How are Transformers Rated in kVA not kW – With Example

How are transformers rated – Every electrical device has their own power rating, either generating or absorbing electric power. Since power is measured in watt or VA, we can use both in our calculation. But keep in mind that transformers are rated in VA or kVA.

Why is that? You need to finish reading this until the end to understand how are transformers rated. You can understand further about:

  • How transformers are rated in VA or kVA
  • Why transformers are rated in VA or kVA
  • Why aren’t transformers rated in watt or kW.

How are Transformers Rated

Just as stated above, transformers are rated in VA or kVA, usually in kVA. As you have known, kVA stands for kilo volt-ampere which is one of the power measurement units. When we are calculating, we ignore the power factor.

The power formula in kVA is written as:

    \begin{align*}\mbox{P(VA)=voltage (V) x current (I)}\end{align*}

From the formula above, the kVA measurement is independent from the power factor. The power in VA is known as apparent power represents the product of the voltage applied to a load and the current drawn by the load. We usually find this unit when having business with power devices.

On the other hand, the kW or kilowatt measurement is affected by power factor since it measures the real power drawn by a load. We usually find this unit when having business with electrical devices that produce mechanical energy like electric motors.

In common understanding, transformers are used to transfer energy from primary side (primary winding) to secondary side (secondary winding). The power loss is zero if we assume it is in an ideal situation. Calculating the power loss on its iron and copper also ignores the power factor if we use VA, and of course we use kVA instead of kW.

Transformers are rated in kVA because the losses in the transformer are independent from the power factor.

Since kVA is the unit of apparent power, it is a combination between real power and reactive power. A transformer is built without future planning whether it is connected to a load (resistive, capacitive, or both) or not. If we still insist on using kW, the transformer needs to be connected with a load since kW is the real power drawn by a load.

Why Transformer are Rated in kVA

After reading a brief explanation about how are transformers rated, we will learn about “why”. Transformers are rated based on their maximum output voltage and current they can deliver. From what we have learned about voltage and current, we now know that transformers are rated in power rating. Just as we know the power is measured in watt or VA. In our case, the transformers are rated in volt-ampere or VA. It is still the product of the maximum output voltage and maximum output current.

For an example, we have a transformer with its specifications as written as:

  • Output voltage: 12 V
  • Output current: up to 10 A.

Hence its power rating is:

12 V x 10 A = 120 VA.

But why do we use VA instead of watt? We can learn about this later along with how are transformers rated.

The filtering properties of the power supply require that the VA rating of the power transformer to significantly exceed the actual power in watts consumed by the load.

A steady and high quality transformer should be capable of producing the necessary current and voltage since it is considered as the critical part in the power supply system. We can say that the transformer is mostly the most expensive part in power supply.

If there is an accident and the transformer burns out, we need to replace it as soon as possible. Replacing it is not as easy as it sounds and of course the cost is not cheap. It is far better for engineers to calculate, design, and choose the most suitable transformer rating when building a power supply.

Choosing the right transformers is not only for cost and efficiency, it is also useful to prevent injury. Choosing the wrong voltage, current, or power of the transformer can lead into serious damage and injury. Even further, the power capabilities of the primary and secondary winding need to be considered. The voltage, current, and power rating specified for the transformer represents the middle point of the respective maximum and minimum rated values.

If you are curious about the maximum rating we can achieve when building a transformer, we need to consider the thickness and type of the insulation we use on it. Thicker insulation is the better and higher voltage maximum rating we can apply to the windings.

It will be different if we are calculating the maximum current rating we can produce. You may see the ampere values when buying electric wires and it is also used when determining maximum current rating on our transformer. The maximum current rating on a transformer depends on its wire’s diameter used for the windings. If the current is higher than the wire’s capability, it will produce heat and burn the insulation or even the wire itself.

This is why we need to take voltage and current specifications into our consideration.

Just as mentioned above, the transformer rating is measured in volt-ampere (VA) or even kilovolt-ampere (kVA). Both primary winding and secondary winding are able to hold the VA or kVA rating written on its specification nameplate.

