Unbalanced motor current with cyclical load

[philly]

We have a 2300hp motor that always shows an unbalance current of about 3-5%. This motor is driving a large roller/grinding mill which I believed can be considered a cyclical load. When looking at the voltage on the bus this motor is connected to it does not seem to be very unbalanced maybe on about 0.1% or so. Also all other motors on this bus as well as other motors in the plant dont show this current unbalance.

We also have a brand new identical motor serving an identical roller Mill that shows the exact same current unbalance characteristics. Since this motor is brand new and exhibits the same unbalance I'm begining to wonder if this unbalance is somehow caused by the load rather then the motor itself. Bus voltage unbalance on the new motor is also about 0.1% or so. Again other motors on this bus do not exhibit any current unbalance.

Is there something about cyclical loads causing the phase currents to jump around that a digital relay will interpret these as unbalanced current?

 

[Jraef]

If by "cyclical" you mean a load that comes on and off periodically, that would not have an effect. If you meant an eccentric load, i.e. an off center attachment like a cam, then maybe. It's fairly well accepted that current signature analysis can be used to detect vibration, and an eccentric load is essentially a very low cycle vibration at some point, so it's entirely possible that an eccentric load could look like a current imbalance.

But that said, 3-5% current unbalance is within the reaches of acceptability. Most motor maintenance info bases unbalanced motor power on unbalanced voltage. 3-5% voltage unbalance would definitely be of concern and most say that the voltage unbalance of 1% or less would be ideal (NEMA MG-1 calls for a motor to perform within spec at a 1% voltage unbalance). But a voltage unbalance relates to a current unbalance percentage that is 6-10 times higher. So working that backwards if you had an acceptable 1% voltage unbalance, that would mean 6-10% current unbalance. You are below that, I wouldn't worry.