ELECTRICAL MACHINE DESIGN SYNCHRONOUS GENERATOR
An electrical machine is a device that converts mechanical energy to electrical energy or viceversa, and changes alternating current from one level to another level.
The machine which produces three-phase power from mechanical power is called an alternator or synchronous generator. An alternator works on the fundamental principle of electromagnetic induction. The Faraday law of electromagnetic induction states that when the flux linking a conductor changes, an emf is induced in the conductor. The alternator has two windings such as
(1) Armature winding and
(2) Field winding.
The armature winding is mounted on a stationary element called stator and field winding is mounted on a rotating element called rotor.
CONSTRUCTION OF ALTERNATOR;
An alternator has 3-phase winding on the stator and a d.c field winding on the rotor. The major parts are
It is the stationary part of the machine and is built up of sheet-steel laminations having slots on its inner periphery. A 3-phase winding is placed in these slots and serves as the armature winding of the alternator. The alternator winding is always connected in star and the neutral is connected to ground. The rotating field-stationary armature constructions offer following advantages:
(i) Better insulation,
(ii) Ease of current collection,
(iii) Increased armature tooth strength,
(iv) More rigid construction,
(v) Reduced armature leakage reactance,
(vi) Lesser number of slip rings,
(vii) Lesser rotor weight and inertia,
(viii) Improved ventilation and heat dissipation.
The rotor carries a field winding which is supplied with direct current through two slip rings by a separate dc source. Rotor construction is of two types, namely,
- Salient pole (or) projected pole type,
- Non-salient pole (or) cylindrical type.
Salient pole type:
The rotor of this type is used almost entirely for slow and moderate speed alternators, since it is least expensive and provides ample space for the field ampere turns. Salient poles cannot be employed in high speed generators on account of very high peripheral speed and the difficulty of obtaining sufficient mechanical strength.
The salient poles are made of thick steel lamination’s riveted together are fixed to rotor by a dove-tail joint. The pole faces are usually provided with slots for damper windings. These dampers are useful in preventing hunting. The pole faces are so shaped that the radial air gap length increases from the pole center to the pole tips so that the flux distribution over the armature is sinusoidal and waveform of generated emf is sinusoidal. The field coils are placed on the pole-pieces and connected in series. The ends of the field windings are connected to a d.c source through slip-rings carrying brushes and mounted on the shaft of the field structure.
Smooth cylindrical pole or non-salient pole:
The rotors of this type are used in very high speed alternators driven by steam turbines. To reduce the peripheral velocity, the diameter of the rotor is reduced and axial length is increased. Such rotors have 2n or 4 poles.
It consists of cylindrical steel forging which is suitably fabricated mechanically and treated thermally. The forging has radial slots in which the field copper, usually in strip form is placed. The coils are held in place by steel or bronze wedges and the coil ends are fastened by metal rings. The slots over certain portions of the core are committed to form pole faces. The regions forming the poles are usually left unslotted.
The frequency of output ac voltage of a synchronous generator is directly proportional to the rotor speed. To maintain constant frequency, the rotor must always move at the synchronous speed.
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