Motor Power Usage

[Jnox]

This is a simple question. I am trying to calculate electricity usage of a large motor:

3 Phase
20,000 HP
Voltage 13800
Idling: 280 Amps
Motor PF Rated: 0.8
Actual PF: 0.2

KW = (13800 * 280 * 1.73 * 0.2)/1000 = 1336.9KW

My question is, power factor plays a big role when it comes to power usage of a motor. A power factor or 0.2 is much worse than PF of 0.8. Wouldn't it make sense for the KW's of motor usage to be greater with worse PF of 0.2?

-Jack

 

[waross]

The KW is load dependent.
The KVAR is fairly constant.
PF describes the relationship between KW KVA and KVAR.
20% PF at idle is common.
When running unloaded the real power is much less than when loaded and the reactive power is about the same unloaded as loaded.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Kilowatts are a measure of real power. So when a motor is idling at no load, it is not using much actual power and much of the current being drawn is simply being used to provide the excitation of the stator to create the magnetic field needed to make the motor operate.

The excitation current is highly inductive, so it has a very low power factor.

The low power factor of 0.2 tells you that the current being drawn is mostly excitation current and lags the voltage by almost 90 degrees. Very little real work is being done in the motor - just enough energy to turn the shaft and overcome the friction, windage and other losses in the motor.

When a load is applied to the shaft of the motor, the torque required goes up, the real power required goes up and the power factor goes up.

 

[Jnox]

Thanks guys, it makes perfect sense now!