- Transformer Rating Definition: Transformer rating is defined as the voltage and current specified for operation, expressed in VA (Volt-Amps).
- Importance of Cooling: The effectiveness of the cooling system impacts the transformer’s rating, with better cooling allowing a higher rating.
- Loss Types: Transformers have constant losses depending on voltage and variable losses depending on current, affecting the overall rating.
- Power Factor Independence: The transformer’s rating in kVA is independent of the load’s power factor since losses do not depend on it.
- Apparent Power Rating in kVA: Transformers are rated in kVA, not kW, to account for the combination of voltage and current without considering power factor.
What is a Transformer Rating?
Manufacturers design transformers based on the needed voltage and current. This information is shown on the transformer’s nameplate in VA (Volt-Amps), which is called the transformer rating. The rating also indicates the maximum voltage and current the transformer can handle safely.
The rating of a transformer depends on temperature rise, which is affected by the transformer’s losses. Proper cooling systems help keep the temperature within safe limits.
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:
- Constant losses or core losses – These depend on V
- 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.
For example, a transformer operating at rated voltage and current with a load power factor of zero delivers zero power to the load but still has its rated kVA output. Therefore, the rating must be 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.
