Topic 2. Development and
Design of DC machines
Contents:
Construction, Voltage formula and Output Equation, Armature
winding and insulation and The magnetic circuit
Armature reaction and field winding design, Commutation and
commutating pole design
Losses, efficiency and temperature rise and Characteristic dc
machine
CONSTRUCTION
Direct current generators and motors may be divided
into three general classes:
1.
The non-commutating-pole machine, use only for
generators and motors for low voltages and small
capacities.
2.
The commutating pole machine is built with small
poles between the main poles and is magnetized by
a winding in series with the armature.
3.
The compensated machine, as a modified
commutating-pole machine.
The type of construction generally used for direct
current generators and motor
Figure 1 Assembly drawing of direct current generator 300 kW, 900 r.p.m., 250 Volt.

The spider of a direct current generator or motor is the frame
upon which the armature laminations are assembled. By
designing the spider with large axial ventilating ducts, good
ventilation of the inside of the armature is obtained and the
weight of the armature is kept small. The spider for large
machine is either a steel casting or is fabricated from rolled
steel.
Figure 2 shows a cast
steel spider of a largediameter, slow speed
machine.
Figure 3 is used, that is, the spider is part
of the armature lamination.

The armature of direct current generators and motors is built up of
electric sheet steel laminations varying in thickness from 0.0141 to 0.025
in. The laminations are punched to correct size by means of dies, carefully
annealed and insulated. The usual method of insulating the armature
punching is that of applying a thin coat of core plate varnish to each side
of the punching. Paper is sometimes used to insulate the armature
laminations from one another. The paper is applied to the sheet steel
before it is punched out. The insulated armature punching is assembled on
the spider between two end plates. The tooth support generally consists of
a piece of rolled steel, spot welded to the end lamination. The armature
coils are placed into the slots with the required amount of insulation
between armature iron and coils, and the slots are sealed with wedges. The
type of wedge generally used is of horn fiber impregnated with paraffin.
a
Figure 4 One segment for large diameter
armature with welded duct spacer.
b
Figure 5.a and b. The position and
thickness of the wedge.
Bands of phosphor bronze or steel wire are used to hold the armature coil end
connections in position. The slots are not always sealed by wedges; they are
sometimes left open and the coils held in place by phosphor bronze or steel
band wires as Figure 6 shows.
Figure 6 Complete armature for 7 1/2 h.p., 1750 r.p.m., 230 Volt, 4 pole, shunt wound motor

The commutator is built up of hard drawn, cooper
segments, insulated from one another by mica. The
thickness of the mica insulation varies from 0.02 to
0.06 in. and depends upon the diameter of the
commutator and the voltage between adjacent
segments.
Figure 7 Armature and commutator assembly, 50 h.p., 850 r.p.m., shunt wound motor.


Field poles
The main poles of most modern
machines ate built up of sheet steel
laminations, usually from 0.025 to 0.05
in. thick. The laminations are riveted
together with no insulation between
them. Figure 8 shows the usual shape
of the laminations, with pole body and
pole shoe punched in one piece. The
objection to the cast steel pole
construction lies in the fact that it is
difficult to obtain castings of uniform
material and free from detects. With
open armature slots, the type generally
used for direct current motors and
generators, cast steel pole shoes can
not be use, because of the excessive
eddy current losses in the pole face due
to the air gap flux pulsations produced
by the armature slots.
Figure 8 Detail drawing of pole
punching, 50 kW, 1200 r.p.m.,
generator


Field Yoke
The yoke is frame to which the field poles are bolted
(see Figure 9). The section of the yoke must have the
required area for the flux and must also have the
required mechanical strength to support the machine.
Figure 9 Field yoke with partially assembled
field poles, 7 1/2 h.p., 4 pole, motor

The bearings of most modern direct current
machines are of the ring oiling type. The
bronze bearing is generally preferred for small
machines; for large motors and for the larger
generators Babbitt bearings are used.
Figure 10 cross section of
sleeve bearing and bearing
housing


Brush Holder and Brush Yoke
Figure 11 shown the brush holders are mounted on
studs or arms which are generally bras rods, from ½
to 1 in. in diameter. The brush studs are pressed into
openings properly spaced in the bearing bracket.
Figure 12 shown one type of brush yoke with brush
arms, brush holders, and brushes.
Figure 11 Brush holders with brush
Figure 12 Brush yoke


Base
All belted type motors and generators are mounted
on a belt- tightener base or on rails. The base or rails
are bolted down, and the machine can be moved on
the base by means of a ratchet device.
Figure 13
Belt tightener base
VOLTAGE FORMULA AND
OUTPUT EQUATION
Voltage Formula :
E
pNn
a  60 108
Volt
where E is the voltage induced in the armature winding between adjacent brushes.
The flux per pole,
Ea  60  108

