How real electric motors work

John Storey

2. "Universal" motors

No home should be without a dozen or so of these as well. They'll be where you want something compact that spins fast (food processor, coffee grinder, electric nose-hair trimmer) or needs variable speed (sewing machine, hand-held electric drill).

Advantages:
• Will turn at any speed you want it to, including really fast
• A lot of power in a small package

Disadvantages
• Horrible
• Arcing brushes create radio interference, ozone, noise.

A universal motor has both a wound field (on the stator) and a wound armature (on the rotor). It cannot use a permanent magnet to create the stator field because it needs to reverse its magnetic polarity every half cycle of the mains. The voltage being fed to the commutator brushes is also changing polarity every half cycle, and in fact is simply in series with the field winding.

This one ran for 30 years in a sewing machine.

Note the brushes at the right hand end of the shaft. To look at, it's almost indistinguishable* from a DC motor. In fact, it will run quite happily on DC. However, unlike a permanent magnet motor, a universal motor will always turn in the same direction, regardless of which way round it’s connected to the DC source. Knowing this simple fact can sometimes enable you to win bets involving small amounts of money.

This motor only has two poles, and so does not have a lot of starting torque. That’s why you sometimes need to turn the big wheel on the end of the sewing machine to get it started.

*Somewhat astute readers will notice that, unlike a conventional DC motor, it has a laminated stator. Totally astute readers will also understand why.