The maximum load rating of the transformer is usually not listed on the nameplate, but we can calculate it easily. The maximum load rating can be measured from the formulas below:

Single phase:

    \begin{align*}\mbox{Full load current}&=\frac{\mbox{VA}}{\mbox{voltage}}\\\mbox{Full load current}&=\frac{\mbox{kVA}\times1000}{\mbox{voltage}}\\\end{align*}

Three phase:

    \begin{align*}\mbox{Full load current}&=\frac{\mbox{kVA}\times1000}{1.73\times\mbox{voltage}}\\\end{align*}

how are transformers rated

Figure 1. Example of Transformer Rating Nameplate

Figure.(1) above is the example of a transformer nameplate. Based on existing guideline of transformer nameplate, below information have to be listed in the nameplate:

  • Manufacturer.
  • Power rating (VA or kVA).
  • Operating frequency.
  • Windings voltage (primary and secondary).
  • Impedance for 25WA and larger.
  • Clearances between transformer and ventilation opening.
  • Insulating liquid used for transformers (amount and type).

how are transformers rated

Table 1. Transformer ratings

Example Why Transformers are Rated in kVA not kW?

If you are still confused about how are transformers rated, let’s observe the example below.

But before that, we need to be clear that a transformer has two types of losses:

  1. Copper losses (winding losses).
  2. Core losses, iron losses, or insulation losses.

Keep in mind that:

The copper losses are calculated by:

    \begin{align*}I^{2}R\end{align*}

where:

I = current flowing through the winding.

The core losses, iron losses, or insulation losses are calculated based on voltage.

Hence the total losses are calculated by voltage (V) and current (I), the unit of Volt-Ampere (VA).

Now let’s proceed to the example:

Assume we have a single phase step-down transformer with the specifications below,

  • Rating: 10 kVA
  • Primary voltage: 220 V
  • Primary current: 50 A
  • Secondary voltage: 110 V
  • Secondary current: 100 A
  • Resistance on secondary: 10 ohm
  • Iron losses: 50 W

For the first case, we use pure resistive load on the secondary side where the power factor is unity (θ = 1).

The total losses for the transformer is the sum of the copper losses and iron losses,

    \begin{align*}P=I^{2}R+\mbox{iron losses}\end{align*}

Insert the values above,

    \begin{align*}P&=(100^{2}\times10)+50W\\&=100050W\end{align*}

The transformer output, using secondary values:

    \begin{align*}P&=V\times I\times\cos\theta\\&=110\times100\times1\\&=11\mbox{kW}\end{align*}

The transformer rating:

    \begin{align*}\mbox{kVA}&=\frac{\mbox{VA}}{1000}\\&=\frac{110\times10}{1000}\\&=11\mbox{kVA}\end{align*}

For the second case, we will use inductive or capacitive load instead of resistive load to the secondary side. Assume the power factor θ = 0.6, then

Total losses of the transformer:

    \begin{align*}P=I^{2}R+\mbox{iron losses}\end{align*}

Insert the values above,

    \begin{align*}P&=(100^{2}\times10)+50W\\&=100050W\end{align*}

The losses for both primary and secondary side is still the same.

But it is different with the transformer output:

    \begin{align*}P&=V\times I\times\cos\theta\\&=110\times100\times0.6\\&=6.6\mbox{kW}\end{align*}

The transformer rating

    \begin{align*}\mbox{kVA}&=\frac{\mbox{VA}}{1000}\\&=\frac{110\times10}{1000}\\&=11\mbox{kVA}\end{align*}

The number 11 kVA means the transformer can handle 11 kVA. We can convert it to 11 kW if we use pure resistive load where power factor θ is unity (1). This is quite an impossible thing to do in the actual field unless you have a method to improve the power factor θ into 1.

The conclusion we can draw from this example are:

  • The losses for both cases are still the same.
  • The Transforming rating for both cases are still the same (11 kVA)
  • The output power for both cases are different (11 kW and 6.6 kW) because of different types of connected load.
  • Since every type of load can be connected every time, the kVA rating is more suitable for transformer rating.

This concludes everything about how are transformers rated.

Frequently Asked Questions

How is a transformer sized or rated?

The transformer is rated with kVA instead of kW. The kVA shows the value of voltage of the transformer and current absorbed by the load.

What is the rating of transformer explain?

Transformer rating shows its capability on carrying load, maximum voltage and current it can provide to the load. Hence transformer is rated in volt-ampere to indicate how much current is drawn by the load.

Why are transformers rated in kVA?

Transformer is rated in kVA because its losses is not affected by the power factor. Apparent power is represented by kVA and a combination of real power and reactive power.

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