How real electric motors work

John Storey

4. “Conventional” DC motors

There are plenty of these in the average household, lurking inside battery powered toys, the cassette player, cordless drill and electric toothbrush. Inside a car, everything from the cooling fan to the windscreen wipers will have a DC motor. In fact, a luxury car with electric headlight washers, electric seat adjustment and remote rear-vision mirrors has more electric motors than you can poke a stick at – an interesting competition is to simply to count them and see who can find the most!

Advantages:
• Reasonably inexpensive
• Easy to control
• Adaptable

Disadvantages:
• Brushes eventually wear out
• Brushes create electrical interference
• Brushes are bad

Most DC motors look something like this. This particular one is beautifully made and probably cost at lost of money.

However, that won't stop us taking it apart.

Starting at the left we have the end cap and the two carbon brushes that contact the commutator, then the rotor, the stator casing and its two C-shaped permanent magnets, the gearbox housing, a little gear-wheel that fell out of somewhere, and the gear-head and output shaft.

The two brushes are solid blocks of graphite, and are pressed against the commutator by the two small coil springs (situated slightly anticlockwise of the brushes). The orange disc is a capacitor that is directly across the power supply to the motor and helps to reduce radio interference caused by sparking where the brushes contact the commutator.

The rotor is very simple…

…and goes in an equally simple housing with two “C” shaped magnets. The housing is made of soft iron and creates the magnetic poles of the stator.

The best thing about this motor is the multi-stage planetary gearbox on the end!