Voltage Imbalance Calculation Method with AVG/MIN/MAX Data
I have a question on how to calculate voltage imbalance.
The data that have contains AVERAGE MINIMUM and MAXIMUM values for each of three phases, with records recorded one time per second.
I know the voltage imbalance percentage formula, and repeat it here:
Formula stating Voltage unbalance=100 x Max. deviation from average voltage/Average voltage
For example, if measured line voltages were 455, 460, and 492, the average would be 469 volts (455 + 460 + 492 = 1407 / 3 = 469). The maximum deviation from that average is 23 volts (492 - 469 = 23). To find the voltage unbalance, solve the equation for the average voltage and the maximum
voltage deviation: Formula stating Voltage unbalance=100 x (23/469) = 4.9%
My question is which voltage values should be used in the calculation.
I think that I should use the AVERAGE of all (AVGab +AVGbc + AVGca)/3, and the maximum deviation being the largest of the AVERAGE from each phase AVG. correct?
I should have mentioned that the average value recorded is not a simple average of the MIN and MAX values, but must be calculated during the one second before recording the value.
Last edited by natfuelbill; 10-18-06 at 01:30 PM.
I have an old motor hand book by Allis-Chalmers that computes the
Originally Posted by natfuelbill
%voltage unbalance the way you have indicated. They use the average of the 3 phase to phase voltages as you suggest. Another interesting point brought out is the the increase in temperature rise in the motor is about
2 times the (% unbalanced)².
Example: For an unbalance of 4%, the temp rise = 2(4)² = 32% increase in temperature for the phase affected by the unbalance.
Is there a chart, table or formula that shows expected motor CURRENT imbalance based on the measured and calculated voltage imbalance?
I have heard that the current versus voltage imbalance can be around 5 to 1.
So if I had a 0.5% voltage imbalance that I might see a 2.5% current imbalance.
Nice to see someone else thinks about this stuff. I always just figured that I should use the same min, max or avg reading for all three. Now I may have to rethink. Kind of depends on what you are looking for or what you want to prove.
Voltage unbalance causes extremely high current unbalance. The magnitude of current unbalance may be 6 to 10 times as large as the voltage unbalance. For a motor, the line currents (at full-load with 2.5% voltage
Originally Posted by Natfuelbilll
unbalance) may be unbalanced by 25%.