Disadvantages of ONAN/ONAF against ONAN Transformer

Life span only. Moving parts do age faster. But the fan will work only when required so not much unnecesary losses and not really much complexity involved

The ONAN/ONAF transformer has 2 MVA ratings: one with ONAN and another with ONAF. The ONAF MVA is always higher than the ONAN MVA, while the ONAN transformer has only one MVA rating.

Based on your MVA requirement, you have two choices to select the ONAN/ONAF transformer:

(1) You can select ONAN/ONAF transformer considering the ONAN rating as your required MVA. In this case, the fan will not be used for the existing load requirements but you have the provision of future expansion or sustaining the peak time over load.

(2) You can select ONAN/ONAF transformer considering the ONAF rating as your required MVA. In this case, the fan will be used most of the time for the existing load requirements. Because of the fans, it has more maintenance, more loss, and more chance of failure.

So, it depends on the nature of the load. The selection can be as follows:

(A) If the loads are mostly continuous (flat load curve) and there is no chance of future expansion, select the ONAN transformer

(B) If the loads are non-continuous, varies with time and there is no chance of future expansion, select the ONAN/ONAF transformer with ONAF rating

(C) If the loads are non-continuous, varies with time and there is a chance of future expansion, select the ONAN/ONAF transformer with ONAN rating

Hope it helps.

- MS

I also suggest to incorporate folloiwnf in the specs before placing order to manufacturer :

Transformenr should be suitable to deliver the rated ONAN rating with specified temperature rise with 2 radiators out of the circuit.

In case of ONAF,

Transformer should have 2 separate Radiator banks with 2 sets of Colling fan. &

Transformenr should be suitable to deliver the rated ONAF rating with specified temperature rise with 2 radiators out of the circuit & any 1 nos of Fan in Off condition in each cooler.

This will help mantainance team to mantain Trafos.