the electric motor
DESCRIPTION
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.TRANSCRIPT
The electric motorElectromagnetic effect
By Kelvin Lam
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
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.
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’.
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.
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’.
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.
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.
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.
Linear motor
(The Shanghai Maglev Train in
China has a top speed of
431km/h, equivalent to 268mph)
Simple Direct current motor
• The motor effect is observed when there is a changing magnetic field.
• Right diagram shows a split-ring commutatorDC motor: earliest and least efficient of all.
• What if the commutatoris short-circuited –stuck in middle of the two brushes?
• 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.
‘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.
AC Current
• The electric current repeatedly changes its direction.
• Single & Three Phase
• Single phase: 360 degrees
• Three phase: 120 degrees
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.
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.
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
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.
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)
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.
Types of motor
DC Motor
• Brushed motor
Stepper, coreless,
pancake
AC Motor
• Induction motor
• Universal motor
• Synchronous (Selsyn)
motor
• Shaded-pole motor
Questions?