Course Name / Code
Degree/Branch
Semester / Section
UNIT I
: Design of Electrical Machines/ EE8002
: B.E/ EEE
: VI
DESIGN OF FIELD SYSTEM AND ARMATURE
Major considerations in Electrical Machine Design – Materials for Electrical apparatus – Design of Magnetic
circuits – Magnetising current – Flux leakage – Leakage in Armature. Design of lap winding and wave
winding.
PART – A
1. Define specific magnetic loading. [M/J 2016]
The average flux density over the air gap of a machine is known as specific magnetic loading.
Total flux around the airgap
P
Bav 

, Wb / m 2 or tesla
Area of flux path of the airgap DL
2. Mention the various duty cycles of a motor. [M/J 2016], [M/J 2017], [N/D 2018]
i) Continuous Duty.
ii) Short time duty.
iii) Intermittent periodic duty.
iv) Intermittent periodic duty with starting.
v) Intermittent periodic duty with starting and braking.
vi) Continuous duty with periodic duty.
vii) Continuous duty with starting and braking.
viii) Continuous duty with speed changes.
3. Define specific electric loading. [N/D 2016]
The number of armature or stator ampere conductors per metre of armature periphery at the air
gap is known as specific electric loading.
I Z
ac  Z , amp.cond / m
D
Where, Iz current through conductor in each parallel path,
Z total number of armature conductors.
D Diameter of armature or stator bore.
4. What are the factors that affect the size of rotating machines? [N/D 2016]
i) Speed
ii) Electric and Magnetic loadings.
5. What are the electrical properties of insulating materials? [M/J 2017], [A/M 2019]
i) Specific resistance.
ii) Thermal resistance.
iii) Dielectric strength or hysteresis.
iv)Mechanical strength.
v) Permittivity.
vi) High insulation resistance.
6. Define short time rating. [N/D 2017]
A rating defining the load that a machine, apparatus, or device can carry for a specified short time.
7. List the standard specifications of transformer. [N/D 2017]
IS : 1180 – 1964: Specifications for outdoor type three phase distribution transformers upto and
including 100KVA and voltage ratings upto 11KV.
IS : 2026 – 1962: Specifications for power transformers beyond 100 KVA.
IS : 2705 – 1964 : Specifications for Current Transformers.
8. What are the major considerations to evolve a good design of electrical machine?
[M/J 2018]
Specific Electric Loading, Specific Magnetic Loading, temperature rise, efficiency, length of air
gap and power factor.
9. What are the different types of heat transfer methods found in electrical machines?
[M/J 2018]
i) Heat Dissipation by conduction
ii) Heat Dissipation by Radiation
iii)Heat Dissipation by convection.
10. What are the different conducting materials used in rotating machines? [N/D 2018]
i)Copper ii) Aluminium iii) Iron and steel iv) Bronze
v)Brass vi)Copper Silver alloy
11) What is meant by peripheral speed? Write the expression for peripheral speed of a
rotating machine. [M/J 2012]
The peripheral speed is translational speed is that may exist at the surface of rotor
while it is rotating.
Peripheral speed = Va = πDrn(m/s)
Dr = Diameter of rotor (m)
n = Speed of rotor (rps)
12) What are the important specifications of a DC machines as per IS 4722:1968? [N/D 2012]
1. Generator or motor, 2. Field excitation, 3. Type of field excitation, 4. Field exciting
voltage and current, 5. Rated output power, 6. Rated voltage, 7. Type of enclosure, 8.
Type of duty, 9.Rated current, 10. Speed
13) How materials are classified according to their degree of magnetism? [M/J 2011]
Classification of based on their degree of magnetism:
Ferromagnetic material
Paramagnetic material
Diamagnetic material
14) Define Space factor. [A/M 2019]
It is defined as the ratio of copper or conductor area to the total winding area.
Copper or conductor area
Space factor(S f ) =
total winding area
15. Write down the Carter’s Coefficient of D.C. machine. [A/M 2019]
The carter’s coefficient is a parameter that can be used to estimate the contracted or effective slot
pitch in case of armature with open or semi closed slots. It is the function of the ratio W 0/lg where,
W0 is slot opening and lg is air gap length.
W
k cs  0
lg
The carter’s gap coefficient (kcs) is the ratio of slot width to gap length. The formula which gives
value of kcs directly is
kcs 
For parallel side open slots,
k cs 
1
 lg 
1  5 
 ws 
2


