A.C. generators or alternators (as they are usually called) operate on the same fundamental principles of electromagnetic induction as D.C. generators.

    Alternating voltage may be generated by rotating a coil in the magnetic field or by rotating a magnetic field within a stationary coil. The value of the voltage generated depends on-

                     

the number of turns in the coil.

                     

strength of the field.

                     

the speed at which the coil or magnetic field rotates.   

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Salient pole type alternator

Main parts of the alternator, obviously, consists of stator and rotor. But, the unlike other machines, in most of the alternators, field exciters are rotating and the armature coil is stationary.

Stator: Unlike in DC machine stator of an alternator is not meant to serve path for magnetic flux. Instead, the stator is used for holding armature winding. The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses.

The Electromotive Force, E_emf, induced in a Coil is proportional to the number of turns, N, in the Coil and the Rate of Change, dΦ / dt, of the number of Magnetic Flux Lines, Φ, passing through the surface (A) enclosed by the Coil.

An Induced Effect is always such as to Oppose the cause that produced it.

Figure 2: Effective Area of the Wire Loop at Different Rotational Angles

The Rate of Change of Φ, dΦ / dt, is the largest at the zero points of the Waveform and is the smallest at the peaks of the Waveform, therefore the Induced E_emf is maximum at the zero points and minimum at the peaks,

The working principle of an alternator or AC generator is similar to the basic working principle of a DC generator.

Above figure helps you understanding how an alternator or AC generator works.  According to the Faraday's law of electromagnetic induction, whenever a conductor moves in a magnetic field EMF gets induced across the conductor. If the close path is provided to the conductor, induced emf causes current to flow in the circuit.

Now, see the above figure. Let the conductor coil ABCD is placed in a magnetic field. The direction of magnetic flux will be form N pole to S pole. The coil is connected to slip rings, and the load is connected through brushes resting on the slip rings.

Now, consider the case 1 from above figure. The coil is rotating clockwise, in this case the direction of induced current can be given by Fleming's right hand rule, and it will be along A-B-C-D.

As the coil is rotating clockwise, after half of the time period, the position of the coil will be as in second case of above figure. In this case, the direction of the induced current according to Fleming's right hand rule will be along D-C-B-A. It shows that, the direction of the current changes after half of the time period, that means we get an alternating current.

Salient pole type alternator

Main parts of the alternator, obviously, consists of stator and rotor. But, the unlike other machines, in most of the alternators, field exciters are rotating and the armature coil is stationary.

Stator: Unlike in DC machine stator of an alternator is not meant to serve path for magnetic flux. Instead, the stator is used for holding armature winding. The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses.

A.C. generator forces electric current to flow through an external circuit. The source of mechanical energy  may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air ,or any other source of mechanical energy.

It is based on the principle of electro magnetic induction, i.e., whenever amount of magnetic flux linked with a coil changes, an e.m.f. is induced in the coil. The direction of current induced is given by Fleming’s right hand rule.

Theory

1.The strong magnetic  field  is  produced  by  a current flow through the field coil of the rotor.

2.The field coil in the rotor receives  excitation  through  the use  of  slip  rings  and  brushes.

3.Two brushes are spring-held in contact with the slip rings to provide the continuous connection between the field coil and external circuit.

4.The armature is contained within the windings of the stator and is connected to the output.

5.Each time the rotor makes one complete revolution, one complete cycle of AC is developed.

6.A generator has many turns of wire wound into the slots of the rotor.

7.The magnitude of AC voltage generated by an AC generator is dependent on the field strength and speed of the rotor.

8.Most generators are operated at a constant speed; therefore, the generated voltage depends on field excitation, or strength.

The A.C. Generator is consist of four main parts :

A rectangular coil ABCD consist of a large number

of turns of copper bound over a soft iron core is called armature. The soft iron core is used to increase the magnetic flux.

It is usually a permanent sponge magnet having concave poles. The armature is rotated of a magnet so that axis of the armature is perpendicular to magnetic field lines.

(3) SLIP RINGS :

Slip rings are the magnetic rings which are connected

in the terminal of the armature. These rings are rotated 

with the coil and these are use to draw the current from 

the generator.

The brushes B1 & B2 are just touch the slip rings. They are not rotating with the coil and these brushes leads to the output of load resistance.

Working of an A.C. Generator

The coil is rotated in anti-clock wise direction. In the first half rotation the arm AB is moving outward and  CD is moving inward. So the e.m.f. is induced in the arm AB from A to B. And in the arm CD from C to D. After half rotation (in the second half). The arm CD is moving outward and AB is moving inward. In this time current is induced in arm  CD from D to C. And in arm AB from B to A. In the second half rotation the current direction is changing so in this generator AC is produced.

1.  Aircraft auxiliary power generation, wind generators, 

    high speed gas turbine generators.

2.  Hybrid electric vehicle (HEV) drive systems, automotive 

    starter generators.

3.An ac generator, or 'alternator', is used to produce ac 

    voltages for transmission via the grid system or, locally,

    as portable generators.

4.All of our household appliances runs on ac current. 

    Ex: Refrigerator, washing machines, oven, lights, fan etc.    

5. The main advantage of AC is ease of power distribution. 

     It is more efficient to use high voltage to distribute power, 

     but it is not safe to have high voltage at home. It is easy to 

     step up (and step down) AC voltage using a transformer.