Induction and generation
COMPTON LECTURE 2: OCTOBER 10, 2009
ERIC SWITZER
Michael Faraday and the dignitaries: “This is all very
interesting, but of what possible use are these toys?"
Faraday responds: “I cannot say what use they may be,
but I can confidently predict that one day you will be
able to tax them."
Summary of physics from lecture 1
Currents in a magnetic field feel a force
Currents produce magnetic fields
Permanent magnets – spontaneously aligned moments
Converting electrical to mechanical energy
Today – induction, or converting mechanical to electrical
energy. (Also, AC motors.)
  The next lecture will describe the grid, and that will
conclude the unit.
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The induction motor
wikipedia
Why so much time on these topics?
  It is estimated that now 2/3 of the electric power in
the U.S. is consumed by electric motors. (Energy at the
crossroads: global perspectives and uncertainties – Vaclav Smil, 2005.)
  Essentially all electricity is generated through
induction.
  Motors and generators: technology for a civilized life
Faraday’s experiments - coil
Faraday’s experiments – W boards
Faraday’s experiments – chopsticks
Faraday’s conclusions
  A changing current in the primary winding
produced induction in the secondary winding
  Relative motion of the primary and secondary
winding caused induction in the secondary
  Relative motion of a magnet near the secondary
winding caused induction in the secondary
Two things first
  Energy in general and in electrical settings
  Magnetic flux
Definition of the work
Constant force over the path: work is the force times distance.
Electric forces
+
wikipedia
Force=charge * electric field
-
Electric forces and potential
Magnetic flux
Flux = strength * area * cos(angle)
Faraday’s induction
Induced voltage = minus the slope of the magnetic flux as a function of time
Can change: the field strength, the angle, or the area to influence the flux.
The rail generator
Flux = area*magnetic field (B) = vtLB
Voltage = - slope of flux = -vLB
The rotating generator
Flux = area*magnetic field (B)*cos(angle)
Schematic of industrial generation
N
S
The “synchronous machine”
AC power
Vrms= square root(average(voltage2))
A small industrial generator
http://www.pgsr.com.au/wp-content/uploads/2008/11/generator-rotor-installation.jpg
Power in remote places
The Atacama Cosmology Telescope
17,030 feet in Chile
A. Hincks
AC induction motors
The squirrel ladder
Wrapping the ladder into a cage
Is this string theory, or your window fan?
Enter: Tesla
wikipedia
Producing a rotating field
Three-phase currents and the rotating magnetic field.
Online course notes
Stator coils
http://www.allaboutcircuits.com/vol_2/chpt_13/7.html
The induction motor
wikipedia
Large induction motors ~90% efficient.
Someone’s got to pay
Not so different
Stott Park Bobbin Mill steam engine (1835)
wikipedia
Conclusion
  Voltage is the energy per charge in a circuit
  Magnetic flux depends upon: field strength,
orientation, and loop dimensions
  Voltage can be produced through a changing
magnetic flux
  Induction: the basis of electrical generation
  AC induction motors are very common, and rely on a
“squirrel cage”
  Remember: Someone’s got to pay
Websites
  http://kicp.uchicago.edu/~switzer/
  The next meeting of the energy physics group will be
Tuesday, Oct. 13th from 12-1 PM in RI 480. Tom Caswell is
hosting and will discuss:
  57th and Ellis, Research Institutes
  "Recent advances in direct solar thermal power generation"
by Yue-Guang Deng and Jing Liu
  J. Renewable Sustainable Energy 1, 052701 (2009)