# imaginary power

Discussion in 'Electronic Basics' started by Irish LaidE, Apr 25, 2004.

1. ### Irish LaidEGuest

i've just started to read about imaginary or reactive power my text
book doesn't seem to cover to much information on this subject.

any additional information on this subject would be greatly
appreciated

2. ### BobGuest

IL,

There probably is a book, somewhere, with an explanation that will make it
clear to you. I've never read one that really explained it well, to me.

Capacitors and inductors (perfect ones, that is) do not dissipate power as
heat (or light), although they will dissipate some power due to radiation
(as a function of the driving source frequency or as a function of their
self oscillations). Ignoring the radiations effects, capacitors and
inductors are energy storage / energy release devices.

When a sinusoidal signal is applied to a capacitor or an inductor, the
resulting current through the device is 90 degrees out-of-phase with respect
voltage across the device. The voltage "lags" the current in a capacitor (it
takes time for the charge to separate across its plates (I * t), so the
resulting voltage (Q/C) builds up as time goes on (it lags). The opposite is
true with an inductor. It takes time for the current to change in an
inductor.

Here are two things to remember:

1) you can never instantaneously change the voltage across a (perfect)
capacitor, and you can never instantaneously change the current through a
(perfect) inductor.

2) a (perfect) capacitor or inductor will never get hot. In the real world,
however, this is not the case -- due to parasitic impurities.

The "so called" real component of power is that component that dissipates
power. The reactive/imaginary component is that which is being transferred
back-and-forth between inductors and/or capacitors in the circuit and its
driving source.

If you apply a sine wave to a circuit of R's and C's and L's, and look at
the resulting source's current's phase (with respect to the driving voltage,
you can always separate the current into two components: the "in phase" part
of the current and a part which is 90 degrees out of phase (either leading
or lagging). The heat generated by the circuit will only be due to the
applied V and its "in phase" component of current. This is the "real"
component of power. The balance is the "imaginary" component of power (the
portion that is transferred back-and-forth to/from the source.

This is probably a pretty good start, for you. Ask more questions after

Bob

3. ### Fred StevensGuest

Reactive or imaginary power is that component of power that is
transferred between energy storage elements (such as capacitors and
inductors) with no dissipation. That is, the dissipative component is
associated with power loss in resistance, whereas the pure reactive
power component is not dissipated, but moves between (multiple)
storage elements or is stored in fields in purely reactive devices
(such as capacitors and inductors).

Fred.

4. ### John PopelishGuest

Real power flows one way, from source to load. Resistors consume
energy in proportion to the applied voltage squared, so they reach
their peak energy consumption at the peaks of the voltage waveform,
but never give any back to the source.

Imaginary power sloshes back and forth between source and load. It
makes both trips twice every AC cycle

Capacitors store peak energy when they reach peak voltage, so a
capacitor receives energy as the voltage rises from zero to the peaks
and starts sending energy back at the voltage peaks of the cycle till
the next zero crossing.

Inductors across an AC source store peak energy when their current is
highest. This occurs after the voltage has had the same polarity the
longest, which is at the voltage zero crossings. The energy flows
into the inductor from the peaks to the zero crossings and returns ot
the source from the zero crossings ot the next peak

If you compare the phase of the current through a capacitor to the
phase of the applied voltage, you will fond that it leads the voltage
by 90 degrees. The current through an inductor lags the applied
voltage by 90 degrees.

5. ### Irish LaidEGuest

ok, so imaginary power all relates back to V leading or lagging I by 90º ?

6. ### RatchGuest

No, the voltage of a inductor is E = -L*di/dt . The derivative of a
sinusoidal current is always 90° out of phase with the current in the
inductance. Since the derivative of the current controls the voltage, that
explains the 90° phase change between voltage current. It does not relate
to 'imaginary power'.

So what is power? Power is the time rate change of energy. The inductor,
capacitor and the generator are constantly swapping energy back and forth
according to the sinusoidal input. This happens in a finite time, so power
is involved. This energy is conserved within the system except for that
which is dissipated by resistance or radiation losses. The electrical
community has given this a secondary name of 'imaginary power'. Bad choice.
The energy stored in a inductor (electromagnetic field) or a capacitor
(electrostatic field) is just as 'real' as the energy that heats a resistor.
The rate which energy stores and depletes in a reactance should only be
called reactive power. Ratch

7. ### John PopelishGuest

Imaginary anything (in mathematics) just refers to a 90 degree
relationship. This right angle just means that two things are
independent, like height and width. It is a silly word (imaginary) to
label a second dimension.

8. ### Irish LaidEGuest

thank you all for you help