Control power transformer derating due to high ambient temp.

[eerjf]

Hello,

I need some help. I am working an accident analysis where we are looking at a high ambient sitution and a motor control circuit. I specifically am looking at the control power transformer within the motor control center starter unit. The transformer is a 100 VA 480/120 volt transformer. We have been given that an ambient temperature of up to 80 degrees C could exist inside the starter unit. I need to determine how to derate the transformer to determine if it will still function in that environment. The transformer has a temperature rating of 105 degrees C with 55 degree C temperature rise at a 40 degree C ambient.

Please help. Thanks.

 

[edison123]

80 deg C inside the starter unit ? Is it located near a heat source like furnaces, hot mills ? Is it properly ventilated ?

The trafos are rated for a maximum of 50 deg C ambient. At your ambient, the derating will be so high that you will need a higher capacity trafo.

 

[itsmoked]

That would be at full load 105C-55C = 50C

That means the box can't be more that 50C inside at full load. That also means you can not then add as much heat via loading of the transformer as the transformer would normally allow. That is, the xformer must be de-rated as edison points out.

Unfortunately you can not just do the proportion because an unloaded transformer heats up some amount that you won't know without specs from the maker.

I would call the maker and state the ambient and the required VA your controls need, including all the ridiculous extras that always get added, (like service outlets). They will tell which transformer you need to pull this off.

Keith Cress
kcress -

http://www.flaminsystems.com