Teaching Notes for Unit 9.3 Motors and Generators Topic of NSW HSC Physics Course

1. Topic 9.3
3.4.1 AC Motors

2. AC Motor Construction
● An AC motor is structurally identical to a DC motor except
that it has a slip-ring commutator rather than the split ring of
a DC motor.
● This is because the current in the coil changes direction
due to its AC nature and doesn't need to be mechanically
flipped.
● This means that the rotation speed is harder to control as
it is related to the frequency of the AC supply.
● It is common in AC motors to use electromagnets (field coils)
to create the stator field.

3. AC Motor Construction

4. Brush-less AC Motors
● As AC electricity changes phase (direction) with a
regular frequency this can be used to set up a rotating
magnetic field around the rotor.
● If the rotor is replaced with a magnet, then a brush-less
motor can be constructed.

5. Induction Motors
● An induction motor consists of a
stator and a rotor.
● The stator consists of a series of
wire coils wound on soft iron cores
that surround the rotor.
● These are connected to the
external power supply in such a
way that they produce a
magnetic field whose polarity
rotates at constant speed in one
direction.
● The rotor consists of coils wound on
a laminated iron armature mounted
on an axle.

6. Induction Motors
● The rotor coils are not
connected to the external power
supply
● An induction motor has neither
commutator nor brushes.
● Instead eddy currents are
induced in the rotor coils by the
rotating magnetic field of the
stator.

7. Induction Motors
● By Lenz's law the eddy currents
produce magnetic fields which
try to oppose the motion of the
rotor.
● However in this case this results
in the rotor trying to keep up
with the stator field (so that
there is no relative motion) and
generating a torque.
● The Shading ring disrupts the
magnetic field and allows the
motor to start spinning.

8. Induction Motors
● The rotor coils are often simplified
to single copper or aluminium
bars capable of carrying a large
current, embedded in the surface
of the soft iron armature.
● The bars are connected at the
ends by a ring or disc of copper
which allows current to flow in a
loop between opposite bars.
● This physical arrangement is
referred to as a squirrel cage
● even though it looks more like
a hamster wheel!

9. Energy Transfers in the Home and
Industry
● There are many many examples of energy
transfers in the home.
● Remember that no energy transfer in the real
world is ever 100% efficient
● energy will always be wasted as thermal
energy (and light and sound) in each change.

10. Energy Transfers in the Home and
Industry
● Good examples to give could
include:
Electrical → Kinetic:
Electrical → Thermal:
Electrical → Light:
Electrical → Chemical:
Electrical → Sound:
● any device with a motor such as a
washing machine.
● any device that heats up such a
kettle or toaster.
● anything that glows or gives off
light: lightbulb?
● Battery re-chargers are good
examples.
● The best example is speakers
(even though we know this goes
via kinetic energy)