ALL ABOUT ELECTRIC MOTORS The Theory And Application Of Electromagnetism Institute Of Electrical And...

ALL ABOUT ELECTRIC MOTORSThe Theory And Application Of

Electromagnetism

Institute Of Electrical And Electronic Engineers, Phoenix SectionTeacher In Service Program / Engineers In The Classroom (TISP/EIC)

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

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This Presentation Includes Material Copied From The Web Sites:

Howstuffworks “How Electric Motors Work,” Http://Www.Howstuffworks.Com/Motor.Htm

(Copyright © 1998-2010 Howstuffworks, Inc.) Wikipedia – Electric Motor,

Http://En.Wikipedia.Org/Wiki/Electric_motorMagnetic Fields,

Http://Www-istp.Gsfc.Nasa.Gov/Education/Wmfield.Html

This Presentation May Only Be Used Free Of Charge And Only For Educational Purposes, And May Not Be Sold Or Otherwise Used

For Commercial PurposesDECEMBER 2010 2IEEE TISP / EIC

http://www.howstuffworks.com/Motor.Htm

http://en.wikipedia.org/wiki/Electric_motor

http://www-istp.gsfc.nasa.gov/Education/wmfield.html

INTRODUCTION

DECEMBER 2010 IEEE TISP / EIC 3

Here's An Interesting Experiment For You To Try: Walk Through Your House And Count All The Motors You Find

Starting in the kitchen, there are motors in: The fan over the stove and in the microwave oven •The dispose-all under the sink •The blender •The can opener •The refrigerator - Two or three in fact: one for the compressor, one for the fan inside the refrigerator, as well as one in the icemaker •The mixer •The tape player in the answering machine •Probably even the clock on the oven In the utility room, there is an electric motor in: •The washer•The dryer•The electric screwdriver•The vacuum cleaner and the Dustbuster mini-vac •The electric saw •The electric drill •The furnace blower Even in the bathroom, there's a motor in: •The fan •The electric toothbrush •The hair dryer •The electric razor

• Your car is loaded with electric motors: • Power windows (a motor in each window) • Power seats (up to seven motors per seat) • Fans for the heater and the radiator • Windshield wipers• The starter motor • Electric radio antennas Plus, there are motors in all sorts of other places: • Several in the VCR • Several in a CD player or tape deck • Many in a computer (each disk drive has two or three, plus

there's a fan or two) • Most toys that move have at least one motor (including

Tickle-me-Elmo for its vibrations) • Electric clocks • The garage door opener • Aquarium pumps In walking around my house, I counted over 50 electric motors

hidden in all sorts of devicesEverything that moves uses an electric motor to accomplish its

movement.

Modern life would be impossible without electric motors!


Some Samples Of Electric

Motor Sizes• Common Nine-volt

battery in the middle front

• Largest motor:Three phase AC induction motor rated with 1 Hp (750 W)

• Next largest: 25 W• Small motors:

– CD player motor– Brushed DC

Electric Motor common as toy motors

– Stepper motor with worm gear for CD pickup-head traversing


MAGNETS AND MAGNETISM


“Lodestones”: Naturally Occurring Magnets

• The ancient Greeks, originally those near the city of Magnesia, and also the early Chinese knew about strange and rare stones (possibly chunks of iron ore struck by lightning) with the power to attract iron

• A steel needle stroked with such a "lodestone" became "magnetic" as well

• Around 1000 the Chinese found that such a needle, when freely suspended, pointed north-south

• The magnetic compass soon spread to Europe


The Magnetic Compass

• Columbus used the early magnetic compass when he crossed the Atlantic ocean

• He noted not only that the needle deviated slightly from exact north (as indicated by the stars) but also that the deviation changed during the voyage

• Around 1600 William Gilbert, physician to Queen Elizabeth I of England, proposed an explanation: o The Earth itself is a giant magnet,

with its magnetic poles some distance away from its geographic ones (i.e. near the points defining the axis around which the Earth turns)


A sketch of Earth's magnetic field representing the source of the field as a magnetThe geographic north pole of Earth is near the top of the diagram, the south pole near the bottomThe south pole of that magnet is deep in Earth's interior below Earth's North Magnetic Pole

Magnetic Fields• People not familiar with magnetism

often view it as a somewhat mysterious property of specially treated iron or steel

• A magnetized bar has its power concentrated at two ends, its poles

• They are known as its north (N) and south (S) poles, because if the bar is hung by its middle from a string, its N end tends to point northwards and its S end southwards

• The N end will repel the N end of another magnet, S will repel S, but N and S attract each other


Magnetic Fields (cont’d)

• The region where this is observed is loosely called a magnetic field

• Either pole can also attract iron objects such as pins and paper clips

• That is because under the influence of a nearby magnet, each pin or paper clip becomes itself a temporary magnet, with its poles arranged in a way appropriate to magnetic attraction


