What’s a “digital motor”?

It’s certainly not just a fancy name for an electric motor, but the term “digital” was added by Dyson (I believe) to make his vacuum cleaners sound more exciting. Before that they were known as “brush-less” DC motors.

In simplistic terms, an electric motor works by having a moving electromagnet attracted to a fixed magnet, with the electromagnet on the spinning bit in the middle. Turn on the current and the electromagnet gets pulled towards the fixed magnet. Yeah! We have motion. However, when the two magnets reach each other they’re going to “stick”, so at that point you need to reverse the polarity of the current in the electromagnet so it changes from North to South, and it will then push itself away from the fixed magnet – carry on spinning.

In a traditional motor this is achieved using a commutator, which contains two contacts (+ve and -ve) called “brushes” which wipe against contacts on the spinning bit and provide power to the electromagnet. As the motor turns 180 degrees, the contacts are arranged so that the polarity is reversed – pull becomes push and vice versa, and the motor continues to rotate another 180 degrees – where it repeats.

To run smoothly you need more than two magnets, so something is always pulling, but that’s the general idea.

These brushes are a PITA. They cause friction, wear away, and make sparks. But it’s 19th Century technology and the best we had for a long time.

A brushless DC motor uses electronics to switch the polarity in the electromagnet. It uses a sensor on the spinning bit to work out which electromagnets need current and when. This can be done with analogue or digital electronics according to taste. Hence someone decided to call it “digital” after a while, because digital was “cool”. However, a digital motor has many advantages – not least of which is getting rid of the hated brushes and the need to replace them periodically. If you have smart electronics the motor can be more efficient by applying exactly the right current for requirements, and to more electromagnets that would be practical otherwise. It’s similar in a way to electronic fuel injection in vehicle engines (if you’re old enough to remember carburettors) – if designed well they can put in exactly the right power at the right time, saving energy and improving performance.

This applies to DC motors only. AC motors are a different matter, and may or may not contain a commutator . There are many types, and some do need a
commutator to activate the electromagnet in the rotor.
The characteristics and uses of these different motor types is beyond this post, but it’s worth noting that the Induction Motor (aka asynchronous motor), that was patented in 1888 by Nikola Tesla. This induces a current to flow in the rotor coils using coils in the casing, and therefore dispenses with the need for a contact. If you can use an induction motor instead of a brushed AC motor, do so. Industrial AC motors often use a commutator as they can generate more power for a given size, at the expense of more maintenance. It was also, at one time, easier to vary the speed of a brushed AC motor.

Update: Well, that’s what I get for trying to write a post on mobile phone using an Android App and “smart” keyboard and a screen so small I can barely see it – it swapped “commutator” for “communicator” in several places in the text (but not all). Thanks to the anonymous Virgin Media user who left a comment to this effect! There are probably plenty more typos of a similar nature if anyone wants to look. It also hyphenated brushless :-)

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