Motor Issue.

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Fordean

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
I have 3 phase 460volt 7.5 HP motor. 480 fed. I have two complete different setups.

#1 Has 5.5, 5.2, 5.6 amps per phase. THis setup is good one.

#2. Has 5.2, 4.3, 3.5 amps. Swapped out motor thinking this was issue but not. This unbalance 3.5 divided by 5.2 it about 35 percent differential. Causing Motor Logic to Trip. Square D says MOtor logics trips anywhere the amperage exceeds 15 or 20 percent indifference.

I finally megger the wires underground. They were deemed bad by megger. I think wires are crushed underground ( but not Direct short) due to the short run and I cant pull the wires out of conduits. from either side (couldnt move a inch) Pipe is 1 in. RGS. 25 feet with only two 90 I believe. Notice RGC is Rotting In other jobsites. that are installed over 35 years in Concrete or underslab.

Why would Pinch cause less amps. If that the case.

I seen this before above ground. Guys tightning clamps to tight not seemingly pierceing insulation. But still Causing trips.
 

kwired

Electron manager
Location
NE Nebraska
You have likely lost a phase, but still have enough continuity through ground to make up enough to get the motor enough torque to run.

If you were to hook up the motor to L1 L2 and N it probably would run, though not as well as if you had L3 in place of N.

Did you check voltage on each line at the motor?

Maybe also disconnect motor and check voltage on each line with a low impedance meter.
 

Phil Corso

Senior Member
Fordean...

I suggest you inter-change phases L1-L2, and then do it again for phases L2 and L3. The double interchange will not alter rotation. If the line curent-imbalance does not shift, then the supply is at fault.

Of-course line-leads are partof the supply!

Regards, Phil Corso
 
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kwired

Electron manager
Location
NE Nebraska
Consider conductor stranding, say 7 strands. What would happen if only one strand is intact and the other strands shorted "open"?

With 7.5 hp motor only loaded to 5 amps or so it probably still runs with no obvious problems with one strand still intact. Throw in some kind of high impedance shunt across an completely opened conductor and it may function somewhat normally, but current imbalance will be more obvious.

If this were a much higher current drawing load and only one strand was left, then you would likely overheat and burn open that strand eventually.
 
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Smart $

Esteemed Member
Location
Ohio
With 7.5 hp motor only loaded to 5 amps or so it probably still runs with no obvious problems with one strand still intact. Throw in some kind of high impedance shunt across an completely opened conductor and it may function somewhat normally, but current imbalance will be more obvious.

If this were a much higher current drawing load and only one strand was left, then you would likely overheat and burn open that strand eventually.
Perhaps... but FWIW let's say its a stranded #12. That's 7 strands that are 0.030" diameter, or between a 20 and 21 AWG conductor. I believe 5A would heat the one strand up sufficiently enough to have a noticeable voltage drop compared to a fully-intact conductor.

Besides, the example was an analogy not intended to identify the true cause of OP currents.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It's more likely that your conductor insulation is compromised and leaking to ground, so not all the current hat is seen by the SSOL is getting to the motor. Measure the motor currents AT the motor peckerhead, I'd be willing to bet they are not as unbalanced.

This by the way has always been a problem with that MotorLogic SSOL from Sq. D., it nuisance trips on imbalance too easily and is not adjustable. Because of this, they sell a slightly different version for pump panels that let's you disable the current imbalance trip feature, because on submersible pumps, the issue of insulation leakage is very common and it's often too expensive to replace the cables if it's not too bad.
 
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kwired

Electron manager
Location
NE Nebraska
Perhaps... but FWIW let's say its a stranded #12. That's 7 strands that are 0.030" diameter, or between a 20 and 21 AWG conductor. I believe 5A would heat the one strand up sufficiently enough to have a noticeable voltage drop compared to a fully-intact conductor.

Besides, the example was an analogy not intended to identify the true cause of OP currents.

How much resistance will it have in comparison to a fuse element sized close to protecting this motor?

