Chapter: Design of Electrical Machines : DC Machines
Important Short Questions and Answers: Design of Electrical Machines - DC Machines
D.C. MACHINES
1. Define gap expansion factor and give the
equation for it.
The ratio of reluctance of flux path
when armature with slot to reluctance of flux path when armature without slot.
Kgs
= Ys / (Ys - Kcs Ws) > 1 slots
Kgd
= L / (L-Kcd nd Wd) >1 ducts
2.
What is the advantage of large
number of poles?
Ø
weight
of iron parts decreases
Ø
weight
of copper part decreases
Ø
length
of commutator reduces
Ø
overall
length of machine reduces
Ø
Distortion
of field form becomes less at full load condition.
3.
Why the interlope is used in a dc
machine.
Ø
To
reduce the armature reaction.
Ø
To
improve commutation.
4.
Why the brush is made up of carbon?
Ø
To
reduce spark between brush and commutator.
Ø
To
conduct electric current.
Ø
To
avoid wear and tear due to rubbing.
5.
Define leakage coefficient and give
the equation for it.
The ratio of total flux per pole to
the useful flux per pole is called leakage coefficient or leakage factor.
C1
= Ф p/Ф=1.08 to 1.25
6. Define iron stacking factor.
It is defined as the ratio of net
length of armature to the gross length of the armature.
Ki
= 0.9 to 0.96
7. What is meant by peripheral speed of
armature?
The distance travel by the armature
per unit time is called as peripheral speed.
n = speed in r.p.s.
D = diameter of armature in m
8. Define armature reaction.
The flux produced due to current
flow to the armature conductors opposes the main flux. This phenomenon is known
as armature reaction.
9.
What are the effects of armature
reaction?
Ø
Reduction
in emf
Ø
Increase
in iron loss
Ø
Sparking
and ring fire
Ø
Delayed
commutation
10.
What does staggering of brushes
mean?
Brushes are provided in different
planes instead of same plane at the surface of commutator to avoid the
formation of ridges. This is called staggering.
11.
Mention the different modes of
operation of a D.C. Machine.
Generator
mode: In this
mode, the machine is driven by a prime mover with mechanical power converted into electrical power.
Ø
Motor mode: The machine drives a mechanical
load with the electrical power supplied converted
into mechanical power.
Ø
Brake mode: The machine works as a generator
and the electrical power developed is either
pumped back to the supply as in regenerative braking.
12.
State use of a yoke in a D.C.
machine.
The yoke serves as a path for flux
in D.C. machine and it also serve as an enclosure for the machine.
13.
What purpose is served by the pole
shoe in a D.C. machine?
The pole shoes serve the following
purposes:
Ø
They
spread out the flux in the air gap.
Ø
Since
they are of larger cross section, the reluctance of the magnetic path is
reduced.
Ø
They
support the field coils.
14.
Mention the factors that affect the
size of rotating machines.
The factors that affect the size of
rotating machines are:
Ø
Speed
and
Ø
Output
co-efficient
15.
What is known as output equation?
The output of a machine can be
expressed in terms of its main dimensions, specific magnetic and electric
loadings and speed. The equation describing this relationship is known as
output equation.
16.
Derive the output equation of a D.C. machine.
Power developed by armature in KW,
Pa = Generated emf *
armature current * 10-3
Pa = (П D L Bav
) (П D ac) n * 10-3
= (П2 Bav ac *
10-3) D2 L n = C0
D2 L n
where C0 = П2 Bav ac * 10-3
D = armature diameter, m L = stator
core length, m n = speed, rps
C0 is the output
co-efficient
16.
How is specific magnetic loading
determined?
The specific magnetic loading is
determined by
Ø
Maximum
flux density in iron parts of machine
Ø
Magnetizing
current and core losses
17.
Calculate the output co-efficient of
a dc shunt generator from the given data. Bg = 0.89 Wb/m2
; ac = 3200 amp.cond/m ; Ψ = 0.66.
