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WHAT IS A GENERATOR?
In power industry, this term refers to a device that produces
electrical energy.
Although electricity does occur naturally, it does not exist in the
forms that currently can be
practically used. For practical use electricity is produced from other
forms of
energy. Since energy cannot be created but can only be transferred from
one form to another, every generator obviously needs an energy source.
In electric generators such source is mechanical energy. The mechanical
energy in turn is converted from so-called primary sources,
such as chemical or nuclear energy in various types of fuel, or
obtained from renewable resources such as wind or falling water. The
machine that converts primary energy into mechanical energy is called
prime mover. Steam turbines, internal-combustion engines, gas
combustion turbines, water and wind turbines are the common types of
prime movers.
HOW IT WORKS.
The operation of electric generators is based on the phenomenon of
electromagnetic induction: whenever a conductor moves relative to a
magnetic field, voltage is induced in the conductor. Particularly, if a
magnet is spinning inside a coil, AC voltage is induced in the coil.
For more details, see How
generators work: a tutorial with an animation that
illustrates the basic operation.
The induced voltage (called electromotive force or emf) will create a
current through an external circuit connected to the coil,
resulting in energy being delivered to the load. Thus, the mechanical
energy that rotates the source of the magnetic field is converted into
electrical energy. Note that the load current in turn creates a
magnetic field that opposes the change in the flux of the coil, so the
coil opposes the motion. The higher the current, the larger the force
that must be applied to the magnet to keep it from slowing down.
In practice, the magnetic field is most often induced by an
electromagnet rather then a permanent magnet. In AC systems, usually
the electromagnet is spinning, and the power-producing armature is
stationary. The armature normally comprises of a set of coils that form
a cylinder. The electromagnet consists of so
called field coils mounted on an iron core. A current flow in the field
coils is required to produce magnetic field. This current may be
obtained from an external source or from the system's own armature.
Most modern AC sources with field coils are self-excited: the current
for field coils is supplied by an additional exciting winding in the
armature. The initial magnetic field is produced by residual magnetism
in the electromagnet's cores. When the prime mover starts turning the
armature, at first the armature rotates in a very weak magnetic field
and produces small emf. This emf creates a current in field coils,
which increases magnetic flux, which in turn increases emf in the
armature. This process continues until the rated output voltage is
reached.
If the electromagnet's core lost its residual magnetism, the rotor will
spin, but no output voltage will be produced. In this case, to start
the device you may need to do so called field flashing.
EMERGENCY BACKUP GENSETS.
In power plants the generators are most often driven by steam or
hydraulic turbines or by diesel engines. The same concept
of converting mechanical energy into electric energy is widely used in
small privately owned units as well. In commercially available
models for home or business use, an alternator is integrated with an
internal-combustion engine into a single appliance. The resulting
device is referred to as an engine-generator set or a genset. It is the most common type
of backup
power source
for home or business. A genset is often casually called just a
generator even though it also includes an engine. There are two main
types of such devices that differ by their connection and activation
methods: standby and portable. Standby
generators for home use
are permanently connected to the house wiring system via a transfer
switch and are also permanently connected to a fuel source, such as a
natural
gas line or a large propane or diesel tank. They cost more than
portables and require professional installation, but they can provide
continuous power for as long as the fuel is available (see standby
brands and ratings). Portable
devices are intended primarily for a temporary connection to several
appliances via extension cords rather than to the whole house. They are
normally fueled from an
on-board tank and therefore need frequent refueling. Some more
expensive portable models
can also be powered from an external source for extended run time (see portable
brands and ratings). A portable unit is generally cheaper
than a standby and does not need a
professional installation unless you want to connect it directly to the
house wiring (in which case you need to install a generator
transfer switch).
Choosing the best device for your application involves selecting the
right type and a proper sizing
based on the amount of power you may need during an emergency.
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