Q3Basic Electrical Engineering
Question
Q.3. Why transformers are rated in kVA?
Answer
Transformers are rated in kVA instead of kW because their primary losses depend strictly on voltage and current, independent of the load's power factor.
The rating of any electrical machine is governed by the maximum temperature rise it can safely withstand, which is directly caused by its internal losses. In a transformer, there are two primary types of losses: Core (Iron) losses and Copper losses. Core losses depend strictly on the applied Voltage, while Copper losses depend entirely on the Current flowing through the windings. Crucially, neither of these losses depends on the phase angle (or Power Factor) of the load connected to the transformer. Since the manufacturer does not know the power factor of the future load, they specify the capacity based solely on the maximum safe voltage and current limits, giving the rating in Volt-Amperes (VA) or kilo-Volt-Amperes (kVA).