Design of series field winding in d.c. machine Design of series field winding Design of series field coil

2. Introduction
 Design of series field winding
 Design of series field coil

3. Design of series field winding
 The series field winding is wound with rectangular
conductors.
 The conductors may be flat wound or wound on
edge.
 The insulation between the turns of series field
depends mainly upon whether the coil is flat-
wound or wound on edge.
 In flat wound copper coils, the inter turn insulation
consists of a tape slightly wider than the copper
strip.
 The tape is composed of dry asbestos paper with
thickness of asbestos varying between 0.2 to 0.4
mm in thickness

4.  In flat-wound coils, which usually consist of two or
more sections, the insulation between section is a
washer of about 0.1 to 0.5 mm thickness,
consisting of micanite or of a composite material
made up from asbestos paper and woven glass
cloth polyester varnish bonded and pressed to
size.
 In order to provide the main insulation, the series
field conductors along with those of shunt field are
epoxide moulded or cast resin insulated.

5.  The mmf to be provided on series field at full load
usually lies between 15 to 25 per cent of armature
mmf and for normal machines it may be taken as
20 per cent.
 The current density used for the series field has a
somewhat higher value than that for the shunt field
owing to better cooling conditions.

6.  In compound machines, each pole carries a shunt field
coil and a series field coil.
 In each pole approximately 80% of the height is
occupied by shunt field coil and 20% by series field
coil.
 The series field coil is provided to compensate for the
reduction in field mmf due to armature reaction.
 In case of series machines each pole carries a series
field coil and all the field all the field coils are
connected in series to form series field winding.
 The ampere turns to be developed by the series field
coil in dc series machine is 1.15 to 1.25 times the full
load armature mmf.

7. Design of series field coil
 The step-by-step procedure for design of series
field coil is given below.
 Step-1 :- Estimate the ampere turns to be
developed by series field coil.
Armature ampere turns at full load (per pole) =
current through a turn No. of armature turn
number of poles
= Iz (Z / 2) = IzZ
p 2p
×
×
×

8.  For compound machines,
Ampere turns to be developed by series field coil
ATse = 0.15 to 0.25 (IzZ /
2p)
 For series machines,
Ampere turns to be developed by series field coil
ATse = 1.15 to 1.25 (IzZ / 2p)
×
×

9.  Step-2 :- calculate the num. of turns in the series
field coil.
Number of turns in series field coil ,
Tse = ATse
Ise
where,
Ise = Ia = current through series field
winding at full load.
(wounded to integer)

10.  Step-3 :- determine the area of cross-section of
the series field conductor
Area of cross-section of series field conductor,
ase = Ise
§ςse
where, §ςse = Current density in series field
conductor.
 The current density is chosen as 2 to 2.3 A/mm .
 For low capacity machines circular conductors are
used and for higher capacity machines rectangular
conductors are used.
2

11.  Step-4 :-Estimate the dimensions of the field coil.
 Conductor area in field coil = Tsease
Also, conductor area in field coil =
Copper space factor Height of coil Depth of coil
= s fse hse dse
Where, s fse = copper space factor for series field
coil.
 When circular conductors are employed the value
of copper space factor is 0.6 to 0.7.
× ×

12.  When rectangular conductors are employed, the
space factor depends on thickness and type of
insulation.
 On equating the two equations area of field coil we
get, s fse hse dse = Tse ase
Height of field coil, hse = (Tse ase ) / (s fse dse)
 Choose a suitable depth and calculate the height
of series field coil using the equation.
 In case of compound machine, the total height of
pole required to accommodate field winding will be
sum of the height of shunt field coil and series field
coil.

13.  Step-5 :- Estimate the resistance of series field
coil.
Resistance of series field coil =
= ρ Lmtse Tse
ase
Where, Lmtse = Length of mean tuen of series
field coil.
 The length of mean turn of series field coil can be
estimated from the dimensions of the pole.
Length of mean turn of series field coil,
Lmtse = 2 ( Lp + bp + 2dse ).

14. Thank u