A diagram of how the PFC is connected would be nice if you have the time. Nameplate on the caps would also be nice..
Draw a symbol for a 3 phase motor. You have lines A ----B-----C to your motor.

Now A---cap-----B--- cap-----C.

Looks Open to me. Normally
NEMA defines % unbalance this way. Average all three values. Calculate the three differences between the original values and the average and throw away the signs. Then multiply by 100 and divide by the average. IEC does it differently and it requires a bunch of engineering math (ratio of the negative sequence to positive sequence). IEC is more accurate but hard to use,

If you have say two high currents and a low one, that missing current creates a negative torque, creating heat rather than output power. If one current is higher the effect is similar...the extra current just contributes to motor heating. Whether the current unbalance is caused externally (voltage unbalance) or internally (weak connections or turn to turn faults) does not matter...the effect is the same.

If you are looking at currents it’s almost linear so if you have say 10% current imbalance, derate the motor 10%. But if you have only voltages a 1% voltage unbalance will cause a 6-8% current unbalance, and NEMA derates accordingly.

So it’s OK if all voltages or all currents are a little high or a little low but if they are even a little bit different phase to phase that’s a huge issue.

This is where electricians are generally taught that anything “close” like say 440 to 500 V is “close enough” or that 1 or 2 A won’t make much of a difference. If all three values are the same, it doesn’t. But small differences have a huge impact.

Also if you are operating the motor at low loads a bad (or missing) capacitor won’t make much difference. Under load though the voltage unbalance will be much more obvious, especially at stall. At low loads the motor flux is most of what you see which is almost purely inductive (all VARs) and low load do voltage drop is small. At name plate current flux will be only 10-15% of the total (the rest is torque...resistive) and voltage unbalance will be much more obvious since most of that 10-15% flux current should be cancelled by the capacitors.

In my area in Dominion and Duke West areas power factor is a big deal and capacitors frequently get ignored and subsequently cause trouble. In Duke East there is almost no value so they aren’t typically used.

Was my Spreadsheet incorrect in my OP?

After correction with caps.
Voltage
A-B 239
B-C 240
C-A 239

Current
A 38.5
B 38.6
C 38.7

No Caps respectively as above
245
245
244

41
42
40

Rotating leads did not change the results.
 
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