Unbalanced current on parallel motor leads.

[thewirenut91]

I have a head scratcher.

The setup is a nearly new part winding starter, so two NEMA 4 contactors in parallel, feeding a 200hp VHS 480V motor. The motor is three wire, probably rewound. It is not PWS, so parallel 1/0 leads are brought to the motor j-box and terminated into the same Polaris tap, two to each motor lead, x3 of course. In the control panel, as is typical, two short MTW 1/0 wires come off the load side of the panel disconnect, each leg, and feed their respective poles on the motor contactors.

The weird part is that on two legs, there is about 15 amps disparity between the parallel runs at full load. There is no voltage drop across either contactor that I detected, and no obvious hot spots or high resistance connections. What could cause this amperage disparity?

Is this not essentially a perfect parallel circuit?

The motor is slightly overloaded so the high legs were causing about a 2 hr trip time on the o-loads.

 

[ptonsparky]

You have a part winding starter setup feeding a motor that is not wired for PWS?

Damaged motor or not rewound correctly.
As far as no VD, examine the contacts. They would be in parallel with a good set and difficult to get a voltage reading. How good is the meter you are using? You need millivolts.

 

[TwoBlocked]

I'd suspect a poor connection at the motor. After it trips, open the motor connection box (guess I shouldn't say "bird beak" head here...) and see if any connections are hotter than the others.

 

[synchro]

The weird part is that on two legs, there is about 15 amps disparity between the parallel runs at full load. There is no voltage drop across either contactor that I detected, and no obvious hot spots or high resistance connections. What could cause this amperage disparity?

The ratio of the two currents would be (inversely) proportional to ratio of the impedances of the two paths. I'm guessing that this 15 amp disparity is because one current is around 14% more than the other?

In the control panel, as is typical, two short MTW 1/0 wires come off the load side of the panel disconnect, each leg, and feed their respective poles on the motor contactors.

The shorter the leads are, the less they will contribute to the total resistance, and therefore the resistances of contacts and connections will be having a higher impact on the balance of currents. It is the ratio of the resistances of the two paths that determines the relative balance of the currents, and not their absolute resistance. And so very small voltage drops across contacts and connections can effect the balance of currents with short wires.

 

[retirede]

Much info not given. If the PW starter has not been modified, you are starting a 200 HP motor with a size 4 contactor.
I’m also guessing that the “parallel” leads to the motor aren’t actually parallel - each set connects to one of the size 4 contactors (or OL relay) in the starter and only joined at the motor end.

If my guesses are in fact correct, the start contactor likely has damaged contacts, thus higher resistance, resulting in the current imbalance.

There is no good way to use a PW starter on a motor that’s not wound for it.

 

[ptonsparky]

Much info not given. If the PW starter has not been modified, you are starting a 200 HP motor with a size 4 contactor.
I’m also guessing that the “parallel” leads to the motor aren’t actually parallel - each set connects to one of the size 4 contactors (or OL relay) in the starter and only joined at the motor end.

If my guesses are in fact correct, the start contactor likely has damaged contacts, thus higher resistance, resulting in the current imbalance.

There is no good way to use a PW starter on a motor that’s not wound for it.

My hope was that they modified the control so both contactors pulled in at relatively the same time. You never know though.

 

[retirede]

My hope was that they modified the control so both contactors pulled in at relatively the same time. You never know though.

IMO, that’s still problematic.

 

[ptonsparky]

IMO, that’s still problematic.

Without a doubt.
It'd be fun to put some meters on.

 

[Jraef]

I'd suspect a poor connection at the motor. After it trips, open the motor connection box (guess I shouldn't say "bird beak" head here...) and see if any connections are hotter than the others.

I have started saying "picker head", after a run in with our HR department on my "use of inappropriate industry slang" during a presentation. There is a case to be made for it. Apparently there was an old machine for picking corn that has a rotating drum with a box on the side, called a "corn picker head" that looks vaguely like a motor with a termination box on the side. The theory is that the "bird-beak head" term we have all used for decades is derived from people mishearing the ORIGINAL source of calling it a "picker head".
Example:

 

[ptonsparky]

I hadn't heard the 'term' until it was used here, MHs. Now that it's been explained, it's no longer peckular, dang.

