Transformer Current Density Selection

Why would you want to get involved in the internal design details of a bought-out transformer? The manufacturer should guarantee the final performance of the product.

I m purchasing so i must know every thing which is related to the item will meet the requirements in better ways & it will help me in future to conclude advantages & disadvantages on my own ....

Pl. get a copy of the ABB Transformer Handbook & go thro the same.

Having read some of your previous posts I doubt very much that you are purchasing as you put it. If you were involved with the specification and purchase you wouldn't be on here asking questions, you would know what's required.

When there is transportation problem it is usual to arrange three single phase transformers so your arrangement goes well.

You have a space limitation; select the transformer that fits.

You are concerned with heating of transformer; Tell he manufacturer to install fans for forced air cooling on the radiator fins (AF), this is a readily acceptable optional requirement and the manufacturer will comply.

You can tell the manufacturer to design for a peak efficiency (iron loss=copper loss) at you estimated loading during normal operation that means less heating and high efficiency.

This transformer is not meant for a low voltage audio amplifier. Power transformer is for use on high voltage, manufacturer has the transformer "type tested" so he will not listen to you on a re-design, sensibly the manufacturer knows his business and you ought to know yours.

When you have mentioned site ambient temperature, altitude to the manufacturer and he offers a t/f , just mention that you will carry out Performance Testing as an acceptance criteria.

_{This transformer is not meant for a low voltage audio amplifier. Power transformer is for use on high voltage, manufacturer has the transformer "type tested" so he will not listen to you on a re-design, sensibly the manufacturer knows his business and you ought to know yours.}

Couldn't have put it better

Hi kvsridher,

I am not quite sure as to what is your concern, however, I try to give an explanation as follows:

For a power transformer design the current density selection is totally dependent on the type of insulation i.e. temperature bearing limit for that selected insulation as well as how efficiently the heat produced in winding can be dissipated.

• The manufacturer has specified OFWF class (quoted by the OP), this you know represents oil forced and water forced cooling method which is highly efficient.
• All high voltage equipment must undergo type testing and is sufficient proof for the adequacy of that equipment under the manufacturer's declared set of criteria.
• For a Hydro power station the inlet water flow pipe (pen stock) limits the turbine to be located in a cavern as described by the OP, however this is not compulsory to put the transformer in that cavern too, place the transformer out of that cavern and use a cable connection to the equipment(s) hence no space and size limitation.

Further I would like to clarify that in case of small transformer like the one used in electronic equipment (Audi Amplifiers etc.) housings:

• The transformers are air cooled.
• insulation is normally class-A
• End user may want the transformer to be of less weight.
• End user is concerned that the heat generated by the transformer will adversely effect the sophisticated electronic circuitry.
• Type testing is not mandatory in LV and the manufacturer tailor makes the transformer to fulfill the current density and the like demands raised by the Customer.

Sincerely Your's.
A.A.Khi

My goodness ! Here i was, trying to give you a compliment and a GA, and out you came with such a long explanation which i certainly don't need. i know all about it already ! Please see #1 which is my comment, identical in meaning to yours, but said in one short line.

Maybe misunderstood since English is not the native language for either of us.

The current density allowable for the copper windings is a somewhat variable figure. Current density refers to the current in Amps per unit of wire area, for example 2.565A/mm² (a reference standard used in Australia and presumably elsewhere as well). Increasing the current density has a major effect - it causes the wire to get hotter for a given current. Side forces caused by the magnetic fields generated between each turn need to be considered in large power distribution transformers, especially under short-circuit conditions where the forces can be destructive. There is no such thing as a "typical" current density, because different manufacturers use different design criteria. In general, it's better to keep current density below 3.0A/mm² and 2.5A/mm² is even better. Naturally, a lower current density means that the transformer is larger and heavier than one operated at a high density, and ultimately it's all a trade-off against temperature rise and cost.
Ultimately, copper losses limit the power available from a transformer. Since all copper loss results in heat, this becomes a limiting factor, so once you reach the point where the temperature rise cannot be limited to a safe value, the size of the core must be increased. This allows the manufacturer to use fewer turns per Volt, and the larger core has more space for the windings. The wire size can therefore be increased, so copper losses are brought back to the point where overheating is no longer a problem. This process continues from the smallest transformers to the largest - each size is determined by the VA rating and allowable temperature rise.