# 3 phase amp imbalance



## leadelecmech (Sep 28, 2012)

I have a new pump station with two 40 HP, 480v motors with VFDs. The motor amp draw is unbalanced running on utility power but runs balanced when running on generator power.


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## HARRY304E (Sep 15, 2010)

leadelecmech said:


> I have a new pump station with two 40 HP, 480v motors with VFDs. The motor amp draw is unbalanced running on utility power but runs balanced when running on generator power.


What are your voltage readings from the utility?

Welcome to the forum..:thumbup:


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## MDShunk (Jan 7, 2007)

Describe the magnitude of the "imbalance". An amp? Couple amps? 10 amps?

I would also expect a large motor to run more "balanced" on gen power because they'll sync themselves to some degree, just like syncing two gensets.


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## leadelecmech (Sep 28, 2012)

*3 phase imbalance*

the voltages phase to ground are 283/288/282, phase to phase 491/492/494. amp draws are 36/34/28 on utility and 36/35/36 on generator. Motor 2 amp draws are 32/34/18 on utility and 28/28/28 on generator power.


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## MDShunk (Jan 7, 2007)

You didn't mention about any voltage readings from the gen.

There's a funny phenomenon about motor currents on a VFD. The current imbalance can be as much as 20 times the line side voltage imbalance. Installing a line reactor will bring this into spec much better. I'm sure you're not running a line reactor with voltage imbalance-to-current imbalances measured like that.


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## leadelecmech (Sep 28, 2012)

voltages from the gen set are 278/278/278..


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## MDShunk (Jan 7, 2007)

leadelecmech said:


> voltages from the gen set are 278/278/278..


Phase to ground is less important on 3-phase motor loads than phase to phase, but I'd have to assume with p-n voltages like that, your p-p voltages are really nice also. 

Since you have about a 1% (3 volts) imbalance in utility supply voltages, the NECA MG-1 rules of thumb will permit you to have about a 7% (2.5 amps) imbalance in the motor currents. It would not be unusual, however, to see up to see 20 times the voltage imbalance as a current imbalance (in this case, 20% or 7.5 amps) in motors operating from a VFD. That's roughly what you have on motor 1. Your one phase to phase reading on motor 2 at 18 amps is a suspected bad reading, in my mind -OR- the motor was doing no work when you measured that. Motors at idle can give some wacky current readings.


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## JRaef (Mar 23, 2009)

If there is a VFD involved and you are reading line currents, it means absolutely nothing as far as the motor goes. The VFD is like a new power source to the motor, it is totally isolated from the line with regards to current. 

But as MDShunk said, it's an indicator that because of the voltage imbalance on the line side, your drive is drawing power unevenly thought the bridge rectifier, which is putting unnecessary thermal stress on some of your diodes, but less on others. This sometimes can lead to early failure of the bridge. His suggestion of a line reactor will help with that, I suggest it too.

Most measurements you take in a power circuit with a VFD are suspect as well, most affordable meters are ill equipped to deal with the complex harmonics.


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## tates1882 (Sep 3, 2010)

leadelecmech said:


> I have a new pump station with two 40 HP, 480v motors with VFDs. The motor amp draw is unbalanced running on utility power but runs balanced when running on generator power.


Where are you measuring current at? The motor, the drive, OCP? If you are on the line side of the drive you are more likely to see an imbalance, the drive should "smooth" out any voltage imbalance. There by eliminating a current imbalance do to voltage differences. An improper aligned shaft connection to the pump can change the air gap inside the motor and cause imbalances before it fries a winding and lets to magic out.


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## wdestar (Jul 19, 2008)

You need a scope. You've got some issues on the utility side. Voltage readings via a multimeter will not tell the whole story. Get a dual input scope and look at the readings. My guess is there's a story to be told on phase "C" so to speak.


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## Jabberwoky (Sep 2, 2012)

If its only a 40HP system can you swap input lines to the drive? Does the imbalance follow the wires or does it stay the same relative to the drive? If it stays the same relative to the drive it may be a phased firing offset of the input rectifier. I have seen this only a couple times and usually its some damn 50 cent cap in the firing circuit. I'm more curious what prompted you to check the line draw in the first place? If you have the make and model of the drive I can usually tell what type of input rectifier used. If the input rectifier is purely diodes this likely isn't a drive issue.


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## JRaef (Mar 23, 2009)

On the diode front end of a 6 pulse drive, the diodes only conduct in one direction of course, but they ALSO only conduct when the voltage into them exceeds the forward bias threshold. If one line voltage is higher than the others, THAT set of diodes will reach that forward bias threshold sooner and begin to conduct earlier than the others. So it will attempt to deliver more of the demanded current to the DC bus, hence a line current imbalance because the diodes are not sharing the load evenly among them. This has nothing to do with the motor load current directly as far as the imbalance goes because the load current is controlled by the drive. But if the motor is only lightly loaded and the line current in that one set of diodes is still below the rated current of the drive, it makes no difference and it could do that forever. If / when the motor is fully loaded, that set of diodes can burn up without the drive noticing, because the _OVERALL_ DC bus current is still within normal and that's often the only thing the VFD reads.

If you put a Line Reactor ahead of the drive, the inductance it adds will slow down the rate of rise in the incoming power, reducing that lead that the one set of diodes will have in conducting. It evens the playing field so to speak.


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## Jabberwoky (Sep 2, 2012)

Oh yeah I have seen a load imbalance on a passive diode rectifier. Usually minor in most cases as the bridges are typically modules now. But on the scr/diode bridges the misfiring SCRs can create a much larger imbalance. Very rare occurance, maybe 2 out of 60 or so drives with this type of front end I have rebuilt. I spent the better part of a day finding the bad part on one of those jobs. A precharge firing section failure on the input rectifier usually faults the newer drives.


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## wdestar (Jul 19, 2008)

JRaef said:


> On the diode front end of a 6 pulse drive, the diodes only conduct in one direction of course, but they ALSO only conduct when the voltage into them exceeds the forward bias threshold. If one line voltage is higher than the others, THAT set of diodes will reach that forward bias threshold sooner and begin to conduct earlier than the others. So it will attempt to deliver more of the demanded current to the DC bus, hence a line current imbalance because the diodes are not sharing the load evenly among them. This has nothing to do with the motor load current directly as far as the imbalance goes because the load current is controlled by the drive. But if the motor is only lightly loaded and the line current in that one set of diodes is still below the rated current of the drive, it makes no difference and it could do that forever. If / when the motor is fully loaded, that set of diodes can burn up without the drive noticing, because the _OVERALL_ DC bus current is still within normal and that's often the only thing the VFD reads.
> 
> If you put a Line Reactor ahead of the drive, the inductance it adds will slow down the rate of rise in the incoming power, reducing that lead that the one set of diodes will have in conducting. It evens the playing field so to speak.


Are you talking about diodes or scrs? If it's diodes, you lost me.


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