3-Phase AC Generator or Alternator

3-PHASE AC GENERATOR OR ALTERNATOR

An alternator works on the principle of electromagnetic induction. If a conductor placed in a moving magnetic field an emf is induced in the stationery conductors per Faraday's first law of electromagnetic induction.

1. Construction

● The two major parts of AC generators are stator and rotor. The construction of ac generator is shown in fig. 2.43

● Stator: The stator consists of a cast iron or mild steel frame, which supports the armature core. This frame acts as an enclosure and provides a closed path for the magnetic flux. The armature core is made of laminated sheets.

● The material for armature core may be special magnetic iron or steel alloys. The inner periphery of the armature core is slotted, in order to accommodate the armature winding.

● The armature winding may be single layer or double layer. The 3-phase armature winding should be a balanced one. The number of turns and size of wire should be equal for all the 3-phase winding and the 3-phase winding are displaced in space by 120° (electrical) between them.

● The three ends of three phase winding are connected together to make the star print, and the other three terminals of three phase windings are brought out of the generator.

● Rotor: The rotor is like a flywheel having alternate N and S poles on its outer periphery. These electromagnets are magnetized by means of low dc voltage of 125 or 250V. As rotor and hence the field magnets are rotating, this dc excitation voltage is given through slip rings which are fixed on the frame.

There are two types of rotors used in ac generators.

(i) Salient or projecting pole type : It is used for engine driven generators, which are run at low and medium speed. The rotor has even number of projecting poles, whose cores are boiled to a heavy magnetic wheel of cast iron. The axial length of rotor is short and the diameter is large.

(ii) Smooth cylindrical type : It is used for steam turbine driven generators or turbo alternators which are run at high speed. The rotor is made up of cast iron and cylindrical in shape. The outer periphery of the rotor is slotted to receive the field windings. The field windings are wound, such that N and S poles occur alternatively. The number of field poles may be two or four. The axial length of rotor is large and its diameter is less.

2. Working of Alternator

The field magnets are magnetized by applying 125 volts or 250 volts through slip rings. The field windings are connected such that, alternate N and S poles are provided. The rotor and hence the field magnets are driven by the prime movers (steam driven turbine or engine driven).

As the rotor rotates, the armature conductors are cut by the magnetic flux. Hence emf is induced in the armature conductors. As the magnetic poles are alternatively N and S pole, the emf acts in of direction and then in the other direction. Hence the alternating emf induced in the stator conductors.

The frequency of induced emf is depends on the number of N and S poles moving past an armature conductor in one second. The frequency of induced emf is given by,

f = PN / 120; 

P → No. of magnetic poles; N→ Speed of rotor in rpm.

The direction of induced emf can be obtained by Fleming's Right hand rule.