DC Electrics - DC Motors — Page 110, Lesson 104

I want to walk you through the basics of DC motors, starting from the very beginning. An electric motor is a machine that converts electrical energy into mechanical energy. That means it takes electricity in and produces motion out. Its function is the reverse of a generator, which does the opposite — it takes mechanical energy and turns it into electrical energy. Interestingly, there is very little difference between the construction of a DC motor and a DC generator. They have essentially the same parts and look alike. In fact, in many cases, a DC machine can be used either as a motor or a generator, depending on how you connect it. Now, let's recall a magnetic principle you've seen before: if a current flows through a wire that is placed in a magnetic field, a force acts on that wire, causing it to move. That is the fundamental principle a motor works on. The direction of rotation of a motor can be determined using Fleming's Left Hand Rule. Let me explain that rule carefully. To use Fleming's Left Hand Rule, you align your first finger with the direction of the magnetic field, from the North Pole to the South Pole. Then, you point your second finger in the direction of the current flowing into or out of the armature — the rotating part of the motor. Your thumb will then indicate the direction of motion. So, first finger for field, second finger for current, thumb for motion. Let's take an example. In the figure, the first finger is aligned with the field. The second finger points in the direction of the current coming out of the red half of the armature — that's the negative half. The thumb points upward, indicating that the motion is upward, which means the armature rotates anticlockwise. On the other side, in the blue half of the armature — the positive half — the current is flowing into the armature. If you keep your first finger aligned with the field and rotate your hand through 180 degrees, your thumb will now point downward, which still confirms anticlockwise rotation of the armature. So both halves of the armature are producing motion in the same direction. Here's an important point: if you reverse either the current direction or the field polarity, the direction of rotation of the motor will reverse. However, if you reverse both — the current and the field — the direction of rotation remains unchanged. So the motor's rotation direction depends on the relationship between current and field, not on either one alone. Now, the simple DC motor I just described, and the simple DC generator you may have seen earlier, are not very practical. They can be improved by adding further armatures — more coils — and by improving the shape of the pole pieces. You can also control the generator's voltage output and the motor's speed by adding field windings. These windings allow you to adjust the strength of the magnetic field. Figure 7.3 shows a sectional view of a practical DC generator, which is very similar to a DC motor in construction. So, to summarise: a DC motor converts electrical energy into mechanical energy using the force on a current-carrying wire in a magnetic field. Fleming's Left Hand Rule gives you the direction of motion. Reversing either current or field reverses rotation; reversing both keeps it the same. And practical motors add more armatures, better pole pieces, and field windings for control.