Power Factor - E Tips BD

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Tuesday, February 13, 2018

Power Factor

 What is power factor?The power factor of an AC electrical power system is defined as the ratio of the real power to the apparent power in the circuit , its value between 0 and 1. OR cosφ is power factor, where φ is angle between voltage and current.


Unity, Leading and Lagging" Power Factors:
 Power factors are usually stated as "leading" or "lagging" to show the sign of the phase angle.

* With a purely resistive load current and voltage changes polarity in step and the power factor will be 1. Electrical energy flows in a single direction across the network in each cycle.

* Inductive loads: Transformers, motors and wound coils - consumes reactive power with current waveform lagging the voltage.

* Capacitive loads :  Capacitor banks or buried cables - generates reactive power with current phase leading the voltage.

What is the effect if power factor is above 1 or Unity Power Factor?
At unity power factor all active power used so it is real power and reactive power will zero, so      losses will be zero. If power factor is above one reverse current will be occurred (Example:   Motors will works as a generators)

The Power- factor at resonance in R-L-C circuit is-

Disadvantages of low power factor.
1) Line losses are 1.57 times those at unity power factor.
2) Larger generators and transformers are required.
3) Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to keep voltage drop within prescribed limits.
4) Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors will have to carry more current at low power factor. This requires a large conductor size.

Methods of improving power factor.
1) Static Capacitors: The static capacitors are connected in parallel with the load operating at lagging power factor.
2) A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.
3) Phase advancers: Are used to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous motor.  .


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