Tuesday 2 September 2014

The Great Vacuum Cleaner Rip-Off

Recent legislation has banned the manufacture or import of vacuum cleaners with a motor power over 1600 watts.  And predictably, idiots who don't understand physics are complaining.

The simple fact is, that manufacturers have been ripping you off.

Let me explain.

A vacuum cleaner turns electricity from the mains into kinetic energy which it imparts to the stream of moving air  (which is what you want it to do).  It also turns electricity into heat  (which you don't want it to do)  and sound  (which you can't avoid, but it isn't much anyway; a screaming toddler is producing about a watt of sound energy.  Yes, loudspeakers really are that inefficient).

You can measure how fast KE is being imparted to the air stream by multiplying the volume flow rate by the pressure change.  Assuming you use the fundamental units throughout, you get metres cubed per second, times Newtons per metre squared = Newtons-metres per second = Joules per second = Watts.  That figure is all the dirt in your carpet cares about.  The difference between air-watts and electrical watts is what is getting wasted.

From the vacuum cleaner manufacturer's point of view, since you already have a moving air stream, you can use this for cooling the motor; so overheating really isn't a huge deal.  You can therefore make your motor inefficient -- using copper wire that is too thin, and/or not enough steel to leave any molecules un-magnetised right through the crests and troughs of each cycle of the mains -- so it turns some electricity into heat that otherwise would have been turned into KE.  Also from the vacuum cleaner manufacturer's point of view, a bigger number on the advertisement attracts more buyers.  So it's actually in the manufacturers' interest to make vacuum cleaners as inefficient as possible, just so as to be able to claim big numbers!

And people have fallen for this hook, line and sinker.  I despair.

(My proposed solution would be to require standardised testing of vacuum cleaners, including ability to extract different types of dirt from different surfaces, percentage efficiency as air-watts / electrical watts and expected lifetime of product.  Of course this will make additional work for the manufacturers; but we consumers outnumber them, and the law is supposed to work for us .....)

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