12.2A Introduction to practical electrical generators

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 crucial 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 e.g. mechanical kinetic energy source.

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.

12.2B Two other things to consider before looking at examples of generators ...

(a) An induced current opposes the change that caused the induction!

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!

 

(b) How can you increase the induced potential difference?, hence the power output of a generator

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.