Download Topic 6 – Generators and Motors

Topic 6 – Generators and Motors
Outcome #1: Investigate technologies that store,
transfer and control electricity.
 Can I construct, use and evaluate devices for transforming mechanical energy
into electrical energy and for transforming electrical energy into mechanical
energy?
In electricity generation, a generator is a device that converts mechanical
energy to electrical energy for use in an external circuit. The source of mechanical
energy may vary widely from a hand crank to an internal combustion engine.
Generators provide nearly all of the power for electric power grids. The reverse
conversion of electrical energy into mechanical energy is done by an electric motor,
and motors and generators have many similarities. Many motors can be
mechanically driven to generate electricity and frequently make acceptable
generators.
Generators
Generators produce electricity because of the relationship between electricity and
magnetism. Magnets pull on electrons which cause the electrons to move in one
direction. This movement is current electricity.
e.g. windmills, hydroelectric dams
On 21 April 1820, during a lecture, Hans Christian Ørsted, a Danish physicist
noticed a compass needle deflected from magnetic north when an electric current
from a battery was switched on and off, confirming a direct relationship between
electricity and magnetism. His initial interpretation was that magnetic effects
radiate from all sides of a wire carrying an electric current, as do light and heat.
Three months later he began more intensive investigations and soon thereafter
published his findings, showing that an electric current produces a circular
magnetic field as it flows through a wire. This discovery was not due to mere
chance, since Ørsted had been looking for a relation between electricity and
magnetism for several years. The special symmetry of the phenomenon was
possibly one of the difficulties that retarded the discovery.
It is sometimes claimed that Italian Gian Domenico Romagnosi was the first person
who found a relationship between electricity and magnetism, about two decades
before Ørsted's 1820 discovery of electromagnetism. Romagnosi's experiments
showed that an electric current from a voltaic pile could deflect a magnetic needle.
His researches were published in two Italian newspapers and were largely
overlooked by the scientific community.
Ørsted's findings stirred much research into electrodynamics throughout the
scientific community, influencing French physicist André-Marie Ampère's
developments of a single mathematical formula to represent the magnetic forces
between current-carrying conductors. Ørsted's work also represented a major step
toward a unified concept of energy.
How to Increase the Current Output from a Generator
a) increase coils of wire—the more coils, the more electrons able to be
moved
b) increase magnet strength--the stronger the magnet, the stronger the pull
on the electrons
c) increase movement --the magnet, or the wires, need to move in order to
get a current to happen (no movement, no current)
Electromagnets
When electricity flows, it aligns the electrons in one direction much like a magnet
therefore, current electricity can be use to create a strong MAGNETIC FIELD!
Michael Faraday is credited with discovering the basic principles of
electromagnetism. He introduced terms such as "ion," "electrode," "cathode" and
"anode." The farad, the unit for stored electric charge is named in his honour. In
1831, Michael Faraday and Joseph Henry independently discovered that a potential
difference is induced in a wire when there is relative motion between the wire and a
nearby magnet.
To create an electromagnet:
1. Use a soft iron core (such as a nail) in the middle
-iron works better than other metals, woods, or plastics
2. Wrap coils of wire around the iron core
-the more wraps of wire, the stronger the magnetic forces created
3. The greater the current, the greater the magnetic field
-the more batteries in series, the more power
*When the current is turned off, the iron core loses its magnetic properties.
(Temporary Magnet)
*If the direction of the current is reversed, the polarity of the magnet is reversed.
Alternating Current Generators are the most common type-they have a coil
of wire that rotates inside stationary magnets. As wires in the coil rotate, electrons
begin to move along the wire in one direction but after half a revolution, each side
of the coil passes near the opposite pole of the magnet and the direction of the
electrons start moving in the other direction so the direction of the current from the
generator changes twice with each revolution. It is called alternating current
because it changes direction or alternates. Generators that produce alternating
current are also called alternators.
Positive and negative
values on the graph
represent the opposite
directions of the
current flow.
Direct Current Generators produce a current in only one direction. This kind
of generator is called a dynamo. Batteries produce direct current but it is a smooth
continuous current rather than a pulsation of current that coincides with a spinning
DC generator.
Electric Motors
Electric motors use electric energy to make a coil of wire spin between the poles of
a magnet.
Parts of the electric motor:
1.
2.
3.
4.
5.
permanent magnets
armature (rotating coil --electromagnet)
split ring commutator (2 metal half rings separated by gaps)
brushes (stationary metal parts connecting the commutator to the power source)
power source connected to the terminals of the motor
DC motors:
Permanent magnets have a north and a south pole. Electromagnets have a north
and a south pole also but these poles can be reversed if the direction of the current
flow through the wire coil is reversed. It is this property of electromagnets that
allows the electric motor to operate.
When the current is flowing, the armature develops a N and a S pole. To determine
which end is the north pole, use the left hand rule (grasp the coil with your left
hand so that your thumb is pointing the direction of electron flow (from the
negative terminal) the direction your thumb is facing is the north pole of the
magnet). Repulsion causes the armature to move away from the permanent
magnet's north pole and rotation is started.
As the armature becomes perpendicular to the permanent magnets, the insulating
gaps in the commutator pass by the brushes stopping the flow of electrons so now
the armature is not magnetized but it continues to turn due to its momentum.
The brushes are once again in contact with the metal part of the commutator so
there is a complete circuit. The left hand rule dictates that now end B is developing
a north pole and end A is the south pole. Notice that the polarity is reversed. As in
the first step, the repulsion of the magnets causes the armature to continue to swing.
The split ring commutator makes DC power.
AC Motors:
Larger appliances such as dishwashers, washing machines and furnaces use
alternating current. AC motors have a rotating core called a rotor which is made up
of a ring of non-magnetic conducting wires held in a laminated steel cylinder. A
stationary stator surrounds the rotor-it is an electromagnet. The attraction and
repulsion between the magnetic poles of the stator and the rotor causes the rotor to
spin.
http://www.edisontechcenter.org/electricmotors.html
http://www.eti.kit.edu/english/1376.php