Why does only one fan spin on my APC Step-Down Transformer (part numbers AP9621, AP9626, AP9627, AP9628 and SYTF2)?
Product working as designed.
APC step down transformers come with two fans installed. One is the primary AC fan that is used for forced air cooling of the transformer. The second is an auxiliary fan which comes on when the internal temperature of the transformer exceeds 100°F. The AC fans are necessary to cool the transformer for proper operation. Two fans were installed to provide redundancy in the event that the primary fan fails. When the auxiliary fan is activated, it increases the air flow in the event that high temperatures are reached within the transformer unit. If the first fan fails due to a mechanical defect or damage, the transformer will be cooled solely by the auxiliary fan.