How real electric motors work

John Storey

3. Advanced AC motors

The compressor motor in an airconditioner is the thing that draws most of the current. Obviously it should be as efficient as possible, and it would be desirable to be able to run it at varying speeds depending on the required load. With its miserable efficiency and lack of enthusiasm for running at anything but one particular speed, a single-phase induction motor is a poor – if inexpensive – choice.

Many modern airconditioners therefore instead use “Inverter” technology. What this means is that they rectify the mains to create DC, then use an inverter, or DC to AC converter, to go back to AC again. However, the AC that is produced is no longer at 50Hz, but is at a variable frequency. Even if we just used a conventional induction motor now we’d be ahead, because we’ll be driving it at the optimum frequency, and hence rotational speed, for the required load. However, we can go one better than this and use a switched reluctance motor (which has a rotor made from soft iron) or a brushless DC motor (which has a permanent-magnet rotor), and achieve even higher efficiencies. It is one of the strange perversities of life that we will now call this a DC motor, even though it’s quite decidedly running off AC!

Advantages:
• Can be optimised for task
• Can be much more efficient
• Can run at variable speed
• Can last forever

Disadvantages:
• Expensive (relative to a simple induction motor)
• Requires complex drive electronics
• Takes longer to explain than simple motors

Look closely and you'll see the word "inverter" on the box.

(I don't yet have any good pictures of the insides of a switched reluctance motor, but I'm working on it ...)