Visualizing Magnetic Fields

• Michael Faraday proposed a widely used method for visualizing magnetic fields

• Imagine a compass needle freely suspended in three dimensions, near a magnet

• We can trace in space (in our imagination, at least!) the lines one obtains when one "follows the direction of the compass needle“

• Faraday called them lines of force, but the term field lines is now in common use


Compasses reveal the direction of the local magnetic field. As seen here, the

magnetic field points towards a magnet's south pole and away from

its north 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; the magnetization of the filings alters the field somewhat


ELECTROMAGNETISM


But What Is Magnetism?• Until 1821, only one kind of

magnetism was known, the one produced by iron magnets

• Then a Danish scientist, Hans Christian Oersted, while demonstrating to friends the flow of an electric current in a wire, noticed that the current caused a nearby compass needle to move


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 the positive and negative ends of the battery for a few seconds

• What you will notice is that the compass needle swings


The Magnetic Field Generated By An Electric Current (cont’d)

• Initially, the compass will be pointing toward the Earth's north pole (whatever direction that is for you), as shown in the figure on the right

• When you connect the wire to the battery, the compass needle swings because the needle is itself a small magnet with a north and south end

• Being small, it is sensitive to small magnetic fields

• Therefore, the compass is affected by the magnetic field created in the wire by the flow of electrons


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 between electric 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 level


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 from the 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 is perpendicular 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 the wire, 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

Magnetic Field Produced By A Coil

• The magnetic field circling each loop of wire combines with the fields from the other loops to produce a concentrated field down the center of the coil

• A loosely wound coil is illustrated to the right to show the interaction of the magnetic field

• The magnetic field is essentially uniform down the length of the coil when it is wound tighter

• The strength of a coil's magnetic field increases not only with increasing current but also with each loop that is added to the coil


Left Hand Rule To Determine Magnetic Field Direction

• To find the direction the magnetic field is going, you can use the “left-hand rule" to determine it

• First, remember that the electrons in the current flow from the negative terminal to the positive terminal

• If you take your left hand and wrap it around the wire, with your thumb pointing in the direction of the electrical current (negative to positive), then your fingers are pointing in the direction of the magnetic field around the wire


An Electromagnet

• If you wrap your wire around a nail 10 times, connect the wire to the battery and bring one end of the nail near the compass, you will find that it has a much larger effect on the compass

• In fact, the nail behaves just like a bar magnet

• However, the magnet exists only when the current is flowing from the battery

• What you have created is an electromagnet!

• You will find that this magnet is able to pick up small steel things like paper clips, staples and thumb tacks


A simple electromagnet

Right Hand Wrapping Rule To Determine Magnetic Poles

• A similar rule to the Left Hand Rule for magnetic fields can be used to determine which end of a coil has the north magnetic pole

• If the fingers of the right hand are wrapped around the coil in the direction the wire has been wound onto the coil from positive to negative, then the thumb points in the direction of the north pole of the electromagnet


Strength Of Electromagnetic Field

• The strength of the electromagnetic field, B, inside a coil of wire is determined by the amount of current, I, the number of coils of wire, N, and the length of the coil, S:

B = 4*∏*N*IS

Unit• The unit of magnetic field is called the tesla (T)• Near a strong magnet the field is 1-T• Another unit used is the gauss, where 104 gauss (10,000) equals 1 teslaCurrent• The strength of the magnetic field is proportional to the current in the wire in Amps• If you double the current, the magnetic field is doubled• Since by Ohm’s Law:

Voltage = Current x Resistance (V = I x R)you can double the current in a wire by doubling the voltage of the source of

electricity


Strength Of Electromagnetic Field (cont’d)Turns of coil• If you wrap the wire into a coil, you increase the magnetic field inside the

coil, proportional to the number of turns• In other words, a coil consisting of 10 loops has 10 times the magnetic

field as a single wire with the same current flowing through it• Likewise, a coil of 20 loops has 2 times the magnetic field than one with

10 loopsCoil length• The magnetic field inside a coil with a fixed number of turns varies

inversely with the length of the coil• For example, if two coils have the same number of turns, the field in a 10

cm long coil is ten times the field in a 100 cm long coil• All this means is, winding an electromagnet coil tightly with closely spaced

turns is more effective than loosely with widely spaced turns– N/S is sometimes called the “turns density,” n, the number of turns

per unit length– Then: B = 4*∏*n*I


ELECTRIC MOTORS

Electromagnets and Motors

• To understand how an electric motor works, the key is to understand how the 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 nail and 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)

• If you were to attach a battery to the electromagnet so that the north end of the nail appeared as shown, the basic law of magnetism tells you what would happen:– The north end of the electromagnet

would be repelled from the north end of the horseshoe magnet and attracted to the south end of the horseshoe magnet

– The south end of the electromagnet would be repelled in a similar way

– The nail would move about half a turn and then stop in the position shown


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 in

an electric motor• The armature is an electromagnet made by

coiling thin wire around two or more poles of a metal core

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

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

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


Armature

Armature, Commutator and Brushes (cont’d)

