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SECTION 2 | General Information
the coil. It is proportional to frequency and inductance and causes the current vector to lag the voltage
vector by Phase Angle (
φ
) = 90°. Capacitive reactance (X
C
) is the property of capacitive elements to oppose
changes in voltage. X
C
is inversely proportional to the frequency and capacitance and causes the current
vector to lead the voltage vector by Phase Angle (
φ
) = 90°. The unit of both X
L
and X
C
is "Ohm" - also
denoted as "Ω". The effects of inductive reactance X
L
to cause the current to lag the voltage by 90° and
that of the capacitive reactance X
C
to cause the current to lead the voltage by 90° are exactly opposite and
the net effect is a tendency to cancel each other. Hence, in a circuit containing both inductances and
capacitances, the net Reactance (X) will be equal to the difference between the values of the inductive and
capacitive reactances. The net Reactance (X) will be inductive if X
L
> X
C
and capacitive if X
C
> X
L
.
Impedance, Z: It is the vectorial sum of Resistance and Reactance vectors in a circuit.
Active Power (P), Watts: It is denoted as “P” and the unit is “Watt”. It is the power that is consumed in the
resistive elements of the load. A load will require additional Reactive Power for powering the inductive
and capacitive elements. The effective power required would be the Apparent Power that is a vectorial
sum of the Active and Reactive Powers.
Reactive Power (Q), VAR: Is denoted as “Q” and the unit is VAR. Over a cycle, this power is alternatively
stored and returned by the inductive and capacitive elements of the load. It is not consumed by the inductive
and capacitive elements in the load but a certain value travels from the AC source to these elements in the
(+) half cycle of the sinusoidal voltage (Positive value) and the same value is returned back to the AC source in
the (-) half cycle of the sinusoidal voltage (Negative value). Hence, when averaged over a span of one cycle,
the net value of this power is 0. However, on an instantaneous basis, this power has to be provided by the
AC source. Hence, the inverter, AC wiring and over current protection devices have to be sized based on the
combined effect of the Active and Reactive Powers that is called the Apparent Power.
Apparent (S) Power, VA: This power, denoted by "S", is the vectorial sum of the Active Power in Watts and the
Reactive Power in “VAR”. In magnitude, it is equal to the RMS value of voltage “V” X the RMS value of current
“A”. The Unit is VA. Please note that Apparent Power VA is more than the Active Power in Watts. Hence, the
inverter, AC wiring and over current protection devices have to be sized based on the Apparent Power.
Maximum Continuous Running AC Power Rating: This rating may be specified as “Active Power” in
Watts (W) or “Apparent Power” in Volt Amps (VA). It is normally specified in “Active Power (P)” in Watts
for Resistive type of loads that have Power Factor =1. Reactive types of loads will draw higher value of
“Apparent Power” that is the sum of “Active and Reactive Powers”. Thus, AC power source should be
sized based on the higher “Apparent Power” Rating in (VA) for all Reactive Types of AC loads. If the AC
power source is sized based on the lower “Active Power” Rating in Watts (W), the AC power source may be
subjected to overload conditions when powering Reactive Type of loads.
Surge Power Rating: During start up, certain loads require considerably higher surge of power for short
duration (lasting from tens of millisecs to few seconds) as compared to their Maximum Continuous
Running Power Rating. Some examples of such loads are given below:
• Electric Motors: At the moment when an electric motor is powered ON, the rotor is stationary (equivalent
to being “Locked”), there is no “Back EMF” and the windings draw a very heavy surge of starting current
(Amperes) called “Locked Rotor Amperes” (LRA) due to low DC resistance of the windings. For example,