M4‐ DC Machines Unit 1 Dr. Pr abodh Bajpai Associate Professor Indian Institute of Technology Kharagpur
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M4‐ DC Machines
Unit 1
Dr. Prabodh Bajpai
Associate Professor
Indian Institute of Technology Kharagpur
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‐
• ‐
– Constructional Features
– .
– Lap
winding
– ave w n ng
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U1‐ Constructional features of DC
machines
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DC machines
• First rotating electrical machines introduced to drive
locomotives in 1839
• Electromechanical energy conversion device – Operate as generator converting mechanical energy from
pr me mover to e ectr ca energy
– Also operate as motor converting electrical energy into
• Various combination of field winding provide wide
ran e of out ut V‐I and s eed‐tor ue characteristics
• Easy speed control, simpler and flexible drive system
makes wide use in Industry, electric traction, cranes
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Construction of DC Machines
• Stator constitute of field poles, yoke and field
• Rotor constitute of armature core and armature
w n ng
• Commutator segments attached to the shaft of the rotor
• A pair of suitably placed stationary carbon
brushes touching the commutator segments
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Stator
• Stator has projected poles with coils wound over it.
• The field winding are wound around pole core such
that they set up alternate north and south poles.• Pole shoe is curved and wider that pole core in order
to spread the flux more uniformly in the air gap.
• Field winding consist of few turns for series field and large no. of turns for shunt field
• Field coil act as electromagnets that produce flux in
t e a r gap nee e to generate an n uce em
• Yoke is part of frame and provide return path for flux
rom one po e o ano er.
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Rotor
• Double layer lap or wave windings are generally used
for armature.
• All the armature coils are connected in series forming a closed armature circuit.
• As the coils are distributed, the resultant voltage
acting in the closed path is zero thereby ensuring no c rcu a ng curren n e arma ure.
• The junctions of two consecutive coils are terminated
. • Stationary carbon brushes are placed physically under
commutator segments.
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DC Machines
• The rotor has a ring‐shaped laminated iron core with slots.
• The commutator consists of insulated copper segments
.
• Two brushes
are
pressed
to
the
commutator
to
permit
current
flow.
• The brushes are placed in the neutral zone, where the
magnetic field is close to zero, to reduce arcing.
•
adjacent coil,
• The switching requires the interruption of the coil current.
• T e su en
nterrupt on
o
an
n uct ve
current
generates
g
voltages .
• The high voltage produces flashover and arcing between the
8
commutator segment and the brush.
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DC Machine Construction
9Sectional diagram of a DC machine
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How a D.C voltage is obtained across brushes
• The generated voltage in a coil when rotated
alternating in nature.
• To convert t is A.C. vo tage into D.C., a num er
commutator
segments attached to the shaft an a pa r o su ta y p ace stat onary car on
brushes touching the commutator segments.
• They are called mechanical rectifier due to
necessar rectification from A.C to D.C.
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Action of commutator segments and brushes
• A simple d.c machine working as generator with
armature occu in various ositions.
• Armature has got a single rectangular coil with sides 1 and 2 whose terminals firmly joined to commutator
segments C1 and C2 respectively.
• Commutator segments made of copper are insulated by mica insulation and rotate along with the armature.
• Magnitude and polarity of the voltages in armature
conductors in different position may be different
• Brushes are suitably placed for obtaining maximum
vo tage, t e magn tu e o t e vo tage across t e
brushes will remain constant
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Action of commutator segments and brushes
• B1 and B2 are stationary carbon brushes placed over the
rotatin commutator in such a wa that the alwa s
make electrical contact with the commutator segments.
• It is from the two brushes, two terminals are taken out
and called the armature terminals.
• Brushes are kept in brush holders with a spring
arrangement.
• Spring tension is so optimally adjusted that brushes
make good contact with C1 and C2 and at the same time
allows the rotor to move freely.
• Fixed brushes B1 and B2 make periodically contact with
both C1 and C2 as rotor rotates.
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Action of commutator segments and brushes
• Let the armature be driven at a constant angular speed
of ω in the ccw direction.
