Basics of Brushed DC Motors

In this blog we will discuss the composition and function of brushed DC motors, along with some extra facts about usage. Brushed DC (BDC) Motors have four main parts: stator, rotor, commutator, and brushes.

• The stator contains either permanent magnets or electromagnets and remains stationary.
• The rotor, also referred to as the armature, is the part of the motor that rotates.
• The commutator is the part of the motor that makes a connection between the rotor and the brushes.
• The brushes are connected to the DC power source.

To fully understand how a brushed DC motor works, let’s explore a basic form. Shown here is a coiled wire (rotor), connected to a commutator with brushes on it, connected to a battery, with magnets (stator) on opposite sides of the wire. The energy given by the battery transforms the wire into an electromagnet that is acting against the force of the magnetic field of the magnets placed on either side.

As the coil rotates, each side of the commutator will eventually make contact with the opposite brush, changing the polarity. This change happens as one side of the coil nears the opposite magnet that is attracting it, then it swaps polarity so the magnet that was originally attracting it repels it away. This keeps the rotation going. However, as you can imagine this rotation is not very smooth. To even out the rotation and keep it smooth, more coils and commutators are added to the rotor.

Now, as the brushes break continuity with each section of the commutator, there is a voltage spike in the reverse direction (or reverse polarity). These spikes cause electrical noise that can cause big problems if the same voltage source is also connected to sensitive electronics. To resolve this, add a bidirectional TVS for motors that change directions, or a unidirectional one for motors that only turn one direction to reduce the generated noise.

Does your device require an immediate break? This can be achieved by shorting the motor leads together, effectively shorting the power being generated by the motor that would otherwise keep it going for a short time. This process, called dynamic braking, forces the armature windings to fight against the magnets and prevents the shaft from rotating. Should the motor be connected to a load with a lot of momentum, dynamic braking can cause the motor to get exceedingly hot and possibly burn out the armature windings. Placing a large wattage resistor in series helps eliminate this problem.

For more information on Brushed DC motors, or to explore other types of motors, take a look at our other videos in the “Another Teaching Moment” series.