AC Generator

 

Introduction -  AC Generator

An Electrical Generator is a device that produces an Electromotive Force (e.m.f.) by changing the number of Magnetic Flux Lines (Lines of Force), Φ, passing through a Wire Coil. Figure 1 is one type of Generators. When the Coil is rotated between the Poles of the Magnet by cranking the handle, an AC Voltage Waveform is produced. Or other word An alternating current (A.C.) generator is an important application of electromagnetic induction. A.C. generator is an electromagnetic device which transforms mechanical energy into electrical energy. It consists of a rectangular coil of wire which can be rotated about an axis. The coil is located between the poles of two permanent magnets. As the coil rotates, the magnetic field through the coil changes, which induces an electromotive force (e.m.f.) between the ends of the coil.

Electrical Generator

Operation principle of a Generator is based on Electromagnetic Induction, which is defined by Faraday’s Law, which states:

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.

In the Generator, the Coil is under a Stationary Magnetic Field. The Magnetic Flux Density, B, is constant and Φ = B x Aeff, so Φ is proportional to the Effective Area, Aeff, of the Loop. As the Loop rotates at different angles, there is a change in Aeff .

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, Figure 3. The Induced E_emf output by the Generator is an AC Voltage and its Waveform is shown in Figure 4.

Different Rates of Change of the Magnetic Flux at Various Rotational Angles

Figure 4: Position of the Rotating Wire Coil Plane to the Magnetic Field Direction and the Induced Electromotive Force

 

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.   

 

 

How Does An AC Generator Work?

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.

Construction Of AC Generator (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.

Why Armature Winding Is Stationary In An Alternator?

§  At high voltages, it easier to insulate stationary armature winding, which may be as high as 30 kV or more.

§  The high voltage output can be directly taken out from the stationary armature. Whereas, for a rotary armature, there will be large brush contact drop at higher voltages, also the sparking at the brush surface will occur.

§  Field exciter winding is placed in rotor, and the low dc voltage can be transferred safely.

§  The armature winding can be braced well, so as to prevent deformation caused by the high centrifugal force.

Rotor:  There are two types of rotor used in an AC generator / alternator:

(i) Salient and (ii) Cylindrical type

1.     Salient pole type: Salient pole type rotor is used in low and medium speed alternators. Construction of AC generator of salient pole type rotor is shown in the figure above. This type of rotor consists of large number of projected poles (called salient poles), bolted on a magnetic wheel. These poles are also laminated to minimize the eddy current losses. Alternators featuring this type of rotor are large in diameters and short in axial length.

2.     Cylindrical type: Cylindrical type rotors are used in high speed alternators, especially in turbo alternators. This type of rotor consists of a smooth and solid steel cylinder havingg slots along its outer periphery. Field windings are placed in these slots.