A presentation done for part of my A-level studies, and also for my personal interest too. Some materials are 'shamefully' from the Internet, so please message me if you find that your copyright is seriously infringed.

1. The electric motor
Electromagnetic effect
By Kelvin Lam

2. Topics:
• Motor introduction
• Motor vs. engines
• Magnetism
• Fleming’s Left/Right
Hand Rule
• Linear motor
• DC Motor (brushed)
• AC Motor (3-phase)
• Alternator
• Eddy current brake

3. What is a motor?
• ‘A device which converts electrical
energy to mechanical torque.’
The asynchronous three-phase
AC traction motor in
the bogie of the Eurostar
train.

4. Motor vs. engines
• An IC engine and an electric motor both produces a
mechanical torque.
But engine converts chemical energy to mechanical
torque via thermal energy;
a motor converts electrical energy to torque.
• The ‘Otto’ engine cycle as illustrated below uses a 4
stroke system: ‘intake, compression, combustion,
exhaustion’.

5. Permanent Magnets
• Ferrimagnetism:
An electron has a ‘spin’
quantum mechanical property.
It defines the rotatory (angular
momentum) of the electron
orbiting around an atom.
• They are orientated randomly.
• At specific temperature (Curie),
the electrons which induces
electromagnetic dipole aligns
itself, causing a magnetic field
as it is polarised. Thus the
metal is magnetised.

6. Electromagnetism
• Electromagnets exhibits
property of magnetism only
when current runs through it.
• As electric current passes
through a ferromagnetic
element the particles become
charged and begins to move
in a path. Again due to the
‘dipole’ it creates a magnetic
field.
• The strength of this field
depends on the cross section
area of the conductor, current
and the frequency ‘of the
change of current’.

7. Fleming’s Left Hand Rule
• We can use a
mnemonic,
‘Fleming’s Left
Hand Rule’, to
understand the
relationship of
electric current and
the ‘thrust of
motion’ caused by
it.

8. Right hand rule
• Maxwell’s Corkscrew
Rule
• Thumb shows the
direction of current; rest
of hand shows direction
of magnetic field.
• As magnetic field is
applied across the flow
of electrons, it affect the
spin of the electrons
which affect the ‘atomic’
magnetic field, causing a
repulsion.

9. Linear motor
• Using the left-hand rule,
having a current
perpendicular to the
magnetic field produces
a linear motion.
• Coils (in loop) produces
an Eddy current field,
producing magnetic
field.
• Used in high-speed
transportation.

10. Linear motor
(The Shanghai Maglev Train in
China has a top speed of
431km/h, equivalent to 268mph)

11. Simple Direct current motor
• The motor effect is
observed when there is
a changing magnetic
field.
• Right diagram shows a
split-ring commutator
DC motor: earliest and
least efficient of all.
• What if the commutator
is short-circuited –
stuck in middle of the
two brushes?

12. • But in reality, DC motor has 3 coils. Each
with 120 degrees angle.
• There are 3 commutators and 2 brushes.
• One of the 3 coils is inactive.

13. ‘DC is now obsolete!’
• Cheap, easy to
operate.
• Excellent for
acceleration/speed
control.
• High precision.
• Maintenance of
mechanical rotating
brushes.
• Friction is enormous:
not efficient for high
speed performance,
i.e. trains.
• Sparks from brushes
may initiate
explosions.
DC Motor are used in new state-of-art
electric aircrafts.

14. AC Current
• The electric current
repeatedly changes
its direction.
• Single & Three
Phase
• Single phase: 360
degrees
• Three phase: 120
degrees

15. Three-phase AC
• It carries 3 alternating current of the same
frequency.
• Each current has a time-separation.
• It gives a constant electrical power to turn
the AC motor.
• Due to the ‘superposition’ of current, it
tends to cancel the p.d. each other so that
it reduce the size of neutral wire.

16. Three-phase AC motor
• Stator produces a varying magnetic field with AC.
• This induces a secondary current in the rotor due to
magnetic flux of different direction.
• Lenz Law: the rotor then induces a magnetic field
that oppose the stator.

18. Motor control
Direct current
• Chopper control
• PWM (Pulse-width
modulation)
• Resistance (Cam shaft)
• Thyristor
• Bridge rectifier
Single/3 Phase AC
• VFD (Variable-frequency
drive)
• Inverter

19. • http://www.youtube.com/watch?v=LpvyrN8
YOBo

20. Regenerative braking
• In railway (or hybrid) vehicles, the vehicle’s inertia
drives the rotor, generating induction current.
• As electricity is generated, this causes a ‘negative’
torque, slowing the vehicle down.
• The produced electricity is either fed into resistor
(dynamic braking) or fed back to the electric
supply.

21. Eddy current brake
• Conventional brake uses friction.
• Rotating disc (which are exposed to
electromagnetic field) induces an eddy
current (opposing current) on the coils.
• This produces a opposing braking
force for the train.
• Magnets are placed 7mm away from
the rail to allow room for the rotating
disc.
• Braking strength controlled by strength
of magnetic field.
• Only usable in high speed situation.
• No energy wasted, no heat, no odour.
(Eddy current brake in
Japanese Shinkansen
700)

22. Dynamo & Generator
• Dynamo is an older term that describe
something that makes direct current.
• Early inventors discovered that
electromagnetic effect could generate AC,
but too complicated to control them.
• It has a commutator.
• The rotating magnets produces a varying
magnetic field, thus generate a varying
current.

23. Types of motor
DC Motor
• Brushed motor
Stepper, coreless,
pancake
AC Motor
• Induction motor
• Universal motor
• Synchronous (Selsyn)
motor
• Shaded-pole motor

24. Questions?