Why is a Transformer Rated in kVA, but Not in kW?

Why are Transformer Always Rated in kVA instead of kW?

As the name suggests, a transformer only transfers power from one circuit to another without changing the value of power and frequency. In other words, it can only step-up or step-down the value of current and voltage, while the power and frequency remain the same. General data on the transformer nameplate is printed for further details, such as rating in VA, single-phase/three-phase (power or distribution transformer), step-up/step-down, connection, accuracy class, etc.

Transformer Losses are Constant & Independent of Power Factor

There are two type of losses in a transformer;

• 1. Copper Losses
• 2. Iron Losses or Core Losses or Insulation Losses

Copper losses (I²R) depend on the current passing through the transformer winding, while iron losses (core losses or insulation losses) depend on the voltage. That is, total losses depend on both voltage (V) and current (I), which are expressed in volt-amperes (VA), not on the load power factor (PF). That’s why the transformer rating may be expressed in VA or kVA, not in watts (W) or kilowatts (kW).

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Load Type is Unknown When Designing a Transformer

Let’s explain in more detail to get the idea of why a transformer is rated in VA instead of kW.

When manufacturers design a transformer, they have no idea which kind of load will be connected to the transformer e.g. they are not sure about the exact applications of transformers in different scenarios. The load may be resistive (R), inductive (L), capacitive (C) or mixed load (R, L and C).

It means, there would be different power factor (P.F) at the secondary (load) side on different kinds of connected loads which additionally depends on R, L and C. This way, the rating of a transformer is expressed in Volt-Amps (VA) instead of  Watts (W) in case of Transformer.

Solved Example:

Let’s clear the rating of the transformer in VA instead of W  with a solved example.

The losses of the transformer will remain constant as long as the magnitude of current or voltage is the same, regardless of the power factor of the load current or voltage.

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Example:

Suppose a single phase step-up transformer with the following ratings:

• Transformer rating in kVA = 11kVA
• Primary Voltage  = 110V
• Primary Current = 100 A
• Secondary Voltage = 220V
• Secondary Current = 50 A.
• Equivalent resistance on Secondary = 5Ω
• Iron losses = 30W

In first scenario, If we connect a resistive load to the secondary of the transformer at unity power factor Φ = 1,

Then total losses of transformer would be copper losses + iron losses, i.e.:

I²R + Iron losses

Putting the values:

(50² x 5 ) + 30W = 12.53kW

i.e. losses on primary and secondary of transfer are still the same. (See below example for secondary losses as well)

The transformer output will be:

P = V x I x Cos ϕ

Again, putting the value from secondary (Same value if we put the values from primary)

P = 220 x 50 x 1 = 11kW.

Now rating of transformer:

kVA = VA ÷ 1000

kVA = (220 x 50) ÷ 1000 = 11kVA.

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Now, In the second scenario, connect a capacitive or inductive load to the secondary of the transformer at power factor Φ = 0.6.

Again, total losses of transformer would be copper losses + iron losses, i.e.:

I²R + Iron losses

Putting the values:

(50² x 5 ) + 30W = 12.53kW

Hence proved that losses in both primary and secondary are the same.

But the output of the transformer will be:

P = V x I x Cos Φ

Again putting the value from secondary (Same value if we put the values from primary)

P = 220 x 50 x 0.6 = 6.6kW.

Now rating of the transformer:

kVA = VA ÷ 1000

kVA = 220 x 50 ÷ (1000) = 11kVA.

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This means that an 11 kVA transformer rating indicates it can handle 11 kVA. It is up to us to transform and utilize the 11 kVA as 11 kW (which can be achieved by improving the power factor to 1 in the case of a pure resistive load), but this is not predictable and even very hard to achieve in the case of inductive and capacitive loads where the power factor can have different values.

From the above example, it is clear that the rating of the transformer is the same (11 kVA), but the power output is different (11 kW and 6.6 kW) due to different power factor values after connecting different kinds of loads. These power factor values are not predictable for transformer manufacturers, as the losses remain consistent in both scenarios.

Thus, these factors illustrate why transformers are rated in kVA rather than kW.

as the losses remain the same in both scenarios.

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