 y 1
 log (1  y 2 ) 
 y 

  tan 1 

16. Define field form factor. [M/J 2016]
It is defined as the ratio of average gap density over the pole pitch to the maximum flux density in
B
B
Pole arc
L

the gap. K f  av  av . Also K f   
Pole Pitch 
Bg
Bm
17. What is real and apparent flux density? [M/J 2016]
The real flux density is due to the actual flux through a tooth. The apparent flux density is d\ue to
total flux that has to be passing through the tooth. Since some of the fluxes pass through slot so that
the real flux density is always less than the apparent flux density.
Apparent flux density Bapp  Total flux in a slot pitch
Tooth area
Real flux density
Breal 
Actual flux in a tooth
Tooth area
PART – B& C
Major considerations in Electrical Machine Design
1. What are the major considerations to evolve a good design of electrical machine? (8) [M/J 2017]
Materials for Electrical apparatus
2. State the different kinds of magnetic materials used in the construction of rotating machines and
transformers. Point out salient features. (8)
3. What are the main groups of electrical conducting materials? Describe the properties and
applications of those materials. (10)
Insulating Materials
4. Classify the insulating materials based on thermal consideration. (8) [M/J 2017], [M/J 2018]
5. Discuss in detail the desirable properties and classification of insulating materials used in rotating
machines. (13) [N/D 2018]
6. Describe the methods of measurements of temperature rise in various parts of an electrical
machine. [A/M 2019]
Choice of Specific Electrical and Magnetic loadings
7. Discuss in detail the factors affecting the choice of specific electric and magnetic loading in
rotating machines. (16) [N/D 2017], [N/D 2018], [A/M 2019]
8. A 350 KW, 500 V, 450 rpm, 6 pole DC generator is built with an armature diameter of 0.87 m
and core length of 0.32 m. The lap wound armature has 660 conductors. Calculate the specific
electric and magnetic loadings. (8) [M/J 2018]
Design of Magnetic circuits – Magnetizing current – Flux leakage – Leakage in Armature
9. Determine the air gap length of a DC machine from the following data. Gross core length=0.12m,
No.of ducts=one of 10 mm width, slot pitch=25mm, slot width=10mm, Carters coefficient for slots
and ducts=0.32, gap density at pole center=0.7T. Field mmf per pole=3900AT, mmf require for iron
parts of magnetic circuit=800AT. [M/J 2016], [M/J 2017], [N/D 2018] (16)
10. Calculate the mmf required for the air gap of a machine having core length 0.32m including 4
ducts of 10 mm each. Pole arc=0.19m, slot pitch=65.4m, slot opening=5mm, air gap length= 5 mm,
flux per pole=52mwb. Given Carter’s coefficient is 0.18 for opening/gap=1 and is 0.28 opening per
gap=2. (13) [N/D 2016], [A/M 2019]
11. Derive the expressions for reluctance of air gap in machines with smooth armature and slotted
armature. (6) [M/J 2017]
12. Derive an expression for the mmf of air gap of a machine with slotted armature and ventilating
ducts. (8) [N/D 2017], [N/D 2018]
13. A 15 KW, 230 V, 4pole DC machine has the following data: Armature diameter=0.25 m;
armature core length=0.125 m; Length of air gap at pole centre=2.5 mm;flux per pole=11.7Х10-3 Wb
Pole arc to pole pitch ratio=0.66. Calculate the mmf required for the air gap i) if the armature surface
is treated as smooth ii) if the armature is slotted and the gap contraction factor is 1.18. (8) [N/D2017]
Net length of Iron –Real & Apparent flux densities
14.
Calculate the apparent flux density at a section of the teeth of an armature of a DC machine
from the following data at that section. Slot pitch=24mm, slot width=tooth width=12mm, length of
armature core including five ducts of 10 mm each=0.38m, iron stacking factor =0.92. True flux
density in the teeth at that section is 2.2T for which the mmf is 70000AT/m. [M/J 2016]
15.
Calculate the apparent flux density at a particular section of tooth from the following data:
Tooth width=12 mm, slot width=10mm, gross core length=0.32m, No.of ventilating ducts=4 with
each 10mm wide, real flux density=2.2wb/m2. Permeability of teeth corresponding to real flux
density=31.4Х10-6 H/m. Stacking factor=0.9. [N/D 2016] (8)
Design of lap winding and wave winding.
16. Write down the procedure for design the lap and wave wining.
UNIT – III – DESIGN OF DC MACHINES
Construction - Output Equations – Main Dimensions – Choice of specific loadings – Selection of
number of poles – Design of Armature – Design of commutator and brushes – design of field
Computer program: Design of Armature main dimensions
PART – A
1. What is real and apparent flux density? [M/J 2016]
The real flux density is due to the actual flux through a tooth. The apparent flux density is d\ue to
total flux that has to be passing through the tooth. Since some of the fluxes pass through slot so that
the real flux density is always less than the apparent flux density.
Apparent flux density Bapp  Total flux in a slot pitch
Tooth area
Real flux density
Breal
Actual flux in a tooth

