Tuesday, 16 July 2013

AC DC fans

We were looking at fans in the electric shop the other day. There were a few new DC fans. The shop shows the rating of each fan, and DC seem to consume about half the electricity. This seems a little counter-intuitive since I thought AC motors were more efficient than DC motors. I guess DC fans are more efficient because the control circuit of a DC fan will change the current electronically. The AC fan, on the other hand, is probably going to use a variable resistor, turning some of the electricity into heat, and running the AC motor at a speed where it's not so efficient. 

But surely, if you're running the AC motor at the design value, it's going to be more efficient?

All electric motors basically work with electromagnets making a rotating magnetic field. AC electric motors can be very simple. Effectively the alternating current goes straight to an electromagnet making a rotating magnetic field.  A fixed magnet on the shaft then rotates. Depending on how well the frequency of the current synchronises with the speed of the motor will change the efficiency. 

I had a record player with a direct-drive AC motor that I brought to Japan many years ago. The UK has mains current at 50 Hz, while Japan has 60 Hz. This made my Bruce Springsteen records sound like Dolly Parton. 

I didn't use it to play heavy metal. 

Eventually I got a 60 Hz motor, then took it back to the UK where I briefly had the opposite problem, although unfortunately no Dolly Parton records. Now it's back in Japan but I'm not sure where the correct motor is.

DC motors can be brushless or with brushes. If they use brushes, the polarity of the electromagnet changes as the shaft rotates. The brushes cause friction which adds to the inefficiency.

Brushless DC motors, also called stepper motors, have the fixed magnet on the shaft, and two sets of electromagnets which are swithed on and off to create a changing magnetic field. 
Another loss of efficiency is in the resistance of the electromagnets, which will be more for DC with its constant current, rather than a current rising as the electromagnet needs more power. The electromagnets are going to be applying their full forcefield the whole time, even when their field is in the same direction as the fixed field and the power is not going to help move the shaft around. AC, on the other hand, is sinusoidal and the power will rise to the challenge of providing torque when it is most effective and most needed. The sine wave in the AC is just circular motion repeated onto the timeline, so it's going to convert easily back into rotation.

So AC motors would seem to be more efficient.

Or maybe the DC fans have AC motors in them, and an electronic inverter converts DC into AC at the optimum frequency, while the AC motors are stuck with the mains frequency.  Even then, the AC fans should be more efficient because there's no conversion from AC to DC in the power supply then back from DC to AC in the inverter.

It may just be that the DC fans aren't more efficient than the AC ones; just less powerful. Apparently they are really good at supplying a gentle breeze.

There's an interesting, but inconclusive, discussion of the efficiency difference between AC and DC in electric vehicles here.