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DC MACHINES
SAJID MEHMOOD ARAIN
09 EL 12
QUEST NAWABSHAH
PAKISTAN
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DC Motor The direct current (dc) machine can be used as
a motor or as a generator.
DC Machine is most often used for a motor. The major advantages of dc machines are theeasy speed and torque regulation.
However, their application is limited to mills,mines and trains. As examples, trolleys and
underground subway cars may use dc motors. In the past, automobiles were equipped with dc
dynamos to charge their batteries.
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DC Motor Even today the starter is a series dc motor
However, the recent development of power
electronics has reduced the use of dc motorsand generators.
The electronically controlled ac drives aregradually replacing the dc motor drives infactories.
Nevertheless, a large number of dc motors arestill used by industry and several thousand aresold annually.
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Construction
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DC Machine Construction
General arrangement of a dc machine
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DC Machines The stator of the dc motor has
poles, which are excited by dccurrent to produce magneticfields.
In the neutral zone, in the middlebetween the poles, commutatingpoles are placed to reducesparking of the commutator.The commutating poles aresupplied by dc current.
Compensating windings aremounted on the main poles.These short-circuited windingsdamp rotor oscillations. .
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DC Machines The poles are mounted on an
iron core that provides aclosed magnetic circuit.
The motor housing supportsthe iron core, the brushes andthe bearings.
The rotor has a ring-shapedlaminated iron core with slots.
Coils with several turns are
placed in the slots. Thedistance between the two legsof the coil is about 180 electricdegrees.
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DC Machines The coils are connected in series
through the commutatorsegments.
The ends of each coil are
connected to a commutatorsegment.
The commutator consists ofinsulated copper segmentsmounted on an insulated tube.
Two brushes are pressed to thecommutator to permit current
flow. The brushes are placed in theneutral zone, where the magneticfield is close to zero, to reducearcing.
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DC Machines The rotor has a ring-shaped
laminated iron core with slots.
The commutator consists of
insulated copper segmentsmounted on an insulated tube.
Two brushes are pressed tothe commutator to permitcurrent flow.
The brushes are placed in the
neutral zone, where themagnetic field is close to zero,to reduce arcing.
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DC Machines The commutator switches the
current from one rotor coil tothe adjacent coil,
The switching requires theinterruption of the coilcurrent.
The sudden interruption of aninductive current generateshigh voltages .
The high voltage producesflashover and arcing betweenthe commutator segment andthe brush.
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DC Machine Construction
|
Shaft
Brush
Copper
segment
Insulation
Rotor
Winding
N S
Ir_dc
Ir_dc
/2
Rotation
Ir_dc
/2
Ir_dc
12
3
4
5
6
7
8
Pole
winding
Commutator with the rotor coils connections.
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DC Motor Operation
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DC Motor Operation In a dc motor, the stator
poles are supplied by dcexcitation current, which
produces a dc magneticfield.
The rotor is supplied bydc current through thebrushes, commutatorand coils.
The interaction of themagnetic field and rotorcurrent generates a forcethat drives the motor
ha t
rush
opper
segment
Insulation
otor
Winding
S
Ir_dc
Ir_dc
/2
Rotation
Ir_dc
/2
Ir_dc
12
3
4
5
6
7
8
ole
inding
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DC Motor Operation
Before reaching the neutral zone,the current enters in segment 1andexits from segment 2,
Therefore, current enters the coilend at slot a and exits from slotbduring this stage.
After passing the neutral zone, thecurrent enters segment 2 and exitsfrom segment 1,
This reverses the current directionthrough the rotor coil, when the coil
passes the neutral zone. The result of this current reversal is
the maintenance of the rotation.
(a) Rotor current flow from segment 1 to 2(slot a to b)
Vdc
30NS
Bv
v
a
b
1
2
Ir_dc
(b) Rotor current flow from segment 2 to 1(slot b to a)
30NS V
dc
a
b
1
2
B
v v
Ir_dc
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DC GeneratorOperation
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DC Generator Operation
The N-S poles produce adc magnetic field and therotor coil turns in thisfield.
A turbine or othermachine drives the rotor.
The conductors in theslots cut the magnetic fluxlines, which induce
voltage in the rotor coils. The coil has two sides:
one is placed in slot a, theother in slot b.
30NS
Vdc
Bv
v
a
b
1
2
Ir_dc
(a) Rotor current flow from segment 1 to 2(slot a to b)
30 NS Vdc
a
b
1
2
vv
B
Ir_dc
(b) Rotor current flow from segment 2 to 1(slot b to a)
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DC Generator Operation
In Figure 8.11A, theconductors in slot a arecutting the field linesentering into the rotorfrom the north pole,
The conductors in slot bare cutting the field linesexiting from the rotor tothe south pole.
