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Corrolation Between % voltage unbalance and % current unbalance

Corrolation Between % voltage unbalance and % current unbalance

Corrolation Between % voltage unbalance and % current unbalance

(OP)

I've looked around at different large motor running on my system buses and notice that each one of them is operating with about 3-5% current unbalance.  In thinking of the common thing amoung all these motors I notice that each of these buses have about .03% - .05% unbalance.

So my question is, what is the corrolation between % voltage unbalance and % current unbalance?  Is there a certain multiplier?  

I'm suspecting that different currents on the feeders throughout the plant are what is leading to the various voltage unbalances, or maybe its possible that my incoming main bus has this voltage unbalance.

I have heard that the maximum voltage unbalance should be 1% while the maximum current unbalance for motors should be between 10-15%.  Do these numbers seem correct?

RE: Corrolation Between % voltage unbalance and % current unbalance

NEMA MG-1 recommends a maximum voltage unbalance of 1% and states 1% can result in current unbalance of 6 to 10%. MG-1 also has a motor derating chart for voltage unbalance up to 5%.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Corrolation Between % voltage unbalance and % current unbalance

Voltage unbalance is often defined as the negative sequence voltage, or V2.  For example, take this to be 1%.

The negative sequence impedance of a motor is the locked rotor impedance or the full load impedance divided by a factor like 5.6, which is designated in a table based on the NEMA design and code letters.

The current unbalance is given by:

%I2 = %V2/%Z2 = %V2 * Code letter factor

So, for a 1% unbalance, and a 5.6 starting current factor, you get

%I2 = 1 * 5.6 = 5.6

Of course, you can get starting current factors higher/lower than 5.6, so NEMA summarizes it by saying it's between 6 and 10%.

I believe it's a linear relationship with % unbalance.  I think the 6 factor is just rounding up the 5.59 or 5.6 multiplier.

 

RE: Corrolation Between % voltage unbalance and % current unbalance

See figure 5.6 here:
http://books.google.com/books?id=tXym7Q7mi-wC&pg=PA138&;dq=%22Effect+of+voltage+unbalance+on+polyphase+induction+motor+currents%22&cd=1#v=onepage&q=%22Effect%20of%20voltage%20unbalance%20on%20polyphase%20induction%20motor%20currents%22&f=false

The figure suggests:
For locked rotor: Current unbalance = (1 to 2) x voltage unbalance
At Full Load: Current unbalance = (5 to 7) x voltage unbalance
At No Load: Current unbalance = (12 to 15) x voltage unbalance

As I recall some references mention the locked rotor impedance (around 7) is similar to the negative sequence impedance and % negative sequence is in the ballpark the same as % unbalance.  So this supports the 5-7 ratio at full load.

I have seen many times that the unloaded motors have a very high current unbalance that reduces as load increases and I presume it is attributable to the voltage unbalance. I believe the magnetizing component of current responds heavily to the voltage unbalance but the load component does not.

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RE: Corrolation Between % voltage unbalance and % current unbalance

Also there was a discussion and spreadsheet providing the relationship between current unbalance and voltage unbalance which is built into the NEMA derating curves (i.e. how are current unbalance  I^2*R considerations translated into the voltage unbalance NEMA derating curves) here:
thread238-209235: Motor Heating

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