Tuesday, February 22, 2011

Power Factor (In Detail) #3

Related Posts
Power Factor #1
Power Factor #2

From our previous discussion we saw that Power Factor can be defined in 3 definitions. ie
  • The cosine of an angle between Voltage (V) and Current (I)
  • True Power / Apparent Power
  • Resistance / Impedance
Please note that in the above expressions, the value of Power Factor can never be greater than 1. A good power factor is always 0.9 or 0.89

In this post we are going to look into detail the Power Factor of angle between Voltage(V) and Current (I)

The power factor PF is mathematically defined as the cosine of the phase angle between voltage and current. True power is defined by the equation . The graphical representation produces a product of the voltage and current. This product is represented as a sine wave, which is twice the frequency of the voltage and current and represents the instantaneous power and its direction. If the power sine wave is completely on the positive side of the axis, it shows that the power in the system is traveling only in one direction, toward the load.

The area under this power sine wave curve represents the amount of energy delivered to the load. If the power sine wave is shifted, the difference in area between the positive and the negative sides represents the power delivered to the load. The negative side of the power sine wave represents the reflected power from a reactive load.

A lagging power factor is one in which the current is lagging behind the voltage and is characteristic of an inductive load.

A leading power factor is one in which the current is leading the voltage and is characteristic of a capacitive load. If the phase angle were to be shifted to be greater than 90 degrees in either direction, the load would effectively become the source.

Power Factor Correction
A power factor of one (1) or "unity power factor" is the goal of any electric utility company since if the power factor is less than one (<1) , 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.

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