The document discusses different types of AC motors, including their construction and operation. It explains that AC motors have a slip-ring commutator rather than split rings, allowing the current in coils to change direction with the alternating current. Induction motors are then described in more detail, having a stator that produces a rotating magnetic field which induces eddy currents in the rotor coils, causing the rotor to spin. The rotor coils in an induction motor form a "squirrel cage" structure. Finally, examples of energy transfers in homes and industries are given, such as electrical to kinetic in motors, electrical to thermal in heating elements, and electrical to sound in speakers.

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)