## Thursday, July 02, 2009

### Is James Dyson held back by the speed of sound?

I was intrigued by a story in the Daily Telegraph about a new electric motor created by Dyson. The DC motor apparently rotates at 104,000 RPM and is to be used in a portable vacuum cleaner.

The motor technology itself is switched reluctance. Essentially, the motor works by turning on and off electromagnets at just the right time to keep the rotor inside the motor spinning.

My immediate thought was 'how fast is the outside edge of the rotor moving if it's spinning at 104,000 RPM?' And shortly after that, 'how close is that to the speed of sound?'

In Electronic Weekly there's an article which states that the motor is 55.8mm across. Now, that's probably not the diameter of the rotor, but given that Dyson is attaching an impeller to the rotor anyway I'm going to take that as the diameter and work my calculations from there.

So the distance travelled in one rotation is π * 55.8mm and there are 104000 / 60 rotations per second. So, the outside is moving at 304ms-1.

The speed of sound at sea level is 340ms-1.

So the impeller is likely operating at near the speed of sound. I wonder if there are any nasty effects of rotating at that speed and if Dyson is close to the theoretical limit of what he can do.

There are two patent applications from Dyson that I believe cover this invention: 20070252551 and 20070278983. Neither mentions the speed of sound.

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