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If field of synchronous motor is under excited power factor will be
Zero
Unity
Lagging
Leading
Lagging
A synchronous motor is said to be under-excited when its field excitation current is lower than that required to produce an induced back EMF (EbтАЛ) equal to the terminal voltage (VtтАЛ). In this condition, the motor consumes reactive power from the supply, which leads to a lagging power factor.
A synchronous motor is said to be under-excited when its field excitation current is lower than that required to produce an induced back EMF (EbтАЛ) equal to the terminal voltage (VtтАЛ). In this condition, the motor consumes reactive power from the supply, which leads to a lagging power factor.
EbтАЛ=4.44f╬жTkwтАЛ тАФ induced EMF formula
P=XsтАЛVtтАЛEbтАЛтАЛsin(╬┤) тАФ power developed by the motor
In a synchronous motor, the power factor is controlled by varying the DC excitation. For a given load, if the excitation is low (EbтАЛ<VtтАЛ), the motor requires reactive VARs to maintain the rotating magnetic field, acting like an inductive load. As excitation increases to the normal level, the motor reaches unity power factor (EbтАЛтЙИVtтАЛ). Increasing excitation further (over-excited) causes the motor to act like a capacitor, providing reactive power to the system and operating at a leading power factor.
Under-excited: EbтАЛ<VtтАЛ, consumes lagging reactive power.
Normal-excited: EbтАЛтЙИVtтАЛ, operates at unity power factor.
Over-excited: EbтАЛ>VtтАЛ, delivers leading reactive power.
The power factor can be adjusted by varying the DC excitation field current.
Ability to operate at leading power factor for power factor correction.
Constant speed operation under varying load conditions.
Not self-starting and requires external starting mechanism.
Requires both AC and DC supply systems.
Industrial drives requiring constant speed.
Power factor correction units in large power systems.
| Feature | Resulting PF | Nature |
|---|---|---|
Excitation Level | Under-excited | Over-excited |
The 'V-curves' of a synchronous motor plot the armature current against field current at various constant loads.
Option A is incorrect because the power factor cannot be zero; zero power factor implies pure inductance/capacitance without real power, while a motor always draws real power.
C is correct тАФ An under-excited synchronous motor acts as an inductive load and therefore operates at a lagging power factor.
Remember: A synchronous motor acts like an inductor when under-excited and like a capacitor when over-excited. Use the mnemonic 'U-L-O-L' (Under-Lag, Over-Lead).