Design of field winding of shunt motor

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Design of field winding of shunt motor

• Introduction

• Field winding

• Types of Field winding

• Design of shunt field winding

Contents

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.

Design of shunt field winding Design 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

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 = πdp2 /4 /Ap4dp

Design of shunt field winding Design of shunt field winding

When rectangular poles employed, length of pole is chosen as 10 to15 mm less than the length of armatureLp=L –(0.001 to 0.015)Net iron length Lpi = 0.9 LpWidth of pole, bp = Ap/Lpi

Height of pole body hp = hf + thickness of insulation and clearance Total height of the pole hpl = hp + hs


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.


Lmt - Calculated using the dimensions of pole and depth of the coil For rectangular coils

Lmt =2(Lp + bp + 2df) or (Lo +Li)/2Where 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


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.


• 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]


Substitute S in Qf ,

Qf = 2Lmt (hf +df).qf

Actual Cu loss in field coil=If2Rf=Ef

2/Rf

Substituting Rf=(Lmt Tf)/ af ,

Actual Cu loss in field coil=Ef2 .af /(Lmt Tf)


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Procedure for shunt field designProcedure for shunt field designStep1 : determine the dimensions of the pole. Assume a suitable

value of leakage coefficient and B = 1.2 to 1.7 T

Фp= Cl. ФAp = Фp/Bp

When circular poles are employed, cross section area will be a circle

Ap = πdp2 /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

Width of pole = Ap/Lpi

Step 2 : Determine Lmt of field coilAssume suitable depth of field windingFor 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/PStep 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

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/ρLmt Tf

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 af by 5% and then proceed again


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.


Check for temp rise:

Actual copper loss = If2 Rf

Surface area = S = 2Lmt (hf + df)

Cooling coefficient C = (0.14 to 0.16)/(1 + 0.1 Va)

m = Actual copper loss X (C/S)

If temperature rise exceeds the limit , then increase the depth of field winding by 5% and proceed again.

Engineering

Design of field winding of shunt motor