Synchronous machine:
An alternator has two main parts:
1) stator
2) rotor
1) stator: the stator is the stationary part of the alternator. It consists of stator core and 3 phase distributed winning. Emf is generated in this winding. Distributed winding is known as armature winding.
The stator core is made up of laminated stamping and to receive the armature winding slots are available on its circumference. This stamping is made up of silicone steel. Varnish is used to provide insulation from each other.
This stamping is housed in the yoke consisting of magnetic material. Yoke is made up of cast iron or silicone steel.
Laminated core is used to reduce the Eddy current losses. The winding is star connected.
2) rotor:
The rotor is a rotating part of the alternator. It produces main magnetic field. Field winding is placed in rotor. A DC current is used in the field winding through two slip rings from the DC source which is known as exciter.
Rotor has N and S pole fixed to its outer rim. It has rotating magnetic poles. These poles are are separately excited from a exciter.
3) exciter:
alternator require a DC winding connected with field winding with the help of slip ring. The function of the exciter is to regulate and supply DC current to the field winding of alternator. If you change the exciter current it results change in terminal voltage. So by controlling excitation current, you can control the terminal voltage of field winding.
Exciter is placed at the shaft of the motor and this nothing but a DC shunt ot compound dynamo. Voltage rating of exciter is generally 110 V to 600 V DC.
Now a day, brush less excitation system is used in large alternator. For this, rectifier circuit is used to convert AC power to DC power. This AC power is supply from the output of the alternator itself and output of rectifier is connected with field windings. So, it will reduce the maintenance cost of alternator and it will reduce brush loss so efficiency also increase.
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