Nnp
The hypothetical total flux,
t 
lines
p
fd
where fd =the field form distribution factor, which it is the ratio of the area under the
true to the area of the hypothetical rectangular field form.
8
Ea

60

10
t =
Nnf d
lines

Output Equation
The armature output of a direct current generator, expressed in kilowatts, is as follows:
Kwa = E Ia X 10-3
[kW]
Substituting with E, so
Kwa =
t Nnf d I a
a  60  1011
The total flux is equal to the product of gap area times the maximum air gap density,
t = πDlBg
lines.
If Q equals the ampere conductors per inch of armature periphery,
So,
Kwa =
Q=
NI a
aD
nf d  2 D 2 QB g l
60  1011
The value of Bg, the maximum air gap density, limited by the permissible value of Bt2, the
maximum tooth density.
Bt2 =
B g D
t 2 k1 S
lines per sq. in.
Air gap Density, Kilo-Lines per sq. in.
Air gap densities that may be used for preliminary design may be
taken from the curve, Figure 14.
70
Scale B
60
50
Scale A
40
30
20
1
2
3
4
5 6 7 8 9 10
2
3
4
100
2
3
4
5 6 7 8 9 1000
2
Kw
 10 3
n
3
4
5 6 7 8 9 100
Scale A
5 6 7 8 9 10000
Scale B
Figure 14. Air gap densities for direct current generators and motors
Average values of Q for commutating pole machines are
given in Figure 15.
1300
1200
Ampere Conductors per inch of
Armature circumference

1100
1000
Scale B
900
800
700
Scale A
600
500
400
300
1
100
2
3
4
5 6 7 8 9 10
2
3
4
2
3
4
5 6 7 8 9 1000
2
Kw
 10 3
n
3
4
5 6 7 8 9 100
Scale A
5 6 7 8 9 10000
Scale B
Figure 15 Ampere conductors per inch of armature circumference for
commutating pole, direct current generators and motors

Average values of the output constant for commutating pole
machines for 50o C. rating may be taken from the curves,
Figure 16.
Output Constant
10 x 104
9
8
7
6
5
4
Scale B
3
2.5
2
Scale A
1.5
1
1.5 2 2.5 3
4
5 6 7 8 9 10 1.5 2 2.5 3
4
100 1.5 2 2.5 3
4
5 6 7 8 9 10001.5 2 2.5 3
Kw
 10 3
n
4
1
5 6 7 8 9 100
Scale A
5 6 7 8 9 10000
Scale B
Figure 16. Output constants for commutating pole, direct current generators and motors









The induced voltage in a motor armature
E = ET – IaRc volts
Multiplying both sides of this equation by Ia gives
EIa = ETIa – Ia2Rc watts
ETIa is the power delivered to the armature, and EIa
is the power developed. Subtracting core loss and
friction and windage losses from the developed
power gives the power available at the shaft.
The developed power,
EIa = TD  n  746 watts
5250
The developed torque equation,
TD = = D2lBgQfd1.16 x 10 -8 lb. ft.


Armature Peripheral Speed
The diameter and length of the armature should be so chosen,
whenever possible, that the peripheral speed of the armature
will not exceed 6000 ft. per min., as high peripheral speed
lead to expensive constructions and commutation difficulties.
For generators for direct connection to steam turbines, the
peripheral velocity of the armature may be 15,000 to 20,000
ft. per min. Such generators require special construction and
very careful design of the commutating field. Except for turbo
generators, the peripheral velocity of direct current generators
and motors is generally from 1200 to 6000 ft. per min.


Armature Diameter and Length
When the output constant is known, the product D2l
is readily found. Either the diameter or the length
may be assumed and the other dimension calculated.
For high speed machine, the diameter is limited by
the peripheral velocity.
D=
3
KwapC
n (1.5 to 3.14)
Number of Poles


In general, the number of poles should be so chosen that good
operating, characteristic are obtained with minimum weight
of active material and minimum cost of construction.
The frequency of the currents in the armature conductors
and of the flux reversals in the armature core is directly
proportional to the number of poles and speed.
Table 1 Medium and High speed
Table 2 Slow Speed Engine Type
Output (kW)
Speed (r.p.m)
No. of Poles
Output (kW)
Speed (r.p.m)
No. of Poles
Up to 2
2 to 100
50 to 300
200 to 600
600 to 1000
Over 1300
Up to 1300
Up to 1000
Up to 600
Up to 500
2
4
4 or 6
6 to 10
8 to 12
35 to 150
200 to 250
250 to 500
225 to 300
135 to 225
100 to 150
6
8
10
Design of the pole shoe


The air gap flux distribution curve must have such shape that the best
possible commutation will result. To obtain good commutation, the flux
density in the air gap must decrease gradually from maximum value under
the center of the pole to zero on the enter line between two poles, and the
flux densities near the neutral point must be low. A field form that drops
off rapidly from maximum value to zero not only leads to commutation
difficulties but may also give rise to magnetic noises in machines with
slotted armatures.
The shape of the field form depends upon the shape of the pole shoe
and the percent pole embrace. The ratio of the pole arc on the armature
surface to the pole pitch on the armature surface, expressed in percent, is
called the percent pole embrace. For direct current machines, 60 to 75
percent pole embrace is generally satisfactory.

