Synchronous generators or alternators or ac generators operate on the
same fundamental principle of electromagnetic induction.
In alternator,
the armature winding is placed on stator and rotor is magnetized by a d.c source to
produce magnetic poles. The d.c source is either bolted on the rotor shaft itself (in
case of small generators) or connected from external circuit by use of slip-ring
brush arrangement (in large synchronous generators).
- The rotor is made to rotate in clockwise direction by a prime mover. Due to rotor rotation, field flux cuts stationary stator conductors, connected either in star or delta. Thus, an emf is induced in them by the law of electromagnetic induction.
- If load is connected to the generated 3 phase output ac (shown in figure), circuits are closed and currents starts flowing in the stator winding.
- As per Lenz's law, the direction of the induced currents must be such as to oppose the very cause of their generation. Here, the cause is the rotor rotation.
- So, the induced coil currents flow in such a direction so that the the magnetic poles formed in stator attract the rotor and tend to stop its rotation.
- Mechanical work must be done in overcoming this attraction and to maintain rotor rotation. This work done is ultimately converted to electrical energy.
