Physical Science - Other

How a DC Electric Motor Works



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Like most good inventions, the DC electric motor is both simple and ingenious. It is a device that takes electrical energy from a power source such as a battery and converts that energy into kinetic energy by means of electromagnetism. This conversion results in a rotating axle that can be used to do all sorts of mechanical work.

The easiest way to understand the workings of a DC motor is to describe the flow of electricity from the power source to the motor and back by means of two wires (an electrical circuit), and examine each stage in turn.

* The battery

Within atoms there are particles called protons, neutrons and electrons. Different kinds of atoms have different amounts of each, but essentially for every proton in the central nucleus there is a corresponding electron orbiting it. Each electron has a negative charge, which means that it is surrounded by a force field called an electrostatic field. The electron's corresponding proton in the atom's nucleus exhibits a force of equal strength but of opposite polarity - it has a positive charge - and because 'opposites attract', electrons and protons hold each other in balance.

However, in some atoms the electrons that orbit furthest away from their protons are only loosely held and can be affected by forces outside the atom. Such electrons can jump from atom to atom and so we call them free electrons. Free electrons normally move randomly, but when a force is used to make them all flow in one direction then we have an electric current. Such a force is called an electromotive force and is best pictured as a form of pressure forcing electrons to flow, much like water pressure forcing water to flow through a garden hose.

A battery is essentially a container in which, due to a chemical process, two separate materials have an imbalance of electrons (one with too many and the other with too few). The excess electrons in one material want to move to the other in order to balance the charge (or equalize the pressure) but to do this they need a means of travel.

The best means is a material that contains free electrons and so allows a current to flow. Such a material is called a conductor and copper wire is a perfect example. When the wire is connected from one terminal to the other, from the negative terminal to the positive, a circuit is created and the electrons flow. The flow of electrons from a power source, around a circuit and back to the source is called direct current (DC). If we add an electric motor to the circuit we can use the flow of electrons, or electric current, to power it.

*The DC motor

When an electric current flows through a wire a magnetic field is created around that wire. If the wire is wrapped around an iron or steel object the free electrons in the object align with the current flow and the object becomes a magnet, an electromagnet. The strength of the magnetic field can be increased by increasing the amount of times the wire is coiled around the object and the polarity of the magnetized object changes when the direction of the current changes. Such is the principle behind an electric motor.

The simplest electric motor can be described very easily. First we take a horseshoe magnet and between the two poles (north and south) we position a perpendicular axle. On the axle is an armature, which is a metal core wrapped in wire and which points to each pole of the permanent magnet. The two leads of a battery (+ and -) are connected to two metal plates called bushes which are situated on either side of the axle. Lastly, the two ends of the wire wrapped around the armature are connected to two separate halves of a circular sleeve called a commutator that is fixed around the axle and with each half in contact with one of the bushes, thus completing an electric circuit.

Now we'll see how the motor works. As the current flows from the negative terminal of the battery it flows through a bush and into the wire of the armature via one half of the commutator, then through the other half, through the second bush and back to the positive terminal of the battery. When the current flows, the armature is magnetized and its north pole is attracted to the south pole of the horseshoe magnet and the armature swings round to align itself accordingly.

And here's the clever part. The commutator is positioned so that when the armature swings round as described above, the two halves of the commutator swing round on the axle and come into contact with the opposite bush and the current is reversed in the armature wire. The magnetic polarity is reversed and the north pole of the armature is now the south pole and visa versa. This means that the armature must swing round another half turn to align with its attractive magnetic opposite. Once this movement is complete the commutator switches bushes again and the process is repeated. The result is a spinning armature and a spinning axle that can be used to do work. Such is an electric motor in action.

In working electric motors the commutators always have more than the two poles described above. This is to prevent the armature sticking if it comes to rest at the half-way point between the two poles of the permanent magnet. However, the basic principle of an electric motor is the same whatever the number of poles on the commutator.

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