Transformer FAQ
What is the difference between ONAN and ONAF transformer cooling?
ONAN uses natural oil circulation and natural air cooling, while ONAF adds forced air from fans around the radiators.
Direct answer
In ONAN operation, transformer oil circulates by natural convection and heat leaves the radiators through natural airflow. In ONAF operation, oil circulation remains natural but fans force air across the radiators. A transformer may carry separate ONAN and ONAF ratings, but the exact capacity increase is a design value stated on the nameplate and datasheet.
ONAN characteristics
ONAN has no cooling fans or oil pumps in operation. It offers low auxiliary power consumption, low mechanical noise and high simplicity.
ONAF characteristics
ONAF uses radiator fans to increase external heat transfer. Fan controls, power supply, redundancy, noise and maintenance become part of the system design.
Selection considerations
Confirm ambient temperature, loading profile, permissible temperature rise, noise limits, fan redundancy and whether the forced-cooling rating is continuous or conditional.
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