A good air gap flux distribution curve is obtained
with the shape of pole shoe shown in Figure17.
Figure 17
Construction of No Load Field Form

The useful flux per pole, in passing from the pole shoe into
the armature, spreads out over the entire pole pitch. The flux
will distribute itself in the air gap in such a way that the total
reluctance will be a minimum. The flux path in the air gap
under the pole may be assumed to be divided in tubes of
force, as shown in Figure 18, each tube being of unit length in
the direction parallel to the shaft.
Figure 18
For this construction the length of the air gap must be known;
it may be estimated with the help of the curve Figure 19.
Air Gap Length In.

0.30
0.25
0.20
0.15
0.10
0.05
0
10
20
30
40
50
60
70
80
90
Armature Diameter In.
Figure 19 Approximate air gap lengths for direct currents generator and motor

For a larger number of squares at the pole ip, the
ratio of the sides of the squares must be multiplied
by the ratio of the number of squares. The flux plot
for a 300 kW, 900 rpm, direct current generator is
shown in Figure 20.
Figure 20 Flux plot for 300 kW generator
Air gap Flux Distribution Factor

The definition of the air gap flux distribution
factor has been given above as the ratio of the
area under the flux distribution curve to the
area of a rectangle having the same base and
maximum ordinate.

Figure 21 shows the air gap flux distribution curve
and the calculations for the average ordinate and flux
distribution constant
1=100
3=100
5=100
7=100
9=100
11=98
B.av
66.5
13=85
fd = ----------- = ------- = 0.665
15=65
B max.
100
17=30
19=12
21=6
23=2
-----798
B.av.= ----- = 66.5
12
D
C
E
100
B
90
80
70
60
50
40
30
20
10
F
24 23 22 21 2019 18 1716 15 14 13 12 1110 9 8 7 6 5 4 3 2 1 0
Figure 21 Flux distribution curve for flux plot shown in Figure 20
Sample Design

A 300 kW, 900 rpm, 230 volt, compound wound,
and direct current generator is to be designed. The
generator is to be part of a synchronous motorgenerator set, to have commutating poles, and be
over compounded to give a full load voltage equal to
250 volts. The efficiency of the generator should not
be less than 92.0 percent at full load and normal
voltage, and is to be calculated from the losses in
accordance with the AIEE Standards. The
temperature rise of no part of the generator should
exceed 50o C. when operating at full load
continuously.
kW
 10 3
n


300
 10 3  333
900
=
The output constant from the curve Figure 16 is 1.97 x 104.
Table 1 show that the best design can generally be obtained with 6 poles for
machine of this size and speed.
D= 3
KwapC
n (1.5 to 3.14)
=
3
300  1.97  10 4  6
900(1.5to3.14)
= 29.8 to 23.1 in.
The corresponding values for l, the length of the armature,
l=
Kwa C
nD
2
300  1.7  10 4
=
900  29.8 2 to23.12
= 7.39 to 12.30 in.
The peripheral speed or 29.8 in. armature diameter is:
3.14  29.8  900
Dn
v=
= 7020 ft. per min.
12
12
and for 23.1 in. armature diameter it is 5450 ft. per min.

In order to avoid expensive constructions, it is generally
desirable to use peripheral of 6000 ft. per min. or less.
Therefore, an armature diameter of 25 in. is chosen for this
design.
The peripheral speed will then be
v=
Dn
=
12
3.14  25  900
12
= 5890 ft. per min
The length of the armature
l=
Kwa C
nD
=
2
300  1.7  10 4
900  25 2
= 10.5 in.
The frequency of the flux reversals in the armature core
f=
pn
2  60
=
6  900
2  60
= 45 cycles per sec.
The pole pitch the armature circumference
D
  25
τ=
= 13.10 in.
p =
6







Choosing 66 percent pole embrace, the pole arc on
the armature circumference
B = τ x 0.66 = 13.10 x 0.66 =8.64; use 8 in.
5
The length of the air gap is taken equal to 0.178 in.
(see curve Figure 19).
The shape of the pole shoe is made the same as
shown in Figure 17.
The Flux plot is shown in Figure 20
And the flux distribution curve in Figure 21.
The air gap flux distribution factor is 0.665.