DC Motor

A motor is an electromechanical device that converts electrical energy into mechanical energy (that is, rotational movement). Its action is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force which forced to move in a direction given by fleming's Left-hand Rule (not within the scope of this topic, so let's leave that for now). There are two types of motor: AC motor and DC motor. The major different between AC and DC motor is the presence of slip rings in AC and split rings DC. AC motors operate on AC electrical power supply while DC motors produces rotational movement only on application of DC electrical power. This rotational movement can then be converted to other types of movement like oscillation, linear movement etc using mechanical device(s). These movements are seen when you inject your CD/DVD drive, when you are opening your motor driven gates, robots and so on.

DC motor which is the focus of this topic is similar to DC generator in terms of construction. In fact, a DC machine can be used as motor or generator. DC motor comes in different ratings like 6V, 12V etc. It has two wires or connectors through which it is connected to supply. When connected to power, its shaft rotates. When the polarity is reversed, the direction of rotation of the shaft changes.

Experiment 1
You can try this experiment out yourself to verify the fact that a DC motor can be used as generator: get a small DC motor of either 6V or 12V connect its connectors to your digital voltmeter just as shown in figure below. Now use your hand or try any other possible means to rotate the shaft as fast as you can and observe the meter display. The display reading represents the generated voltage as a result of turning the shaft. The generated voltage is proportional to angular velocity (speed of rotation) of the shaft. One more test before you disconnect your experiment: change the direction of rotation of the shaft (that is, turn it in other direction) and notice the reading on the meter display. Results of the experiments will show negative sign in one and no such negative sign in other. This shows how polarity and shaft direction are related in a DC motor.

Experiment 2
Before you go, connect your DC motor to a battery as shown in the figure below and observe the direction of rotation of the shaft. Now! Change the polarity and also observe the direction of the shaft. The result you see is the evidence how polarity reversal cause the shaft to change direction.

What I have done is just to let you have basic knowledge of a simple DC motor so that you can under those DC motor related circuits on this site as I am not going to take you through the construction or how the magnetic field cause the shaft to rotate. So we will use electronic circuits control speed and direction of rotation of DC motor.