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Displacement Power Factor

What is Displacement Power Factor?

Displacement Power Factor is caused by a reactive component in the load. If there is an inductive compinent in the load, then there will be an inductive current flowing in addition to the resistive current. The inductive curent follows the voltage waveform by 90 degrees. Likewise a capacitive component causes a capacitive current that leads the voltage waveform by 90 degrees. The vector sum of the reactive (capacitive and/or inductive) current(s) and the resistive current results in a single current with a phase angle before (leading) or after(lagging) the voltage waveform. The displacment power factor value is the cosine of the angle between the voltage waveform and the resultant current waveform.
Displacement power factor is typically decreased by inductive loads such as induction motors, transformers and lighting ballasts.

 

Power Factor of an Induction Motor

Reference to the equivilent circuitof an induction motor, shows that the induction motor draws current from the supply that is made up of resistive components and inductive components. The resistive components are:
    1)  Load current.
    2)  Loss current.
and the inductive components are:
    3)  Leakage reactance.
    4)  Magnetizing current.

The current due to the leakage reactance is dependant on the total current drawn by the motor, but the magnetizing current is independent of the load on the motor. The magnetizing current will typically be between 20% and 60% of the rated full load current of the motor. The magnetizing current is the current that establishes the flux in the iron and is very necessary if the motor is going to operate. The magnetizing current does not actually contribute to the actual work output of the motor. It is the catalyst that allows the motor to work properly. The magnetizing current and the leakage reactance can be considered passenger components of current that will not affect the power drawn by the motor, but will contribute to the power dissipated in the supply and distribution system. Take for example a motor with a current draw of 100 Amps and a power factor of 0.75 The resistive component of the current is 75 Amps and this is what the KWh meter measures. The higher current will result in an increase in the distribution losses of (100 x 100) /(75 x 75) = 1.777  or a 78% increase in the supply losses.