Download Physics_A2_41_BackEMF

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
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!
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
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.....
If we take our earlier equation: V -  = IR
and multiply throughout by current :
VI - I = I2R
Which equates to :
Electrical power supplied (IV) =
Electrical power transferred to mechanical power (I)
+
Electrical power wasted due to circuit resistance (I2R)
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
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