A synchronous motor is operating on no-load at unity power factor. If the field current is increased, the pf will becomea)leading and the current will increase.b)lagging and the current will decrease.c)leading and the current will decrease.d)lagging and the current will increase.Correct answer is option 'A'. Can you explain this answer?

Question

Answer

Explanation:
When a synchronous motor operates at no-load, the power factor is unity, which means the current and voltage are in phase. This is because there is no mechanical load on the motor shaft, so it only needs to overcome the losses and supply the magnetizing current to the field winding.

Effect of increasing field current:
When the field current of a synchronous motor is increased, it strengthens the magnetic field produced by the rotor. This has the following effects:

1. Change in power factor:
When the field current is increased, the motor starts to operate at a leading power factor. This is because the increased magnetic field produced by the rotor pulls the stator current ahead of the voltage waveform. This leading power factor can be adjusted by controlling the field current.

2. Increase in current:
Increasing the field current also affects the stator current. As the magnetic field becomes stronger, it induces a higher electromotive force (EMF) in the stator windings. This causes the stator current to increase to maintain the equilibrium condition.

Summary:
So, when the field current of a synchronous motor operating at no-load is increased, the power factor becomes leading and the current increases. This is because the stronger magnetic field leads to a leading power factor and induces a higher stator current.

Answer

In a synchronous motor operating at no-load and unity power factor, the power factor is equal to 1. This means that the current and voltage are in phase, resulting in a purely resistive load.

When the field current of a synchronous motor is increased, it strengthens the magnetic field produced by the rotor. As a result, the motor's excitation increases, causing the voltage to lead the current.

With an increased field current, the power factor of the synchronous motor will shift to a leading power factor. This means that the current will lead the voltage, resulting in a leading power factor.

Additionally, when the field current is increased, the current drawn by the motor will also increase. This is because a stronger magnetic field requires more current to maintain.

Therefore, the correct answer is A: leading and the current will increase.

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