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Synchronous motor can be used as synchronous capacitor when it is
Under loaded
Over loaded
Under excited
Over excited
Over excited
A synchronous motor, when over-excited, operates with a leading power factor, allowing it to behave like a capacitor. In this state, it draws leading reactive current from the supply system to compensate for lagging reactive power loads, hence the name 'synchronous capacitor' or 'synchronous condenser'.
A synchronous motor, when over-excited, operates with a leading power factor, allowing it to behave like a capacitor. In this state, it draws leading reactive current from the supply system to compensate for lagging reactive power loads, hence the name 'synchronous capacitor' or 'synchronous condenser'.
EbтАЛ>V тАФ Condition for over-excitation
Q=VIaтАЛsin(╧Х) тАФ Reactive power produced
The excitation of a synchronous motor determines its power factor. When the motor is over-excited, the back EMF (EbтАЛ) becomes greater than the supply voltage (V), causing the motor to draw a leading current. This surplus of reactive power delivery acts as a shunt capacitor connected to the grid, which is why it is used for power factor correction.
Over-excited synchronous motor acts as a source of reactive power (leading VARs).
It is primarily used for voltage regulation and power factor improvement in power systems.
Synchronous condensers are often used at the receiving end of long transmission lines.
Unlike static capacitors, the reactive power output can be varied by controlling the DC excitation current.
Smooth and continuous control of reactive power.
Can also provide mechanical power if required.
Helps in maintaining voltage stability.
Higher maintenance costs due to rotating parts.
Higher losses compared to static capacitors.
Generates acoustic noise.
Power factor improvement in large industrial plants.
Voltage support in transmission systems.
Grid stability during sudden load changes.
A synchronous motor is under-excited when EbтАЛ<V, acting like an inductor with a lagging power factor.
Option C is incorrect because under-excitation causes the motor to consume reactive power rather than generate it.
D is correct тАФ A synchronous motor acts as a synchronous capacitor when it is over-excited, providing leading reactive power to the network.
Always remember: Over-excited synchronous motors generate reactive power (capacitive), while under-excited synchronous motors absorb reactive power (inductive).