Transformer Ratings: What Are They (And Why Are They in kVA)?

What is a Transformer Rating?

Manufacturers design transformers based on the voltage and current required for the transformer’s operation. They then specify this on the transformer’s nameplate in terms of VA (Volt-Amps). This is referred to as the rating of a transformer. The transformers rating can also be the maximum voltage and current that can be safely applied to the transformer.

The rating of a transformer depends upon temperature rise, which depends on the transformer’s losses. However, the temperature can be maintained within permissible limits by using the proper cooling system.

The greater the effectiveness of the cooling system, the higher the rating of the transformer (and vice versa). For a given cooling system, the rating of an electrical machine is indirectly determined by the losses present in the machine.

In a transformer, there are two main types of losses:

  1. Constant losses or core losses – These depend on V
  2. Variable losses or ohmic (I2R) losses – These depend on I

Hence total losses depend on V and I. Since the rating of a transformer depends upon losses and losses depends upon V and I, the rating of a transformer depends on V×I, which is also termed VI ratings.

As losses are independent of the power factor of load, the rating of the transformer is also independent of load and can be only decided based on losses.

That’s why the transformer is generally specified with apparent power rating (VA or kVA) and not in kW.

Lets us take an example, a transformer working on rated voltage and rated current with load power factor equal to zero; as load power factor is zero, it will deliver zero power to load, but it has rated kVA output. Hence rating must be expressed in kVA.

For any transformer rated input in kVA at the primary = rated output in kVA at secondary + losses.

As the transformer runs on very high efficiency, its losses can be neglected and hence rated input in kVA at the primary = rated output in kVA at secondary.

We can say that rated kVA marked on the nameplate of the transformer refers to both the windings. i.e. kVA rating for both the primary and secondary winding is the same.

The kVA rating mentioned on the transformer’s nameplate is load side kVA but only at full load condition.

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