FWIW, I grew up around CPHs and don't remember any looking like the PC version of a motor PH.

 

[TwoBlocked]

Having been in the service, seemed like EVERTHING had a vulgar nickname. I never gave it a thought (and don't intend to) that PH could be a mis-pronunciation.

 

[thewirenut91]

Much info not given. If the PW starter has not been modified, you are starting a 200 HP motor with a size 4 contactor.
I’m also guessing that the “parallel” leads to the motor aren’t actually parallel - each set connects to one of the size 4 contactors (or OL relay) in the starter and only joined at the motor end.

If my guesses are in fact correct, the start contactor likely has damaged contacts, thus higher resistance, resulting in the current imbalance.

There is no good way to use a PW starter on a motor that’s not wound for it.

I just noticed my post was generating a thread. The starter panel is modified to pull in both at once. Customer has a thing about soft starts and vfd's, so just working with what i got, which is a PWS starter modified to start ATL, due to the fact the existing motor is a 3 lead motor, not a six lead PW. So okay the size 4 contactors are in parallel, not the motor leads, if that's more accurate.

 

[ptonsparky]

I just noticed my post was generating a thread. The starter panel is modified to pull in both at once. Customer has a thing about soft starts and vfd's, so just working with what i got, which is a PWS starter modified to start ATL, due to the fact the existing motor is a 3 lead motor, not a six lead PW. So okay the size 4 contactors are in parallel, not the motor leads, if that's more accurate.

The both at once could be a problem if they aren't quite, or are defective.

 

[thewirenut91]

The both at once could be a problem if they aren't quite, or are defective.

Welp. I dont disagree. Im not trying to convince anyone to like it. Im trying to thread a needle between the utility company, the customer, the NEC, best practices, and no appreciable AHJ. This is oilfield adjacent ag work. Or maybe ag adjacent oilfield work. My question was more theoretical but ill take the critique.

 

[thewirenut91]

The ratio of the two currents would be (inversely) proportional to ratio of the impedances of the two paths. I'm guessing that this 15 amp disparity is because one current is around 14% more than the other?



The shorter the leads are, the less they will contribute to the total resistance, and therefore the resistances of contacts and connections will be having a higher impact on the balance of currents. It is the ratio of the resistances of the two paths that determines the relative balance of the currents, and not their absolute resistance. And so very small voltage drops across contacts and connections can effect the balance of currents with short wires.

Thanks this might be what im looking for.

 

[retirede]

I just noticed my post was generating a thread. The starter panel is modified to pull in both at once. Customer has a thing about soft starts and vfd's, so just working with what i got, which is a PWS starter modified to start ATL, due to the fact the existing motor is a 3 lead motor, not a six lead PW. So okay the size 4 contactors are in parallel, not the motor leads, if that's more accurate.

The differences in current is most likely due to differences in contact resistance. And it’s likely to get worse.
The contactors will never pull in at exactly the same time.

 

[thewirenut91]

The differences in current is most likely due to differences in contact resistance. And it’s likely to get worse.
The contactors will never pull in at exactly the same time.

Sure. I was unable @ full load to detect any VD across any leg of the contactors, (Fluke 1587), which is the most likely place to find contact resistance. I suspected the Polaris taps in the "Pickerhead," but they weren't obviously warm, and my thermal camera didn't pick up any hot spots.
Motor contactor contact pads have always appeared ridiculously small to me, and one cycle already produces material transfer.

 

[Ziyad Alahrbi]

If you are using single core cable for long distance then this might be the problem.
The installation method of parallel runs can cause unbalanced impedance and thus current imbalance.

It is better to refer to your company/country standards and regulations with installation method and grounding.

 

[texie]

I just noticed my post was generating a thread. The starter panel is modified to pull in both at once. Customer has a thing about soft starts and vfd's, so just working with what i got, which is a PWS starter modified to start ATL, due to the fact the existing motor is a 3 lead motor, not a six lead PW. So okay the size 4 contactors are in parallel, not the motor leads, if that's more accurate.

So, the way I am understanding this, you are using a PWS starter (acting in paralell) to run a single winding motor. Yes, it will work if as you say, the starter has been modified to pull in both contacters at the same time but this is not an NEC compliant method on a number of levels. I've seen far worse in the oil fields though.