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# Electricity from Magnetism

Rural Technology

## Electricity from Magnetism -

Do you know that electricity can be generated by moving a wire through a magnetic field. As long as there is relative motion between the conductor & the magnetic field, electricity is generated. If there is no relative motion between the conductor & the magnetic field, electricity is not generated. The generated electricity is actually a voltage, called an “ Induce Voltage”, & the method of generating this voltage by cutting a magnetic field with a conductor is called “ Induction”.

## Principle of operation of generator -

You already know that you can generate electricity by having a conductor cut through a magnetic field. This is essentially the principle of operation of any generator from the smallest to the giants which produce kilowatt of power.

fig. 1 operation of generator An elementary generator consists of a loop of wire placed so that it can be rotated in a stationary magnetic field to cause an induced current in the loop. Sliding contacts are used to connect the loop to an external circuit in order to use the induced e.m.f. The pole pieces are the north & south poles of the magnet which supplies the magnetic field. The loop of wire which rotates through the field is called the armature. The ends of the armature loop are connected to rings called “slip rings”, which rotate with the armature. Brushes ride up against the slip rings to pick up the electricity generated in the armature and carry it to the external circuit. Self - excited DC generator - Self excited generators use part of the generators output to supply excitation current to the field. These generators are classified according to the type of field connection used. In a “series” generator, the field coils are connected in series with the armature, so that the whole armature current flows through both the field & the load.

fig. 2 Shunt generator

If the generator is not connected across a load the circuit is incomplete and no current will flow to excite the field. The series field contains relatively few turns of wire. “Shunt” generator field coils are connected across the armature circuit, forming a parallel or “shunt” circuit. Only a small part of the armature current flows through the field coils, the rest flowing through the load. Since the shunt field and the armature form a closed circuit independent of the load, the generator is excited even under “ no load ” conditions - with no load connected across the armature. The shunt field contains many turns of fine wire. Fig 3 Compound generator A “ compound ” generator has both a series a shunt field, forming a series- parallel circuit. Two coils are mounted to each pole piece, one coil, series connected and the other shunt connected . The shunt field coils are excited by only a part of the armature current, while the entire load current flows through the series field. Therefore as the load current increases, the strength of the series field is increased.

### 1] In Text Question-

Fill in the blanks.
1. Electricity can be generated by moving a wire through a ______.
2. The generated electricity called an ____________.
3. The method of generating voltage by cutting a magnetic field with a conductor called_________.
4. The ends of the armature loop are connected to rings called ____.
5. ____ are classified according to the type of field connection used. State true or false.
1. Electricity is generated when there is no motion between conductor & magnetic field.
2. The pole pieces having the north pole and south pole.
3. Shunt generator field coils are connected across the armature circuit only parallel circuit.

Rural Technology