Basic Principle of Alternators
Basic Principle of Alternators:
Fig: 1 Sectional view of Alternator
- A.C. generators or alternators (as they are usually called) operate on the same fundamental principles of electromagnetic induction as d.c. generators.
- They also consist of an armature winding and a magnetic field. But there is one important difference between the two.
- Whereas in d.c. generators, the armature rotates and the field system is stationary, the arrangement in alternators is just the reverse of it.
- In their case, standard construction consists of armature winding mounted on a stationary element called stator and field windings on a rotating element called rotor. The details of construction are shown in Fig. 1. The stator consists of a cast-iron frame, which supports the armature core, having slots on its inner periphery for housing the armature conductors.
- The rotor is like a flywheel having alternate N and S poles fixed to its outer rim. The magnetic poles are excited (or magnetised) from direct current supplied by a d.c. source at 125 to 600 volts. In most cases, necessary exciting (or magnetising) current is obtained from a small d.c. shunt generator which is belted or mounted on the shaft of the alternator itself.
- Because the field magnets are rotating, this current is supplied through two sliprings. As the exciting voltage is relatively small, the slip-rings and brush gear are of light construction. Recently, brushless excitation systems have been developed in which a 3-phase a.c. exciter and a group of rectifiers supply d.c. to the alternator. Hence, brushes, slip-rings and commutator are eliminated.
- When the rotor rotates, the stator conductors (being stationary) are cut by the magnetic flux, hence they have induced e.m.f. produced in them. Because the magnetic poles are alternately N and S, they induce an e.m.f. and hence current in armature conductors, which first flows in one direction and then in the other.
- Hence, an alternating e.m.f. is produced in the stator conductors (i) whose frequency depends on the number of N and S poles moving past a conductor in one second and (ii) whose direction is given by Fleming's Right-hand rule. An alternator operates on the same fundamental principle of electromagnetic induction as a d.c. generator i.e., when the flux linking a conductor changes, an e.m.f. is induced in the conductor.
- Like a d.c. generator, an alternator also has an armature winding and a field winding. But there is one important difference between the two. In a d.c. generator, the armature winding is placed on the rotor in order to provide a way of converting alternating voltage generated in the winding to a direct voltage at the terminals through the use of a rotating commutator.
- The field poles are placed on the stationary part of the machine. Since no commutator is required in an alternator, it is usually more convenient and advantageous to place the field winding on the rotating part (i.e., rotor) and armature winding on the stationary part (i.e., stator).