Why DC is used not AC for excitation of Synchronous Alternator and Motor?

Excitation of Synchronous Machine

We know that the Synchronous Alternator and Synchronous Motor are almost the same in construction. You may notice DC supply is given for the excitation of the synchronous alternator and motor. So today we will discuss  Why DC is used not AC for the excitation of the Synchronous Alternator and Motor?

Also, we will discuss what happens if we give AC supply to the synchronous machine for excitation?

First of all, let's take a look at the construction of Synchronous Alternator(generally called Alternator) and Synchronous Motor.

Synchronous Motor Construction

The synchronous motor has the winding arrangement as same as the synchronous alternator. In fact, we know that during fault condition a synchronous alternator run as a synchronous motor.

In the synchronous motor also, armature winding is the static part and main field winding is the rotating part of the machine.

To operate the synchronous motor we have to need to give the main AC supply to the static armature winding and an auxiliary DC supply to the rotating field winding. The interaction of both windings will create a torque to rotate the motor.

Synchronous Alternator Construction

Synchronous alternator or Alternator has the stator and rotor arrangement opposite of a DC generator. Remember that we are talking about the three-phase alternator.

Generally, In a DC machine, the armature is placed in the rotor, and field winding is placed in the stator that means in a DC machine Armature is the rotating part and main field winding is the static part. But in the case of the phase alternator, the Armature is the static part and the main field is the rotating part. In a DC motor, the power supply is applied to both armature winding and field winding but in an induction motor, the power supply is applied to the armature winding only.

Why DC is given to the field winding of Synchronous Motor?

When AC supply is given to the stator winding or static armature winding of the synchronous motor it creates a rotating magnetic field. The speed of the rotating magnetic field is called synchronous speed.

So the rotor will rotate at synchronous speed if a magnetic locking is created between the stator field and rotor field.

If we give DC supply to the field winding or rotor winding of the synchronous motor then the pole created in the rotor will be constant.

Now we can use a prime mover to rotate the rotor initially to create the magnetic locking between the stator field and rotor field. Once the magnetic locking between the stator field and rotor field is created we can remove the prime mover.

What happens if we give an AC supply to the field winding of the Synchronous Motor?

Now a question is raised What happens if we give AC supply to the field winding of the Synchronous Motor?

The simple answer is if we give AC supply to the field winding of the synchronous motor then the magnetic locking between the stator field and rotor field will not be created.

We know that the opposite poles can attract each other. So the locking will be created if the stator and rotor pole are opposite to each other. So if we give an AC supply the pole created in the rotor winding will be alternating and it not create any magnetic locking with the stator field.

So if we give an AC supply, the rotor will stop rotating after removing the prime mover.

This was the reason Why DC supply is used not AC for excitation of Synchronous Motor.

Why DC is used instead of AC for the excitation of the Synchronous Alternator?

In rotating machines(both generator and motor), Dynamically Induced EMF works not Statically induced EMF. When the flux is constant and the conductor changes its position then the induced emf is called Dynamically induced emf. When the conductor is constant and the flux is changing then the induced emf is called statically induced emf.

In the alternator, as the rotor conductor changes its position as the rotor rotates so constant flux is to be produced in the rotor winding and it is possible only when the DC supply is given to the rotor winding.

So the AC creates alternating flux and DC creates Constant flux. As we need constant flux that is why DC is used instead of AC for excitation of Synchronous Alternator.

Read Also:

• What is Synchronous Speed and Asynchronous Speed in Induction Motor?
• Induction Motor Advantages and Disadvantages Explained
• Advantages of both STAR and DELTA connection in Induction Motor
• Why the Rotor of Slip Ring Induction Motor always Star Connected?
• Actual Difference Between Synchronous Motor and Induction Motor

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