Temp rise, is this the max. operating temp of the transformer.
Reason for question, I measured a trans, 480x208/120y, that was operating at 25-30% load, and the internal temp was 296-300F degrees. It stated on the nameplate a temp rise of 150C, which closely puts it at 330F.
Is that the max operating temp? The insulation class is listed at 220C.
There are classes of insulation systems for different temperatures as defined by NEMA and ANSI. Insulation classes are rated in °C rise above a specific ambient of 40°C maximum. A transformer having a specific class of insulation, for example Class 220, can have an average winding temperature rise of 150°C with a maximum hot spot temperature rise of 180°C. If the room ambient temperature is 40°C, then the total temperature of the hottest spot would be 220°Dry type distribution transformers are designed to operate at rated load and voltage in maximum room ambient temperatures of 40°C, average room ambient temperature not to exceed 30°C, and at altitudes not to exceed 3300 feet in accordance with NEMA standards.
Transformers' temperature rise values are rated from a maximum ambient of 40ºC with an additional 30ºC for the coil "Hot Spot" allowance. Adding the 150ºC max. temperature rise is why transformers have 220ºC rated insulation...but to operate the transformer at these temperatures will reduce is life considerably.
To overcome the transformer's max. temperature tolerance and increase its life, the transformer's nameplate load rating should be decreased by 8% for every 10ºC above 40ºC ambient.
Additionally, the transformer should be derated 0.3% for every 100 meters (330 feet) for altitudes over 1000 meters (3300 feet). And derated if there are any involved motors that require starting at least twice an hour by 20% for service factors below 1.2 and equal to the service factor above 1.2.
The benefit of derating the transformer (or versely increasing its nameplate size) is to reduce heat via larger coils and less work.
I hope this helps in understanding how the max. temperature rise is utilized.
Volts has an excellent transformer sizing module that goes into more detail about derating and operating conditions and sizing.
edited for spelling errors
[This message has been edited by DiverDan (edited 08-20-2005).]
Diverdan, Good stuff! My xmr application notes that I got from an old-timer DTDT design engineer indicated that one can expect a 30year life based upon a 35% avg. loading per accepted "industry standards" on loading. My notes also indicate that there is a sever penalty that is paid for 100% loading, expect 3-5 years based upon 30degC avg. ambient w/40degC max. (My notes don't specify but I would assume that it's for a 150degC transformer.) Also, for each 10degC above 30degC reduces the life of the transformer by 50%. Dave
At a given loading, the transformer is putting out X btu per hour. But this is not sufficient to determine winding temperature. btu is a measure of _heat_, and it is heat which causes temperature to change, but the actual temperature of the transformer depends upon the balance of heat gained versus heat lost.
The heat gained is pretty constant for a given load. The heat lost changes all over the place, depending upon such things as air flow, and particularly depending on the temperature; the hotter something is, the more it heats the surrounding air up, and thus the more heat lost to the surroundings. So when you load the transformer, it heats up until the heat loss naturally balances the heat gain and you reach equilibrium.
If you were to take a fan and force air through the transformer, it would run cooler. The heat gain would be the same, but more heat would be lost at any given temperature. The ambient temperature factors in to this; the hotter the surroundings, the less heat lost at any given temperature, and the higher the coil temperature.
You see the same things with conductor ampacity and temperature. The more current that flows through the wire, the more heat generated, and the hotter the wire has to run to dissipate this heat. The higher the ambient temperature, the hotter the wire has to run in order to dissipate the heat. This is why conductors need to be de-rated for operation in high temperature environments.
The transformer that you describe is running at 90% of its rated temperature rise, but at only 30% load. This sounds wrong to me, and suggests really high core losses. But I don't really have the experience to back up this hunch; I do design work in a related field (motors), but not much with transformer design. It is plausible that a particular transformer will have similar losses at both low and high load, but at least with motors the losses go up as the load goes up. I wonder if the supply voltage is on the high side, or if there is a harmonics current problem.