Tooth area
2. Define field form factor. [M/J 2016]
It is defined as the ratio of average gap density over the pole pitch to the maximum flux density in
B
B
Pole arc
L

the gap. K f  av  av . Also K f   
Pole Pitch 
Bg
Bm
3. Write the expression for output coefficient of DC machine. [N/D 2016]
Output coefficient Co=Π2Bav ac Х 10-3.
Where, Bav Specific Magnetic Loading ac Specific Magnetic Loading
4. Mention guiding factors for the selection of number of poles. [N/D 2016]
i) The frequency should lie between 25 to 50 Hz.
ii) The value of current per parallel path is limited to 200A, thus the current per brush armature
should not be more than 400A.
iii) The armature mmf should not be too large. The mmf per pole should be in the range 5000
to 12500 AT.
iv) Choose the largest value of poles which satisfies the above three conditions.
5. Distinguish between real and apparent flux densities in the tooth section of slot. [M/J 2017]
Apparent Flux Density
It is defined as, Bapp 
Total flux in a slot pitch
Tooth area
Bapp=Breal+4Π Х 10-7(Ks-1)
Where Ks=(L ys)/(LiW t)
Real Flux Density
It is defined as, Breal 
Actual flux in a tooth
Tooth area
Since some of the flux passes at real (Ks-1)
through slot, the real flux density is always less
than the apparent flux density.
6. Write down the expression for brush friction losses. [M/J 2017]
Brush friction loss Pbf = µ Pb P Ab Vc .
Where, µ Coefficient of friction
Pb Brush contact pressure on commutator.
P Number of poles
AbEffective area af all brushes
Vc Peripheral speed of commutator.
7. Define specific electric and magnetic loading. [N/D 2017], [N/D 2018]
Specific Electric Loading:
The number of armature or stator ampere conductors per metre of armature periphery at the air
gap is known as specific electric loading.
I Z
ac  Z , amp.cond / m
D
Where, Iz current through conductor in each parallel path,
Z total number of armature conductors.
D Diameter of armature or stator bore.
Specific Magnetic Loading:
The average flux density over the air gap of a machine is known as specific magnetic loading.
Bav 
Total flux around the airgap
P

