Electricity generation, How
does an alternator work? How does a dynamo work?
How can you increase the power output
from a generator?
A generator is a practical way of producing a
continuous supply of electricity.
There are two main types -
(a) the a.c. alternator - the
current direction is changing,
(b)
the d.c. dynamo - the current only flows in one direction.
Both types make use of the generator effect to induce
current, and the two simple generators described below both involve
rotating coils of wire in a fixed magnetic field cutting through lines
of force.
They both involve a similar circuit construction BUT
there are some differences, so take care!
Both types described here involve rotating the coil in a
fixed magnetic field, so the coil is continuously cutting through the
magnetic flux lines of force.
Diagram illustrating the generator effect
All generators must have a source of
power to rotate the coil of wire.
As the coil spins, it cuts through
the magnetic field and a current is induced in the coil.
Dynamos are d.c. generators and
alternators generate an a.c. current.
Electromagnetic induction - inducing
a current in a rotating coil cutting through a magnetic field.
The direction of rotation of the coil can be
predicted from Fleming's right-hand rule (NOT on the GCSE
specification?)
Consider the right side of
the coil for clockwise motion (could be part of simple dc dynamo generator)
The thuMb represents the direction the
force acts
-
direction of motion - downwards (emphasise the M).
The
First Finger represents the direction of the
magnetic field N => S (phonetically emphasise the F).
The SeCond finger
predicts the direction of the
induced convention current (emphasise the 'hard' C).
Repeat for the left side of the
coil, moving upwards, and you should predict the current will be
flowing in the opposite direction.
You should eventually appreciate
the following:
(i) a d.c. motor and a d.c.
generator are essentially of the same construction, and,
(ii) an a.c. motor and an a.c.
generator are essentially of the same construction,
because they are constructed
of similar components, so,
in an electric motor the
electrical current energy is converted into kinetic energy, and,
in an electrical generator,
the kinetic energy is converted into electrical energy.
We have seen that a change in magnetic field induces a
current in a conductor e.g. a copper wire.
BUT, when a current flows through a wire, a magnetic
field is created around the wire.
So, we are now dealing with two magnetic fields.
The magnetic field produced by the induced current
in the wire always acts against the change that made it,
AND, it doesn't matter whether the induction is due to
the movement of the wire or the movement of the magnetic field.
It's as if the 'system' is trying to change back to
where it started i.e. the induced current opposes the change that
made it.
At first you might think - how can we continuously
extract electrical energy from the induction system?
BUT, remember, a generator requires a constant
input of kinetic energy from e.g. a diesel engine or steam/water
turbine.
You are building up an electrical energy
store from another energy store!
You can't get energy for nothing!
It is important to know how change the size of the
induced p.d. or current flow.
To change the size of the induced pd you must change
the rate at which the magnetic field changes.
You are usually interested in increasing the pd or
current or both at the same time e.g.
Increasing the speed of rotation (motion)
- increasing the kinetic energy input
Increased rotation of the
coil or magnet means more magnetic lines of force are cut per unit
time.
For a given strength of magnetic field the
density of the lines of force are constant, but with increased
motion you move through them faster.
Increasing the strength of the magnetic field
with a more powerful magnet
The greater magnetic flux
density means more magnetic lines of force are cut per unit time.
Remember - the greater the strength of the
magnetic field from a stronger magnet, the closer together are the lines of force.
Increasing the number of turns
of wire on the coil
The greater the density of
the coils, the more of the conductor the magnetic lines of force
cut through.
