|A practical generator. In this diagram the iron core
that fills the space between the axle and the rotating coils has been
removed for clarity. The magnetic field is provided by the two electromagnet
coils ('field windings') which also have iron cores.
Also called a dynamo, a generator is a device for converting
mechanical energy into electrical energy.
Generators originated with the discovery of induction by Michael Faraday
in 1831; the considerable advantages of electromagnets over permanent magnets
were first exploited by E. W. von Siemens in 1866.
|The top picture shows a single coil of wire placed
in a magnetic field. As the coil is turned it cuts across the lines
of force and (if it forms part of a complete circuit) a current is
produced. When the coil is in the position shown in the second picture
it is moving along the lines of force without cutting them. No current
is produced here. In the last two pictures the red side of the coil
again cuts lines of force but this time it is moving upward, so the
direction of current is reversed.
Traditional forms are based on inducing electric
fields by changing the magnetic field
lines through a circuit (see electromagnetic
induction). All generators can be, and sometimes are, run in reverse
as electric motors.
The simplest generator consists of a permanent magnet
(the rotor) spun inside a coil of wire (the stator); the magnetic field
is thus reversed twice each revolution, and an AC voltage is generated at
the frequency of rotation (see also magneto).
Equivalent to this is rotating a coil of wire between the poles of a permanent
magnet, as shown immediately below and in the illustration to the right.
|A simple generator. On the right the coil is seen
from the end making one complete revolution. The size of the current
in each of the eight stages varies as shown by the curve. At 'e' (coil
vertical) the current reverses.
In practical designs (see top illustration), the rotor is usually an electromagnet
driven by a direct current obtained by rectification of a part of the voltage
generated, and passed to the rotor through a pair of carbon brush/slip ring
contacts. The use of three sets of stator coils 120° apart allows generation
of a three-phase supply (see also armature).
Direct current generation
Simple DC generators consist of a coil rotating in the field of a permanent
magnet: the voltage induced in the coil alternates at the frequency of rotation,
but it is collected through a commutator – a split-ring broken into
two two semicircular parts, to each of which one end of the coil is connected,
so that the connection between the coil and the brushes is reversed twice
each revolution – resulting in a rapidly pulsating direct voltage.
A steadier voltage can be achieved through the use of multiple coil/commutator
arrangements, and except in very small generators, the permanent magnet
is again replaced by an electromagnet driven by part of the generated voltage.
|A generator can be made to give a 'one-way' or direct
current by connecting the ends of the coil to the two halves of a
split-ring or commutator. This device neatly puts whatever is the
'outgoing' end of the coil onto the same brush at the moment the coil
comes up to the vertical and reverses the flow. In the first of the
diagrams above, the red side of the coil is moving downward and the
current produced in it flows out of the coil into the right hand brush.
In the second diagram the red side of the coil is moving upward and
now the current produced in it flows into the coil. But by this time
the red side of the coil is connected to the left hand brush. So the
current still flows out of the right hand brush, the rough the lamp
and re-enters the generator at the left hand brush as it did in the
|Although the commutator ensures that current always
flows in the same direction, it does not prevent the current from
falling to zero each time the coil reaches the vertical position.
No current is produced when the coil is vertical because it moves
along the lines of force instead of cutting across them. With a number
of coils it is possible to have the current in one reaching a maximum
when the current in another is zero. The commutator in that case consists
of several pairs of segments arranged around the axle instead of the
two halves of the split ring. The segments are insulated from each
other, and the ends of each coil are connected to opposite segments.
For large-scale generation, the mechanical power is usually derived from
steam turbines, or from dam-fed water turbines,
and the process is only moderately efficient. The magnetohydrodynamic generator
avoids this step and has no moving parts either. A hot conducting fluid
(treated coal gas, or reactor-heated liquid)
passes through the field of an electromagnet, so that the charges are forced
in opposite directions producing a DC voltage. In another device, the electrogasdynamic
generator, the voltage is produced by by using a high speed gas stream to
pump charge from an electric discharge, against the electric field, to a