Motor Unbalance Current

Friends,

The indicated current imbalance is a little over 5%. This is generally considered OK. Most comments on this topic will correctly link the current imbalance to an underlying voltage imbalance on the incoming mains. Look at a 2-yr old discussion on this topic, along with a very helpful method for determining if the cause is within or outside the motor:

http://maintenanceforums.com/eve/forums/a/tpc/f/7161085912/m/9781005762.

Don't forget, however, that an imbalance in motor voltage/current will cause a greater internal heating of the motor, so NEMA publishes a recommended derating curve for this condition. Perhaps an even better discussion of this is in this paper from Pacific Gas & Electric:

http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_unbalance_rev2.pdf

When the current imbalance becomes too great, damage to the motor becomes a very real problem, because of circulating magnetic fields that exceed the design limits of the motor. This happened once at a former employer's site, with the total loss of a 500Hp motor due to a ~5% voltage imbalance on the incoming mains (5-hours of troubleshooting by 3 crews of utility linemen found the fault 1-mile away, a failed splice on the incoming primary).

JMM