Power factor
All electrical installations will have a power-factor, a relationship between the power (Kw) and the apparent power (Kva). This relationship will either have a lagging power factor, unity power factor or a leading power factor. Except in very specific instances the power factor will not be leading.
A lagging power factor.
This is where the current in a circuit reaches the maximum and minimum values later than the applied voltage has reached its maximum and minimum values. this circumstances is brought about by the presents of an inductance in the circuit. In industrial and commercial applications the most common inductive device is the AC motor. This is because by its construction it is built up of coils, it retards the growth and decay of current in the circuit, which causes a current to be out of phase with that applied voltage.
Power factor correction
Power factor is the ratio of watts to voltage amperes in a circuit, or power to apparent power.
Power Factor = Watt / Volts x Amperes = Cos q
Every load will require a specified power input and the general aim is to supply this while keeping the volt-ampere demand to a minimum, i.e. the power factor to a maximum of as near unity as possible.
The main cause of a low power factor in industrial plant is the induction motor which is the most widely used industrial drive. Particular note should be taken for the values at reduced load. For this reason as well as others, motor should not be oversized on installation, and motors under loaded due to cyclic loads will have a similar effect. The reason for this is that the cause of lagging power, is the magnetizing current drawn by the motor, which is constant, irrespective of load. On light load, therefore, the proportion of lagging current to the total current is increased with a consequent reduction in power factor.
Methods of Correction
Compensation may be effected by either rotary or static equipment and the following is a list of the general methods available. However, for the vast majority of installations static capacitors would be the automatic choice. For that reason while giving the full list the discussion is confined to static capacitors.
Static capacitors,
Synchronous induction motors,
Salient pole synchronous induction motors,
Induction motors with phase advancers
Synchronous condensers.
Monday, March 16, 2009
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