Output co-efficient , C0 = П 2 Ψ Bg ac *
10-3
=
П2
* 0.66 * 0.89 * 3200 * 10-3
=
185.5 KW / m3 – rps.
18.
What is the range of specific
magnetic loading in D.C. Machine?
The usual range of specific magnetic
loading in dc machine is 0.4 to 0.8 Wb/m2.
19.
What are the factors to be considered for the selection of number of poles in
dc machine?
The factors to be considered for the
selection of number of poles in dc machine are:
Ø
Frequency
Ø
Weight
of iron parts
Ø
Weight
of copper
Ø
Length
of commutator
Ø
Lab
our charges
Ø
Flash
over and distortion of field mmf
20.
What are the quantities that are
affected by the number of poles?
Weight of iron and copper, length of
commutator and dimension of brushes are the quantities affected by the number
of poles.
21.
List the disadvantages of large number of poles.
The large number of poles results in
increases of the following:
Ø
Frequency
of flux reversals
Ø
Labour
charges
Ø
Possibility
of flash over between brush arms
22.
Mention guiding factors for the
selection of number of poles.
Ø
The
frequency should lie between 25 to 50 Hz.
Ø
The
value of current per parallel path is limited to 200A, thus the current per
brush arm should not be more than 400A.
Ø
The
armature mmf should not be too large. The mmf per pole should be in the range
5000 to 12,500 AT.
Ø
Choose
the largest value of poles which satisfies the above three conditions.
23.
What are the losses at the
commutator surface?
The losses at the commutator surface
are the brush contact losses and brush friction losses.
24.
Write down the expression for brush friction losses.
The brush friction loss is given as
Pbf = µ Pb AB Vc
Where µ = co-efficient of friction
Pb = brush contact
pressure
AB = total contact area
of all brushes, m2
Vc = Peripheral speed of
commutator, m/s
25.
What are the advantages of carbon
brushes?
Ø
They
lubricate and polish the commutator
Ø
If
sparking occurs, they damage the commutator less than with the copper brushes.
Ø
They
provide good commutation.
26.
What is the height occupied by
series field coil in a field pole?
In a field pole of compound machine,
approximately 80% of the height is occupied by shunt field coil and 20% by the
series field coil.
27.
How the Ampere turns of the series field coil is estimated?
In compound machines, the ampere
turns to be developed by the series field coil are estimated as 15 to 25% of
full load armature mmf. In series machines, the ampere turns to be developed by
the series field coil are estimated as 1.15 to 1.25 times of full load armature
mmf.
28.
Mention the factors to be considered
for the design of shunt field coils.
Ø
Mmf
per pole and flux density
Ø
Loss
dissipated from the surface of field coil
Ø
Resistance
of the field coil
Ø
Current
density in the field conductors
29.
State the use of interpoles.
The interpoles are used in D.C.
machines to neutralize the cross magnetizing armature mmf at the interpolar
axis and to neutralize the reactance voltage in the coil undergoing
commutation.
30.
State the relation between the
armature and the commutator diameter for various ratings of D.C. machines. The diameter of the commutator is
chosen as 60 to 80% of armature diameter. The limiting factor is the peripheral speed. The typical choice of
commutator diameter for various voltage ratings are listed here:
Where D is the armature diameter.
31.
Why is the value of magnetizing current not a series design consideration in
D.C.machines?
The value of magnetizing current is
not a series design consideration in D.C.machines as there is sample space on
salient poles to accommodate the required number of field turns.
32.
What should be the peripheral speed of the commutator?
The commutator peripheral speed is
generally kept below 15 m/s. Higher peripheral speeds upto 30m/s are used but
should be avoided wherever possible. The higher commutator peripheral speeds generally
lead to commutation difficulties.
33.
How is the length of commutator
designed?
The length of the commutator is
designed based upon the space required by the brushes and upon the surface
required to dissipate the heat generated by the commutator losses.
Length of commutator, Lc
= nb ( Wb + Cb
)+ C1 + C2
Where nb = number of
brushes per spindle
Wb = width of each brush
Cb = clearance between
the brushes
C1 = clearance allowed for staggering the
brushes
C2 = clearance for allowing the end play
34.
