September 15, 2011

Discuss basic principles of DC generator, AC generator (Alternator) & Motor.



DC Generator Principle
An electrical generator is a machine which converts mechanical energy (or power) into electrical energy (or power).


The energy conversion is based on the principle of the production of dynamically (or motionally) induced e.m.f.  As seen from Fig. 26.1, whenever a conductor cuts magnetic flux, dynamically induced e.m.f. is produced in it according to Faraday’s Laws of Electromagnetic Induction.  This e.m.f. causes a current to flow if the conductor circuit is closed.

Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux.

Basic Principle of Alternator
A.C. generators or alternators (as they are usually called) operate on the same fundamental principles of electromagnetic induction as d.c. generators. They also consist of an armature winding and a magnetic field.  But there is one important difference between the two.  Whereas in d.c. generators, the armature rotates and the field system is stationary, the arrangement in alternators is just the reverse of it.  In their case, standard construction consists of armature winding mounted on a stationary element called stator and field windings on a rotating element called rotor.
  
The stator consists of a cast-iron frame, which supports the armature core, having slots on its inner periphery for housing the armature conductors.  The rotor is like a flywheel having alternate and S poles fixed to its outer rim.  The magnetic poles are excited (or magnetised) from direct current supplied by a d.c. source at 125 to 600 volts. In most cases, necessary exciting (or magnetising) current is obtained from a small d.c. shunt generator which is belted or mounted on the shaft of the alternator itself. Because the field magnets are rotating, this current is supplied through two slip rings. As the exciting voltage is relatively small, the slip-rings and brush gear are of light construction.  Recently, brushless excitation systems have been developed in which a 3-phase a.c.  exciter and a group of rectifiers supply d.c. to the alternator.  Hence, brushes, slip-rings and commutator are eliminated. When the rotor rotates, the stator conductors (being stationary) are cut by the magnetic flux, hence they have induced e.m.f. produced in them.  Because the magnetic poles are alternately N and S, they induce an e.m.f.  and hence current in armature conductors, which first flows in one direction and then in the other. Hence, an alternating e.m.f. is produced in the stator conductors (i) whose frequency depends on the number of N and S poles moving past a conductor in one second and (ii) whose direction is given by Fleming's Right-hand rule.


DC Motor Principle
An Electric motor is a machine which converts electric energy into mechanical energy. Its action is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s Left-hand Rule and whose magnitude is given by F=BIl Newton.




Constructionally, there is no basic difference between a d.c. generator and a d.c. motor.  In fact, the same d.c. ma- chine can be used interchangeably as a generator or as a motor.  D.C. motors are also like generators, shunt-wound or series-wound or compound-wound.

In Fig.2 a part of multipolar d.c. motor is shown. When its field magnets are excited and its armature conductors are supplied with current from the supply mains, they experience a force tending to rotate the armature.  Armature conductors under N-pole are assumed to carry current downwards (crosses) and those under S-poles, to carry current upwards (dots).  By applying Fleming’s Left-hand Rule, the direction of the force on each conductor can be found.  It is shown by small arrows placed above each conductor.  It will be seen that each conductor can be found.  It will be seen that each conductor experiences a force F which tends to rotate the armature in anticlockwise direction. These forces collectively produce a driving torque which sets the armature rotating.

It should be noted that the function of a commutator in the motor is the same as in a generator. By reversing current in each conductor as it passes from one pole to another, it helps to develop a continuous and unidirectional torque.



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