With increasing concern for both the
supply and cost of energy comes a corresponding concern for
efficiency in its use. Although electric energy can be converted to
mechanical energy with great efficiency, achieving maximum efficiency
requires both careful design of the electric machinery and proper
matching of machine and intended application.
Clearly, one means to maximize the
efficiency of an electric machine is to minimize its internal losses.
For example, the winding I2R losses can be reduced by increasing the
slot area so that more copper can be used, thus increasing the
cross-sectional area of the windings and reducing the resistance.
Core loss can be reduced by decreasing
the magnetic flux density in the iron of the machine. This can be
done by increasing the volume of iron, but although the loss goes
down in terms of watts per pound, the total volume of material (and
hence the mass) is increased; depending on how the machine design is
changed, there may be a point beyond which the losses actually begin
to increase.
Similarly, for a given flux density,
eddy-current losses can be reduced by using thinner iron laminations.
One can see that there are trade-offs
involved here; machines of more efficient design generally require
more material and thus are bigger and more costly. Users will
generally choose the "lowest-cost" solution to a particular
requirement; if the increased capital cost of a high-efficiency motor
can be expected to be offset by energy savings over the expected
lifetime of the machine, they will probably select the
high-efficiency machine.
If not, users are very unlikely to
select this option in spite of the increased efficiency. Similarly,
some types of electric machines are inherently more efficient than
others. For example, single-phase capacitor-start induction motors
are relatively inexpensive and highly reliable, finding use in all
sorts of small appliances, e.g., refrigerators, air conditioners, and
fans.
Yet they are inherently less efficient
than their three-phase counterparts. Modifications such as a
capacitor-run feature can lead to greater efficiency in the
single-phase induction motor, but they are expensive and often not
economically justifiable.
To optimize the efficiency of use of
electric machinery the machine must be properly matched to the
application, both in terms of size and performance. Since typical
induction motors tend to draw nearly constant reactive power,
independent of load, and since this causes resistive losses in the
supply lines, it is wise to pick the smallest-rating induction motor
which can properly satisfy the requirements of a specific
application.
Alternatively, capacitative
power-factor correction may be used. Proper application of modern
solid state control technology can also play an important role in
optimizing both performance and efficiency.
There are, of course, practical
limitations which affect the selection of the motor for any
particular application. Chief among them is that motors are generally
available only in certain standard sizes. For example, a typical
manufacturer might make fractional-horsepower ac motors rated at 1/8
, 1/6 , 1/4 , 1/3 , 1/2 , 3/4 , and 1 hp(NEMAs tandard ratings).
This discrete selection thus limits the
ability to fine tune a particular application; if the need is 0.8 hp,
the user will undoubtedly end up buying a 1-hp device and settling
for a somewhat lower than optimum efficiency. A custom-designed and
manufactured 0.8-hp motor can be economically justified only if it is
needed in large quantities.
It should be pointed out that an
extremely common source of inefficiency in electric motor
applications is the mismatch of the motor to its application. Even
the most efficient 50-kW motors will be somewhat inefficient when
driving a 20-kW load.
Yet mismatches of this type often occur
in practice, due in great extent to the difficulty in characterizing
operating loads and a tendency on the part of application engineers
to be conservative to make sure that the system in question is
guaranteed to operate in the face of design uncertainties. More
careful attention to this issue can go a long way toward increasing
the efficiency of energy use in electric machine applications.
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