Transcript About Electric Motors

ALL ABOUT ELECTRIC MOTORS
The 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”
COPYRIGHT NOTICE
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_motor
Magnetic 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 Purposes
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INTRODUCTION
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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
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•
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 devices
Everything that moves uses an electric motor to accomplish
its movement.
Modern life would be impossible
without electric motors!
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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
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MAGNETS AND MAGNETISM
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“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
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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)
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A sketch of Earth's magnetic
field representing the source of the
field as a magnet
The geographic north pole of Earth is
near 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
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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
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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
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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
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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.
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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
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ELECTROMAGNETISM
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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
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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
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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
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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
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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 crosssection 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)
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Magnetic field of a wire
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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
Magnetic field of a wire
One loop's magnetic field
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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
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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
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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
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A simple electromagnet
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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
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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*I
S
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 tesla
Current
• 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
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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
loops
Coil 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:
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B = 4*∏*n*I
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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
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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
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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
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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
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Armature
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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
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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
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Commutator and brushes
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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)
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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)
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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
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In this figure, the armature
winding has been left out so
that it is easier to see the
commutator in action
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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
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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
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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
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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
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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
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Let’s Build An Electric Motor!
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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
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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
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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
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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
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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
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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)
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This Chart Summarizes The Different Types Of Motors
Type
Advantages
Disadvantages
Typical Application
Typical Drive
AC Induction
(Shaded Pole)
Least expensive
Long life
high power
Rotation slips from
frequency
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 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
Uni/Poly-phase AC
AC Synchronous
Rotation in-sync with freq
long-life (alternator)
More expensive
Industrial motors
Clocks
Audio turntables
tape drives
Stepper DC
Precision positioning
High holding torque
High initial cost
Requires a controller
Positioning in printers and
floppy drives
DC
Brushless DC
Long lifespan
low maintenance
High efficiency
High initial cost
Requires a controller
Hard drives
CD/DVD players
electric vehicles
DC
Brushed DC
Low initial cost
Simple speed control
Maintenance (brushes)
Medium lifespan
Treadmill exercisers
automotive motors (seats,
blowers, windows)
Direct DC or PWM
Pancake DC
Compact design
Simple speed control
Medium cost
Medium lifespan
Office Equip
Fans/Pumps
Direct DC or PWM
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