Hi pf, I have been reading about back EMF and just had a few questions. I found this article on wiki (http://en.wikipedia.org/wiki/Counter-electromotive_force) and the last paragraph talks about the effects of back emf when you plug a vacuum cleaner into your wall. It says that you can notice an incandescent bulb dim which I tried and it does indeed do this. I understand the concept of back emf and that when you first switch on the vacuum it draws a large current as the back emf is zero at this point. How does this cause a momentary voltage drop in your household? The only thing I can think of is that this same effect happens in a car due to the internal resistance of the car battery but there is no battery in a household circuit so what is responsible for this voltage drop? Is there a source of internal resistance at the voltage input to a house? The other question I had it what is it about the design of a motor that keeps the input voltage above the back emf. If you consider a motor which starts to spin then the back emf grows and grows as the motor spins faster and faster. So shouldn't it keep doing this until it reaches a point where the back emf equals the input emf which would mean there would be no net emf to keep driving the motor? Thanks for any help offered!