Electric Generators

Electric generator diagram

Fig. 2: A diagram to show how an electric generator is used to help convert wind power into useful electric power for the national grid.

The diagram above shows how an electric generator can help us generate electricity using a wind turbine. Firstly, the turbine is forced to spin by the wind. The gearbox is used to transform the slow (but high torque) rotation of the turbine, into the rapid (but low torque) rotation required for the coil of wire in the generator. The turbine has converted wind power to mechanical power, and the generator has converted mechanical power into electric power for use in the national grid.

How does an electric generator function?

It is important to understand how a wire placed inside a magnetic field induces a current across it to generate electricity. Let's say that you have two magnets placed next to each other with a constant magnetic field between them as shown in the diagram below. A wire that is bent into a coil shape is placed between the magnetic field. Initially, the wire is at rest, so nothing is induced in or around the wire.

Fig. 3: A stationary wire produces no current.

As soon as we move the wire through the magnetic, a potential difference will be induced between the ends of the wire. This is because when the wire is moved through the magnetic field, the area of the wire exposed to the magnetic field changes.

When the area exposed to the magnetic flux changes, a potential difference is induced in the wire. As soon as the wire stops moving through the magnetic field, the potential difference also disappears. Keep in mind that a stationary wire cannot generate a potential difference across its ends.

Fig. 4: A wire moving through a magnetic field experiences a potential difference because the area exposed to the magnetic field changes.

Another thing to note is if you look at the figure above, the potential difference changes polarity every time the direction of the movement of the wire is reversed. The positive and negative signs at the ends of the wire switch with the change of direction of motion.

For the previous setup, only the potential difference will be induced as this is a piece of wire with two open ends, so there will be no flow of electrons (current). If we instead replace the piece of wire with open ends with a wire having its ends joined, we would get a complete loop. Now the movement of this wire in the magnetic field will also generate a current as the electrons have a complete path to flow around the wire. The direction of the current reverses depending on the direction of motion of the wire.

Fig. 5. A coil of wire moving through a magnetic field experiences a current because its area exposed to the magnetic field changes.

So far, we have discussed the motion of a wire moving vertically inside a magnetic field. What if you move a wire horizontally inside a magnetic field as shown in the diagram below? Will the magnetic field be induced if the wire moves forwards or backward inside a magnetic field?

Fig. 6. When the area exposed to the magnetic field is not changing, no current will be induced.

For the case depicted in the above diagram, the wire will not induce any current as the branches of the wire are parallel to the magnetic field. When the wire is moving parallel to the magnetic field, there is no change in the area that is cutting the magnetic field lines. The same amount of wire is cutting the magnetic field if the wire moves back and forth in the magnetic field which is why there is no change in the area and hence, there is no potential difference induced in the wire.

The same effect of induced current can be experienced if you move the magnets vertically instead of the wire. This is because the area of the wire through which the magnetic field passes still changes, which is the core concept behind electromagnetic induction.

Fig. 7. Alternatively, if the magnet moves instead of the wire, a magnetic field would still be induced in the wire.

Factors affecting the induced current

To alter the size of the induced potential difference which ultimately leads to the generation of current, we can do three things:

• Changing the strength of the magnetic field by having stronger magnets can lead to the wire cutting more field lines as it moves through it, so it would increase the potential difference induced.

• Moving the wire or magnetic field more rapidly can also increase the induced potential difference because the wire would have to pass through more field lines in a given time.

• Instead of a single wire, we can have a coil with multiple turns which would generate a stronger potential difference. Thus, the more turns a coil has, the bigger the potential difference it will induce.

Fig. 9. A coil with multiple turns is called a solenoid, Wikimedia Commons.

Coils and Magnets

We need to understand how the concept of electromagnetic induction works in a coil or solenoid. A magnet can be moved into and out of a coil of wire which induces a potential difference as depicted in the diagram below. When a magnet is pushed into the coil, a current is generated in the solenoid whose direction depends on whether the north pole or the south pole entered first. When the magnet is pulled out of a solenoid, the direction of the current reverses. This reversal of potential difference is indicated on the voltmeter.

