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

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

YOBo

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?