10 | SAMLEX AMERICA INC.
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 Power (S), 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 specied as “Active Power” in Watts (W)
or “Apparent Power” in Volt Amps (VA). It is normally specied 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.
Starting Surge Power Rating: Certain loads require considerably higher Starting Surge 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 starting current (Amperes)
called “Locked Rotor Amperes” (LRA) due to low DC resistance of the windings. For example, in motor driven
loads like Air-conditioning and Refrigeration Compressors and in Well Pumps (using Pressure Tank), LRA may
be as high as 10 times its rated Full Load Amps (FLA) / Maximum Continuous Running Power Rating. The
value and duration of LRA of the motor depends upon the winding design of the motor and the inertia /
resistance to movement of mechanical load being driven by the motor. As the motor speed rises to its rated
RPM, “Back EMF” proportional to the RPM is generated in the windings and the current draw reduces
proportionately till it draws the running FLA / Maximum Continuous Running Power Rating at the rated RPM.
• Transformers (e.g. Isolation Transformers, Step-up / Step-down Transformers, Power Transformer in Microwave
Oven etc.):At the moment when AC power is supplied to a transformer, the transformer draws very heavy
“Magnetization Inrush Current” for a few millisecs that can reach up to 10 times the Maximum Continuous
Rating of the Transformer.
SECTION 1.2 | Denitions
