Environmental Engineering Reference
In-Depth Information
Eciency vs. Distance
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0
0
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Distance (cm)
FIGURE 6.21
Experimental efficiency of a WPT over a range of separation distances.
less electrical power was transferred wirelessly over to the load. Conversely,
when the two coils became closer to each other, the mutual coupling between
the coils became stronger; at the same time, the counter-emf effect rose. The
counter-emf is the voltage, or electromotive force, that pushes against the cur-
rent that induces it. It is caused by a changing electromagnetic field, which
is represented by Lenz's law of electromagnetism. The voltage's polarity is
at every moment the reverse of the input voltage. When a rapidly changing
magnetic field induces an emf in a coil, a current caused by this emf flows.
This current flow would in turn generate a magnetic field in the coil that op-
poses the original magnetic field that created it, and this would ultimately
reduce the induced emf in the coil. As such, it can be seen in
Figure 6.21
that
as the distance between the two coils decreased, the efficiency of the WPT
system also decreased due to the counter-emf effect.
6.2.3.3 Experimental Efficiency versus Load
Once the resonant frequency and separation distance of the WPT system were
fixed at 2.05 MHz and 20 cm, respectively, a load resistance ranging from 10
to 10 k
was connected to the load coil to determine the characteristic of the
WPT system.
Figure 6.22
shows the efficiency plot of the WPT system under
different loading conditions.
Referring to
Figure 6.22
, it is observed that a maximum WPT efficiency of
75% was attainable at a matching load resistance of 220
. However, for other
loading conditions, shifting away from the internal resistance of the load coil
of the WPT system, either very light or heavy electrical loads, the efficiency
of the system dropped significantly.
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