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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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OVERALL DIMENSION OF A TRANSFORMER
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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OVERALL DIMENSION OF A TRANSFORMER
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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