ECEg439:-Electrical Machine II
2.2 Main Structural Elements of
DC Machine
By Sintayehu Challa
Construction of DC Machines
A DC machine consists of two main parts
1. Stationary Part (Stator):-It is designed mainly for producing a
magnetic flux. The stationary part of a DC Machine consists of
a)
b)
c)
Main poles:-Designed to create the magnetic flux
Commutating poles interposed between the main poles and designed to
ensure sparkless operation of the brushes at the commutator
Frame (Yoke):The Frame is the stationary part of the machine to which are fixed the
main and commutating poles and by means of which the machine is
bolted to its bed plate.
The Yoke is the ring shaped portion which serves as the path for the
main and commuting pole fluxes
2.
Rotating Part (Rotor):-It is called the armature, where the
mechanical energy is converted into Electrical(as Generator) or
conversily,Electrical Energy into mechanical (as Motor).
Air-Gap :-The stator and Rotor are separated from each other by small
air gap.
By Sintayehu Challa
ECEg439:Electrical Machine II
DC Machine Construction
By Sintayehu Challa
ECEg439:Electrical Machine II
3
DC Machines
? The stator of the dc motor has poles, which are
excited by dc current to produce magnetic fields.
? In the neutral zone, in the middle between the
poles, commutating poles are placed to reduce
sparking of the commutator. The commutating
poles are supplied by dc current.
? Compensating windings are mounted on the main
poles. These short-circuited windings damp rotor
oscillations.
? The object of the Field System (Series and Shunt)
is to create a uniform magnetic field, within
which the armature rotates.
By Sintayehu Challa
ECEg439:Electrical Machine II
4
DC Machines
? The poles are mounted on an iron core
that provides a closed magnetic circuit.
? The motor housing supports the iron
core, the brushes and the bearings.
? The rotor has a ring-shaped laminated
iron core with slots.
? Coils with several turns are placed in the
slots. The distance between the two legs
of the coil is about 180 electric degrees.
By Sintayehu Challa
ECEg439:Electrical Machine II
5
DC Machines
? The coils are connected in series through the
commutator segments. The ends of each coil
are connected to a commutator segment.
? The commutator is a form of rotating
mechanical switch placed between the
armature and the external circuit and so
arranged that it will reverse the connection
with the external circuit at the instant of
each reversal of current in the armature. The
commutator consists of insulated copper
segments mounted on an insulated tube.
? The function of brushes is to collect current
from the commutator and supply it to the
external load circuit.
By Sintayehu Challa
ECEg439:Electrical Machine II
6
DC Machines
?
?
?
?
The rotor has a ringshaped laminated iron
core with slots.
The commutator consists
of insulated copper
segments mounted on an
insulated tube.
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.
By Sintayehu Challa
ECEg439:Electrical Machine II
7
DC Machine Construction
Rotation
Ir_dc/2
Brush
Ir_dc/2
Ir_dc
Shaft
Pole
winding
|
1
2
8
N
3
7
6
S
4
5
Insulation
Rotor
Winding
Ir_dc
Copper
segment
Figure Commutator with the rotor coils connections.
By Sintayehu Challa
ECEg439:Electrical Machine II
8
DC Machine Construction
Figure Details of the commutator of a dc motor.
By Sintayehu Challa
ECEg439:Electrical Machine II
9
DC Machine Construction
Figure DC motor stator with poles visible.
By Sintayehu Challa
ECEg439:Electrical Machine II
10
DC Machine Construction
Figure Rotor of a dc motor.
By Sintayehu Challa
ECEg439:Electrical Machine II
11
DC Machine Construction
Figure Cutaway view of a dc motor.
By Sintayehu Challa
ECEg439:Electrical Machine II
12