A Short Illustrated Primer on Brushed DC Motors

brushed DC motor is an electromechanical motor driven by a DC power source. The basic parts of a brushed DC motor are:

  • Case, bearing and stator magnets (stator, i.e. stationary),
  • Motor shaft and washers,
  • Armature / rotor,
  • Commutator (and sometimes a Varistor), and
  • Brushes and terminals or leads.

The diagram below shows this layout in an exploded view of a typical DC motor – in this case, a small Ø12mm vibration motor.

Exploded Small Vibrator Motor

Exploded Small Vibrator Motor

The Armature is mounted on the shaft and has windings terminated to a commutator (sometimes with a varistor to reduce electromagnetic interference from commutator sparking); motor terminals (or leads) are connected to the motor windings through the motor brushes.

The stator magnets will have at least two permanent magnet poles. The electromechanical motor is designed such that opposite magnetic fields of the energized windings and the stator magnets causing the shaft to rotate. When the armature is aligned with the stator magnets, the brushes, which are also fixed to the motor case, will connect to the next commutator segment and thereby energize the next winding. This will change the magnetic field the armature, which causes the motor to continue rotating. The diagram below shows the stator, armature and commutator geometry in more detail.

Armature, Stator and Commutator Geometry on a 3-pole DC Motor

Armature, Stator and Commutator Geometry on a 3-pole DC Motor

Two-pole brushed motors have two big problems. The first is that when the rotor poles and the stator poles are in line the torque is zero and the motor cannot start from this position.  The other problem is that during the motor revolution, there is a moment where the brushes touch both the commutator sections. This leads to a short circuit of the inputs, condition that can damage the power supply and reduce efficiency.

For this reason, most small electromechanical DC motors have a minimum of three poles. The rotor has three windings that are wired in a triangle (delta) configuration to the three sections of the commutator. The brushes touching two opposite sides of the commutator and, depending on the rotor angle, will energize the three coils in turn. The wiring and the position of the windings are designed to always create a force moving the rotor in a particular direction given a constant brush polarity.

There are two main types of construction for small brushed DC motors: coreless and iron core. The use of an iron core rotor is very common for motors above 10mm of diameter. The coils are winded on a laminated iron core; this gives them a rigid support and helps to dissipate heat. Drawbacks are the inertia generated from the iron core mass and the high coil inductance that limit brushes and commutator life.

Motors of diameter 10mm or smaller are usually realized using a coreless construction – an exploded diagram of this construction is detailed below. In this case, the windings by themselves create the rotor structure. The rotor is hollow and the stator magnet can be placed in the centre, saving space, reducing the inductance and increasing the heat dissipation (since the windings are closer to the case/outside of the motor).

Exploded Coreless Mini Vibrating Motor

Exploded Coreless Mini Vibrating Motor