Generator, its working principle and its types

• A generator is a machine which converts mechanical energy into electrical energy.
• It consists of a coil moving in a magnetic field. As the coil rotates, an emf is induced in the coil, which generates current (induced current) through the coil. All the electricity supplied to our homes is generated by this method.
• It works on the principle of electromagnetic induction.
• There are two types of generators:
  • AC generator
  • DC generator

Faraday’s law of electromagnetic induction

• The process of inducing current in a closed coil due to the relative motion between the magnet and the coil is called electromagnetic induction.
• The relative motion between the coil and the magnet changes the magnetic flux associated with the coil and hence induces an emf (or current) in the coil, called induced emf or induced current.
• The induced emf in a circuit is numerically equal to the rate of change of magnetic flux through the circuit. This is called Faraday’s law of induced emf.

Induced emf (E) = -dφ/dt

• Magnetic flux (φ) is measured in Weber (Wb) or Newton-meter per ampere. Hence, the unit of dφ/dt is,

=(Newton-meter)/(ampere-second)

=joules/coulomb

=volts

• The negative (-) sign indicates that the emf induced in the coil acts to oppose any change in the magnetic flux. This is because according to Lenz’s law, the induced emf generates a current that sets up a magnetic field which acts to oppose the change in magnetic flux.

Direction of the induced current:

• The direction of induced current in a coil across a magnetic field is given by Fleming’s right hand rule.
• It states that if the thumb, the fore finger and the middle finger of the right hand are stretched such that they are mutually perpendicular to one another, with the thumb pointing to the direction of motion of conductor, the fore finger pointing towards the direction of magnetic field (i.e. North to South), then the middle finger points to the direction of the induced current.

1. AC Generator:

• • An AC generator generates alternating current or AC.
  • It is used in hydropower station or thermal power station to generate AC in large scale.

Construction:

• A rectangular coil of copper wire having a number of turns called armature is kept within a uniform magnetic field produced by pole pieces N and S.
• Strong magnetic field can be produced by passing a DC around it. This magnet is called a field magnet.
• To produce a high voltage, the coil is wound on an iron core, so, the flux linked with the coil is increased.
• The two ends of the coil are connected to circular rings known as slip rings (or collecting rings). Slip rings are used to prevent the twisting and tangling of wires in a circuit during the rotation of the armature.
• Two carbon (graphite) brushes make contact on these rings to complete the circuit with the external load resistance R.
• In some of the generators, emf can be induced by rotating the field magnet instead of the armature coil.

Working:

• When the armature coil is rotated (e.g. using a water turbine in a hydropower station of by high pressure steam in thermal power station), the magnetic flux through it continuously changes inducing a current that flows via the carbon brushes, through the external load R.
• The magnitude and the direction of this current are not constant. The direction of current (as given by Fleming’s right hand rule) varies in a sinusoidal way.
• The current in the coil flows in one direction for the first half of a revolution and the direction is opposite for the next half of the revolution of the coil. Hence, this current is called alternating current (AC).
• One complete revolution of the coil produces one complete cycle of AC. The number of such cycles which the current goes through in one second is called frequency of AC. Frequency simply means the total number of rotations of the armature coil in one second.
• In most of the countries in the world, the frequency of AC supply is 50Hz.
• The induced emf in the generator can be increased by increasing the:
  • Number of turns of the armature coil
  • Area of the armature coil
  • Strength of the magnetic field
  • Speed of the rotating turbine

2. DC generator:

• • A DC generator produces direct current.
  • The flow of direct current through an external load resistance R is always unidirectional.
  • Dynamo is an example of a DC generator which is used to produce current in small scale.

Construction:

• It is similar to AC generator but the ends of the armature coil are connected to the split rings
• The carbon brushes are in physical contact with the split rings that complete the circuit for the flow of induced current through the external load resistance, R.
• Split rings help in the unidirectional flow of current through the external circuit. Hence, the split rings are also called commutators.

Working:

• When the armature coil rotates between the pole pieces N and S, an alternating current is induced in the coil.
• But, after each half cycle of the revolution of the coil, the contact of the coil to the outer circuit reverses from left to right and vice-versa with the help of commutators via the carbon brushes.
• Therefore, at the instant when the current reverses in the coil, the connection to the outer circuit reverses sending current in the same direction through the load R.
• Thus, AC is induced in the armature coil but current through the external load is always in the same direction (unidirectional current).
• In this way, a DC generator uses commutators to actually convert AC induced in the armature coil to DC flowing through the external load.

Generator, its working principle and its types