Answer Posted / ganeesh jain
We know that reactive loads such as inductors and capacitors
dissipate zero power, yet the fact that they drop voltage
and draw current gives the deceptive impression that they
actually do dissipate power. This “phantom power” is called
reactive power, and it is measured in a unit called
Volt-Amps-Reactive (VAR), rather than watts. The
mathematical symbol for reactive power is (unfortunately)
the capital letter Q. The actual amount of power being used,
or dissipated, in a circuit is called true power, and it is
measured in watts (symbolized by the capital letter P, as
always). The combination of reactive power and true power is
called apparent power, and it is the product of a circuit's
voltage and current, without reference to phase angle.
Apparent power is measured in the unit of Volt-Amps (VA) and
is symbolized by the capital letter S.
As a rule, true power is a function of a circuit's
dissipative elements, usually resistances (R). Reactive
power is a function of a circuit's reactance (X). Apparent
power is a function of a circuit's total impedance (Z).
Since we're dealing with scalar quantities for power
calculation, any complex starting quantities such as
voltage, current, and impedance must be represented by their
polar magnitudes, not by real or imaginary rectangular
components. For instance, if I'm calculating true power from
current and resistance, I must use the polar magnitude for
current, and not merely the “real” or “imaginary” portion of
the current. If I'm calculating apparent power from voltage
and impedance, both of these formerly complex quantities
must be reduced to their polar magnitudes for the scalar
arithmetic.
REVIEW:
Power dissipated by a load is referred to as true power.
True power is symbolized by the letter P and is measured in
the unit of Watts (W).
Power merely absorbed and returned in load due to its
reactive properties is referred to as reactive power.
Reactive power is symbolized by the letter Q and is measured
in the unit of Volt-Amps-Reactive (VAR).
Total power in an AC circuit, both dissipated and
absorbed/returned is referred to as apparent power. Apparent
power is symbolized by the letter S and is measured in the
unit of Volt-Amps (VA).
These three types of power are trigonometrically related to
one another. In a right triangle, P = adjacent length, Q =
opposite length, and S = hypotenuse length. The opposite
angle is equal to the circuit's impedance (Z) phase angle.
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