• In position(i), ωt = 0°, – plane of the coil is vertical i.e., along the reference line
– No induced voltage across B1 and B2 as no flux density
component
is
perpendicular
to
the
tangential
velocity
of
.
• In position(ii), ωt = 45°,
– ,
it will be . Similarly conductor 1 being under S pole, polarity
of the induced voltage in it will be .
– Therefore across B1 and B2 we will get a voltage with B1 being
+ve and B2 being ‐ve.
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Action of commutator segments and brushes
• In position (iii) & (iv), The polarity of the voltage in
conductors 2 and 1 does not change because 2 remains
under
N
pole
and
1
remains
under
S
pole.
• In position (v) to (viii), conductor 2 comes under S pole
and conductor 1 under N pole.
• Therefore polarity of voltage in conductor 1 is while polarity of voltage in conductor 2 is .
• B1 now makes contact with C1 and B2 makes contact
w t .
• Thus polarity of B1 remains +ve as before and that of B2
rema ns –ve una ere .
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Action of commutator segments and brushes
• Polarity of voltage across C1 and C2 will periodically
. C1C2 .
• Polarity of voltage across B1 and B2 will not change with
+ –
V B1B2 always remains unidirectional .
• Brushes are not associated with a fixed conductor but
they make contact with different conductors when they
come at some fixed position in space.
• Value of B is not constant under a pole, it is sinusoidally
distributed therefore magnitude of V C1C2 and V B1B2 does
not remain constant, since e = Blv
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VC1C2
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Armature winding terminology
• A coil has two coil sides occupying two distinct specified
slots and spacing between them is Coil span, defined as
ration
of
S
and
P• Double layer winding house two coil sides in each slot
belonging to two different coils
• For a double layer winding total number of coils must be equal to the total number of slots.
• Coil sides of a coil are numbered depending on the slot
num ers n w c t ese are p ace
• Number of commutator segments must be equal to S,
num er o s o s
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Armature winding terminology
• Separation of coil sides of a coil in terms of number of
commutator se ments is commutator itch .
• For lap winding y c = ±1 and for wave winding PS yc
12
• Complete windings diagram shows
– Positions of coil sides in slots,
– interconnection of the coils through commutator segments
using appropriate numbering of slots,
–
– commutator segments
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Armature winding types
• Practical D.C. machine armature have large number of
slots housing many coils along with a large number of
commu a or segmen s.
• Each coil ends are terminated on two commutator
.
• Armature windings may be of different types (lap/ wave)
–
segments, lap connected armature winding is obtained.
– when ends of a coil are terminated on segments apart by
approx. two pole pitch, a wave connected winding results.
• Number of parallel paths in armature,
– a = P for LAP winding.
– a = 2 for WAVE winding
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Lap type Armature winding
• Second coil 2 ‐ 6' is in the lap of the first coil 1 ‐ 5', hence
the winding is called lap winding.
• Winding proceeding from left to right (assuming y c =+1) called progressive simplex lap winding.
• For y c = ‐1, winding would proceeded from right to left
• A winding table have information of number of coil sides
where the free ends of the coil sides will be terminated
• Number of brushes must be equal to number of poles.
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The spatial distance between two coil sides of a coil should be
one pole pitch apart
,
equal to the number poles P.
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Wave type Armature winding
• In this winding coil sides of a coil is not terminated in
adjacent commutator segments, i.e., y c ≠1.
• Instead y c is selected closely equal to two pole pitch in terms of commutator segments. Mathematically y c ≈
2S/P
• Windin ro resses like a wave hence called wave
winding
• ,
multiple of P. It will be progressive wave winding if +ve
si n is taken and retro ressive wave windin if –ve si n
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Simplex wave winding
For a P polar simplex wave winding, between any two
consecutive commutator segments P/2 coils will be present
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Comparison of Lap and wave winding
• For a given current density and conductor cross section
– Total armature current obtained in lap winding is P/2 times
a o a ne n wave w n ng
• For given number of armature conductors
– No. of conductors connected in series per path in wave
winding is P/2 times that in lap winding
–
brushes in wave winding is P/2 times that in lap winding
machines