PREPARED BY – S. K. MISHRA P G T (PHYSICS) JNV NONGSTOIN

STUDENTS WILL LEARN-  ABOUT CONSTRUCTION, THEORY AND WORKING OF A. C. GENERATOR. UUSE OF GENERATOR IN DIFFERENT APPLICATIONS. DDIFFERENCE BETWEEN A. C. AND D. C. GENERATOR.

Students have knowledge of 1.Magnetic flux 2.Flemegin’s right hand rule 3.Faraday’s law of electromagnetic induction 4.Induced current.

 A A.C. CONSISTS OF FOLLOWING PARTS –  F FIELD COILS  A ARMATURE COIL  S SOFT IRON CORE  S SLIP RINGS  C CARBON BRUSH BBEBARINGS

DIAGRAM

A simple AC generator consists of: (a) a strong magnetic field, (b) conductors that rotate through that magnetic field, and (c) a means by which a continuous connection is provided to the conductors as they are rotating (Figure 3). The strong magnetic field is produced by a current flow through the field coil of the rotor.

Consider a coil of wire rotating in a uniform magnetic field, of flux density, B, as shown below.

Fleming’s right hand rule When the coil is in the position shown in the diagram, side 2 is moving down and side 1 is moving up. We can use Fleming’s right hand rule to decide that end q will (at that instant) be the positive terminal of the generator.

When the coil has rotated through half a turn, end p will be the positive terminal. Therefore, a coil of wire rotating in a magnetic field has an alternating emf induced in it. To connect the coil to a light bulb (or any other component) brushes made of carbon make contact with slip rings made of brass, as shown in the next diagram.

Consider a coil having N turns, rotating in a magnetic field of uniform flux density, B. The area of the coil is A and the angle between the normal to the plane of the coil and the field is a. flux linkage = NABcosa If the angular velocity is w, then a = wt, so the flux linkage at any time, t, is given by flux linkage = NABcoswt

Therefore, the induced emf at any instant is given by represents the slope of a graph of cos wt against t. Where

It can easily be shown that the slope of a graph of cos  t against t is equal to –  sin  t so the induced emf is given by

Induced E.M.F.Vs Time Graph

The magnitude of AC voltage generated by an AC generator is dependent on the field strength and speed of the rotor. Most generators are operated at a constant speed; therefore, the generated voltage depends on field excitation, or strength.

The frequency of the generated voltage is dependent on the number of field poles and the speed at which the generator is operated, as indicated.

AC Generator The turning of a coil in a magnetic field produces motional emfs in both sides of the coil which add. Since the component of the velocity perpendicular to the magnetic field changes sinusoidally with the rotation, the generated voltage is sinusoidal or AC.

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GENERATION OF INDUCED CURRENT IN SINGLE COIL

The field coil in the rotor receives excitation through the use of slip rings and brushes. Two brushes are spring-held in contact with the slip rings to provide the continuous connection between the field coil and the external excitation circuit.

The armature is contained within the windings of the stator and is connected to the output. Each time the rotor makes one complete revolution, one complete cycle of AC is developed. A generator has many turns of wire wound into the slots of the rotor.

Magnetic flux is the product of the average magnetic field times the perpendicular area that it penetrates. It is a quantity of convenience in the statement of Faraday's Law and in the discussion of objects like transformers and solenoidssolenoids. In the case of an electric generator where the magnetic field penetrates a rotating coil, the area used in defining the flux is the projection of the coil area onto the plane perpendicular to the magnetic field.

It is used for generation of electricity for domestic as well as commercial purpose. In all types power plants only A. C. generators are used. The principle of working of A. C. motors are reciprocal to that of A. C. generators.

HYDRO-ELECTRICITY GENERATION

HYDRO-ELECTRICITY GENERATION

SUMMARY When a coil of N turns, rotates in Uniform magnetic field with constant angular velocity ω.the induced current developed in the coil is taken as out put through slip rings.

QUERRY ?

1.What is function of field coils? 2.What is the principle of A.C. Generator? 3.How can you convert A.C. Generator in to D.C. Generator? 4.Why radial magnetic field is necessary?

1.Draw labeled diagram of A.C. Generator. 2.Write theory and working of A.C. Generator. 3.Draw labeled diagram of D.C. Generator. 4.Write two names each of thermal, hydro, nuclear and wind power stations in India.

BIBILOGRAPHY The following reference Books and web sites are consulted for preparation of this lesson plan- 1.Concept of Physics By- H.C. Verma 2.Fundamentals of Physics By-D.Holliday& R.Resinik 3. erator/ac.html 4. _e.htm