Saturday, 9 June 2007

Lenz's law

Explain this: I just connected my 1300W vacuum cleaner to a power meter. When running unobstructed it consumes 1100W. When I block the tube so the motor is audibly under more load the power drops to 900W. Power factor remains close to 1. Weird motor?


  1. Well I worked it out. When you block the air inlet to a fan its load actually goes down and it spins faster. It counter intuitive but I think what happens is that when a fan is blowing it is accelerating a constant supply of low speed air so it is doing work. When the air supply is blocked it ends up just spinning the same piece of air round and round at constant velocity so very little work is done.

  2. When the inlet is blocked the turbine is in greater vacuum, thus less load & higher revs

  3. The fan has way less resistance (its not moving air out anymore only around its own case)So there is no load anymore for the engine => less ampère (amps?) =>...