So the back emf is formed due to eddy currents. Eddy currents are swirling currents in bulk pieces of metal.

This is how back emf works:

1. Coils start rotating in the external magnetic field when the motor starts up

2. Lenz's law is what causes the induced back emf

(Remember that Lenz's law is when there is a change in flux an emf is induced. A current is then induced in which a flux opposing the original change in flux is produced).

The opposing flux is what causes the induced back emf.

3. So the formula for Net emf is Supply emf - back emf

So the supply emf is due to the change in flux (EMF = Electromotive Force) as the coil is rotated in the magnetic field.

Therefore, as the motor speed increases the eddy currents and the resulting back EMF also increases due to the increase in change in flux and induced emf (supply emf).

Hope that helped!

If you need help on other stuff you can visit my blog, I will be posting once a week giving tips and doing some model band 6 answers for phys/chem!

http://hscsynergyeducation.weebly.com/blog/