Book Reference : Pages 129Book Reference : Pages 129

1.1. To understand that a spinning motor is also a To understand that a spinning motor is also a generatorgenerator

2.2. To understand how electric motors behave To understand how electric motors behave when lightly or heavily loadedwhen lightly or heavily loaded

Previously we have seen the followingPreviously we have seen the following

1.1.Current carrying conductors in magnetic fields Current carrying conductors in magnetic fields experience a force. The ultimate embodiment of this is experience a force. The ultimate embodiment of this is an an electric motor electric motor (a spinning coil in a magnetic field or (a spinning coil in a magnetic field or similar)similar)

2.2.Conductors in a Conductors in a changingchanging magnetic field (due to magnetic field (due to movementmovement or electromagnetic variation) will have an or electromagnetic variation) will have an EMF induced in them. A EMF induced in them. A generatorgenerator is the ultimate is the ultimate embodiment of this (a spinning coil in a magnetic field or embodiment of this (a spinning coil in a magnetic field or similar)similar)

Does an electric motor fit the second point & Does an electric motor fit the second point & have an induced EMF? Discuss!have an induced EMF? Discuss!

A spinning coil in an electric motor will have an A spinning coil in an electric motor will have an EMF induced in it because it sees changing flux EMF induced in it because it sees changing flux linkagelinkage

This EMF is called This EMF is called “Back EMF” “Back EMF” because it follows because it follows Lenz’s law and opposes the potential difference Lenz’s law and opposes the potential difference (V) supplied to the motor(V) supplied to the motor

At any moment :At any moment : V - V - = IR = IR

Where V is the supplied PD, Where V is the supplied PD, is the back EMF, I is is the back EMF, I is the current through the coil and R is the the current through the coil and R is the resistance of the coilresistance of the coil

Like any generator, the induced back EMF will be Like any generator, the induced back EMF will be proportional to the spin speed.proportional to the spin speed.

1.1. At low speed the current is high since the At low speed the current is high since the induced back EMF is smallinduced back EMF is small

2.2. At high speed the current is low since the At high speed the current is low since the induced EMF is highinduced EMF is high

Focussing on current in our earlier equation.....Focussing on current in our earlier equation.....

If we take our earlier equation:If we take our earlier equation: V - V - = IR = IR

and multiply throughout by current :and multiply throughout by current :

VI - VI - I = II = I22RR

VI = VI = I + II + I22RR

Which equates to :Which equates to :

Electrical power supplied (IV) =Electrical power supplied (IV) =

Electrical power transferred to mechanical power (Electrical power transferred to mechanical power (I)I)

++

Electrical power wasted due to circuit resistance (IElectrical power wasted due to circuit resistance (I22R)R)

An unloaded motor will spin with a high speed, An unloaded motor will spin with a high speed, the induced back EMF will be high and the the induced back EMF will be high and the resulting current is low. The speed is limited by resulting current is low. The speed is limited by resistive forces (bearing friction and air resistive forces (bearing friction and air resistance). Little power usedresistance). Little power used

In contrast a loaded motor will spin with a low In contrast a loaded motor will spin with a low speed, the induced back EMF will be low and the speed, the induced back EMF will be low and the resulting current is high. Power is transferred resulting current is high. Power is transferred from the voltage source to mechanical power in from the voltage source to mechanical power in the load and wasted heat due to resistancethe load and wasted heat due to resistance

An electric motor is connected to a battery and An electric motor is connected to a battery and an ammeter. It is connected to a variable loadan ammeter. It is connected to a variable load

Explain why the motor current is low when the Explain why the motor current is low when the load is zeroload is zero

Explain why the motor current increases when Explain why the motor current increases when the load is increasedthe load is increased