- 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?
A transformer rating is the nameplate apparent power, in VA or kVA, together with the rated voltages. Manufacturers size the windings for those voltages and for the current that follows from kVA = V × I (times √3 on a three-phase unit). Those figures are the continuous thermal duty, not a short-circuit withstand.
The rating of a transformer is limited by temperature rise from losses. Cooling must keep that rise within the insulation class.
Better cooling (for example ONAN to ONAF) can raise the allowed kVA of the same core and coils. For a given cooling method, losses set how much current and voltage the machine can carry continuously.
In a transformer, there are two main types of losses:
- Constant losses or core losses – These depend mainly on V and frequency
- Variable losses or ohmic (I2R) losses – These depend on I
Hence total losses depend on V and I. Since the rating of a transformer is a thermal limit from those losses, planners state it as a VI product: the VA or kVA rating.
Copper loss at a given current is almost independent of the load power factor. So the kVA rating is independent of power factor. The same figure is still a load limit: the maximum continuous apparent power.
The transformer is therefore specified in apparent power (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.
IEC 60076 treats the rated power as output apparent power. Input kVA is a little higher because of losses, which are watts, not a kVA add-on. At high efficiency the two kVA figures are close, so the same nameplate kVA is used for both windings.
Because efficiency is high, losses are often neglected in a first estimate, and rated primary kVA is taken as equal to rated secondary kVA.
The rated kVA on the nameplate of the transformer is that throughput. Primary and secondary currents differ by the turns ratio, but they share the same kVA rating.
The nameplate kVA is the continuous capability at rated voltage and rated conditions. Partial load uses less than rated kVA; the rating is not a full-load-only figure.
