Parts of a Synchronous Machine

The synchronous machine is used widely in industry both as a generator for power generation as well as a motor for driving industrial equipment such as conveyors, pumps, crushers etc.

The synchronous machine has the following parts

Stator

The Stator of the Synchronous Machine consists of the Core.  The core is made of electrical steel.  It is made in the form of insulated laminations which are stacked together.  This prevents the flow of eddy currents in the core.  The core has slots.  The windings of the core are placed in the slots. The windings are made of copper.  The winding are preformed and placed in the slots.

Rotor

The rotor of the Synchronous Generator consists of a number of poles.  The number of poles depends on the speed and frequency of the machine.  The relation between the number of poles and speed or the frequency is N=120 x f /p ( where N is the speed in rpm, f is the frequency and p is the number of poles).

The rotor of the synchronous machine can be either salient or non-salient in construction.  In salient pole rotors, the poles are protruding from the rotor while in non-salient pole construction, the rotor windings are placed in slots machined in the rotor (See article on Salient and Non-salient Rotor Construction).Alternator Block Diagram

Excitor

The excitor can be imagined to be a small generator placed in the rotor.  The excitor provides the excitation power for the the excitation.  The excitor consists of a field winding in its stator.  The armature winding of the excitor is placed in the the rotor of the machine. 

Rotating Rectifiers

The output of the excitor is a 3 phase AC supply.  This supply is rectified by means of the rotating rectifiers which are fitted on the rotor shaft.  This rectifier assembly converts the AC power into DC.  The rectified DC is then supplied to the main rotor windings. 

Operation

When the machine is first started, residual magnetism in the excitor field winding induces a voltage in the excitor stator.  This 3 phase supply is rectified and fed to the field windings of the main rotor poles.  These poles get magnetized and induce the output voltage in the stator.

The control of the machine output voltage is done by a device called the AVR ( Automatic Voltage Regulator).  The output voltage of the machine is connected through a potential transformer to the AVR.  The AVR then regulates the excitation input based on the feedback received.  This a closed loop control.


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