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CONSTRUCTION OF DC MACHINE
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Armature Core or Stack
The armature stack is made up thin magnetic
steel laminations stamped from sheet steel
with a blanking die. Slots are punched in the
lamination with a slot die. Sometimes these
two operations are done as one. The
laminations are welded, riveted, bolted orbonded together.
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Armature Winding
The armature winding is the winding, which
fits in the armature slots and is eventually
connected to the commutator. It either
generates or receives the voltage depending
on whether the unit is a generator or motor.
The armature winding usually consists ofcopper wire, either round or rectangular and
is insulated from the armature stack.
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Field Coils
The field coils are those windings, which are
located on the poles and set up the magnetic
fields in the machine. They also usually consist
of copper wire are insulated from the poles.
The field coils may be either shunt windings
(in parallel with the armature winding) orseries windings (in series with the armature
winding) or a combination of both.
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Yoke
The yoke is a circular steel ring, which
supports the field, poles mechanically and
provides the necessary magnetic path
between the pole. The yoke can be solid or
laminated. In many DC machines, the yoke
also serves as the frame.
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Poles and pole shoe
IF POLE SHOE ENLARGES, LARGER INDUCEDEMF
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Commutator
The commutator is the mechanical rectifier,
which changes the AC voltage of the rotating
conductors to DC voltage. It consists of a
number of segments normally equal to the
number of slots. The segments or commutator
bars are made of silver bearing copper and areseparated from each other by mica insulation.
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Brushes and Brush HoldersBrushes conduct the current from the
commutator to the external circuit. There aremany types of brushes. A brush holder isusually a metal box that is rectangular in shape.The brush holder has a spring that holds thebrush in contact with the commutator. Eachbrush usually has a flexible copper shunt whichextends to the lead wires. Often, the entire
brush assembly is insulated from the frame andis made movable as a unit about thecommutator to allow for adjustment.
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Interpoles
Interpoles are similar to the main field polesand located on the yoke between the main
field poles. They have windings in series with
the armature winding. Interpoles have thefunction of reducing the armature reaction
effect in the commutating zone. They
eliminate the need to shift the brush
assembly.
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Frame, End Bells, Shaft, and Bearings
The frame and end bells are usually steel,
aluminum or magnesium castings used toenclose and support the basic machine parts.
The armature is mounted on a steel shaft,
which is supported between two bearings.The bearings are either sleeve, ball or roller
type. They are normally lubricated by grease
or oil.
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Back End, Front End
The load end of the motor is the Back End.
The opposite load end, most often the
commutator end, is the Front End of themotor.
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Armature Windings
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WINDING TERMINOLOGIES
LAP WINDING
LAP WINDING CONNECTED TO
COMMUTATOR BARS
LAP WINDING
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When the end connections of the coils arebrought to adjacent bars a lap or parallel winding
is formed. In this type winding, there are as manypaths through the armature as there are poles onthe machine. Therefore, to obtain full use of thistype winding, there must be as many brushes as
there are poles, alternate brushes being positiveand negative. The outermost connecting linesrepresent the end connections on the back of thearmature and the inner connecting lines represent
the connections on the front or commutator endof the armature. The lap winding is best suitedfor low voltage, high current ratings because ofthe number of parallel paths.
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LAP WINDING IN CIRCULAR FORMLap Winding,
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Wave Winding
WAVE WINDING
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The WAVE winding is best suited for high
voltage, low current ratings because of the
number of parallel paths.
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E.M.F Equation
Let
= flux/pole in weber
Z = total number of armature conductors
= No.of slots x No.of conductors/slotP = No.of generator poles
A = No.of parallel paths in armature
N = armature rotation in revolutions per minute (r.p.m)
E = e.m.f induced in any parallel path in armature
Generated e.m.f Eg = e.m.f generated in any one of the
parallel paths i.e E.Average e.m.f generated /conductor = d/dt volt (n=1)
Now, flux cut/conductor in one revolution d = P Wb
No.of revolutions/second = N/60
Time for one revolution, dt = 60/Nsecond
Hence, according to Faraday's Laws of Electromagnetic
Induction,
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E.M.F generated/conductor is
For a simplex wave-wound generator
No.of parallel paths = 2=A
No.of conductors (in series) in one path = Z/2
E.M.F. generated/path is
For a simplex lap-wound generator
No.of parallel paths = P=A
No.of conductors (in series) in one path = Z/P
E.M.F.generated/path
In general generated e.m.f where A = 2 - for simplex wave-winding
= P - for simplex lap-winding
=e = Rate of cutting the flux
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A1-A2 ENDS OF ARMATURE
F1-F2 FIELD WINDING
SYMBOL REPRESENTATION OF DC GENERATOR.
