physics dc gen
TRANSCRIPT
-
8/14/2019 Physics Dc Gen.
1/20
103/23/09
Welcome to
Lectures on D.C.Generators
-
8/14/2019 Physics Dc Gen.
2/20
203/23/09
DIRECT CURRENT
MACHINESDC machine Construction
The major advantages of DCmachines are: easy speed andtorque regulation.
The stator of the DC motor haspoles, which are excited by DCcurrent to produce magnetic fields.
The rotor has a ring-shapedlaminated iron-core with slots.
Coils with several turns are placed inthe slots. The distance between thetwo legs of the coil is about 180electric degrees.
N S
Stator with
with polesBrush
Rotor
Field
-
8/14/2019 Physics Dc Gen.
3/20
3
DC Generators.
Basic operation of the generator As the loop rotates, the magnetic
flux through it changes withtime
This induces an emf and acurrent in the external circuit
The ends of the loop areconnected to slip rings thatrotate with the loop
Connections to the externalcircuit are made by stationary
brushes in contact with the sliprings
-
8/14/2019 Physics Dc Gen.
4/20
4
DC Generators, cont
The output voltage alwayshas the same polarity
The current is apulsatingcurrent
To produce a steady current,many loops andcommutators around the axisof rotation are used
The multiple outputs aresuperimposed and theoutput is almost free offluctuations
-
8/14/2019 Physics Dc Gen.
5/20
5
DC Generators
If we increase the number
of turns and arrange them
as shown, we can smooth
out the variations in
voltage.
In the old days, autos used
DC generators to recharge
the battery.
-
8/14/2019 Physics Dc Gen.
6/20
6
DC Generators
Replace the brushes with
slip rings and you get a
DC generator. Notice the
voltage is not steady. The
coil is shown at the top
and bottom of the shaft.
-
8/14/2019 Physics Dc Gen.
7/20703/23/09
D. C. Generator
Principle of Operation:When a conductor rotates in a magnetic field,
dynamically induced e.m.f is produced in it
according to Faradays law of electromagnetic
induction.
-
8/14/2019 Physics Dc Gen.
8/20803/23/09
Generated EMF
Let
Ei.epathsparalleltheofoneanyingeneratedEMFEEMFGenerated
ArmatureinpathparallelanyininducedEMFE
rpminrotationArmatureN
ArmatureinPathsParallelofNo.A
polesGeneratorofNo.P
SlotperConductorsofNo.SlotsofNo.
ConductorsArmatureofNo.TotalZ
WbinFlux/Pole
g =
====
===
-
8/14/2019 Physics Dc Gen.
9/20903/23/09
secN
60dt,revolution1forTime
N/60s/secRevolutionofNo.
WbPd,revolution1intorcut/conducFluxNow,
voltdt
dConductorGenerated/EMFAverage
=
==
=
Hence, according to Faradays Laws of electromagnetic induction,
volt60
PN
60/N
P
volt
dt
dconductorGenerated/EMF
==
=
-
8/14/2019 Physics Dc Gen.
10/201003/23/09
For Wave wound Generator
No. of Parallel Paths = 2
No. of Conductors (in series) in 1 path = Z/2
volt
120
ZPNvolt
260
PNpathGenerated/EMF ==
Z
-
8/14/2019 Physics Dc Gen.
11/201103/23/09
For Lap wound Generator
No. of Parallel Paths = P
No. of Conductors (in series) in 1 path = Z/P
volt60
ZNvolt
P60
PNpathGenerated/EMF ==
Z
-
8/14/2019 Physics Dc Gen.
12/201203/23/09
In General,
Generated EMF
windinglapforP
windingwavefor2Awhere,
volt
A
P
60
ZNEg
==
=
-
8/14/2019 Physics Dc Gen.
13/201303/23/09
Types of D.C. Generators
According to the way in which their fieldsare excited, generators are classified into :
i) Separately excited D.C. Generator
ii) Self- excited D.C. Generator
-
8/14/2019 Physics Dc Gen.
14/201403/23/09
i) Separately excited D.C. Generators are those whose field
magnets are energized from an independent source of D.C. Current
Eg
L
O
A
D
+
-
I
Field Winding
R
Brush
Brush
V
If
DropsBrushIREV ag =Ra = Armature Resistance
-
8/14/2019 Physics Dc Gen.
15/201503/23/09
ii) Self excited D.C. Generators are those whose field
magnets are energized by the currents produced by the generators
themselves.
There are 3 types of self-excited D.C. Generators named according to
the manner in which their field coils are connected to the armature.
These are :
Shunt wound D.C. Generator Series wound D.C. Generator
Compound wound D.C. Generator
-
8/14/2019 Physics Dc Gen.
16/201603/23/09
i) Shunt wound D.C. Generator
Eg
L
O
A
D
+
-
I
Shunt Field V
Ia
Ish
Rsh
Ra
Rsh Shunt Field Resistance
Ra - Armature Resistance
sha III +=DropsBrushRIEV aag =
shshRIV =
-
8/14/2019 Physics Dc Gen.
17/20
-
8/14/2019 Physics Dc Gen.
18/201803/23/09
iii) Compound wound D.C. Generator
Eg
L
OAD
I
Shunt Field
V
Ia
Ish
Series Field
Short Shunt
-
8/14/2019 Physics Dc Gen.
19/201903/23/09
Eg
L
O
A
D
I
Shunt FieldV
Ia
Ish
Series Field
Long Shunt
-
8/14/2019 Physics Dc Gen.
20/2020
03/23/09
Portable D. C. Generator
http://upload.wikimedia.org/wikipedia/commons/7/7d/Portable_electrical_generator_angle.jpg