DC Motors & Speed Control – B1P21


Let’s learn about motors and let’s build a simple speed control circuit.


A motor is a device that converts current to rotary motion. BBox 1 contains a 3V DC motor, which means that applying 3V DC to it will cause it to spin at the rated speed. For DC motors, you can vary the applied voltage to vary the speed – within limits depend on the motor.

Motor parts

This is what the symbol for a motor looks like and as can be seen, it has two terminals. The motor itself is made up of different components as shown above.

Motor magnet

We have the permanent magnets which create a strong magnetic field at the centre.

Motor armature

We then have the part that rotates which is called the armature. It is made up of a number of coils or windings that are ultimately connected to the motor terminals.

Motor brush

The third component is called the commutator which consists of brushes that are connected to the motor terminals.

When DC voltage is applied to the armature, it produces a magnetic field, and this causes the armature to align itself depending on the field of the permanent magnets. In order to cause the armature to rotate, we need to vary this field constantly which would create a varying magnetic field giving rise to rotation.

Motor armature slots

This is the reason why we have these slots on the armature, which come in contact with the commutator brushes. The slots and coil arrangements are designed such that they cause the magnetic field to vary which causes the armature to rotate continuously.

Moto speed control - schematic

This is what the circuit looks like. We have the motor and we have also placed a diode across its terminals. We’ve learnt about this diode in the project about relays, and it is necessary to add this to allow a path for current to flow when the motor is switched OFF. This diode is also called a freewheeling diode.

The transistor we will be using here is slightly different and has a different pin configuration. This transistor can handle higher current compared to the BC547 that we have been using previously. The rest of the circuit operation is pretty straight forward. A trimpot is used to vary the base voltage/current which changes the operating point of the transistor and this controls the amount of current that can flow through the motor, changing its speed.

Moto speed control - layout

Let’s use the breadboard layout to build the circuit. The motor does not have any wires attached to it so you please wrap jumper wires across its terminals. Instead of using a trimpot, you can also use something called Pulse-Width-Modulation or PWM to control the motor speed. We will look at this in BBox 2.

Moto speed control - demo

Let’s move on to the next project.