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Induction motor magnetising current and power factor at duty point

Induction motor magnetising current and power factor at duty point

Induction motor magnetising current and power factor at duty point

(OP)
We have several Squirel cage induction motors running on our plant at well below their power rating   e.g. 345kW 3.3kV four pole 50Hz motor full load current 75 amps. with a power factor of 0.8 at full rated power. I understand that if you run a motor at well below its rated capacity
(in this case absorbed power 146 kW ) The power factor drops markedly . can anyone help with the formulae and calculations to work out the power factor at duty point and the magnetising current.

RE: Induction motor magnetising current and power factor at duty point

Use the equivalent circuit of the motor. You'll find the circuit in books on power system analysis. The circuit and a program for the parameters can also be found at http://pp.kpnet.fi/ijl/prog.htm

Alternatively, use a computer program for power system analysis to calculate the current etc, for example at http://pp.kpnet.fi/ijl
 

RE: Induction motor magnetising current and power factor at duty point

Simply put, the magnetizing current is constant when voltage is constant.

It can be calculated as In*sqrt((1-cos(phi)^2). In your example the magnetizing current is 75*sqrt(1-.8^2) = 75*.6 = 45 A while the torque producing current is 75*.8 = 60 A.

This current stays roughly constant when shaft load changes so, at 146 kW, your torque producing current will be 60*146/345 = 25.4 A. Your total current will then be around 65 A and the PF is 25.4/65 or around .4 not so good.

This is a rough calculation, but the result should not be too far off the actual value.

 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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