Understanding Transformers and kVA Ratings

Transformers Overview

  • Definition & Rating

    • Transformers are rated in kVA (kilo-Volt-Amps).

    • The rating in kVA is used instead of Watts due to the nature of electrical loads connected to transformers.

Concept of Inductive Load

  • Inductive Characteristics

    • Transformers operate on the principle of mutual induction, which results in them being classified as inductive loads.

    • An inductive load does not produce purely TRUE power; it also induces REACTIVE power.

  • Difference between Resistive and Inductive Loads

    • A resistive load allows voltage and current to be in-phase, meaning their sine waves peak at the same time, e.g., incandescent lighting or heating.

    • In contrast, in an inductive load such as a transformer, the current lags behind the voltage, which creates reactive power in the circuit.

Power Relationships in Transformers

  • Power Ratings

    • Only resistive loads are measured in Watts; for transformers, we must account for the inductive elements, hence the use of kVA instead.

    • The kVA rating reflects the maximum current a transformer can deliver without overheating.

  • Power in AC Circuits

    • For further insights, examine the sections on Power in AC Circuits and AC Theory.

Calculating kVA of Transformers

  • kVA Calculation

    • The kVA rating for a transformer is calculated using the formula:
      kVA = V x A/1000

    • Both the primary and secondary windings of a transformer share the same kVA rating.

  • Example Problem

    • Consider the scenario where a single-phase transformer supplies a trace heating system fed from a 230V / 50Hz AC supply with an output voltage of 25V and a secondary current of 150A.

    • Given Values:

    • Output Voltage (Vs) = 25V

    • Secondary Current (Is) = 150A

    • Calculation:

    • Substitute the given values into the kVA formula:

    • Calculate: kVA = V x A/1000

      • kVA = 3.75

    • Thus, this transformer has a power rating of 3.75 kVA.

Key Points to Remember

  • Transformers are rated in kVA, not kW (Watts).

  • The kVA rating indicates the current that a transformer can safely deliver.

  • Transformers are not 100% efficient and induce reactive power due to mutual induction, meaning they have a specific power factor.

  • Calculate kVA as the product of voltage and current: kVA = V x A/1000.

  • It does not matter if you use the secondary (Vs, Is) or primary (Vp, Ip) winding values; the kVA remains the same.