How Does Back EMF Cause a Voltage Drop in Household Circuits?

[Jimmy87]

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!

 

[milesyoung]

How does this cause a momentary voltage drop in your household?

The wiring in your household has resistance. If your bulb and vacuum cleaner are on the same circuit, then the bulb will see a momentary voltage drop due to the large inrush current flowing to the vacuum. There's more to it than just resistance, though.

The other question I had it what is it about the design of a motor that keeps the input voltage above the back emf.

There's always a practical limit to what voltage you can apply to the motor, so, as you say, there will always be some speed at which you can't drive current into it due to its back-EMF.

The motor doesn't have a say in what voltage you apply to it, however.