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max difference between phase currents during uncoupled run

max difference between phase currents during uncoupled run

max difference between phase currents during uncoupled run

(OP)
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)

RE: max difference between phase currents during uncoupled run

(OP)
This is 2500hp horizontal sleeve bearing motor.

RE: max difference between phase currents during uncoupled run


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%?]
  

RE: max difference between phase currents during uncoupled run

(OP)
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).

RE: max difference between phase currents during uncoupled run


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.}
  

RE: max difference between phase currents during uncoupled run

(OP)
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.

RE: max difference between phase currents during uncoupled run

'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.

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