CBSE Class 10 Answered

An 'AC generator ' or 'dynamo ' is a machine which produces AC from mechanical energy. Actually, it is an alternator which converts one form of energy into another.

Principle

AC Dynamo is based on the phenomenon of electromagnetic induction. That is, when the relative orientation between the coil and the magnetic field changes, the flux linked with the coil changes and this induces a current in the coil.

As the armature coil rotates, the angle Q changes continuously. Therefore, the flux linked with the coil changes.

= NBA cos wt

E = + NBA w sin wt

Construction

Armature

ABCD is the armature coil consisting of a large number of turns of the insulated copper wire wound over a laminated soft iron core I. The coil can be rotated about the central axis.

Magnets

N and S are the pole pieces of a strong electromagnet in which the armature coil is rotated.

Slip rings

R₁ and R₂ are two hollow metallic rings to which both ends of the armature coil are connected. These rings rotate with the rotation of the coil.

Brushes

Brushes B₁ and B₂ are two flexible metal plates or carbon rods. These brushes are used to pass current from the coil to the external load resistance.

Working

To start with, suppose the plane of the coil is perpendicular to the plane of the paper in which the magnetic field is applied, with AB at the front and CD at the back, the flux linked with the coil is maximum in this position. As the coil rotates clockwise, AB moves inwards and CD moves outwards. According to Fleming 's right hand rule, the current induced in AB is from A to B, and in CD, from C to D. In the external circuit, current flows from B₂ to B₁. After half of the rotation of the coil, AB is at the back and CD is at the front. Therefore, AB starts moving outwards and CD inwards. The current induced in AB is from B to A, and in CD, from D to C. The current flows from B₁ to B₂ through the external circuit. We therefore see that the induced current in the external circuit changes direction after every half rotation of the coil, and hence is alternating in nature.