The cutting of the field
lines generates voltage inthe conductors.
The voltages generated inthe two sides of the coilare added.
30NS
Vdc
Bv
v
a
b
1
2
Ir_dc
(a) Rotor current flow from segment 1 to 2(slot a to b)
30 NS Vdc
a
b
1
2
vv
B
Ir_dc
(b) Rotor current flow from segment 2 to 1(slot b to a)
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DC Generator Operation
The induced voltage isconnected to the generatorterminals through thecommutator and brushes.
In Figure 8.11A, the induced
voltage in b is positive, and ina is negative.
The positive terminal isconnected to commutatorsegment 2 and to theconductors in slot b.
The negative terminal isconnected to segment 1 andto the conductors in slot a.
30NS
Vdc
Bv
v
a
b
1
2
Ir_dc
(a) Rotor current flow from segment 1 to 2(slot a to b)
30 NS Vdc
a
b
1
2
vv
B
Ir_dc
(b) Rotor current flow from segment 2 to 1(slot b to a)
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DC Generator Operation
When the coil passes theneutral zone: Conductors in slot a are
then moving toward thesouth pole and cut flux linesexiting from the rotor
Conductors in slot b cut theflux lines entering the inslot b.
This changes the polarityof the induced voltage inthe coil.
The voltage induced in ais now positive, and in b isnegative.
30NS
Vdc
Bv
v
a
b
1
2
Ir_dc
(a) Rotor current flow from segment 1 to 2(slot a to b)
30 NS Vdc
a
b
1
2
vv
B
Ir_dc
(b) Rotor current flow from segment 2 to 1(slot b to a)
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DC Generator Operation
The simultaneously thecommutator reverses itsterminals, which assuresthat the output voltage(V
dc
) polarity isunchanged.
In Figure 8.11B
the positive terminal isconnected to commutatorsegment 1 and to the
conductors in slot a. The negative terminal is
connected to segment 2 andto the conductors in slot b.
30NS
Vdc
Bv
v
a
b
1
2
Ir_dc
(a) Rotor current flow from segment 1 to 2(slot a to b)
30 NS Vdc
a
b
1
2
vv
B
Ir_dc
(b) Rotor current flow from segment 2 to 1(slot b to a)
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Generator
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DC Generator Equivalent circuit The magnetic field produced by the stator poles induces a
voltage in the rotor (or armature) coils when the
generator is rotated.
This induced voltage is represented by a voltage source.
The stator coil has resistance, which is connected in
series.
The pole flux is produced by the DC excitation/field
current, which is magnetically coupled to the rotor
The field circuit has resistance and a source
The voltage drop on the brushes represented by a battery
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DC Generator Equivalent circuit
Equivalent circuit of a separately excited dc generator.
Rf
Ra
Vbrush
VdcE
agV
f
*max
If
Iag
Load
Mechanical
power in
Electrical
power out
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DC Generator Equivalent circuit The magnetic field produced by the stator poles
induces a voltage in the rotor (or armature) coils
when the generator is rotated. The dc field current of the poles generates a
magnetic flux
The flux is proportional with the field current if
the iron core is not saturated:
fag IK1!*
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DC Generator Equivalent circuit The rotor conductors cut the field lines
that generate voltage in the coils.
The motor speed and flux equations are :
vBNE grag N2!
2
gDv [! ggag DBN!*
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DC Generator Equivalent circuit The combination of the three equation
results the induced voltage equation:
The equation is simplified.
[[[agrggr
g
grgrag NDBND
BNvBNE *!!
!! NNN
222
[[[ fmfragrag IKIKNNE !!*! 1
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DC Generator Equivalent circuit When the generator is loaded, the load current produces
a voltage drop on the rotor winding resistance.
In addition, there is a more or less constant 13 V voltagedrop on the brushes.
These two voltage drops reduce the terminal voltage ofthe generator. The terminal voltage is;
brushaagdcag VRIVE !
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Motor
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DC Motor Equivalent circuit
Figure 8.13 Equivalent circuit of a separately excited dc motor
Equivalent circuit is similar to the generator only the currentdirections are different
Rf
Ra
Vbrush
VdcE
am
Vf
*max
If
Iam
Mechanical
power out
Electrical
power in
DC Power
supply
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DC Motor Equivalent circuit The operation equations are:
Armature voltage equation
brushaamamdc IEV !
The induced voltage and motor speed vs angular
frequency
[fmam IKE !mnT[ 2!
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DC Motor Equivalent circuit The operation equations are:
The combination of the equations results in
The current is calculated from this equation.The output
power and torque are:
mamdcamfm RIVEI !![
amamout IEP ! fammout II
PT !!
[
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