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TYPES OF DC GENERATORS
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Flemings Right hand rule
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SEPARATELY EXCITED GENERATOR
The field wdg is supplied from external,
separate dc supply (i.e.) excitation of field wdg
is separate, then the generator is called
separately excited generator.
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SEPARATELY EXCITED GENERATOR
DC SUPPLY
Ia=IL
F1
F2
A1
A2
IF
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Ia= IL
Emf e is not equal to Vt.
IaRais minimum.
Some voltage drop at the contacts of the brush.
So voltage equation is
E=Vt
+Ia
Ra
+Vbrush
E=(PNZ)/(60A)
Vbrushis negligible.
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SELF EXCITED GENERATOR
1. Though Generator does not work, without anycurrent through field wdg, possess somemagnetic flux. This is residual flux and property isresidual magnetism.
2. Voltage building processsmall emf drives smallct through field wdg, further flux producesincreasing field ct and flux. The process iscumulative and continues till generator develops
rated voltage across armature.3. Three typesShunt , series and compound
Generator.
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SHUNT GENERATOR
F1
F2
IaA1
A2
Ish
Vt
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VOLTAGE AND CURRENT RELATIONS
Ia=IL+Ish
Ish=Vt/Rsh
Induced emf still supply IaRavoltage drop and
brush contact drop.
E=Vt+IaRa+Vbrush
E=(PNZ)/(60A)
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Series Generator
IaA1
A2
S1 S2
Vt
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VOLTAGE AND CURRENT RELATIONS
Ia=Ise=IL
E=Vt+IaRa+IaRse+Vbrush
E=Vt+ Ia(Ra+Rse)+Vbrush
E=(PNZ)/(60A)
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Compound Generator
Long shunt Compound Generator
Ia=Ise
Ia=Ish+IL
Ish=Vt/Rsh
Rsh=Resistance of shunt field wdg
E=Vt+IaRa+IaRse+Vbrush
Rse=Resistance of series field wdg.
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Long Shunt Compound Gr
S1
S2
A1
A2
F1
F2
Ish
Ise
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Short shunt Compound Gr
F1
F2
A1
A2
Ia
S1 S2
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Ia=Ise+Ish
Ia=IL+Ish
Ise=IL
Drop across shunt field wdg is drop across armature only not
on Vt. The drop is E-IaRaIsh=(E-IaRa)/Rsh
E=Vt+IaRa+IseRse+Vbrush
Ise=IL
E=Vt+IaRa+ILRse+VbrushE=Vt+IaRa+ILRse
E-IaRa=Vt+ILRse
Ish=(Vt+ILRse)/Rsh
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Cumulative and Differential Compound
Generator
shse
Differential compound Generator
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T=sh+se
sh=Flux produced by shunt winding
se= Flux produced by series , field winding
Depending on the direction of the winding of the poles,
two fluxes produced by shunt and series field may helpor oppose each other.
If the two fluxes help each other, then it is cumulativecompound generator.
If two fluxes oppose each other, then it is differentialcompound generator.
T=sh-se
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Applications of DC generator
1. Shunt generators are extensively used for general light andpower supply, and for charging of batteries, since, in conjunctionwith a field regulator, a constant terminal voltage can bemaintained at all loads.
2. Series generators are mainly used as animation boosters in dc
transmission system, in order to compensate for the drop of voltagedue to the resistance of transmission conductors.
3. compounded generators find use in dc transmission, since it ispossible to keep on a constant voltage at the load end, bygenerating a larger voltage so as to overcome the line drop.
Cumulative compound generator- used for domestic lighting
purposes and to transmit energy over long distances.
differential compound generatorare very rare and used forspecial application like electric arc welding.
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DC Motor
A DC motor is a device which converts
electrical energy into mechanical energy.
D.C. motors are motors that run on Direct
Current from a battery or D.C. power supply.
Direct Current is the term used to describe
electricity at a constant voltage.
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The principle of operation iswhen a current
carrying conductor is placed in a magnetic
field, it experiences mechanical force.
