Why is the power factor important?

Question

Why is the power factor important?
about the phase relationship of voltage and current

Answer 1

Power factor is important because it is one of the terms in he equation for AC power:
Power (watts) = Voltage (volts) X Current (amps) X Power Factor

Power factor is a number between 0 and 1 that indicates how much of the apparent power (volt-amperes or VA) is real power. Part of the power that flows between the source and load may actually be stored by the load for a fraction of a cycle and then returned to the source. Energy is stored by inductance and capacitance in loads. Loads that are purely resistive do not store energy. Motors are responsible for most of the energy storage.

Since most of the cost of generating energy is due to the real power, utilities measure and charge for real power rather than volt-amperes. Since high volt-amperes cuts into the capacity of the utility to generate real power and reduces the efficiency of power generation and transmission, the utility may charge a low power factor penalty to customers that have low power factor loads. Customers can avoid the penalty by installing capacitors to allow power to circulate locally between the capacitors and motors rather than between the utility and the motors.

Answer 2

Power companies sell power, that is, they sell real power not imaginary power. Real power (measured in kW) is power where the voltage and current are in phase with each other, just as it is when it leaves the generator at the power plant.

Imaginary power (measured in kVA) occurs when the current is out of phase with the voltage, and happens when the power customer uses a lot of high-inductance motors and other high inductive loads (heaters would be another example).

When the customer starts breaking apart the phase of the current with the voltage, the power company cannot measure it accurately (they measure real power on kW-hr meters). The power company doesn't like that because they are sending more real power than the customer is paying for. The amount of kVA used divided by the kW measured is called the power factor, and it is always less than 1.

So, if the power factor is say, 0.8, that means the customer is only paying 80% of the amount they should be paying for real power, even though their motors are still spinning and using 100% of kVA-hr energy.

The power factor is important to the power company. They need to know what power factor is being caused by the customer and all his inductive loads, so they can know how to correct the power bill with the proper sur-charge -- or correcting it by adding the proper number of capacitors on the power lines.

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