max difference between phase currents during uncoupled run 1

[electricpete]

31.6A, 30A, 28.4A. Would you consider it abnormal?
No, we haven't checked the voltages.

(Moved same clamp-on probe to all phases several times to establish repeatbility)

 

[electricpete]

This is 2500hp horizontal sleeve bearing motor.

 

[busbar]

That's ~5½% deviation...doesn't seem drastic. I'd note it and check under load.

[By using the reverse logic of some recent discussion here, would that imply that terminal voltage is balanced to within 1%?]

 

[electricpete]

5.5% deviation or 11% range, depending on how you look at it.

Let us assume the voltage is balanced within 1/2% range. I think that is typically what we see at our power plant (absent a power system problem).

 

[busbar]

Excerpt from IEEE 141 §3.8.2 Measurement of phase-voltage unbalance
The simplest method of expressing the phase-voltage unbalance is to measure the voltages in each of the three phases: [Method listed in ANSI C84.1 and NEMA MG-1]

voltage unbalance = maximum deviation from average / percent unbalance

{Also in IEEE 141} The amount of voltage unbalance is better expressed in symmetrical components as the negative sequence component of the voltage:

voltage unbalance = negative-sequence voltage / positive-sequence voltage

{No mention is made of current-imbalance relationships.}

 

[electricpete]

Thanks busbar. I think there are several standards we talk about geared toward ensuring the motor does not overheat due to current unbalance at full load.

I believe that motors may tend to have higher unbalance when unloaded. I am really looking to see what are typical values during solo runs.

 

[jbartos]

Suggestion: Visit

http://www.energyautomation.com/html/methods/electricalsystemwork.php

etc. for more info

 

[DanDel]

'pete, I would not consider those values a problem for no load, but I would definitely check the currents with load(and the voltages, also).
But you knew that.