, Wb / m 2 or tesla
Area of flux path of the airgap DL
8. State the advantages of having larger number of poles in a DC machine. [N/D 2017]
When increasing the number of poles in machine which reduces the weight of iron parts, weight
of copper, Length of commutator and distortion of field form.
9. Mention the two types of armature winding used in dc machine and compare. [M/J 2018]
i) Lap winding
ii) Wave winding
10. What factor decides the minimum number of armature coils? [M/J 2018]
The maximum voltage between adjacent commutator segments decides the minimum number of
coils.
Minimum Number of coils C= (EP)/15.
11. What are the constituents of magnetic circuits in a DC machine? [N/D 2018]
The various elements in the flux path of salient pole machines are poles, poles shoes, air gap,
armature teeth, armature core and yoke. The various elements in the flux path of non-salient pole
machines are stator core, stator teeth, air gap, rotor teeth and rotor core.
12. What are the factors to be considered in the design of commutator of a DC machine? [A/M
2019]
 Peripheral speed
 Voltage between adjacent segments
 Number of coils in the armature
 Number of brushes
 Commutator losses
PART – B & C
Output Equations
17.
Derive the output equation of a dc machine and point out the salient features. (8) [M/J 2016],
[A/M 2019]
Main Dimensions - Choice of Specific Electric and Magnetic Loading
18. Determine the main dimensions of a 80 KW, 4 pole, 600 rpm dc shunt generator, the full load
terminal voltage being 220 V. The maximum gap density is 0.75 Wb/m2 and ampere conductors per
metre are 27000. Assume a square pole face. (8) [M/J 2017]
19.
Find the main dimensions and the number of poles of a 37KW, 230V, 1400 rpm shunt motor,
so that a square pole face is obtained. The average gap density is 0.5 wb/m 2 and the ampere
conductors per meter are 22000. The ratio of pole arc to pole pitch is 0.7 and the full load efficiency
is 90%. [M/J 2016], [M/J 2018] (16)
20.
State and explain the factors which govern the choice of specific magnetic loading in a DC
machine. [M/J 2016] (8)
21.
A design is required for a 50KW, 4 pole, 600 rpm, dc shunt generator, the full load terminal
voltage is 220V. If the maximum gap density is 0.83wb/m2 and the armature ampere conductors per
metre are 30000. Calculate suitable dimensions of armature core to give a square pole face. Assume
that the full load armature voltage drop is 3 percent of the rated terminal voltage and the field current
is 1 percent of rated full load current. Ratio of pole arc to pole pitch is 0.67.
22.
A 5KW, 250V, 4 pole, 1500 rpm DC shunt generator is designed to have a square pole face.
The specific Magnetic loading and specific electric loading are 0.42wb/m2 and 15000 ac/m
respectively. Find the main dimensions of the machines. Assume full load efficiency is 87% and
pole arc to pole pitch ratio is 0.66. (8) [A/M 2019]
Selection of number of poles
23. What are the advantages and disadvantages of large number of poles in a dc machine? (5)
[M/J2018]
24. Explain the procedure for the selection of number of poles in dc machine. (16) [N/D 2016]
25. Find the main dimensions, number of poles and length of air gap of a 600KW, 500V, 900 rpm
generator. Assume average gap density is 0.6wb/m2 and ampere conductors per metre 35000.
The ratio of pole arc to pole pitch is 0.75 and the efficiency is 85%. The following are the
design constraints: Peripheral speed should not be greater than 40 m/s, frequency of flux
reversal should not be greater than 50Hz, current per brush arm should not be greater than
400A, armature mmf per pole should not be greater than 7800AT. The mmf required for air gap
is 60percent of armature mmf and gap contraction factor.
Design of Armature
26. For a preliminary design of a 50HP, 230 V, 1400 rpm, dc shunt motor. Calculate the armature
diameter and core length, the number of poles and peripheral speed. Take Bav=0.5 wb/m2,
ac/m=25000, efficiency=0.9.(13) [N/D 2016]
27. Determine the diameter and length of armature core for a 55KW, 110 V, 1000 rpm, 4 pole shunt
generator assuming specific electric and magnetic loading of 26000 ampere conductor per metre and
0.5Wb/m2 respectively. The pole arc should be about 70% of pole pitch and length of core about 1.1
times the pole arc. Allow 10 ampere for the field current and assume a voltage drop of 4 V for the
armature circuit. Specify the winding used and also determine the values of the number of armature
conductors and number of armature slots. (16) [N/D 2017], [N/D 2018]
28. Explain the various steps involved in the design of armature winding of DC machine. (15) [A/M
2019]
Design of commutator and brushes
29. Design a suitable commutator for a 350 KW, 600 rpm, 440 V, 6 pole dc generator having an
armature diameter of 0.75 m. The number of coils is 288. Assume suitable values whenever
necessary. (8) [M/J 2018]
30. The armature of a 10 pole 1000KW, 500 V, 300 rpm DC generator has a diameter of 1.6m.
There are 450 coils. Determine suitable length and diameter of the commutator, giving details of
brushes having regard to commutation conditions and temperature rise. The design limitations are;
peripheral speed of commutator ≤ 20 m/s, pitch of segments ≥ 4, current per brush ≤ 70 A,
Temperature rise ≤ 400C. The other data given are: The brushes span three segments approximately,
brush contact drop 1.5 V, coefficient of friction 1.5, brush pressure 20 KN/m2. Cooling
coefficient=0.012/(1+0.1Vc). Make suitable assumptions for clearance between brushes, staggering
of brushes and end play. (15) [N/D 2018]