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Electric Generators and Motors 1
Electric Generators and
Motors
Electric Generators and Motors 2
Introductory Question

If you connect an ordinary electric motor in a
circuit with a lightbulb, is there a chance that
spinning the motor will cause the lightbulb to
light up?
A.
Yes
No
B.
Electric Generators and Motors 3
Observations about Electric
Generators and Motors
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A generator requires a mechanical power
A generator provides electric power
A motor requires electric power
A motor provides mechanical power
Generators aren’t common in daily life
But motors are widely used
Electric Generators and Motors 4
4 Questions about Electric
Generators and Motors
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How can a moving magnet produce electricity?
How can electricity produce a moving magnet?
How can a motor operate on DC power?
How can some motors use aluminum rotors?
Electric Generators and Motors 5
Question 1

How can a moving magnet produce electricity?
Electric Generators and Motors 6
Generating Electricity

A changing magnetic field
produces an electric field
 and can induces current in a conductor.
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Moving magnets have changing magnetic fields
and can propel currents in coils,
 so that mechanical power becomes electrical power.
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Electric Generators and Motors 7
An AC Generator
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An AC Generator resembles a transformer
except that it has no primary coil
 and instead uses a spinning magnet
 to induce current
in the secondary coil.
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Magnet spin rate sets
AC frequency (60 Hz).
Magnet strength and
coil turns set voltage.
Electric Generators and Motors 8
Question 2

How can electricity produce a moving magnet?
Electric Generators and Motors 9
An AC Motor

An AC Generator resembles a transformer
except that it has no secondary coil
 and instead uses a spinning magnet
 to extract power from
the primary coil.
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AC frequency (60 Hz)
sets magnet spin rate.
Electric Generators and Motors 10
Introductory Question (revisited)

If you connect an ordinary electric motor in a
circuit with a lightbulb, is there a chance that
spinning the motor will cause the lightbulb to
light up?
A.
Yes
No
B.
Electric Generators and Motors 11
Question 3

How can a motor operate on DC power?
Can a motor consist only of permanent magnets?
 Are permanent magnets an endless source of
energy?

Electric Generators and Motors 12
A Permanent Magnet Motor?
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A motor comprised only of permanent magnets
will move as like poles repel and opposites attract
 and then settle down at minimum potential energy.
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Unless something alters its poles, that motor will
never move again.
Electromagnets operating on DC power behave
like permanent magnets, so DC alone can’t keep
a motor turning for long.
Electric Generators and Motors 13
A DC Motor
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Switches can reverse a motor’s poles
so that a DC motor with switches can work!
The simplest switch system is a commutator,
which reverses the magnetic poles whenever the
motor approaches minimum potential energy.
Its commutator allows
a DC motor to turn
endlessly.
Electric Generators and Motors 14
Question 4

How can some motors use aluminum rotors?
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Aluminum is nonmagnetic, so how can it work?
Electric Generators and Motors 15
The Lorentz Force

A charge moving through a magnetic field
experiences the Lorentz force,
 a force that pushes it perpendicular to its velocity
 and perpendicular to the magnetic field:
Lorentz force = charge· velocity· magnetic field·
sine(angle)
where the “angle” is that between
the velocity and the magnetic field.
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Electric Generators and Motors 16
Induction and Lorentz Force
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When a magnetic field moves past a conductor,
it induces current in that conductor
 and that current experiences the Lorentz force.
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The current and conductor experience a force
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that pulls them with the moving magnetic field.
A moving magnet tends to drag metals with it.
This effect can be used to make a motor.
Electric Generators and Motors 17
AC Induction Motor
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Stationary AC electromagnets can produce a
magnetic field that circles a conducting rotor.
Currents induced in that rotor
experience the Lorentz force
 and are pushed along with the rotating field.
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The rotor spins almost as fast as the field circles.
This system is known as an AC induction motor.
Electric Generators and Motors 18
Summary about Electric Generators
and Motors
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Generators using moving magnets to turn
mechanical power into electrical power
Motors use moving magnets to turn electrical
power into mechanical power
Induction motors depend on the Lorentz force
to allow electromagnets to spin a nonmagnetic
metal rotor.