Avionics News September 2019 - 48

THEORY & PRACTICE
Continued from page 47

get a false answer. This answer is called reactive power,
and we don't call it watts. Instead, we call it volt-amps,
or VA. True power is calculated using current and
resistance (I2 x R). If a circuit is purely inductive, then
R will be 0 Ω, and the true power dissipation would be
0 W. The apparent power would be calculated using Z or
X, instead of R.
For example, let's consider a purely inductive
circuit, with 10 Ω of inductive reactance and zero Ω
of resistance connected up to 10 VAC. Keep in mind
with no resistance, X and Z will be equal. Using Ohm's
law, we can determine the amperage to be (E/R) 1 amp.
With R equal to zero, any way of calculating true power
using R results in zero watts. If we calculate using
the inductive reactance of 10 Ω, then we see that the
apparent power (I2 x Z) is 10 VA.
Add resistance to the circuit, and we will get three
types of power. We will get true power based on the

48

avionics news

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september

2019

heat dissipated by the resistor. We get reactive power
based on XL. Finally, we will get apparent power based
on Z. Since the resistance limits the current through the
inductor, the reactive and true power will be equal, and
the apparent power will be a higher value.
Admittedly, apparent power is a name that is slightly
misleading. Motors, which present an inductive load
to a power supply, are rated in VA. Apparent power
does represent the load placed on the system and is an
indication of the amperage the motor will draw. So it is
a real useful number.
In AC circuits, inductors have many uses. As
mentioned above, they can be used as part of a filter.
They can be used as part of a transformer. They are an
important component of motors and generators. Radio
signals will induce a voltage drop across the inductor,
so of course, they are used as part of radio receivers or
transmitters. Their usefulness increases when combined
with a capacitor. We will examine capacitors in AC
circuits in next month's Theory & Practice column. Stay
tuned! q



http://www.latitudetech.com

Avionics News September 2019

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