Control transformers are classified by their rating in which unit?

Control transformers are classified by their rating in which unit?

• A. Watts
• B. Volt-amperes (V.A.)
• C. Horsepower
• D. Amperes

Answer: (B)

Control transformers are rated by apparent power, measured in volt-amperes (VA), because the heating and safe operating limits of the windings depend on the current at the rated voltage, not on real power alone. In AC circuits, loads like relays and solenoids are often inductive and have varying power factors. The transformer must be sized to handle the maximum current it could deliver at the rated voltage, regardless of how that current translates into real watts. That’s why the VA rating provides a consistent standard: it encapsulates the voltage and the maximum current the windings can safely carry. If we used watts, we’d need to know the power factor to determine current, which isn’t a reliable fixed rating for a transformer’s thermal limit. So the standard designation is volt-amperes. For example, a 24 V secondary with a 40 VA rating can supply up to about 1.67 A at 24 V. The actual real power drawn (watts) depends on the power factor of the load, which can vary, but the current capability—and thus the heating risk—follows the VA rating. The other units (watts, horsepower, amperes alone) don’t capture this combination of voltage and allowable current in a single, consistent rating.

Control transformers are rated by apparent power, measured in volt-amperes (VA), because the heating and safe operating limits of the windings depend on the current at the rated voltage, not on real power alone. In AC circuits, loads like relays and solenoids are often inductive and have varying power factors. The transformer must be sized to handle the maximum current it could deliver at the rated voltage, regardless of how that current translates into real watts. That’s why the VA rating provides a consistent standard: it encapsulates the voltage and the maximum current the windings can safely carry. If we used watts, we’d need to know the power factor to determine current, which isn’t a reliable fixed rating for a transformer’s thermal limit. So the standard designation is volt-amperes.

For example, a 24 V secondary with a 40 VA rating can supply up to about 1.67 A at 24 V. The actual real power drawn (watts) depends on the power factor of the load, which can vary, but the current capability—and thus the heating risk—follows the VA rating. The other units (watts, horsepower, amperes alone) don’t capture this combination of voltage and allowable current in a single, consistent rating.