Introduction Design of field winding of shunt motor Field winding Types of Field winding Design of shunt field winding

1. Presentation
on
Design of field winding of shunt motor

2. • Introduction
• Field winding
• Types of Field winding
• Design of shunt field winding
Contents

5. Design of field windingDesign of field winding
 Consists of poles, pole shoe and field winding.
 Types:
 Shunt field winding
 Series field winding
 Shunt field winding – have large no. of turns made of thin
conductors ,because current carried by them is very low.
 Series field winding is designed to carry heavy current and so it is
made of thick conductors/strips.
 Field coils are formed, insulated and fixed over the field poles.

6. Design of shunt field windingDesign of shunt field winding
 Involves the determination of the following information regarding
the pole and shunt field winding
 Dimensions of the main field pole ,
 Dimensions of the field coil ,
 Current in shunt field winding,
 Resistance of coil,
 Dimensions of field conductor,
 Number of turns in the field coil ,
 Losses in field coil.
 Dimensions of the main field pole
 For rectangular field poles
o Cross sectional area, length, width , height of the body
 For cylindrical pole
o Cross sectional area, diameter, height of the body

7.  Area of the pole body can be estimated from the knowledge of
flux per pole , leakage coefficient and flux density in the pole.
 Leakage coefficient (Cl) depends on power output of the DC
machine.
 Bp in the pole 1.2 to 1.7 wb/m2
 Фp = Cl. Ф
 Ap = Фp/Bp
 When circular poles are employed, cross section area will be a
circle
 Ap = πdp
2
/4 π= /Ap4dp
Design of shunt field windingDesign of shunt field winding

8.  When rectangular poles employed, length of pole is chosen as
10 to15 mm less than the length of armature
Lp=L –(0.001 to 0.015)
Net iron length Lpi = 0.9 Lp
Width of pole, bp = Ap/Lpi
 Height of pole body hp = hf + thickness of insulation and
clearance
 Total height of the pole hpl= hp + hs
Design of shunt field windingDesign of shunt field winding

9.  Field coils are former wound and placed on the poles.
 They may be of rectangular or circular cross section depends
on the type of poles.
 Dimensions – Lmt, depth, height, diameter.
 Depth(df) – depends on armature.
 Height (hf) - depends on surface required for cooling the coil
and no. of turns(Tf).
 hf, Tf – cannot be independently designed.
Design of shunt field windingDesign of shunt field winding

10.  Lmt - Calculated using the dimensions of pole and depth of the
coil
 For rectangular coils
Lmt=2(Lp + bp + 2df) or (Lo +Li)/2
Where Lo – length of outer most turn & Li – length of
inner most turn
 For cylindrical coils
Lmt = π(dp +df)
 No. of turns in field coil: When the ampere turns to be
developed by the field coil is known, the turns can be estimated
 Field ampere turns on load, ATfl= If. Tf
 Turns in field coil, Tf = ATfl/If
Design of shunt field windingDesign of shunt field winding

11. Power Loss in the field coil:Power Loss in the field coil:
• Power loss in the field coil is copper loss, depends on
Resistance and current
• Heat is developed in the field coil due to this loss and it is
dissipated through the surface of the coil
• In field coil design , loss dissipated per unit surface area is
specified and from which the required surface area can be
estimated.
• Surface area of field coil – depends on Lmt, depth and height
of the coil.
Design of shunt field windingDesign of shunt field winding

12. • Lmt – estimated from dimensions of pole
• Depth – assumed (depends on diameter of armature)
• Height – estimated in order to provide required surface area
Heat can be dissipated from all the four sides of a coil. i.e, inner ,
outer, top and bottom surface of the coil
 Inner surface area= Lmt (hf – df)
 Outer surface area = Lmt (hf + df)
 Top and bottom surface area = Lmt df
Total surface area of field coil, S= Lmt (hf – df)+ = Lmt (hf + df)+
Lmt df+ Lmt df
S= 2Lmt hf +Lmt df= 2Lmt (hf +df)
Permissible copper loss, Qf=S.qf [qf-Loss dissipated/ unit area]
Design of shunt field windingDesign of shunt field winding

13. Substitute S in Qf,
Qf= 2Lmt (hf +df).qf
Actual Cu loss in field coil=If
2
Rf=Ef
2
/Rf
Substituting Rf=(ρLmt Tf)/ af ,
Actual Cu loss in field coil=Ef
2
.af/(ρLmt Tf)
∴
Design of shunt field windingDesign of shunt field winding
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14. Procedure for shunt field designProcedure for shunt field design
Step1 : determine the dimensions of the pole. Assume a suitable
value of leakage coefficient and B = 1.2 to 1.7 T
Фp= Cl. Ф
Ap = Фp/Bp
When circular poles are employed, cross section area will be a
circle
Ap = πdp
2
/4 : dp =Ѵ(4Ap/π) When rectangular poles employed,
length of pole is chosen as 10 to15 mm less than the length of
armature
Lp=L –(0.001 to 0.015)
Net iron length Lpi= 0.9 Lp

15. Step 2 : Determine Lmtof field coil
Assume suitable depth of field winding
For rectangular coils
Lmt =2(Lp + bp + 2df) or (Lo +Li)/2
For cylindrical coils Lmt = π(dp +df)
Step 3: Calculate the voltage across each shunt field coil
Ef = (0.8 to 0.85) V/P
Step 4 : Calculate cross section area of filed conductor
Af= ρLmt ATfl/Ef
Step 5:Calcualate diameter of field conductor
dfc =Ѵ(4af/π)
Diameter including thickness dfci= dfc + insulation thickness
Copper space factor Sf = 0.75(dfc/dfci)2
Procedure for shunt field designProcedure for shunt field design

16. Step 6 : Determine no. of turns (Tf) and height of coil (hf)
They can be determined by solving the following two
equations 2Lmt(hf+ df) = Ef
2
af/ρLmtTf
Tf.af = Sf.hf.df
Step 7 : Calculate Rf and If : Rf = Tf. ρLmt /af
If= Ef/Rf
Step 8 : Check for δf
δf= If / af
δf – not to exceed 3.5A/mm2
.
If it exceeds then increase a by 5% and then proceed again
Procedure for shunt field designProcedure for shunt field design

17. Step 9 : Check for desired value of AT
ATactual= If.Tf
ATdesired- 1.1 to 1.25 times armature MMF at full load
When ATactual exceeds the desired value then increase the
depth of field winding by 5% and proceed again.
Procedure for shunt field designProcedure for shunt field design

18. Check for temp rise:
Actual copper loss = If
2
Rf
Surface area = S = 2Lmt (hf + df)
Cooling coefficient C = (0.14 to 0.16)/(1 + 0.1 Va)
θm= Actual copper loss X (C/S)
If temperature rise exceeds the limit , then increase the depth
of field winding by 5% and proceed again.