Electric Motors

Having covered Fleming’s Left-Hand Rule previously, we will now apply its general principles to the specific application of an electric motor. In particular, we will look at how the sideways motion due to a linear force, as predicted in Fleming’s Left-Hand Rule, can be used to produce continuous rotation.

We will be focusing on a simple DC (direct current) electric motor, as shown in the diagram below.

The blue arrows show that the magnetic field is running from top to bottom across the poles of a horseshoe magnet. (Two separate magnets, with oppoiste poles facing each other, could also be used.) The current, supplied by the cell, flows around the wire loop: it therefore moves in opposite directions on opposite sides of the coil, as shown by the black arrows.

Thinking about the closest side of the coil, we can use Fleming’s Left-Hand Rule to predict that the force acting on the wire will be out of the screen. To prove this, you need to twist your left hand so that your first finger (field) is pointing downwards and your second finger (current) is pointing right-to-left; your thumb (force) should now be pointing towards you. The front edge of the coil will therefore try to move forwards, out of the screen. This is shown by the small green arrow at the bottom of the coil.

Now look at the far side of the coil. The magnetic field is still going from top to bottom but the current is flowing in the opposite direction (left-to-right). This means the force will also be in the opposite direction, moving the rear edge of the coil into the screen, as shown by the small green arrow at the top of the coil.

These two forces will tend to make the coil settle in a horizontal plane, with the front edge as far forwards as it can go and the rear edge as far backwards as it can go. In theory, the coil would then simply stay in that position but two factors prevent this from happening.

The first factor is momentum. This comes from Newton’s Second Law of Motion and tells us that objects take time to change their state of motion. Think of a pendulum: ideally it will settle in its rest position (vertically) but once it has started moving it swings right through its rest position and keeps on going (until friction, including air resistance) dissipates its energy. A similar thing happens in our simple motor, causing the coil to over-shoot its stable horizontal position.

If nothing else happened, the coil would then vibrate back and forth until it dissipated its energy (like the pendulum) and came to rest horizontally.

But here is where things get clever. The coil isn’t connected directly to the cell: instead it goes via a device called a split-ring commutator, which is shown by the blue and red semi-circles below the component labelled Brush (which is the engineering term for any connector that is used between stationary and moving parts).

The split-ring commutator reverses the direction of the current when the coil crosses the horizontal plane. That in turn reverses the forces acting on the coil. So, instead of the coil being pulled back when it over-shoots the horizontal plane, the reversed force pushes the coil to keep it turning. This is harder to explain than it is to see so I highly recommend that you view the original animation then watch its accompanying video, which is linked from the animation page or can be viewed directly by clicking here.

Note that the current flowing out of the cell is always in the same direction (conventional current goes from the positive terminal to the negative terminal) but the commutator automatically reverses the direction of the current flowing through the coil. You might spot the fact that if we had used an AC (alternating current) power supply then there would be no need for a commutator – but that’s a completely different story!

To reinforce your understanding, I strongly recommend that you also play with the excellent animation on Tom Walsh’s oPhysics website. This is a particularly useful learning tool because it allows you to change some of the parameters to investigate their effect on the behaviour of a DC motor. These are things that you could be asked about in the exam! For example, you will see that increasing the number of turns on the coil produces a faster rate of rotation. (What does that tell you about the magnitude of the force that has been generated – and can you explain why?)

Finally, remember that an electric motor can be used “in reverse”. That is to say, an electric motor takes electrical energy as its input and transfers kinetic energy (rotation) as its output. The same device can be turned by hand (or by the wind, for example) so that kinetic energy is the input and electrical energy is the output. When used like this “in reverse”, the device is known as a generator. (To be more exact, if a generator is producing DC electricity then we call it a dynamo, but it is still an example of a generator.)

Generators are used in electric cars as part of regenerative braking systems. This relies on the fact that when electricity is drawn out of a generator, the axle becomes harder to rotate and that increased force can be used to reduce the car’s speed at the same time as electricity is being returned to the battery (where it is stored as chemical energy).

In a normal (internal combustion) car, the energy transfers are from chemical to kinetic when the car accelerates, then from kinetic to heat during braking to make the car decelerate.

In an electric car, the energy transfers are from chemical, via electrical to kinetic when the car accelerates, then from kinetic, via electrical back to chemical through regenerative braking to make the car decelerate.

You should be able to recall and explain these two situations as they could be used in the exam to illustrate the applications of motors and generators to achieve improved efficiency. Bear in mind, however, that electric car design is actually much more complicated than this!