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18.10.2016

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Transformer's power and ambient temperature  

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 Sn should be 500 VA, although the load's effective output Pn differs.

 

Cos φ

Pn

Sn

Heating resistor

 1

500 W

 500 / 1 = 500 VA

Electric motor

 0.8

400 W

 400 / 0.8 = 500 VA

Discharge lamp (uncompensated)

 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.

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