In the above figure,
- EMF applied produces a current in the rod
- Current interacts with field, producing a force to the right (F = IlB)
- Speed of rod (v) increases
- However, the rod is cutting through field lines
- Therefore, an EMF is induced (Faraday’s law)
- This EMF is induced in the opposite direction to the applied EMF!
- EMF back = lvB
- Hence, the EMF pushing current is equal to: EMF applied – EMF back
- As v and EMF increase, EMF current decreases, and so therefore I and F decrease, so acceleration of the rod stops at max velocity.
Back EMF in DC motors
- As a DC motor spins, EMF back is produced, since the coil is cutting through field lines.
- Therefore, the voltage available to produce current in the coil, and therefore torque, is equal to
EMF applied – EMF back
EMF back is the instantaneous EMF induced = 2NlvBsinθ
- If motor speed increases, current in coil decreases, and so the applied torque decreases
- Simple AC motors are simple AC generators in reverse
- All usual equations apply.