Electrical Machine Design

Legend: : Formula | : Limits, Range | : Assumption, etc.

Item 24. Dimensions of window, inch, H X D
H X D = 2 (TI) / D x sf

The height, H is usually 2½ to 3½ times the width D

Item 25. Total Flux in Iron, maxwells, F
F = E x 10⁸ / (4.44 x f x T)

For correction later, when number of turns, item 2, is finalized.

Item 26. Flux density in core under windings, lines per square inch, B"_limbs
B"_limbs = F / CS_limbs
For 50 or 60 cps, Distribution transformers: 70,000 to 85,000 maxwells per sq. in. and for Power transformers: 80,000 to 95,000 maxwells per sq. in
Use the lower limits for the trial density in the limbs under the windings. For correction when flux and the dimensions of the stampings have been finalized.

Flux Density in Core End Portions, lines per square inch, B"_ends
B"_ends = F / CS_ends
For 50 or 60 cps, Distribution transformers: 70,000 to 85,000 maxwells per sq. in. and for Power transformers: 80,000 to 95,000 maxwells per sq. in
Use the lower limits for the trial density in the limbs under the windings. For correction when flux and the dimensions of the stampings have been finalized.

Note: Go to Item 31 for the calculation of Watts Lost in Iron.

Item 27. Cross section of iron in core under windings, CS_limbs
Use CS_limbs = sf x L x S = F / B".

Cross Section of Iron in Core End Portions, CS_ends
Use CS_ends = sf x M x S.

Item 28. Width of stampings in core under windings, L
The thickness of the stampings, S is usually from 1.5 to 3.0 times the width of the stampings, L.

Item 29. Gross thickness of core, S
For initial value, use S = 1.5L.

Width of the End Portions of the Core, M
For initial value, use M = L. Also, assume the same cross section of iron and the same flux of density in the end portions of the core as in the limbsunder the windings. Changes should be made later if the core losses are too high.

Item 30. Total Weight of Iron in core, pound (lb), Wt_Fe
Wt_{Fe @ limbs} = 0.28 (sf x S x L) x (H x 2) lbs.
Wt_{FE @ ends} = 0.28 (sf x M x S) x 2(2L + D) lbs.

Note: Go back to Item 26 for the solution of Flux Densities.

Item 31. Watts Lost in Iron, W_Fe
From Fig. 154, find the watts per pound, w/lb for the selected B", then the total core loss, W = w/lb x Wt_Fe.. The total Watts Lost in Iron is calculated from W_Fe = W_{Fe @ limbs} + W_{Fe @ ends}
Compare this value to the Guaranteed Core Loss, item d, above.

Note: Solve the approximate Copper Losses, before proceeding further. Thus, go back to Item 06.

Item 32. Total full-load losses, watts, WL_F.L.
WL_F.L. = W_Cu + W_Fe
Should be within the guarantee, that is less than that value, see Item c

Items 33 - 35