Fig. 10. A falling magnet inside a solenoid would generate a potential difference as seen by the deviation in the voltmeter, Wikimedia Commons CC BY-SA 4.0

Electric Generator types

Typically, in most modern electric generators used in power stations, the coil or coils are fixed and mounted outside the magnet. It is the magnet that rotates to induce a potential difference. Although moving coils are still preferred in smaller-scale generators.

We can use the generator effect to produce an alternating current (AC) and a direct current (DC). We will discuss an alternator that generates AC current and a dynamo that generates DC current and the electric generator diagrams.

Alternator

An alternator is a coil of wire moving in a magnetic field connected with two metal rings called commutators. The purpose of commutators is that they allow the current to pass out of the coil so that it can be used in various applications such as powering our homes.

Fig. 12. A diagram of an alternator.

The image below shows the anticlockwise direction of the motion of the coil inside the magnetic field. Commutator A is connected to the left side of the wire (red color) and commutator B is connected to the right side of the wire (blue color). The potential difference graph in the figure below depicts the potential difference across the commutators with respect to time.

Fig. 13. An alternator has the highest potential difference when it is horizontal because it is sweeping through the wire at the fastest rate.

When the coil is completely horizontal and therefore parallel to the magnetic field lines, the maximum field lines would be cut at this point in time, hence the maximum potential difference will be induced as indicated by the graph.indicates the maximum potential difference andis the time.

As the wire moves, eventually, it will move through the position shown in the figure below. In this position, if you look at the coil from the side, the coil is perpendicular to the magnetic field so no magnetic field lines are cut and the potential difference at this position is zero as shown in the graph below.

Fig. 14. When the alternator is in the vertical position its potential difference is zero because the coil is moving parallel to the field.

Although the current is zero when the wire is vertical, the momentum of the coil continues to rotate it. As depicted in the figure below, the potential difference reverses when the wire is horizontal again. We get this reversal of potential difference because the two sides of the coil are in the opposite orientation than when we started the movement.

Fig. 15. When the wire is horizontal again, there is a maximum potential difference but in an opposite direction because the coils are moving in a different direction than before.

Once again, because the coil is vertical the induced potential difference would be zero again as shown in the figure below. The wire is moving parallel to the magnetic field and so the potential difference induced would be zero.

Fig. 16. The potential difference is once again zero when the wire is vertical.

The key point after all this discussion about alternators is that because the wire is connected to two different commutator rings, an alternator produces an alternating current as a result of the alternating potential difference. This alternating current can be increased by:

• Increasing the strength of the magnetic field.

• Increasing the number of turns on the coil.

• Increasing the rotational speed of the coil.

• Wrapping the wire around a soft iron core.

A dynamo

The key features of a dynamo are that it has a split ring commutator and it produces a direct current (DC). The split ring is divided into two gaps A and B which are connected to the sides of the coil.

Fig. 17. A diagram of a dynamo.

The working of a dynamo is similar to that of an alternator, the only difference is the single ring commutator instead of two solid metal commutators as in an alternator. Hence, the direction of the current and the potential difference does not change after one complete rotation of the coil and as a result, we get a direct current.

The figure below shows a graph of a potential difference with respect to time in a dynamo circuit. We get two peaks for one whole rotation of the coil because each side of the coil passes through the magnetic field twice during each cycle of rotation.

Fig. 18. The direction of the potential difference and the current do not reverse because of the split-ring commutator, Adapted from image by Wikimedia Commons.

Keep in mind that the above graph is a DC current graph of a dynamo and it should not be confused with a DC graph of current vs time as shown in the figure below. A DC graph is a constant line indicating a constant current over time.

Fig. 19. A DC current with respect to time graph is a straight horizontal line, Wikimedia Commons.

Example of an electric generator

The example of the coal-fired power station will be discussed, as it is arguably the simplest to understand. Coal-fired power stations generate electricity by first combusting (burning) coal as a source of fuel. This burning of coal heats the water in the boiler which is converted into steam, which then rises to the turbines.

The steam is kept under high pressure and high temperature, and directed towards the blade of the turbine, forcing the turbine to rotate. The turbine moves an electromagnet inside a copper coil which in turn produces electricity. This is an excellent example of an electric generator because the turbine's mechanical energy is converted into electrical energy.

Fig. 20. Diagram of how coal is used as a fuel to generate electricity, Wikimedia Commons CC BY-SA 3.0.