112.5 KVA Transformer heat rejection

[Sahib]

If temp rating of a transformer changes, its efficiency may also change. So if one knows efficiency, power factor etc of a transformer per OP and post #4, it is possible to calculate the heat rejection of the transformer using the following identity:

{(1-output/input)*[output/(output/input)]}=input-output...................(1)

Here input-output= losses of transformer=Heat rejection by transformer under steady state,

output/input=Efficiency of transformer,

output=KVA*Powerfactor.

Substituting values from post#4,

Output=112.5*0.9=101,25KW

Output/Input=0.95 on the left hand side of (1), the heat rejection is

(1-0.95)*(101.25/0.95)=5.33kJ/s or approximately 5 kW.

 

[Besoeker]

If temp rating of a transformer changes,

Transformers are built for a specific temperature rise.
Why would that rating change?

 

[Sahib]

Transformers are built for a specific temperature rise.
Why would that rating change?

If the buyer prefers one temp rating over another.

 

[Besoeker]

If the buyer prefers one temp rating over another.

That doesn't make its rating change.

 

[Sahib]

That doesn't make its rating change.

Take, for example, a dry type transformer. It operates at a higher temperature than an oil cooled transformer and so has a lower efficiency.

 

[Besoeker]

Take, for example, a dry type transformer. It operates at a higher temperature than an oil cooled transformer and so has a lower efficiency.

Whilst it is generally true that ONAN transformers are more efficient than dry types it isn't just a simple temperature difference. Core and coils differ between the two. Yes, i²R losses go up if you run at a higher temperature but they also go up if you reduce the cross sectional area of the conductor.

 

[Haji]

Whilst it is generally true that ONAN transformers are more efficient than dry types it isn't just a simple temperature difference. Core and coils differ between the two. Yes, i²R losses go up if you run at a higher temperature but they also go up if you reduce the cross sectional area of the conductor.

Here the discussion is about the OP choosing a transformer of particular temperature rating and so efficiency and not about design aspects of the transformer.

 

[Besoeker]

Here the discussion is about the OP choosing a transformer of particular temperature rating and so efficiency and not about design aspects of the transformer.

Efficiency is a design aspect.

 

[Bob Morris]

THe Dept of Energy requires a minimum efficiency of 99.01% per attached table.

Dept of Energy issued these required efficiencies on 10/12/07 for dry type transformers:

Single-Phase kVA​	Efficiency​	Three-Phase kVA​	Efficiency​
10​	99.62​	15​	98.36​
15​	98.76​	30​	98.62​
25​	98.91​	45​	98.76​
37.5​	99.01​	75​	98.91​
50​	99.08​	112.5​	99.01​
75​	99.17​	150​	99.08​
100​	99.23​	225​	99.17​
167​	99.25​	300​	99.23​
250​	99.32​	500​	99.25​
333​	99.36​	750​	99.32​
500​	99.42​	1000​	99.36​
667​	99.46​	1500​	99.42​
833​	99.49​	2000​	99.46​
		2500​	99.49​

 

[templdl]

At what load is the chart based upon?

 

[topgone]

At what load is the chart based upon?

IIRC, testing is done at 50% of nameplate rating per DOE test procedure.