Introduction to Motors and Generators
Reference
[ 1] Chapter 6 Introduction to Motors and Generators of Electrical Power and Controls, by Timothy L.
Skvarenina and William E. DeWitt, 2004, Prentice Hall
[ 2] Industrial Electricity, 8th Edition, Michael Brumbach, published by Delmar/Cengage Learning, ISBN13: 978-1-4354-8374-3
[ 3] Youtube Demos
Topics
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Electromechanical Energy Conversion
o Types of Losses
 Copper losses
 Mechanical losses
 Magnetic Core losses
2
 Eddy current loss 𝑃𝑒 = 𝑘𝑒 𝑓 2 𝐵𝑚
 Stray losses
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Magnetism
Magnetic Field, http://en.wikipedia.org/wiki/Magnetic_field
Fields around Current Carrying Conductor and Coil
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Some Electromagnetic Principles
o EMF (electromotive force) => Produce a current that is proportional to the conductivity,
σ, of the path. (Ohm’s Law I = E/R)
o Self-Induced EMF
 Inductance (L - henry) of a coil
 Back emf of a relay coil is e = -L i/t
 Self Inductance and a Solenoid
o Electric Current => MMF (magnetomotive force) that produces magnetic flux
proportional to the permeability, µ, of the path. (Ohms’ Law for the magnetic circuits)
 Magnetic field strength H
 Magnetic flux density B
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o
o
o
A magnetic field will exert a force on a current-carrying conductor. (the basis for motor
action)
 F=BxIxL
A conductor moving through a magnetic field have an EMF induced in it (E = B L v),
Similarly, if a magnetic flux passing through a coil changes with time, an EMF, will be
induced in the coil. (Faraday’s law – the basis for generator action)
 e = N /t (volts)
 Faraday’s Law, http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html
The polarity of the EMF induced in a coil will cause a current that will oppose any
change in the magnetic flux. This is a corollary of Faraday’s law, known as Lenz’s law (e =
-N dΦ/dt).
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Electromagnetic Induction
Electric Motors and Generators,
http://www.animations.physics.unsw.edu.au/jw/electricmotors.html#mandg
Motors
o
o
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In 1834, the German scientist H.F.E. Lenz stated that the induced emf
(electromotive force) in a conductor moving perpendicular to a magnetic field
tends to oppose that motion.
The induced emf causes a current to flow in a closed circuit in such a direction
that its magnetic effect will oppose the change that produces it.
An electric current interacts with a magnetic field to cause an electromagnetic torque
(Tem), which drive some load (e.g., a pump)
The load provides an mechanical load (Tmech) which opposes the electromagnetic torque
Generators
o
o
A mechanical torque is applied to turn conductors through a magnetic field and
generate electric current.
The mechanical torque is opposed by an electromagnetic torque that results from the
interaction of the current with the magnetic field
Magnets and Magnetic Materials
 Magnets - ability to retain magnetism and ability to attract iron and steel
 Permanent and Temporary (soft-iron) magnets
 Alloy materials: steel, nickel, cobalt, etc
 Permeability - the degree of easiness for a material to be magnetized
o Low permeability (soft-iron) easier to magnetize
o High permeability (steel) harder to magnetize
Magnetic Fields
 N-S Poles
 Flux lines (from north to south)
 Magnetic flux
o SI unit, Weber (wb)
 Flux Density
o SI unit, Tesla (T) = 1 wb/m2
 Magnetic circuit: reluctance
 Magnetic Attraction and Repulsion
 Nature of Magnetism
 Field Around a Current Carrying Conductor
 Field Around a Coil (solenoid)
 Magnetomotive Force (MMF)
Magnetism, http://www-istp.gsfc.nasa.gov/Education/Imagnet.html
 Forces between electric currents, observed in 1821 by a Danish scientist, Hans Christian Orested
Induced EMF
 Principle of DC and AC generators
 Induced Current
 Equation of Induced EMF
o e = - N /t (volts)
o Faraday’s Law of Induction:
o Lenz’ Law of Induction (direction of induced voltage)
 Self-Induced EMF: Inductance (L - henry)
o e = -L i/t
 Mutual Inductance
 Mutual Induction Devices:
o Ignition coil
o Transformer (High side/low side)
Youtube Demos
 DC Motors and Generators, Dept. of Defense 1961 – PIN 29942 – Design, Application, and
Operation of Basic, Series, Shunt, and Compound Wound Type DC Generators and Motors, 34:
11 minutes, http://www.youtube.com/watch?v=OpL0joqJmqY
 AC Motors and Generators, Dept. of Defense 1961 – PIN 29943, 23:51 minutes,
http://www.youtube.com/watch?v=07uXnc1C5CA
 Construction of DC Machine, 8:06 minutes, http://www.youtube.com/watch?v=oI-O9FCDqmg
 DC Motor, Working Principle of DC Motor, 5:48 minutes,
http://www.youtube.com/watch?v=fWyzPdyCAzU
 Construction and Working of DC Motors, 5:49 minutes, http://www.youtube.com/watch?v=ICPWxtcirI
 Construction of DC Generator, 3:53 minutes,
http://www.youtube.com/watch?v=gW45N2WpD64
 Brushless Motor Construction, 9:59 minutes, http://www.youtube.com/watch?v=mbU-uOdKgOc
 Brushless DC Motors & Control – How it Works (Part 1 of 2), 10:32 minutes,
http://www.youtube.com/watch?v=ZAY5JInyHXY
 Brushless DC Motors & Control – How it Works (Part 2 of 2), 7:23 minutes,
http://www.youtube.com/watch?v=WyQInXjpGwU
 Hybrid Electric Motor Magnetic Field Strength Demonstration, 4: 25 minutes,
http://www.youtube.com/watch?v=u1n_W7wGn1U
Web References
 Electricity and Magnetism, http://www.mip.berkeley.edu/physics/bookddx.html
 Direct Current Electric Motor, Java applet: http://www.walterfendt.de/ph14e/electricmotor.htm
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Electric Motors and Generators,
http://www.animations.physics.unsw.edu.au/jw/electricmotors.html
AC Motor: Electric Machines Animation,
http://educypedia.karadimov.info/electronics/javamotor.htm