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The a.c. generator or alternator
A simplified form of the a.c. generator is shown in
Figure 1(a).
A coil (the rotor) is rotated between the poles of a d.c.
electromagnet (energised by the field coils), except in
the case of a bicycle dynamo where a permanent
magnet is used, and the e.m.f. generated is taken
from the ends of the coil. These are connected to
sliding contacts known as slip rings on the axle, and
contact is made with these by two pieces of carbon
(the brushes) which press against the slip rings. As
the coil rotates it cuts through the lines of magnetic
flux producing an induced e.m.f., the variation of
which with time is shown by Figure 1(a). A much
smoother output is obtained by having a number of
coils wound on an iron core which is laminated to
reduce eddy currents. The output of such a generator
is shown in Figure 1(b).
Figure 1(a)
coil
magnet
slip rings
brushes
Figure 1(b)
e.m.f
time
In generators where the output current may be very
large, as in a power station, it is the magnet that
rotates while the coil remains at rest. A simplified
version of this is shown in Figure 1(c). The advantage
of this is that the slip rings and brushes have to carry
only the small current needed to magnetise the
rotating electromagnet while the current produced the
static field coils may be many hundreds of amps. In
modern alternators installed in a power station the
e.m.f. generated will be some 25 kV and the current
produced over 1000 A!
a.c
output
Effect of changing the speed of rotation of the coil
on the induced emf
Figure 1(c)
If the speed of rotation of the coil is changed two
things happen (See Figures 2(a) and 2(b):
(a) since the rate of cutting of magnetic flux is increased the output emf will be incresead also in line
with Faraday’s law
(b) the frequency of the output emf will be increased as well since the coil makes a revolution in a
shorter time
Induced emf
Induced emf
Figure
2(a)
Figure 2(b)
time
time
Slow coil rotation speed
Faster coil rotation speed
1
Student investigation
The induction motor relies on the phenomenon of electromagnetic
induction for its operation and a simple investigation will demonstrate
this (Remember that to obtain an induced e.m.f. either the conductor
or the magnetic field may move.)
Set up the apparatus as shown in Figure 2 and switch on the electric
drill. As the copper disc speeds up, observe and explain the effect on
the magnet.
thread
magnet
copper disc
plastic
plate
electric drill
Figure 2
2