1. Book Reference : Pages 129 1.To understand that a spinning motor is also a generator 2.To understand how electric motors behave when lightly or heavily loaded

2. Previously we have seen the following 1.Current carrying conductors in magnetic fields experience a force. The ultimate embodiment of this is an electric motor (a spinning coil in a magnetic field or similar) 2.Conductors in a changing magnetic field (due to movement or electromagnetic variation) will have an EMF induced in them. A generator is the ultimate embodiment of this (a spinning coil in a magnetic field or similar) Does an electric motor fit the second point & have an induced EMF? Discuss!

3. A spinning coil in an electric motor will have an EMF induced in it because it sees changing flux linkage This EMF is called “Back EMF” because it follows Lenz’s law and opposes the potential difference (V) supplied to the motor At any moment :V -  = IR Where V is the supplied PD,  is the back EMF, I is the current through the coil and R is the resistance of the coil

4. Like any generator, the induced back EMF will be proportional to the spin speed. 1.At low speed the current is high since the induced back EMF is small 2.At high speed the current is low since the induced EMF is high Focussing on current in our earlier equation.....

5. If we take our earlier equation:V -  = IR and multiply throughout by current : VI -  I = I 2 R Which equates to : Electrical power supplied (IV) = Electrical power transferred to mechanical power (  I) + Electrical power wasted due to circuit resistance (I 2 R)

6. An unloaded motor will spin with a high speed, the induced back EMF will be high and the resulting current is low. The speed is limited by resistive forces (bearing friction and air resistance). Little power used In contrast a loaded motor will spin with a low speed, the induced back EMF will be low and the resulting current is high. Power is transferred from the voltage source to mechanical power in the load and wasted heat due to resistance

7. An electric motor is connected to a battery and an ammeter. It is connected to a variable load Explain why the motor current is low when the load is zero Explain why the motor current increases when the load is increased