Ws All About Electric Motors Baseline April 2011

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ALL ABOUT ELECTRIC MOTORSThe Theory And Application Of Electromagnetism

Institute Of Electrical And Electronic Engineers, Phoenix Section

Teacher In Service Program / Engineers In The Classroom (TISP/EIC)

“Helping Students Transfer What Is Learned In The Classroom To The World Beyond”



MAGNETS AND MAGNETISM

DECEMBER 2010 IEEE TISP / EIC 6



The Magnetic Compass

• Columbus used the early magnetic

compass when he crossed the Atlanticocean

• He noted not only that the needledeviated slightly from exact north (asindicated by the stars) but also thatthe deviation changed during thevoyage

• Around 1600 William Gilbert,physician to Queen Elizabeth I ofEngland, proposed an explanation:

o The Earth itself is a giant magnet,

with its magnetic poles somedistance away from its geographicones (i.e. near the points definingthe axis around which the Earth

turns)


A sketch of Earth's magnetic

field representing the source of the

field as a magnet

The geographic north pole of Earth isnear the top of the diagram, the south

pole near the bottom

The south pole of that magnet is deep

in Earth's interior below Earth's North

Magnetic Pole



Visualizing Magnetic Fields (cont’d)

• Field lines of a bar magnet are

commonly illustrated by iron filings

sprinkled on a sheet of paper held

over a magnet

• The mutual attraction of opposite

poles of the iron filings results in the

formation of elongated clusters of

filings along "field lines“

• The field is not precisely the same

as around the isolated magnet; themagnetization of the filings alters

the field somewhat




The Magnetic Field Generated By An Electric Current

• The part about the magnetic field might be a

surprise to you, yet this definitely happens in all

wires carrying electricity

• You can prove it to yourself with the following

experiment. You will need:

– One AA, C or D-cell battery

– A piece of wire

– A compass

• Put the compass on the table and, with the wire

near the compass, connect the wire between thepositive and negative ends of the battery for a

few seconds

• What you will notice is that the compass needle

swingsDECEMBER 2010 IEEE TISP / EIC 15



The Magnetic Field Generated By An Electric Current

(cont’d)

• The new phenomenon was studied

in France by Andre-Marie Ampere,who concluded that the nature of

magnetism was quite different

from what everyone had believed

•It was basically a force betweenelectric currents: two parallel

currents in the same

direction attract, in opposite

directions repel

• Iron magnets are a very special

case, which Ampere was also able

to explain

– It involves currents at the atomic levelDECEMBER 2010 IEEE TISP / EIC 17



Coiling A Wire To Strengthen The Magnetic Field

• The figure on the right shows the shape of

the magnetic field around the wire• In this figure, imagine that you have cut

the wire and are looking at it end-on

• The green circle in the figure is the cross-

section of the wire itself• A circular magnetic field develops around

the wire, as shown by the circular lines in

the illustration

•

The field weakens as you move away fromthe wire (so the lines are farther apart as

they get farther from the wire)


Magnetic field of a wire



Coiling A Wire To Strengthen The Magnetic Field

(cont’d)

• You can see that the field isperpendicular to the wire and that the

field's direction depends on which

direction the current is flowing in the

wire• The compass needle aligns itself with

this field (perpendicular to the wire)

• Because the magnetic field around a

wire is circular and perpendicular to thewire, an easy way to amplify the wire's

magnetic field is to coil the wire, as

shown to the right


One loop's magnetic field

Magnetic field of a wire



ELECTRIC MOTORS



Electromagnets and Motors

• To understand how an electric motor works,

the key is to understand howthe electromagnet works

• An electromagnet is the basis of an electric

motor

• Say that you created a simple electromagnet

by wrapping 100 loops of wire around a nailand connecting it to a battery

• The nail would become a magnet and have a

north and south pole while the battery is

connected.

•Now say that you take your nail electromagnet,run an axle through the middle of it and

suspend it in the middle of a horseshoe

magnet as shown in the figure to the right




Electromagnets and Motors (cont’d)

• You can see that this half-turn of motion is simply due to the way

magnets naturally attract and repel one another• The key to an electric motor is to then go one step further so that, at the

moment that this half-turn of motion completes, the field of the

electromagnet flips

• The flip causes the electromagnet to complete another half-turn of

motion

• You flip the magnetic field just by changing the direction of the

electrons flowing in the wire (you do that by flipping the battery over)

• If the field of the electromagnet were flipped at precisely the right

moment at the end of each half-turn of motion, the electric motor

would spin freely




Armature, Commutator and Brushes

• Consider the image on the previous page

•

The armature takes the place of the nail inan electric motor

• The armature is an electromagnet made bycoiling thin wire around two or more polesof a metal core

• The armature has an axle, and thecommutator is attached to the axle

• In the diagram to the right, you can seethree different views of the same armature:front, side and end-on

• In the end-on view, the winding is eliminatedto make the commutator more obvious


Armature



Armature, Commutator and Brushes (cont’d)

• The diagram at the right shows how

the commutator and brushes worktogether to let current flow to theelectromagnet, and also to flip thedirection that the electrons areflowing at just the right moment

• The contacts of the commutator areattached to the axle of theelectromagnet, so they spin with themagnet

•

The brushes are just two pieces ofspringy metal or carbon that makecontact with the contacts of thecommutator


Commutator and brushes



Parts Of An Electric Motor• An electric motor is all about magnets

and magnetism

•A motor uses magnets to createmotion

• We have already learnt thefundamental law of all magnets:

– Opposites attract and likes repel

• So if you have two bar magnets withtheir ends marked "north" and "south,"then the north end of one magnet willattract the south end of the other

• On the other hand, the north end of

one magnet will repel the north end ofthe other (and similarly, south willrepel south)




Parts Of An Electric Motor (cont’d)

• Inside an electric motor, these

attracting and repelling forcescreate rotational motion

• In the diagram to the right, you can

see two magnets in the motor:

–

The armature (or rotor) is anelectromagnet

– The field magnet is a permanent

magnet (the field magnet could

be an electromagnet as well, but

in most small motors it isn't inorder to save power)




Putting It All Together (cont’d)

• Almost always, however, the rotor will have three poles rather than the

two poles as shown in this diagram• There are two good reasons for a motor to have three poles:

– It causes the motor to have better dynamics

– In a two-pole motor, if the electromagnet is at the balance point,perfectly horizontal between the two poles of the field magnet whenthe motor starts, you can imagine the armature getting "stuck" there

– That never happens in a three-pole motor.

– Each time the commutator hits the point where it flips the field in atwo-pole motor, the commutator shorts out the battery (directlyconnects the positive and negative terminals) for a moment

– This shorting wastes energy and drains the battery needlessly

– A three-pole motor solves this problem as well

• It is possible to have any number of poles, depending on the size of themotor and the specific application it is being used in




Shunt (Parallel) Wound Electric Motors

• In a shunt motor the field windings are

connected in parallel (shunt) with thearmature windings

• Once you adjust the speed of a dc shunt

motor, the speed remains relatively

constant even under changing load

conditions• One reason for this is that the field flux

remains constant

• A constant voltage across the field makes

the field independent of variations in the

armature circuit




Series Wound Electric Motor

• This type of motor develops a very large

amount of turning force, called torque, from a

standstill

• Because of this characteristic, the series dc

motor can be used to operate small electric

appliances, portable electric tools, cranes,

winches, hoists, car starters, etc.

• Another characteristic is that the speed varies

widely between no-load and full-load

• Series motors cannot be used where a

relatively constant speed is required under

conditions of varying load

• A final advantage of series motors is that they

can be operated by using either an ac or dc

power source

• You will be building a series wound electric

motorDECEMBER 2010 IEEE TISP / EIC 39



Compound Electric Motor

• A compound motor has two field

windings

• One is a shunt field connected in

parallel with the armature

• The other is a series field that is

connected in series with the armature• The shunt field gives this type of

motor the constant speed advantage

of a regular shunt motor

•

The series field gives it the advantageof being able to develop a large

torque when the motor is started

under a heavy load




Let’s Build An Electric Motor!




Dissecting A Commercial Electric

Motor



Inside A Toy Motor

• The motor being dissected here isa simple electric motor that youwould typically find in a toy

• A similar motor was used in thesolar cars you previously built inthe “Here Comes The Sun” lesson

• You can see that this is a smallmotor, about as big around as adime

• From the outside you can see thesteel can that forms the body ofthe motor, an axle, a nylon end capand two battery leads

• If you hook the battery leads of the

motor up to a flashlight battery,the axle will spin

• If you reverse the leads, it will spinin the opposite direction




Inside A Toy Motor

(cont’d)


• Here are two other views of the

same motor

•

Note the two slots in the side ofthe steel can in the second shot --

their purpose will become more

evident in a moment



Inside A Toy Motor

(cont’d)

•

The nylon end cap is held inplace by two tabs that are part

of the steel can

• By bending the tabs back, you

can free the end cap and

remove it

• Inside the end cap are the

motor's brushes

• These brushes transfer power

from the battery to the

commutator as the motor spins




More Motor Parts

• The axle holds the armature andthe commutator

• The armature is a setof electromagnets, in this casethree

• The armature in this motor is aset of thin metal plates stackedtogether, with thin copper wirecoiled around each of the threepoles of the armature

• The two ends of each wire (onewire for each pole) are solderedonto a terminal, and then each ofthe three terminals is wired to

one plate of the commutator• The figures to the right make it

easy to see the armature,terminals and commutator




This Chart Summarizes The Different Types Of Motors

Type Advantages Disadvantages Typical Application Typical Drive

AC Induction(Shaded Pole)

Least expensiveLong life

high power

Rotation slips fromfrequency

Low starting torque

Fans Uni/Poly-phase AC

AC Induction

(split-phase capacitor)

High power

high starting torque

Rotation slips from

frequency

Appliances

Stationary Power ToolsUni/Poly-phase AC

Universal motorHigh starting torque,

compact, high speed

Maintenance (brushes)

Medium lifespan

Drill, blender, vacuum

cleaner, insulation blowersUni-phase AC or Direct DC

AC SynchronousRotation in-sync with freq

long-life (alternator)More expensive

Industrial motorsClocks

Audio turntables

tape drives

Uni/Poly-phase AC

Stepper DCPrecision positioning

High holding torque

High initial cost

Requires a controller

Positioning in printers and

floppy drivesDC

Brushless DC

Long lifespan

low maintenanceHigh efficiency

High initial cost

Requires a controller

Hard drives

CD/DVD playerselectric vehicles

DC

Brushed DCLow initial cost

Simple speed control

Maintenance (brushes)

Medium lifespan

Treadmill exercisers

automotive motors (seats,

blowers, windows)

Direct DC or PWM

Pancake DCCompact design

Simple speed control

Medium cost

Medium lifespan

Office Equip

Fans/PumpsDirect DC or PWM

Ws All About Electric Motors Baseline April 2011

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