What is the purpose of slot insulation?
The conductors are placed on the
slots in the armature. When the armature rotates, the insulation of the
conductors may damage due to vibrations. This may lead to a short circuit with
armature core if the slots are not insulated.
35.
State any three conditions in
deciding the choice of number of slots for a large D.C.machine.
Ø
The
slot loading should be less than 1500 ampere conductors.
Ø
The
number of slots per pole should be greater than or equal to 9 to avoid
sparking.
Ø
The
slot pitch should lie between 25 to 35 mm.
36.
What are the factors that influence
the choice of commutator diameter?
Ø
Peripheral
speed
Ø
The
peripheral voltage gradient should be limited to 3V/mm
Ø
Number
of coils in the armature
37.
What type of copper is used for
commutator segment?
The commutator segments are made of
hard drawn copper or silver copper (0.05% silver)
38.
What are the materials used for
brushes in D.C.machine?
Ø
Natural
graphite
Ø
Hard
carbon
Ø
Electro
graphite
Ø
Metal
graphite
39.
What are the points to be considered
while fixing up the dimensions of the slot?
Ø
Excessive
flux density
Ø
Flux
pulsations
Ø
Eddy
current loss in conductors
Ø
Reactance
voltage
Ø
Mechanical
difficulties
40.
Mention the factors that govern the
choice of number of armature slots in a d.C.machine.
Ø
Slot
pitch
Ø
Slot
loading
Ø
Commutation
Ø
Suitability
for winding
Ø
Flux
pulsations
41.
What is back pitch?
The distance between top and bottom
coil sides of a coil measured around the back of the armature is called the
back pitch. The back pitch is measured in terms of coil sides.
42.
When are the pulsations and
oscillations of air gap flux reduced to minimum?
The pulsations and oscillations of
air gap flux reduced to minimum when,
Ø
The
number of slots under the pole shoe is equal an integer plus ½.
Ø
The
number of slots per pole is equal to an integer plus ½.
43.
What factor decides the minimum
number of armature coils?
The maximum voltage between adjacent
commutator segments decides the minimum number of coils.
44.
Explain how depth of armature core for a D.C. machine is determined.
Let Ǿ = Flux/pole ; Li =
Net iron length of armature;
Ǿc = Flux in armature
core ; dc = depth of armature core ;
Bc = Flux density in the
armature core ; Ac = Area of cross-section of armature core.
Now Ǿc = Ǿ/2 and Ac
= Ǿc / Bc
Also Ac = Li dc dc = Ǿ / 2 Li Bc
45.
List the characteristics of wave
winding.
Ø
The
number of parallel paths is two.
Ø
The
current through a conductor is Ia / 2 , where Ia is the
armature current.
Ø
The
winding will have less number of conductors with larger area of cross-section
Ø
The
emf induced in both the parallel paths will be always equal
46.
What are the applications of D.C.
special motors?
The D.C. special motors are used in
closed loop control system as power actuators and to provide linear motions.
They are also used as clutches, couplings, eddy current brakes, very high speed
drives, etc.,.
47.
Why square pole is preferred?
If the cross-section of the pole
body is square then the length of the mean turn of field winding is minimum.
Hence to reduce the copper requirement a square cross-section is preferred for
the poles of D.C.machine.
48.
Distinguish between lap and wave windings used in D.C. machine.
The lap and wave windings primarily
differ from each other in the following two factors:
Ø
The
number of circuits between the positive and negative brushes, i.e., number of
parallel paths.
Ø
The
manner in which the coil ends are connected to the commutator Segments.
49.
What are dummy coils?
The coils which are placed in
armature slot for mechanical balance but not connected electrically to the
armature winding are called dummy coils.
50.
What are the different types of commutation?
The different types of commutation
are:
Ø
Resistance
commutation
Ø
Retarded
commutation
Ø
Accelerated
commutation
Ø
Sinusoidal
commutation
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