Transformer class rating

[steve holt]

How can you determine the class rating of a transformer? The only information given on the nameplate is "Class AA". Does this reference the insulation rating of the transformer? I am looking for something that will tell me if the transformer is rated at 155 or higher.

 

[templdl]

Insulation Rating Transformer Rating Max. Ambient Temperature Hot Spot Allowance

Class 105 55?C Rise 40?C 10?C
Class 150 80?C Rise 40?C 30?C
Class 180 115?C Rise 40?C 30?C
Class 220 150?C Rise 40?C 30?C

 

[templdl]

Insulation Classes:
Y = 90? C
A = 105? C
E = 120? C
B = 130? C
F = 155? C
H = 180? C
Class A: No more than 55o Celsius winding temperature rise, at 40o Celsius (maximum) ambient air temperature.
Class B: No more than 80o Celsius winding temperature rise, at 40o Celsius (maximum)ambient air temperature.
Class F: No more than 115o Celsius winding temperature rise, at 40o Celsius (maximum)ambient air temperature.
Class H: No more than 150o Celsius winding temperature rise, at 40o Celsius (maximum)ambient air temperature.

As an example encapsulated transformers are rated 180 or 185 deg insulation with 115degC rise. 115+40+30(must be added for hot spot)= 185degC
A ventilated dry type would be 150+40+30=220degC
If one were to go with 115deg C for a ventilated with 220 deg insulation it would provide for a 15% continuous over load capacity without sacrificing transformer live. At 80deg C rise that same transformer provides a 30% overload capacity.
Encapsulated with an 80 deg C rise provides a 15% overload capacity.
So class H isulation is total temperature of 220deg C (150+40+30) and F is 185degc (115+40+30).

 

[zog]

How can you determine the class rating of a transformer? The only information given on the nameplate is "Class AA". Does this reference the insulation rating of the transformer? I am looking for something that will tell me if the transformer is rated at 155 or higher.

The class AA is the cooling class, or the type of cooling system used, in this case air, natural circulation.

What you are looking for is the max rise, which should be on the name plate. Why do you need to know this?

 

[templdl]

Regarding cooling class,
AA is Air to Air, with not forced air.
750kva AA would provide 750kva at its rated temperature rise.
A 750kva AA/FFA is rated 750kva but fans can be added later (FFA= Future Fans to Air) can boost the KVA 33-1/3% to 1000kva.
Then there is 750/1000kva AA/FA that includes a fan package which is 750kva w/o fans running an 1000kva when the fans are running.
One very important thing to note is that fan packages are not common on smaller transformers so they are simply AA. You can't simply add a fan package to an existing transformer expecting to get 1/3 additional capacity out of it unless it has been designed and rated to do so. On larger transformers where fan packages are common the transformer's cooling ducts designed with sufficient ducting for the additional cooling capacity. Also, the bus will be sized for the additional ampacity. Should you add fans to a transformer that is not rated for it the transformer could over heat even with fans and the bus would be overloaded.