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The objective of this case study is to compare the cost effectiveness of a shell-type design against core-type design for a single-phase 50 kVA overhead transformer.

Our findings indicate that subject 50kVA shell-type design is more cost efficient costing 5% less than its core-type counterpart. The cost effectiveness of shell-type is 2.73 and for core-type it is 3.56. Shell-type uses 18% more core but 21% less conductor material than core-type. Total core & coil weights for the two types are similar. No visible non-tangible manufacturing savings favor one design. Our shell-type design's marginal cost advantage makes it the preferred design of choice in comparison to core-type.

This case study analyzes the cost competitiveness of a 50kVA core-type versus shell-type transformer design.

Single-phase core-type transformers consist of two windings and one core; shell-type consists of one winding and two cores. Shell-type provides a higher degree of mechanical protection to the winding because it is surrounded by core. On the other hand, core-type provides better insulation between the primary and secondary because of its inherent two windings.

This analysis is based on 50kVA shell-type and core-type overhead pole-mount transformer designs. The idea is to compare the total material cost of a shell-type design against a core-type design. For this comparison to be meaningful, we made equivalent core-type and shell-type designs with similar losses and impedance values. Our analysis and discussion here forth is based on these two designs.

Table-1   Design specifications for 50kVA Core-type and Shell-Type Transformers

Our data for the subject two types of transformers is tabulated in Table-1 above. It shows that both design types have similar core & coil weights, though core-type costs 5% more. The shell-type core & coil cost is $790; the core-type in comparison is $848. The two types are also comparable in size requiring same size tank and cooling liquid quantity.

Our shell-type design is more cost efficient. The cost effectiveness of shell-type is 2.73 and for core-type it is 3.56. In other words, every dollar spent on material in our shell-type design gives us an evaluated worth of 2.73. More bang for our buck!

Some other noteworthy observations from our Table-1 are that shell-type design is slightly more compact, about 3% smaller in length than the core-type. This marginally small core & coil is of no significance because standards govern same standard tank size for both. Since shell-type design uses two coils it uses approximately 18% more core than the core-type. However, since shell-type has one coil its conductor requirements are 21% less than the core-type. Insulation usage in shell-type is roughly half the amount of that in core type.

From a quick glance at our table it may appear that considerable non-tangible savings can be realized in manufacturing, such as savings in labor and downtime costs. These savings are difficult to quantify, however, we can make some quick observations. Use of one core-stack in the core-type rather than two stacks (as in shell-type) should translate in savings in coring and material handling labor. It should also eliminate some of the downtime in apparatus setup such as lifting and setting of core stacks during coring, etc.

However, as apparent from Chart-1 and Chart-2, the non-tangible savings of core-type are offset by less winding and insulation requirements of shell-type. Less winding time should mean less winding labor, less machine wear and tear, and energy savings in terms of power bills, etc. No real visible benefits in labor and non-tangible costs are therefore gained in choosing one design over the other.

Our 50 kVA overhead shell-type design appears to be more cost effective than shell-type by $42 or 5% less. Our shell-type design weight is the same as core-type but it is a more compact design, 3% smaller in length. In shell-type core is the most expensive part, while in core-type conductor is the costliest factor. From a manufacturing standpoint, core-type and shell-type designs are very similar.