Notes on AC Generator

Introduction

• AC Generator: Converts mechanical energy to electrical energy (alternating EMF or alternating current).

Working Principle

• Based on electromagnetic induction.
• Relative motion between a coil and magnetic field induces electric current or EMF in the coil.

Components of an AC Generator

1. Armature (ABCD):
   • Rectangular coil with many turns around a soft iron core.
2. Shaft:
   • Can be rotated rapidly.
3. Field Magnet:
   • Strong permanent magnet with concave poles.
4. Slip Rings (S1 and S2):
   • Connected to the armature and rotate with it.
5. Brushes (B1 and B2):
   • Provide electrical contact with the slip rings.
6. Load (e.g., Galvanometer):
   • Used to measure current.

Working of an AC Generator

• When the armature rotates between the poles of the field magnet:
  • Magnetic flux linked with the armature changes continuously.
  • EMF is induced in the armature, producing electric current.
  • Current flows through the galvanometer and slip rings.
• Galvanometer Indication:
  • Needle swings between negative and positive values, indicating alternating current.

Direction of Induced Current

• Initial position: Armature ABCD is vertical (A B up, C D down).
• First Half Rotation:
  • Arm A B moves down, C D moves up.
  • By Fleming's right-hand rule, current flows in direction DCBA (from B1 to B2).
• Second Half Rotation:
  • Arm A B moves up, C D moves down.
  • Current flows in direction ABCD (from B2 to B1).

Graphical Representation of Induced EMF

• Time taken for one complete rotation = T seconds.

• At T = 0:

  • Armature is vertical; induced EMF is zero (flux change is zero).

• First Quarter Rotation (0 to T/4):

  • Induced EMF increases.

• At T/4:

  • Armature is horizontal; induced EMF is maximum.

• Second Quarter Rotation (T/4 to T/2):

  • Induced EMF decreases, returns to zero at T/2.

• Third Quarter Rotation (T/2 to 3T/4):

  • Induced EMF increases, but opposite polarity.

• At 3T/4:

  • Maximum negative induced EMF.

• Fourth Quarter Rotation (3T/4 to T):

  • Induced EMF decreases to zero at T.

• Graph Shape: Sinusoidal.

Summary

• An AC generator converts mechanical energy into electrical energy (alternating EMF or current) based on electromagnetic induction.
• Key components: armature, shaft, field magnet, slip rings, brushes.
• Induced EMF varies sinusoidally over time.