Why is the transformer rated in kVA, not in KW?
Hint: The iron losses in a transformer depend on the voltage, and copper losses depend on the current. Consequently, a transformer’s heat loss is based on volt-amperes (VA) rather than the power factor, which is why the rating is in kVA rather than kW.
Complete Solution:
Transformers are designed to transfer energy between circuits without altering the power or frequency. They can step up or step down current and voltage while keeping energy and frequency constant. Transformers display details on a nameplate, including VA rating, configuration (single-phase or three-phase), step-up/step-down capability, and connection type.
Manufacturers cannot know what type of load a transformer will handle, as it could be resistive (R), inductive (L), capacitive (C), or a combination of these (R, L, and C), each with a different power factor. Due to this variability, the transformer is rated in VA rather than in watts.
Transformers incur core and copper losses:
- Core Losses: These depend on the input voltage.
- Copper Losses: These depend on the current flowing through the winding’s.
Thus, the total losses are determined by both voltage and current but not by the power factor, leading to the rating of transformers in kVA, not kW.
Note: Temperature in the transformer rises due to these losses. Copper losses, which vary based on load, are proportional to the square of the current, while core losses (largely constant) depend on voltage. The overall loss is affected only by the magnitude of current, regardless of the load’s power factor.