Topic 8

How do generators work?

The Generator Effect

In the last topic we saw that if we got an electric current to interact with a magnetic field, we got movement.  Can we instead get an electric current if we move the wire through a magnetic field?  The answer is yes, as long as the wire is connected to an outside circuit.  If the wire is NOT connected to an outside circuit, there is a potential difference (voltage) instead.  This is called the generator effect.

The picture shows a carbon rod connected to a very sensitive voltmeter that can detect tiny voltages.

The carbon rod is moving.

Question 1  Is there a voltage?  How can you tell?  ANSWER

Question 2  What would happen to the voltage if you moved the rod from right to left?  ANSWER

Question 3  What happens if the rod is stationary?  ANSWER

We could keep the rod still and move the magnet from left to right.

Question 4  What would you see on the voltmeter this time?  ANSWER

This time, instead of moving the carbon rod from left to right, we move it perfectly vertically up and down, as in the picture:

This is easier said than done.

Question 5  Is there a reading on the voltmeter this time?  ANSWER

For there to be a reading on the voltmeter, the wire has to cut through magnetic field lines.  Therefore, if the rod is moved vertically, it does not cut field lines, so there is no voltage.

We can increase the voltage by:

• moving the wire quicker;

• having a stronger magnetic field;

• using a coil of wire consisting of two or more turns.

If we increase the area of the coil going through the magnetic field lines, we also increase the voltage.

Both wires in the picture are travelling at the same speed.  The magnetic field lines are all going into the screen.

Question 6  Why is the voltage increased in Wire 2?  ANSWER

A magnet moving through a coil of wire also produces a voltage. 

The voltage can be increased by:

• increasing the strength of the magnet;

• increasing the area of the coil;

• increasing the number of turns in the coil;

• making the magnet move faster.

The Generator

An engine moves a wire in a magnetic field to make a voltage.  If the wire is connected to an outside circuit, a current will flow.  The picture below shows a simple generator.

It is similar to the simple motor we looked at in Topic 7.  It has two magnets and an armature.  However there is an important difference in this generator.  Instead of a commutator, there are two slip-rings.  Each brush is connected to a slip-ring. 

We will now see what happens as the armature is turned in the magnetic field.  The voltage produced will be displayed on both a centre-zero voltmeter and a computer programmed to show a voltage time graph.  The voltage is on the vertical axis, while the time is on the horizontal axis.

Let's start with the armature vertical.  It is turning all the time.

Question 7.  What would happen if the armature were stationary?  ANSWER

There is a little red spot on the armature to show us how it turns:

Question 8  Why is the voltage zero?  ANSWER

Now let's look what happens 1/4 turn later.  The little red dot has disappeared behind the brushes.

Question 9  Why is the voltage at a maximum value?  ANSWER

Notice how the computer has plotted the rise in voltage from zero to a maximum positive value.  It is positive because of the way the coil is wired.

Now look what happens 1/4 turn later:

Question 10  What is the value of the voltage now?  ANSWER

On the computer you can see how half a cycle has been plotted on the graph.

Now 1/4 turn later, we have a maximum negative value of the voltage.

Question 11  Why is the voltage negative?  ANSWER

Finally, 1/4 turn later, the generator has turned a full circle.  You can see a complete cycle on the computer screen.

Question 12  What kind of current is this generator producing?  ANSWER

In this generator we have shown a coil of wire turning in a magnetic field.  It is, of course, possible to have a magnetic field turning and the coils of wire stationary.

A bicycle dynamo works in this way. 

Generators Small and Large

One of the smallest generators, still in relatively common use, is the pick-up cartridge on an old-fashioned record-deck (for playing vinyl LPs).

This cartridge moves a tiny magnet between coils of very fine wire with many turns.  It is also possible to get moving coil cartridges which have a tiny coil that moves in a strong magnetic field.   Both types generate a continually varying signal that is amplified to the musical sounds that we listen to through the loudspeakers.

Large generators called alternators have an electromagnet spinning.  This is called the rotor.  The fixed coils in which the electricity is generated form the stator.  In a car alternator, a current is passed to the rotor from the battery.

The alternating current from the alternator is converted to direct current for the battery by diodes.  This is called rectification.

In power station alternators there is usually a smaller generator on the end of the shaft that provides current for the electromagnets in the rotor.  This generator is called the exciter.

The steam turbine is at the far end.  The alternator is in the middle, while the exciter is in the foreground.

Question 13 (Harder)  If you turn a simple d.c. motor (as shown in Topic 7) what kind of output do you get?  Is it like the output of a battery?  ANSWER

Now answer Question 14 on the Motor Effect and the Generator Effect.

Now try the Topic Quiz

Home            Unit Index