overall dimensions

8/10/2019 Overall Dimensions

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Design of Transformers

D.Raja

AP/EEE/SMVECPuducherry


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Design of Transformers

Selection of design constants

Window dimension

yoke design

overall dimension


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SELECTION OF DESIGN CONSTANTS

Bm = m flux density decidesarea of cross section ofAi core & core loss

If Bm increases, Ai decreases

(1) Smaller core area(2) Lesser cost,

(3) Reduction in length of mean turn windings.

Disadvantages:

Iron loss and higher temperature rise

Core will saturate.

Large magnetizing current which contain objectionable

Harmonics .

Choice of flux density (Bm):


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Choice of flux density depends on

1. Service condition (i.e. distribution or transmission)

2. Material used for the core- Hot rolled silicon steel.

- Cold rolled silicon steel

Hot rolled Silicon Steel:Bm = 1.1 to 1.4 wb/m^2 for distribution transformers

Bm = 1.2 to 1.5 wb/m^2 for power transformers

Cold rolled Silicon Steel:

Bm = 1.55 wb/m^2 (for transformers with voltage ratingupto 132 KV)

Bm = 1.6 wb/m^2 (132 KV to 275 KV)

Bm = 1.7 wb/m^2 (275 KV to 400 KV)


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Choice of Window space factor (KW):

Kw = Copper Area in window (Ac)

Total window Area (Aw)

(Amount of copper & insulation used will influence the

Kw value)

Choice of Kw depends on KVA rating & voltage rating

Kw is smaller for small transformers

Kw is larger for large transformers

Total window Area (AW) = Copper Area + Insulation area

+ Air or oil spaces.


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Window Dimension

Total window Area, Aw = Hw x Ww

Kw = AC

Aw

Where,

Awwindow height.

Ww - window width.

Narrow window, Hw increases when windings are long and thin.

Distance between adjacent limbs and windings is less.

- Therefore leakage reactance reduces.

When Hw decreases, Ww increases and leakage reactance alsoincreases.

Usual value of Hw =2 to 4

Ww


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YOKE DIMENSIONS

No winding comes over the yoke portion.

Yoke area, Ay=1.15 to 1.25 times of limb section.

Yoke may be rectangle or stepped.

For a rectangular yoke ,

Ay = Dy * Hy Where,

Ayyoke area

Dydepth of yoke

Hyheight of yoke.

Dy=a= depth of yoke = Width of largest core stamping

Ay = 1.15 to 1.25 Agi


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OVERALL DIMENSION OF A TRANSFORMER

SINGLE PHASE CORE TYPE TRANSFORMER

Hwheight of window

Ww

width of window

awidth of largest

stamping (width of limb)

ddiameter of

circumseribility circle.

D

distance between corecenters.

Hyheight of yoke

Dydepth of yoke

Hoverall height of

transformer frame

Woverall width of

transformer frame.


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OVERALL CORE DIMENSIONS

From the diagrams,Single phase core type transformer

Distance between core center

D = Ww+d

Depth of yoke,

Dy = a

Frame height,

H = Hw+Hy+Hy

H = Hw+2Hy

Overall width,

W = D+a/2+a/2

W = D+a


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THREE PHASE CORE TYPE TRANSFORMER

Hwheight of window

Ww

width of window

awidth of largest

stamping (width of limb)

ddiameter of

circumseribility circle.

D

distance between corecenters.

Hyheight of yoke

Dydepth of yoke

Hoverall height of

transformer frame

Woverall width of

transformer frame.


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THREE PHASE CORE TYPE

TRANSFORMER

Distance between core centers,

D = Ww+d/2+d/2

D = Ww+d

Depth of yoke,

Dy = a

Overall height,

H = Hw+2Hy

Overall width,

W = 2D+a


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SINGLE PHASE SHELL TYPE TRANSFORMER

a- Width of outer limbs,

b

depth of outer limbs 2awidth of central limb.

Overall width,

W = 2Ww+2a+2a

W = 2Ww+4a

Overall height,

H = Hw+2Hy

H = Hw+2a

(since Hy = a)

Gross core area of central

limb = 2a*b


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Choice of current density

If , Heating ,

= I Ai d, lAi

Space for cooling part is large.

Permissible values of current density are, For standard distribution & small power transformers

(Oil immersed, self cooled)- = 1.5 to 2.6 A/mm^2

For medium & large power transformers (oil cooled or Airblast)

- = 2.4 to 3.4 A/mm^2 For large power transformers

(Oil immersed with force circulation of oil or with water

cooling coils)- = ^


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