I refer to Flame proof transformers that are quite small in construction hight and are designed using 5 limb cores in order to achieve this. Can anyone tell me how this works? and what is the relation to the impedance of the transformer? These transformers are used in coal mining and range between 1250 KVA - 2000 KVA.

 

Stuart... 5-limb construction is used to meet height restrictions in mines, tunneling machines, and problems associated with overhead clearances for rail-transport.

For simplicity assume the winding is a flat rectangular conductor, wound around a magnetic core (limb.) The area of the core varies with the Volts/Turn parameter. Coil height, hence core height, is a function of conductor width and the number of turn. Thus, kVA capacity is proportional to the volume of both core and coils.

When height is restricted, then to retain a certain capacity, core-volume must be increased. But, this increase is disproportionate with kVA capacity. Designers, came up with the 5-limb construction. The three primary and secondary phase coils are wound around three of the cores, and the other two provide the needed core volume.

Please note: the effects of the top and bottom 'yokes' connecting the 5-limbs is excluded in this discussion.

Regarding your impedance question. I am unaware of any deleterious restrictions due to 5-limb construction.

Regards, Phil Corso ([email protected])

 

Stuart... additional information can be found at:
http://control.com/thread/1297944362

Regards, Phil Corso ([email protected])

 

Thanks for your response Phil.

Would it then be correct to say that one would use less number of turns to reduce the coil height, and increase the area of the core (by adding two extra limbs) in order to maintain the correct flux density?

Regards Stuart

 

Stuart… actually it is more complicated than just the two modifications you suggested. In fact, there are some twenty steps in the design of a power transformer!

A key design-parameter is called the Area Product Ap, which is proportional to the core's Window Area, Wa, times the core's Cross-Sectional Area, Ac!

The formula is Ap ~ Wa x Ac, which yields a
parameter (inches or centimeters) raised to the 4th power. For the case of a 5-limb transformer, the result is a squatter configuration.

Regards, Phil