The following devices take leading current from the supply line: 1. Static Capacitor or Condenser 2. Over-Excited Synchronous Motor 3. Phase Advancer.
Device # 1. Static Capacitor or Condenser:
Static capacitor takes a current which leads the applied voltage by about 90°. Thus, it acts in direct opposition to inductance, neutralizes the inductive influence of the circuit and improves the power factor.
There are two methods of connecting capacitor in the circuit. In one method every individual inductive load is provided with a capacitor of suitable capacitance in order to improve the power factor of that particular load circuit. In another method a big delta-connected capacitor bank (three similar capacitors connected in delta) is installed, usually at the supply mains, to improve the power factor of the whole installation.
Although the former method involves greater cost than the latter, it is more popular system as it provides greater flexibility in operation and helps to maintain power factor always at the desired value.
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The power loss in a capacitor is negligible and it is much less in first cost than any other device used for the improvement of power factor. Therefore among all the power factor correction devices, it has the largest application in different electrical circuits and installations.
Device # 2. Over-Excited Synchronous Motor:
When the direct current flowing through the field coil of a synchronous motor exceeds the normal rated value (for which the e.m.f. induced per phase in the armature is equal to voltage applied per phase at the motor terminals), the motor is said to be over excited. Under this condition the armature of the motor draws a leading current from the a.c. bus-bars, and as a result the power factor of the circuit is improved.
In a factory or in an electrical installation where large number of induction motors are used, it is economical to use one or two synchronous motors for constant-speed drive as well as for power factor correction. Since the over-excited synchronous motor takes a leading current from the supply line like a capacitor, it is also called Synchronous Condenser or Synchronous Capacitor.
Device # 3. Phase Advancer:
Phase advancer is seldom used for the improvement of power factor in an a.c. circuit, as this method involves very high cost. It is used occasionally with a big induction motor where the phase advancer supplies the reactive component of current taken by the motor. As a result the motor draws only the active component of current from the supply line and thereby the power factor of the motor circuit is raised almost to unity.