Two fluxes are present.
i) Flux produced by permanent magnet is
called main flux
ii) Flux produced by current carrying conductor
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Principles of Operation
Force in DC Motor
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Magnetic Field in DC Motor
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Torque in DC Motor
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Current Flow in DC Motor
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The magnitude of force experienced byconductor in a motor is given by
Force, F = B lInewton
Where B is the flux density due to flux producedby field winding in weber/m2.
Iis the current in amperes and
lis the length of the coil in meter.
The force, current and the magnetic field are allin different directions.
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Flemings Left hand rule:
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Back EMF in DC motor.
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In generating action, conductor cuts the magnetic flux,emf is induced in conductor.
In DC motor, after motoring action, armature rotatesand armature conductors cut the main flux. So this is
generating action in motor after motoring action.
Induced emf in rotating armature conductors accordingto Faradays law of electromagnetic induction.
Induced emf in armature always act in oppositedirection to supply voltage.
This is Lenz lawdirection of induced emf is always soas to oppose the cause producing it.
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In DC motor, electrical i/p, supply voltage is causefor armature current and in motoring action,induced emf opposes supply voltage.
Emf induced sets up the current through
armature which is in opposite directionsupplyvoltage is forcing through conductor.
Emf always opposes supply voltage - back emf,Eb.
The magnitude is determined byEb= (PNZ)/(60A).
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Symbol of Ebin DC mtotr
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Voltage equation of DC motor
V=Eb+IaRa+Brush drop
Ia=(V-Eb)/Ra
Si ifi f b k f
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Significance of back emf Due to back emf, DC motor is a regulating machine.
Back emf N
When there is load, motor slows down. So the speed of motorreduces, so Ebalso reduces.
The net voltage across armature (V-Eb) increases and motor drawsmore armature current.
Due to increased armature current, force experienced by conductorsand torque on armature increases.
The increase in torque satisfy increase in load.
When load of motor is decreased, speed of motor increases. So Ebincreases.
(V-Eb) cause to reduce, reducing Ia.
The motor speed stops increasing when Ia is enough to produce lesstorque by new load.
So back emf regulates flow of Ia and Ia meets load requirement.
This is the significance of back emf.
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Power Equation of DC motor
V=Eb+IaRaMultiplying by Iaon both sides,
VIa=EbIa+Ia2Ra
VIa=Net electrical power i/p to armature
Ia2Ra=Power loss due to resistance of armature
called armature copper loss.
Difference between Via and Ia2Rais InputLosses givesoutput of armature
EbIais equation of gross mech. Power developed.Power input to armaturearmature copper loss = Gross
mech power developed in armature
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Torque equation of DC motor
The turning or twisting force about an axis iscalled torque.
T= F*R
The wheel rotating at a speed of N rpm,then angular speed is
= (2N)/60 rad/sec.
The work done in one revolution isW= F*Distance travelled in one revolution
=F*2R Joules
P = Power developed = Work
done/
Time
(=F* 2R )/time
for 1 rev.
=(F* 2R )/(60/N)=(F*R)*((2N)/60 )
P=T*watts
T = Torque in Nm
= Angular speed in rad/sec.
Ta-gross torque by armature of motor. It isalso called as armature torque.
The gross mech. Power developed in armature isEbIa.
The speed of motor is N rpm, then
Power in armature = Armature torque
*
EbIa=Ta*((2N)/60 )
Ebin motor is, Eb= (PNZ)/(60A)
(PNZ)/(60A) *Ia =Ta*((2
N)/60 )Ta= (1/2)Ia*(PZ)/A = 0.159
Ia*(PZ)/ANm
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Types of torque in Motor:
There is power loss due tofriction, windage and ironlosses.
The torque which overcomethese losses is called losttorque denoted as T
f.
These losses are also called asstray losses.
The torque at the shaft doesuseful work is called Loadtorque or shaft torque
denoted as Tsh. Ta= Tf+Tsh
Ta = armature torque
Tsh
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No Load condition of Motor
On no load, Tsh= 0 So motor can rotate at a
speed N0rpm on no load.
The motor draws armature
current Ia0. Ia0= (VEb0)/Ra
Eb0back emf on no load to speed N0
TaIa. Flux is present and Iais
present ; Ta0exists on noload (armature torque)
Ta= Tf+Tsh
Tsh= 0
Ta0= Tf
Power developed (Eb0*Ia0) =
Friction , windage and ironlosses.
The stray losses Eb0Ia0isconstant though the load onmotor changes from 0 tofull capacity of motor.
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