Power and ambient temperature of the transformer

A transformer’s load capacity is given as apparent power (S) and its unit is volt-ampere (VA).

If the transformer’s feeding load is purely resistive, the effective output of the transformer is equal to its apparent power.

Effective output (P): P = S cos φ

If the load has reactive components such as inductance (motors, inductors, discharge lamps), the transformer must be chosen taking into account the load’s power factor (cos φ).

Example:

In all the below cases the transformer’s apparent power Sshould be 500 VA, although the load’s effective output Pn differs.

 Cos φ PN SN Heating resistance 1 500 W 500/1 = 500 VA Electric motor 0,8 400 W 400/0.8 = 500 VA Discharge lamp (non-compensated) 0,5 250 W 250/0.5 = 500 VA

The transformer’s power is stated as apparent nominal power in nominal ambient temperature. Due to this, the installation site’s temperature should always be taken into account when choosing a transformer. When the ambient temperature increases, the load capacity of the transformer decreases. The transformer’s load capacity decreases almost exponentially relative to the temperature increase.