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Answer Posted / rajesh mohanan
Looking at different electrical equipment you will notice that the power ratings are either expressed in kVA (kilo Volt Ampres) or sometimes in kW (kilo Watts). The next question is what’s the difference? Both are measurements of power however they are truly different. To understand the differences consider how each are derived. kW is called actual or real power, or simply the amount of power that is available to do real work. kVA. On the other hand kVA is known as “apparent” power. This is because only a portion of the available kVA may be available to do real work. The remainder is simply excess current.
The difference between kVA and kW really depends on the power factor (PF). When the voltage and current are in phase with each other in an AC circuit, the power factor is 1.0 or “unity”. The more the voltage falls out of phase with the current the lower the power factor becomes and the less “real power” or kW the device uses; however it may still be using the same amount of current as a device with a higher
power factor. Power factor will be "leading" or "lagging" depending on which way the load shifts the current’s phase with respect to the voltage’s phase. Inductive loads cause current to lag behind voltage, while capacitive loads cause current to lead voltage hence leading or lagging power factor.
The next question is why express the kVA value of a device and what use is
kVA if only part of it can do work? The reason kVA is used as a rating is
because the rating of an electrical device is dependent on the amount of
current they can sustain. A kW rating does not indicate how much current
a device can carry (as the power factor may not be known) where kVA
does indicate the maximum current at a specific voltage level.
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