PASCO SE-8658A PERMANENT MAGNET MOTOR - Manual (Page 17)

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012-07210A Permanent Magnet Motor
EQUIPMENT NEEDED OPTIONAL EQUIPMENT
• Permanent Magnet Motor • Voltage Sensor
• multimeter or galvanometer • computer interface
• patch cords • small strips of masking tape
Purpose
The purpose of this experiment is to detail the operation of an AC generator and a DC generator
in terms of basic concepts of electromagnetism.
Theory
Motors and generators may be regarded as devices that convert energy from one form to another
(e.g., transducers). A motor converts electrical energy into mechanical energy. Many designs of
motors work as generators as well: when mechanical energy is input by spinning the shaft,
electrical energy is produced. More than one line of reasoning may be used to predict the
magnitude and direction of the electrical current that is produced. At the most fundamental
level, electrical charges moving across a magnetic field experience a force that is at right angles
to both the direction of motion and the direction of the magnetic field, according to the vector
equation:
F=qv x B
Conductors, of course, contain charges, and moving a conductor sideways across a magnetic
field exerts a force on the charges that make the charges flow the length of the conductor if it is
part of a circuit. The force on the charges can be seen from the equation to be proportional to
both the speed and the strength of the magnetic field.
From this reasoning you can derive Faraday’s law of electromagnetic induction, which states
that a change in the magnetic flux linking a closed circuit will result in an electromotive force (or
electric current) in the circuit that is instantaneously proportional to the time rate of change of
the linking flux; however, it is easier to understand Faraday’s law by observing the action of a
generator. In a generator, an electromotive force (emf) that is proportional to the rate of change
is induced in a loop of wire that is in a field of changing magnetic flux. (The coils of the
armature may be thought of as many loops connected in series.)
Surprisingly, the direction of induced current can be determined from the law of conservation of
energy: due simply to friction, work must be done to rotate a generator. If the generator is
connected to a load and producing electric current, additional work must be done to turn the
shaft. This reasoning led to Lenz’s law: the induced current is in such a direction as to produce a
magnetic field that opposes the original magnetic field.
You can demonstrate Lenz’s law yourself by determining the direction of the magnetic field of
the permanent magnets and by detecting the direction of the induced electric current with a
galvanometer (or multimeter) as you move the armature through the magnetic field.
Experiment 2: Operation of AC and DC Generators