ALTERNATING CURRENT GENERATOR

This is the device which is used to produce an A.C from mechanical energy
Principle :

    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.   

    

CONSTRUCTION OF A.C GENERATOR

A.c generator is made up of the following parts.

1.Poles -this produces the magnetic fluxes 2.Rectangular coil -Rotates when the mechanical energy is applied to cut the magnetic fluxes passing perpendicular with it  3.Slip rings-rotates with the rotating coil and rubs the brushes to transfer the alternating current to external circuit  4.Carbon/ graphite brushes-Carry the current from the coil by means of slip rings to the external circuit  5.Galvanometer-detect the EMF induced and the direction whether its a Direct Current or Alternating Current.

WORKING MECHANISM

    According to the faraday’s law of Electromagnetic Induction which states that “Whenever there is is a change in a magnetic flux linkage with a circuit,there is an induced EMF” The direction of the induced EMF is always obtained by using Flemings right hand rule which states that”When the right hand is extended mutually at right angle(90 degrees) the thuMb will point the direction of the motion of the conductor,the first finger will point the direction of the Field an the second finger will point the direction of the Current”

When the eternal mechanical energy is applied the rectangular coil(Armature)will start rotating.The rotation of the rectangular coil will then cut the magnetic fluxes produced by the permanent bar magnets.Thisproduced flues are perpendicular to rotating rectangular coil.According to the faraday’s law of electromagnetic induction,Emf will be induced.Themagnitude of the the EMF induced would be directly proportional to the rate of change of magnetic fluxes.

The Emf produced will flow through the coil then flow to the eternal circuit by means of two Slip rings and Four carbon brushes.SLIP RINGS rotates with the rotating rectangular coil while rubs the carbon brushes.Here the brushes will correct the alternating current from the coil to the eternal circuit.

GRAPH FOR OUTPUT  VOLTAGE

Consider a rectangular coil having N turns and rotating in a uniform magnetic field with an angular velocity of w radian/second. Maximum flux Øm is linked with the coil when its plane coincides with the X-axis. In time t seconds, this coil rotates through an angle q = wt. In this deflected position , the component of the flux which is perpendicular to the plane of the coil is Ø= Ømcos wt. Hence flux linkage at any time are NØ=NØm cos wt.

   According to Faraday’s Laws of Electromagnetic Induction, the e.m.f. induced in the coil is given by the rate of change of flux linkage of the coil. Hence the value of the induced e.m.f. is

                                                      e = – d(NØ)/dt volt

                                                                       = – N d(Øm cos wt) / dt volt

                                                                       = – NØm w(-sin wt) volt

                                                                       = wNØm sin wt volt

                                                                       = w NØm sin q volt    ———————–   (i)

    When the coil turned through 90º i.e. when q = 90º, then sin q = 1, hence e has maximum value, say Em. Therefore from Eq(i) we get

                                                                Em = wNØm 

                                                     =  w NBmA = 2pfNBmA volt

                             where Bm = maximum flux density in Wb/m2.

                                        A = Area of the coil in m2.

                                         f  = frequency of rotation of the coil in rev/second. 

            Substituting this value of Em in Eq(i), we get

                                         e = Em sin q   = Em sin wt

    Similarly, the equation of the induced alternating current is

                                          i = Im sin wt