Motor-Generator Set, Power Factor, True Power (kW )

[Friction1]

Hi, looking for thoughts on estimating Power Factor (and kW) for a synchronous motor in a Motor-Generator (MG) set.

This involves a Motor Generator set used in steel milling process. 1350 HP AC synchronous motor (General Electric):
900 RPM, 4160 V, 1380 KVA, PF 0.80, 3 Phase, 60 Hz, 187 Armature Amps.

Two verified data points:
1. Full load. Amps = 150.
2. No load. Amps = 112.

During no load motor and generator are running, but end-use mill load is not.

Obviously nameplate lists 0.80, but doesn't PF approach unity if you're running a synchronous motor at full speed.
I've assumed an 80% load. Facility has sub-station in-house at 21,000V and steps down to 4160V with transformers.

kW = V x I x PF x Root-3. I've had a hard time tracking down good data for Power Factor in synchronous motors of this size.

 

[aolalde]

The power factor (PF) for any Synchronous motor is a function of the load and field excitation. For a given fix load the excitation current will modify the power factor.
The locus developed by the line current (IL) compared to the field excitation (IF) is known as “V-Curve” of a synchronous motor.
Fix a certain load (HP) and reduce the field DC current, the AC line current will start to increase. That is because the motor PF is “lagging”, such a line current increase will continue while weakening the field until the motor reaches the pullout condition, stalling the rotor and the current increases drastically, at this point the over current protection should trip the motor off line.
Now start increasing the excitation and the line current starts to decrease until it reaches a minimum value and then it starts increasing again. The PF now is “leading”.
When the power factor is “lagging”; the motor consumes reactive power (kVAR) from the line, if the PF is “leading” the motor delivers reactive power (kVAR) to the line. At the minimum line current the PF is unity and the motor has zero reactive power.All this performance is due to the strength of the magnetic field being proportional to the excitation current.

A synchronous motor always runs at synchronous speed (ws); ws = 120*f/p, f = line frequency (HZ), p = motor poles.

IL = HP*746 / (1.732*Vll*EFF*PF)

PF = HP*746 / (1.732*Vll*EFF*IL)

 

[rbulsara]

I am not sure what type of protection you have on this machine.

At minimum adding a power factor meter (or a electronic meter reading all parameters) will be a good investment for this size of machine and also at major disribution boards.

Better yet consider upgrading the motor protection with a modern microproncessor based multi-function motor protection relay which will proide a better protection plus all metering functions.