Power Factor Control in Alternators and Synchronous Motors
The sources provide a detailed explanation of how the power factor of an alternator and a synchronous motor can be controlled.
Alternators
- The power factor of an alternator is affected by the load power factor.
- For a unity power factor load, the armature reaction is cross-magnetizing, meaning it distorts the main field flux but doesn’t weaken or strengthen it.
- For a lagging power factor load (inductive load), the armature reaction weakens the main field flux.
- For a leading power factor load (capacitive load), the armature reaction strengthens the main field flux.
- The change in terminal voltage due to armature reaction depends on the armature current and the load power factor.
Synchronous Motors
- The power factor of a synchronous motor can be controlled by adjusting the field excitation.
- Under-excited: If the rotor excitation is less than normal, the motor operates at a lagging power factor and draws reactive power from the supply.
- Normal excitation: If the rotor excitation is such that the back EMF (Eb) equals the supply voltage (V), the motor operates at unity power factor and does not draw reactive power from the supply.
- Over-excited: If the rotor excitation is greater than normal, the motor operates at a leading power factor and delivers reactive power to the supply.
Phasor Diagrams for Synchronous Motors
The sources provide phasor diagrams to illustrate the relationship between the back EMF (Eb), the supply voltage (V), the armature current (Ia), and the power factor angle (φ) for different excitation levels in a synchronous motor.
Under-excited:
- Eb is smaller than V.
- Ia lags behind V.
- The motor operates at a lagging power factor.
Normal excitation:
- Eb equals V.
- Ia is in phase with V.
- The motor operates at unity power factor.
Over-excited:
- Eb is larger than V.
- Ia leads V.
- The motor operates at a leading power factor.
Synchronous Condenser
- An over-excited synchronous motor operating at no load is known as a synchronous condenser.
- A synchronous condenser acts like a capacitor and can be used to improve the power factor of a system.
- When a synchronous condenser is connected in parallel with an inductive load, the leading reactive power it supplies partially neutralizes the lagging reactive power of the load, improving the overall power factor.
The sources provide illustrations with phasor diagrams to explain how a synchronous condenser improves the power factor of a system