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Resources: Magnetic Induction
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PowerPoint Lecture Notes: Magnetic Induction: Motors, Generators and Transformers

UVA Virtual Lab presentation on "DC and AC Motors" (link):


Induction Motors (a.k.a. "brushless" motors):

www.LearnEngineering.org (link)


Real Transformers vs. Two Simple Side by Side Coils

Using a "ferromagnetic pole piece" to increase the coupling of magnetic field between the coils:

Ferromagnetic bar draws out the magnetic field so that rather than just brushing the second coil

it now extends all of the way through it:


A design with even better magnetic coupling:

Here the ferromagnetic shape causes the magnetic field lines to swing around,

so that almost all of them pass clear through the second coil


Finally, a compact, strongly coupled design (with no stray magnetic fields) = Common TRANSFORMER:

Coil #2 is inside coil # 1 (getting the full benefit of it's core magnetic field)

Ferromagnetic shape then traps the rest of oscillating field,

which then can't move charges elsewhere (wasting electrical energy)


Minimizing heating of "pole pieces" to increase energy efficiency

Looking more closely at what happens within a pole piece:

Permanent magnet without pole pieces =>

Permanent magnet with pole pieces =>

AC Powered electromagnet with pole pieces:

Pole pieces STRETCH OUT the magnetic field


But inside the pole piece, the magnetic field sets up loops of current

And current means electrons bumping into atoms, heating them up (wasting power)

Growing / Shrinking magnetic field MOVES through the pole pieces

INDUCING loops of current (here called "eddy currents" - in blue)


To reduce eddy currents (and power lost to heating):

Make pole pieces out of stacks of metal plates (insulated from one another)

Breaking up paths over which large eddy currents want to flow

(It's easy to insulate: Just paint both sides with insulating lacquer!)