How to calculate KW losses in a unbalanced motor
How to calculate KW losses in a unbalanced motor
(OP)
I Know that if the current is unbalanced in a three phase motor , heat develops and the life of the winding coil is reduced. The formula to calculate the loss in KW is I x I x R but how do I calculate easy the R value in a motor , is it not a table available where one can read the % losses if one knows the HP of the motor and the % of current unbalanced ?
Appreciate your comments. thanks
Appreciate your comments. thanks





RE: How to calculate KW losses in a unbalanced motor
First, take an average of the 3 voltages (ex., 220, 208, 210 = 212.7 Vac). Find the largest deviation from the avg.(220-212.7 = 7.3). Divide the deviation by the avg.( 7.3/212.7 = .034) and multiply by 100 = 3.4%.
It's a fairly linear derate curve up to 5%.
4% = .85 derate factor
2.5% = .95
1% = .99
Don't run a motor with more than 5% voltage deviation
RE: How to calculate KW losses in a unbalanced motor
RE: How to calculate KW losses in a unbalanced motor
Not sure if I can answer your question any more accurately, however I hope the information below, used together with that provided by Baldor will prove useful to you.
The standard specification for LV motors appears to be; for every 1% line voltage error you can expect between 6 & 10% line current error. As there is a relationship between current and voltage imbalance (albeit not fixed), I believe the formula given by Baldor is a good guide.
On looking at a derating curve provided to me by a motor manufacturer, derating is as follows:
1% voltage error, derate motor to approx 0.99
2% voltage error, derate motor to approx 0.95
3% voltage error, derate motor to approx 0.87
4% voltage error, derate motor to approx 0.80
5% voltage error, derate motor to approx 0.75
Regards,
GGOSS
RE: How to calculate KW losses in a unbalanced motor
Usefull link containing derating tables.
http://www.elongo.com/pdfs/voltages.PDF
regards,
GGOSS
RE: How to calculate KW losses in a unbalanced motor
hope I have explained myself. thank you anyway.
RE: How to calculate KW losses in a unbalanced motor
I'm curious how you propose to control individual phase currents. Are you suggesting balancing the currents by having some form of regulation on each phase and perhaps a controller to keep the imbalance to a minimum?
Sounds like a lot of work for limited gain, and almost certainly would reduce the reliability compared with living with the imbalance. Am I completely missing your point?
regards,
Scotty.
RE: How to calculate KW losses in a unbalanced motor
RE: How to calculate KW losses in a unbalanced motor
RE: How to calculate KW losses in a unbalanced motor
Sorry is not that easy as “ I^2*R.”or a table of resistances.
Probably you will need a computer program to foresee the performance of a motor under unbalanced Voltage (the result of unbalanced voltage or winding or both is unbalanced current). Such a program probably has been developed at a University or by a Motor manufacturer.
I will try a brief explanation but I can’t attempt a calculation.
The worst resultant of unbalance is the creation of a negative sequence voltage system. The higher is the unbalance, the larger is the negative sequence. (If you are not familiar with electric circuits a negative sequence rotates in reverse direction as compared with the three phase main power phase sequence).
Then a rotating synchronous flux is created by the motor winding but rotating in reverse direction, the rotor bars cut this flux at a rate twice of the line frequency , resulting in high induced current in the rotor bars in spite of this negative sequence having low flux .
The I^2*R of the rotor increases generating heat and a negative torque which acts like a break. The stator winding reflects this rotor current with an increased current too and now the stator winding losses increase too.
The temperature rise increases and the conductors resistance increases too, then the losses increase.
Some times the rotor bars are melted.
My advice, “Try to keep as balanced phases as possible on lines feeding electric motors”
RE: How to calculate KW losses in a unbalanced motor
It referred to tests on a 100 hp motor at 75% load with a 1% voltage unbalance. It predicted current unbalance in the range of 6 to 10%. I observe conditions like that often. The efficiency loss was only 0.1%.
The same motor at 2.5% voltage unbalance had a 1.3% efficiency loss. In the absence of excessive failure rates, this is beginning to justify corrective action.
However, in my experience, voltage unbalance as large as 2.5% is uncommon. (It is typically accompanied by 25% current unbalance.)
The main thrust seems to remain motor life. Am I to conclude that unless we have a high failure rate there is no reason to worry about phase current unbalance?
RE: How to calculate KW losses in a unbalanced motor