xaktly | Electricity & magnetism

DC motors

Using electric current to do work

Electric motors are one very common way we do work with electricity. Motors allow us to convert the motion of electrons in wires into forces that can produce a mechanical twisting force (torque), which in turn can be used to do all kinds of useful things.

This section will show you how a basic direct current (DC) motor works. In further sections we'll look at other devices that eploy electromagnetic induction.

The first thing you'll need to do is to familiarize yourself with the basic anatomy of an electric motor. See the diagram below for that.

The armature is a piece of metal that can be magnetized – a ferromagnetic metal like iron.

Wrapped around it are windings of wire to form an electromagnet. What's special about it are a set of contacts, one connected to each end of the windings, called the commutator. They form a circle, but the contacts (black and gray in the figure below, green and magenta in the other figures that follow) are separated by a small gap. That gap is a little wider than the brushes.

The brushes (so called because in older motors they were often made of metal fibers) contact the poles of the battery through the poles of the commutator. The commutator arrangement ensures that the polarity of the armature is switched ever half turn of the motor.

*The term "brushes" for the commutator comes from a time when they were made of flexible, conductive steel brush bristles. More modern conducting materials have replaced those and improved motors, but brush-commutator motors are still out there.

The DC motor cycle

In the eight steps below you can follow a single cycle of a DC motor in detail. See if you can work through the logic of how the motor works. It'll be worth it.

You can read down the left side of these steps, or down the right side. Each expresses the steps a little differently, so try them both. See which works for you.

1. We'll begin with the motor in this position. The armature is turned just a little bit clockwise from the horizontal. The negative pole of the battery is connected to the green contact of the electromagnet coil and the positive to the magenta contact, creating the poles of the armature magnet that are the commutator: north on the left and south on the right.

Notice that in this position, the armature would experience a clockwise force as the north poles and south poles of the armature and permanent magnet repel each other. So already our motor is turning.

2. Now we'll assume that the armature axle is well-oiled and that it turns pretty freely. Its inertia will sweep it around past the half-way point. At this point the battery is still connected to the electromagnet coil in the same way, brush to contact point.

The north pole of the armature is now attracted to the south pole of the permanent magnet, and the south pole of the armature is attracted to the north pole of the permanent magnet; the turning force continues.

3. At this point, the most important thing is that the battery is now disconnected from the electromagnet because of the gaps between the green and magenta contacts. The brushes no longer make contact there.

The inertia of the armature will now carry it through to the next step, in which the brushes will contact the poles of the electromagnet in the other of the two ways they can.

4. Here the brushes have switched contacts, and re-energized the electromagnet to switch its poles. Now north is on the left, south on the right, and we're right back in that position where the repulsive magnetic force is turning the armature clockwise.

5. The repulsion continues as the armature continues to rotate to the right. Remember that the armature is a magnet because the brushes, connected to poles of the battery, are touching either end of the electromagnet coil.

6. The magnet continues to rotate.

7. Now attraction between the poles of the armature and the permanent magnet is the dominant force rotating the armature.

8. At this point the brushes disengage again because of the gaps between the green and magenta contacts. The inertia of the armature will continue its rotation, the brushes will switch contacts, and we're right back at the beginning.

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