• You can see that the commutator is simply a pair of plates attached to the axle

• These plates provide the two connections for the coil of the electromagnet

• The "flipping the electric field" part of an electric motor is accomplished by two parts: the commutator and the brushes


Armature

Armature, Commutator and Brushes (cont’d)

• The diagram at the right shows how the commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment

• The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet

• The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator


Commutator and brushes

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

and magnetism• A motor uses magnets to create

motion• We have already learnt the

fundamental law of all magnets:– Opposites attract and likes repel

• So if you have two bar magnets with their ends marked "north" and "south," then the north end of one magnet will attract the south end of the other

• On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south)


Parts Of An Electric Motor (cont’d)

• Inside an electric motor, these attracting and repelling forces create rotational motion

• In the diagram to the right, you can see two magnets in the motor:– The armature (or rotor) is an

electromagnet– The field magnet is a permanent

magnet (the field magnet could be an electromagnet as well, but in most small motors it isn't in order to save power)


Putting It All Together• When you put all of these parts together, what you

have is a complete electric motor• The key thing in the diagram is that as the armature

passes through the horizontal position, the poles of the electromagnet flip

• Because of the flip, the north pole of the electromagnet is always above the axle so it can repel the field magnet's north pole and attract the field magnet's south pole

• If you take apart a small electric motor, you will find that it contains the same pieces described above:– Two small permanent magnets– A commutator– Two brushes, and– An electromagnet made by winding wire

around a piece of metal


In this figure, the armature winding has been left out so that it is easier to see the commutator in action

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 when the 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 a

two-pole motor, the commutator shorts out the battery (directly connects 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 the motor and the specific application it is being used in


Field Coils

• The power of a motor that has permanent magnets is limited by the fixed strength of the magnets

• In bigger motors the permanent magnets are replaced by electromagnets, called the field coils

• Since the strength of the field magnets depends on the number of turns of wire in the coils and on the magnitude of the current in the coils, this allows the motor to be made more powerful

• The field coils and the armature coils can be connected together and to the voltage supply in three different ways:– In parallel– In series– A combination of the two


Shunt (Parallel) Wound Electric Motors

• In a shunt motor the field windings are connected in parallel (shunt) with the armature 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 motor


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 advantage of 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 is

a simple electric motor that you would typically find in a toy

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

• You can see that this is a small motor, about as big around as a dime

• From the outside you can see the steel can that forms the body of the motor, an axle, a nylon end cap and 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 spin in 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 of the 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 in place 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 and

the commutator• The armature is a set

of electromagnets, in this case three

• The armature in this motor is a set of thin metal plates stacked together, with thin copper wire coiled around each of the three poles of the armature

• The two ends of each wire (one wire for each pole) are soldered onto a terminal, and then each of the 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


More Motor Parts (cont’d)

• The final piece of any DC electric motor is the field magnet

• The field magnet in this motor is formed by the can itself plus two curved permanent magnets

• One end of each magnet rests against a slot cut into the can, and then the retaining clip presses against the other ends of both magnets


ALL Electric Motors Work The Same Way• There are many types of electric motors:

– AC– DC– Universal– Stepper– Synchronous– Induction– Brushless– Etc., etc.

• However, they ALL work on the same principles we have discussed• What varies are the details of the physical implementation:

– the electrical supply (AC or DC)– the field coil (permanent magnets or electromagnets)– The commutator (segments on the axle, or transistor circuit, or induced

magnetism)– The armature (wound coils, or induced poles, or permanent magnets)


This Chart Summarizes The Different Types Of Motors


Type Advantages Disadvantages Typical Application Typical Drive

AC Induction(Shaded Pole)

Least expensiveLong lifehigh power

Rotation slips from frequencyLow starting torque

Fans Uni/Poly-phase AC

AC Induction(split-phase capacitor)

High powerhigh starting torque

Rotation slips from frequency

AppliancesStationary Power Tools Uni/Poly-phase AC

Universal motor High starting torque, compact, high speed

Maintenance (brushes)Medium lifespan

Drill, blender, vacuum cleaner, insulation blowers Uni-phase AC or Direct DC

AC Synchronous Rotation in-sync with freqlong-life (alternator) More expensive

Industrial motorsClocksAudio turntablestape drives

Uni/Poly-phase AC

Stepper DC Precision positioningHigh holding torque

High initial costRequires a controller

Positioning in printers and floppy drives DC

Brushless DCLong lifespanlow maintenanceHigh efficiency

High initial costRequires a controller

Hard drivesCD/DVD playerselectric vehicles

DC

Brushed DC Low initial costSimple speed control

Maintenance (brushes)Medium lifespan

Treadmill exercisersautomotive motors (seats, blowers, windows)

Direct DC or PWM

Pancake DC Compact designSimple speed control

Medium costMedium lifespan

Office EquipFans/Pumps Direct DC or PWM

ALL ABOUT ELECTRIC MOTORS The Theory And Application Of Electromagnetism Institute Of Electrical And...

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