MAGNETIC EFFECTS OF ELECTRIC CURRENT
ELECTRIC
GENERATOR
The principle of electromagnetic induction can be applied to produce large currents for use in homes and industry.
In an electric generator, mechanical
energy is used to rotate a conductor in a magnetic field to yield electricity.
An electric generator consists of
a rotating rectangular coil ABCD placed between the two poles of a
permanent magnet. The two ends of this coil are connected to the two rings
R1 & R2. Their inner sides are
insulated.
The two conducting stationary brushes
B1 & B2 are kept pressed separately on the
rings R1 & R2, respectively.
R1 & R2
are internally attached to an axle. The axle may be mechanically rotated from
outside to rotate the coil inside the magnetic field.
Outer ends
of the brushes are connected to galvanometer.
When the axle attached to the two
rings is rotated such that the arm AB moves up (and the arm CD moves down) in
the magnetic field produced by the permanent magnet.
The coil ABCD is rotated
clockwise. By applying Fleming’s right-hand rule, the induced currents are set
up in these arms along the directions AB and CD. Thus an induced current flows
in the direction ABCD. If there are larger numbers of turns in the coil, the
current generated in each turn adds up to give a large current through the
coil. Thus the current in the external circuit flows from B2 to B1.
After half a rotation, arm CD
starts moving up and AB moving down. As a result, the directions of the induced
currents in both the arms change, giving rise to the net induced current in the
direction DCBA. The current in the external circuit now flows from B1
to B2. Thus after every half rotation the polarity of the current in
the respective arms changes. Such a current, which changes (reverses) direction
after equal intervals of time, is called an alternating current (AC).
This device is called an AC generator.
The current which always flows in
one direction is called Direct current (DC). To get a DC, a split-ring
type commutator is used. With this arrangement, one brush is at all
times in contact with the arm moving up in the field, while the other is in
contact with the arm moving down. Thus, a unidirectional current is produced.
This device is called a DC generator.
Most power stations constructed
these days produce AC. In India, the AC changes direction after every 1/100
second, that is, the frequency of AC is 50 Hz.
Advantage of AC over DC:
Electric power can be transmitted over long distances without much loss of
energy.
