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An over excited synchronous motor takes
Leading current
Lagging current
Both of above
None of above
Leading current
An over-excited synchronous motor acts as a synchronous condenser, which draws leading current from the source. This behavior is fundamentally due to the excitation current exceeding the value required for unity power factor operation, causing the motor to supply reactive power to the system.
An over-excited synchronous motor acts as a synchronous condenser, which draws leading current from the source. This behavior is fundamentally due to the excitation current exceeding the value required for unity power factor operation, causing the motor to supply reactive power to the system.
EbтАЛ=4.44f╬жTpтАЛhKwтАЛ тАФ Back EMF equation
IaтАЛ=ZsтАЛVтИТEbтАЛтАЛ тАФ Armature current relationship
In a synchronous motor, the back EMF (EbтАЛ) depends on the field excitation. When the motor is over-excited, EbтАЛ becomes greater than the supply voltage (V). To maintain equilibrium, the motor draws a leading current component to produce a demagnetizing effect, effectively making the motor behave like a capacitor.
Over-excitation means field current (IfтАЛ) is greater than normal excitation.
Synchronous motors can provide power factor correction for industrial loads.
The armature current IaтАЛ leads the terminal voltage V by an angle ╧Х.
Under-excited motors draw lagging current (behave like an inductor).
Improves overall power factor of the distribution system
Helps in maintaining constant terminal voltage at the load end
Additional cost for excitation system
Requires complex starting mechanism
Power factor improvement in large industrial units
Voltage regulation in long transmission lines
| Feature | Lagging Current | Leading Current |
|---|---|---|
Excitation Level | Under-excited | Over-excited |
Normal excitation: EbтАЛ=V, power factor is unity.
Option B (Lagging current) occurs when the motor is under-excited (EbтАЛ<V).
A is correct тАФ An over-excited synchronous motor operates with a leading power factor, drawing leading current from the supply.
Remember the mnemonic 'OLD': Over-excited = Leading, Lagging = De-excited (Under-excited).