motor unbalance current vs load 3

[electricpete]

Let's say a motor is exposed to a small voltage unbalance of 1% or less. How does the current unbalance vary as load is increased from no-load to full-load?

Constant percentage of total current? Or decreasing percentage of current?

I have seen a few things to make me suspect that the "percentage-imbalance" decreases as load decreases. Interested if others believe the same. Maybe it depends upon the source of the unbalance (unbalanced voltage or unbalanced motor windings)?

 

[electricpete]

correction:
..I have seen a few things to make me suspect that the "percentage-imbalance" decreases as load increases...

 

[OperaHouse]

The imbalance "problem" is basically because the motor acts as a rotating transformer and tries to create voltage to correct the imbalance. This causes the windings to draw more current and heat up the motor. At 5% if I remember it has 25% higher heat loss. Given that the slip increases under load, I would be inclined to say the current imbalance might decrease. I'm just a contrarian and most people would be thrilled to have a 1% imbalance.

 

[jbartos]

Suggestion: Visit

http://www.moeller.co.uk/ht_motor_protection.htm

for a new protection against the current imbalance. Check with the manufacturer regarding the voltage imbalance versus current imbalance dependences with the increasing load.

 

[buzzp]

I have seen this as well. I am not sure I am prepared to apply it as a general rule since there are way too many variables. I do know the unbalanced currents cause negative sequence currents to flow to oppose the flux rotation. In turn, the negative sequence current causes additional heat as well as the additional current draw needed on the three lines. If the motor is lightly loaded it will not be operating very efficiently, as you know and when it is pulled into the best efficient point (full load) then the current imbalance is not as bad as when it is lightly loaded. What you state is likely true but some boundaries would have to be applied as far as percent of full load vs. load for this rule to apply. In the short time I have, I can not think of a real reason for this but it sort of makes sense to me. It will be interesting to see if someone has a theorhetical reason for this. (sorry for ramblin on)

 

[jbartos]

Suggestion: Contact the manufacturer rep for the device theory since they may have more info available.

 

[GusD]

Hello Pete

I would say that current umbalances would decrease with an
increase in load.This is true for some motor designs, that
have an assymetrical magnectic field.They have large current umbalances when idling but fairly well balanced phases when fully loaded.As for the 1% voltage umbalance
it is indeed great,but the old rule still applies.A small voltage unbalance may cause a ten fold current umbalance .
"Remarks to a previous post dealing with current unbalances".It is true Pete.I read it somewhere and so did you.

GusD

 

[buzzp]

Yes that rule has been around for quit some time (longer than I am old). I think Petes trying to understand the theory why the imbalance seems to decrease (from light load) as the motor approaches full load.

 

[jbartos]

Suggestion: The motor unbalanced current will depend on its origin. If the origin is in the magnetizing branch of the motor electrical equivalent circuit, then it will be about the same with the increasing motor load. If the origin is not in the motor magnetizing branch, then it will increase with the increasing motor load. This is where the unbalanced current relay setting comes into picture.