Why do we give DC excitation to AC 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. … 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.

What is role of DC exciter in synchronous motor?

Synchronous motor excitation refers to the DC supply given to rotor which is used to produce the required magnetic flux. … This resultant air gap flux is established by the co operation of both AC supply of armature winding and DC supply of rotor winding.

What is the use of DC excitation in AC generator?

A generator has a prime mover like a turbine or diesel generator. The excitation system creates the electromagnetic field in the rotor. The stator has the armature winding that has the electrical energy induced. The stronger the magnetic field created, the stronger the electrical power produced.

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Why do we have to provide a DC excitation voltage to the synchronous generators rotor?

DC current is a steady, unchanging current. When we apply it to the rotor, it creates a steady, predictable magnetic field. An increase or decrease in the excitation voltage increases or decreases the strength of the magnetic field. This allows voltage regulation.

Why DC supply is necessary to start a synchronous motor explain in details?

In synchronous motors, the stator has 3 phase windings and is excited by 3 phase supply whereas the rotor is excited by DC supply. The 3 phase windings provide rotating flux whereas the DC supply provides constant flux. … Due to this, the motor cannot start on its own.

What is DC excitation system?

What is an Excitation System? Definition: The system which provides DC to the synchronous machine field winding to perform protective & control functions of the power system. This system consists of exciter, PSS (Power System Stabilizer), AVR (Automatic Voltage Regulator), processing unit, and measuring elements.

Why pilot exciter is used?

A pilot exciter is a small AC generator with a permanent magnet mounted on the rotor shaft and the three-phase winding on the stator. It provides the field current of the exciter. … Thus, the use of a pilot exciter makes the excitation of the main generator completely independent of external supplies.

Why do synchronous generators need excitation?

Excitation in an electrical machine means production of magnetic flux by passing current in the field winding. In order to excite the field winding on the rotor of the synchronous machine the direct current is required.

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What happens when DC is cut off from synchronous motor?

In case the field excitation of Generator is available when the steam supply to the turbine is cut off, the Synchronous Generator will draw power from the Grid and will behave as a Synchronous Motor rotating in the same direction.

What is AC excitation?

The excitation system on a AC alternator refers to the way the alternators voltage is initially built when rotated and controlled while in use. The excitation system is responsible for supplying the field current to the main rotor.

Why does DC excitation is used rather than AC excitation?

So if we give AC supply the pole created in the rotor winding will be alternating and it not be created any magnetic locking with the stator field. So if we give AC supply, the rotor will stop rotating after removing the prime mover. This was the reason Why DC is used not AC for excitation of Synchronous Motor.

What is excitation in DC motor?

The magnetic flux in a d.c machine is produced by field coils carrying current. The production of magnetic flux in the device by circulating current in the field winding is called excitation.

How is excitation obtained in an AC generator?

An electric generator or electric motor consists of a rotor spinning in a magnetic field. The flexible excitation of a rotating electrical machine is employed by either brushless excitation techniques or by the injection of current by carbon brushes (static excitation). …

Why does synchronous motor runs at synchronous speed?

Because of the magnetic coupling between the stator poles and rotor poles the motor runs exactly at synchronous speed.

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Why a synchronous motor is not self starting?

Starting methods

Above a certain size, synchronous motors are not self-starting motors. This property is due to the inertia of the rotor; it cannot instantly follow the rotation of the magnetic field of the stator.

Why are synchronous motors not started under load?


Synchronous motors are inherently not able to self-start on an AC power source with the utility frequency of 50 or 60 Hz. As stated in Chapter 1, this is because synchronous motors can develop a torque only when running at the synchronous speed.