Electromechanical Energy Conversion
The chief advantage of electrical energy over other forms of
energies is the relative ease and high efficiency with which it can be
transmitted over a long distance. Electric energy is seldom available naturally
and is rarely used directly. Its main use is in the form of a transmitting link
for transporting other forms of energy e.g. light, mechanical, heat, sound etc.
From one physical location to another obviously two kinds of energy conversion
devices are needed to convert form one form of energy to the electric form and
to convert it back to original or any other desired form. Our aim in this
article is to understand the devices used for electromechanical energy
conversion.
These devices can be
-
Transducers
for processing and transporting low-energy signals.
-
Production
of force or torque with limited mechanical motion like electromagnets,
actuators, relays etc.
-
Continuous
energy conversion devices like motors, generators for bulk energy conversion
and utilisation.
We concentrate on the principles of electromechanical energy
conversion and the analysis of the devices which accomplish this function.
Emphasis is placed on the analysis of systems which use magnetic fields as the
conversion medium. However, the analytical techniques for electric field
systems are quite similar. The purpose of this analysis is three fold:
1. To assist us in understanding low
energy conversion takes place.
2. To provide us with the techniques for
designing and optimizing the devices for specific requirements, and
3. To show how to develop models of
electromechanical energy conversion devices that can be used in analysing their
performance as components n engineering systems.
The principle of conservation of energy states that “energy
is neither created nor destroyed; it is merely changed in form”. One can
account for energy transfer as
(Energy input from electrical source) = (mechanical energy
output) + (increase in energy stored in magnetic field) + (energy converted
into heat i.e. losses)
The electrical and mechanical energy terms have positive
values for motor action whereas for generator these terms than simply have
negative values in the above energy balance equation.
In this systems the conversion of energy into heat occurs by
such mechanisms as ohmic heating due to current flow in the windings of the
electrical terminals and the mechanical friction due to the motion of the
system components forming the mechanical terminals. It is mathematically
possible to separate these loss mechanisms from the energy storage mechanism.
Here the device can be represented as a loss less magnetic energy storage
system with electrical and mechanical terminals as shown in the fig. the loss
mechanism can then be represented by external elements connected to these
terminals. Resistances can be represented to the electrical terminals and
mechanical dashpots to the mechanical terminals. It is quite general in the
sense that there is no limit to the number of electrical or mechanical
terminals. For this type of system, the magnetic field serves as the coupling
medium between the electrical and mechanical terminals
The ability to identify a lossless energy storage system is
the essence of the energy method. It is important to recognize that this is
done mathematically as part of the modelling process. It is not possible to
take the resistance out of windings or the friction out of bearings. Instead we
are making use of the fact that a model in which this is a valid representation
of the physical system. In summary magnetic and electric fields are seats of
energy storage. Whenever the energy in the field is influenced by the
configuration of the mechanical parts constituting the boundaries of the field,
mechanical forces are created which tend to move the mechanical elements so
that energy is transmitted from the field to the mechanical system.
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