Electric Motors and Generators
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V.Sezer (*)
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(*) Istanbul Technical
University
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Summary
In this section we will learn what an electric motor is, how it operates,
how to select the appropriate electric motor, what an electric generator
is and the relationship between an electric motor and a generator.
Electric Motors
In basic terms an electric motor is a machine which converts electrical
energy into mechanical motion.
Operation
of Electric Motors
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When the coil is powered, a magnetic field is
generated around the armature. The left side of the armature is
pushed away from the left magnet and drawn toward the right, causing
rotation.
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The armature continues to rotate
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When the armature becomes horizontally aligned,
the commutator reverses the direction of current through the coil,
reversing the magnetic field. The process then repeats itself.
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Most electric motors work by electromagnetism,
but motors based on other electromechanical phenomena, such as electrostatic forces
and the piezoelectric effect,
also exist. The fundamental principle upon which electromagnetic motors
are based is that there is a mechanical force on any
wire when it is conducting electricity while contained within a magnetic
field. The force is described by the Lorentz
Force Law and is perpendicular to both the wire and the
magnetic field. In a rotary motor, there is a rotating element, the rotor.
The rotor rotates because the wires and magnetic field are arranged so
that a torque is developed about the rotor's axis.
Most magnetic motors are rotary, but linear types also exist. In a rotary
motor, the rotating part (usually on the inside) is called the rotor,
and the stationary part is called the stator.
The motor contains electromagnets that are
wound on a frame. Though this frame is often called the armature,
that term is often erroneously applied. The armature is that part of the
motor across which the input voltage
is supplied or that part of the generator across which the output voltage
is generated. Depending upon the design of the machine, either the rotor
or the stator can serve as the armature.
Selecting An Appropriate Electric Motor
Electric motors, both ac motors and dc motors, come in many shapes and sizes.
Some are standardized electric motors for general-purpose applications.
Other electric motors are intended for specific tasks. In any case, electric
motors should be selected to satisfy the dynamic requirements of the machines
on which they are applied without exceeding rated electric motor temperature.
Thus, the first and most important step in electric motor selection is determining
load characteristics -- torque and speed versus time or torque versus speed.
Electric motor selection is also based on mission goals like available power
and cost.
Electric
Generators
An electric motor converts electrical
energy into mechanical motion. The reverse task, that
of converting mechanical motion into electrical energy, is accomplished
by a generator or
dynamo. Faraday discovered that if an electric conductor, like
a copper wire, is moved through a magnetic field, electric current will
flow (be induced) in the conductor. So the mechanical energy of the moving
wire is converted into the electric energy of the current that flows in
the wire.
This operation uses the Lorentz Force Law again but in the reverse
direction. Some applications use a single device that can act both as
an electric motor or an electric generator. The best example of this is
an electrical car with regenerative braking. During the acceleration phase,
electrical cars take their mechanical energy from the electric motors
but when they need braking, the electric motor changes into an electric
generator and converts the mechanical braking energy into electrical energy
stored in the batteries.
References
Robert H. Bishop, (2002), “The Mechatronics Handbook”
Links
http://en.wikipedia.org/wiki/Electric_motor
http://www.ieee-virtual-museum.org/collection/tech.php?id=2345877&lid=1
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