Electrical Machines  Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.) 
8. Transformer 
8.2. Operational behaviour of a transformer 

Efficiency represents the ratio of power output to power input.
_{}
Efficiency is determined by the separate loss indication.
P_{in} = P_{out} + P_{VFe} + P_{VW}
P_{out} 
rated power of the transformer given 

cos j = 1 (resistive load) 
P_{VW} 
metered winding losses in shortcircuit trial 
P_{VFe} 
metered iron losses in noload trial 
_{}
The following equation applies for determining efficiency for partially inductive or capacitive load:
_{}
Example:
Given:
S_{n} = 100 kVA
P_{VFe} = 570 W
P_{VW} = 2.1 kW
cos j = 0.85
Sought: h
Solution:
_{}