what is the power factor in an AC motor?

Question

and how to improf it

Answer 1

Power Factor

For a DC circuit the power is P=VI, and this relationship also holds for the instantaneous power in an AC circuit. However, the average power in an AC circuit expressed in terms of the rms voltage and current is

where is the phase angle between the voltage and current. The additional term is called the power factor

Importance of Power Factor

A power factor of one or "unity power factor" is the goal of any electric utility company since if the power factor is less than one, they have to supply more current to the user for a given amount of power use. In so doing, they incur more line losses. They also must have larger capacity equipment in place than would be otherwise necessary. As a result, an industrial facility will be charged a penalty if its power factor is much different from 1.

Industrial facilities tend to have a "lagging power factor", where the current lags the voltage (like an inductor). This is primarily the result of having a lot of electric induction motors - the windings of motors act as inductors as seen by the power supply. Capacitors have the opposite effect and can compensate for the inductive motor windings. Some industrial sites will have large banks of capacitors strictly for the purpose of correcting the power factor back toward one to save on utility company charges.

You know that the voltage across a capacitor lags the current because the current must flow to build up the charge, and the voltage is proportional to that charge which is built up on the capacitor plates.

From the phase or diagram for AC impedance, it can be seen that the power factor is R/Z. For a purely resistive AC circuit, R=Z and the power factor = 1.

Answer 2

The power factor of an AC electric power system is defined as the ratio of the active (true or real) power to the apparent power.

Active (Real or True) Power is measured in watts (W) and is the power drawn by the electrical resistance of a system that does useful work.

Reactive Power, is measured in volt-amperes reactive (VAR). Reactive Power is the power stored in and discharged by the inductive motors, transformers or solenoids.

Apparent Power is measured in volt-amperes (VA) and is the voltage on an AC system multiplied by all the current that flows in it. It is the vector sum of the true and the reactive power.

When real and apparent power are equal, the circuit is said to have a power factor of 1 (unity). This is the ideal situation.

In actual practice, there is no such thing as a true unity power factor circuit. Inductive loads consume VARS, causing the power factor to drop below one. This makes the circuit less efficient, as more VA is required to produce the same amount of work (W). Most A.C. motors are inductive, which means they have power factors less than 1. The exact power factor varies from motor to motor. Very efficient motors have power factors of 0.9 or higher. Less efficient motors may be as low as 0.7.

Power factor can be improved by adding capacitors or switching to synchronous motors.

Answer 3

The powerfactor in an unloaded motor is less than one always.

so to imporve the power factor you have two choices. One keep motors as small as possible so as to keep them fully under rated load, that will produce a PF of unity. The second one is to correct the PF by adding capacatance to the circuit. (add capacator)