3 Phase Motor Current Imbalance

We have 4 Inbound Carton Conveyors (same design) in our warehouse. 3 have been running for years with minimal issues. All have a Gapper Bed run by a 1HP 460V motor (1.7 FLA). VFDs are Powerflex 40, with 150 ohm Dynamic Brake Resistors. Overload current is set to 2.1A (125% of FLA)



Recently, #4 has been getting frequent JAMs, and has been faulting. PF40 Fault Queue shows 4, 5, 4 - Undervoltage and Overvoltage. Also getting occasional Current Overload. VFD failed and we replaced it. Same symptoms. Replaced the motor. Same symptoms.



Meggered the motor (500V), got 200+ Megohms per phase. Lifted the motor leads at the VFD, and meggered there. Similar readings.


PF40 HMI in current mode shows total motor current ranging from 2 Amps up to 4 Amps, with some excursions up to 4.3 Amps, followed by a fault.



Here is the interesting part: At the main panel, 100 ft away, we used a current coil ammeter, and got a reading on each of the 3 phases: 1.4A - 0.8A - 2.2A - So clearly, the VFD is giving us good Amp readings.



Question is, what could be the cause of such an extreme spread of current per phase? Especially given the VFD and motor were replaced with new? TIA - Jim

you may want to post this on an electrical website such as mikeholt.com.
james

Mechanical problems on the #4 conveyor?

100 feet distance for yourself? What’s the distance for the motor leads from VFD?

MTB_Hardtail said:

We have 4 Inbound Carton Conveyors (same design) in our warehouse. 3 have been running for years with minimal issues. All have a Gapper Bed run by a 1HP 460V motor (1.7 FLA). VFDs are Powerflex 40, with 150 ohm Dynamic Brake Resistors. Overload current is set to 2.1A (125% of FLA)



Recently, #4 has been getting frequent JAMs, and has been faulting. PF40 Fault Queue shows 4, 5, 4 - Undervoltage and Overvoltage. Also getting occasional Current Overload. VFD failed and we replaced it. Same symptoms. Replaced the motor. Same symptoms.



Meggered the motor (500V), got 200+ Megohms per phase. Lifted the motor leads at the VFD, and meggered there. Similar readings.


PF40 HMI in current mode shows total motor current ranging from 2 Amps up to 4 Amps, with some excursions up to 4.3 Amps, followed by a fault.



Here is the interesting part: At the main panel, 100 ft away, we used a current coil ammeter, and got a reading on each of the 3 phases: 1.4A - 0.8A - 2.2A - So clearly, the VFD is giving us good Amp readings.



Question is, what could be the cause of such an extreme spread of current per phase? Especially given the VFD and motor were replaced with new? TIA - Jim

Click to expand...

-There is nothing else between the load side of the VFD and the motor? Usually it could be a bad disconnect switch or wires that are corroded or nicked and shorting to ground somewhere in the conduit or conveyor.

-I'm not sure how difficult it is to decouple the load, but another idea is to swap two motors if they're identical and see if the current imbalance moves, stays, or disappears.

-Check voltage on each phase to ground on the load side of the VFD, and then also at the motor side and see if there are any voltage drops (indicating resistance in the circuit somewhere).

-When I had a problem like this with a conveyor a while back, I pulled three new wires and ground through the conduit (about 100ft) and the problem didn't come back. I think possibly the spikes from the VFD caused wear in the wire insulation.

-Overvoltage alarm is usually from the motor regenerating. Check carefully the resistor settings are programmed properly for the dynamic brake resistor and that it is wired properly and not open or burnt out.

-Why 100ft away? Why not just check current at the VFD and at the motor with a True RMS meter? Maybe I'm missing something here but the readings could be goofy further out, especially if you're not using a load reactor at the VFD. I've seen odd current readings as well like that further out.

-I think you might be chasing your tail here. It's probably mechanical issues with the conveyor. Try to check the currents with a Fluke 323 Ammeter (which are known for being more accurate with VFDs).

Why is the overload set at 125% of FLA, it should be set to motor FLA the overload works out the tripping time via the FLA.

MTB_Hardtail said:

Overload current is set to 2.1A (125% of FLA)

Click to expand...

I have never seen a motor manufacturer advise anything higher than 100% when running on a VFD.

MTB_Hardtail said:

Recently, #4 has been getting frequent JAMs, and has been faulting. PF40 Fault Queue shows 4, 5, 4 - Undervoltage and Overvoltage.

Click to expand...

At the moment of the over voltage fault is the motor supposed to be stopping? And is this when your "JAM" happens?

MTB_Hardtail said:

Meggered the motor (500V), got 200+ Megohms per phase. Lifted the motor leads at the VFD, and meggered there. Similar readings.

Click to expand...

I hope this means that you meggered the motor by lifting the motor leads from the VFD. So that you measured the motor and the leads at the same time.

MTB_Hardtail said:

PF40 HMI in current mode shows total motor current ranging from 2 Amps up to 4 Amps, with some excursions up to 4.3 Amps, followed by a fault.

Click to expand...

When this happens is the conveyor "JAMED"? because it couldn't stop fast enough? Has the gearbox been changed from a worm to a helical?

I read your question and I answer about my knowledge , three phase equipment such as induction motor with unbalance in its windings. If the reactance of three phases is not same, it will result in varying current flowing in three phases and give out system unbalance. – With continuous operation, motor's physical environment cause degradation of rotor and stator windings. Thank You!!

Cheap Chinese motor? Have had a load of issues with those.

I thought a 460v motor should be meggered at 1000 volt

nehpets said:

I thought a 460v motor should be meggered at 1000 volt

Click to expand...

Yes, that is correct. 1000 volts for 480V motors, and 500V for 240V motors.

I like your signature (octal 31 = decimal 25)

I hope the OP is not chasing his tail over something that's probably a simple mechanical issue.