I don't honestly know, but seems logical that one strand will have fairly significant amount of conductivity to run a 5 amp load, and it probably will do so at an elevated temperature. So would a fuse element sized at 115% of motor current.

If I put in one 115% fuse and two 200% fuses would I see a similar current imbalance? I could be wrong but I am guessing probably not.
 

Smart $

Esteemed Member
Location
Ohio
How much resistance will it have in comparison to a fuse element sized close to protecting this motor?

I don't honestly know, but seems logical that one strand will have fairly significant amount of conductivity to run a 5 amp load, and it probably will do so at an elevated temperature. So would a fuse element sized at 115% of motor current.

If I put in one 115% fuse and two 200% fuses would I see a similar current imbalance? I could be wrong but I am guessing probably not.
I honestly don't know either, and I'm not going to research t just to give you a more accurarate assessment that would still be speculative. FWIW, I was basing my "analogy" on an allowable ampacity of 6A for 18AWG fixture wire. But in defense of your position :eek:hmy:, the fully intact conductor to each side of the single strand would act as a heat sink.
 

kwired

Electron manager
Location
NE Nebraska
I honestly don't know either, and I'm not going to research t just to give you a more accurarate assessment that would still be speculative. FWIW, I was basing my "analogy" on an allowable ampacity of 6A for 18AWG fixture wire. But in defense of your position :eek:hmy:, the fully intact conductor to each side of the single strand would act as a heat sink.

To add a little more to that, the 18AWG fixture wire would not have the "heat sink" on each side of a very short section of wire. You could likely push much more than 6 amps through a small 18 AWG link if there is something adjacent to sink the heat, and remember that that 6 amp ampacity is to prevent overheating the insulation, higher temp insulation means higher ampacity.
 

kwired

Electron manager
Location
NE Nebraska
No. If the motor is running on single phasing condition, voltages across its leads would be equal in magnitude due to generation of back emf by the motor.
So why bother running all three phases to the motor then?

True you will get generation of back emf, but a loaded motor will still lose a lot of torque and voltage and current will be far from balanced.
 

kwired

Electron manager
Location
NE Nebraska
Yes. That is why it is not possible to check loss of phase by checking voltages at the motor terminals.



To start the motor, of course.

I agree checking for loss of phase can be difficult to determine from motor terminal voltage alone. How difficult will depend on motor loading conditions, more load there is on the motor the more obvious the readings will likely be.
 

Haji

Banned
Location
India
I agree checking for loss of phase can be difficult to determine from motor terminal voltage alone. How difficult will depend on motor loading conditions, more load there is on the motor the more obvious the readings will likely be.

You may be correct.

Loss of phase would generate negative sequence voltages.

So the voltages at the motor terminals may or may not be balanced and the unbalanced case may be detected by a true RMS meter only.
 

Besoeker

Senior Member
Location
UK
Yes. That is why it is not possible to check loss of phase by checking voltages at the motor terminals.
The OP has already determined that all three phases are taking current. If there was a loss of one phase owing to it having gone open circuit somewhere between the motor and a known good supply, that phase would have no current. On that basis, loss of a phase would seem to be an unlikely contender for the current unbalance being observed.
So what are other possibilities? Since it's usually easy to do, measuring the voltage at source and at the motor end of the conductors with and without running* would seem like an eminently sensible thing to do.

*Yes, this would mean disconnecting the conductors from the motor at its terminals

To start the motor, of course.
Is that an arrangement you often come across?
 

kwired

Electron manager
Location
NE Nebraska
I still think OP has lost a phase, probably in his questionable underground conductors. That lost phase maybe is grounded on the motor side of the break therefore motor does run, but there is less voltage on that phase and results in unbalancing of current. If supply side of break is grounded then he should be blowing fuses or circuit breakers. If it is a high resistance ground on the supply side of the break, he may very well measure current that is questionably close to appearing reasonable normal running current. Which coincidentally is just what he has measured.

Clamping meter around all three leads will give a reading of any current returning via other paths.
 
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