ECEg439:-Electrical Machine II
2.1.General Arrangement of DC Machine
By Sintayehu Challa
Objectives
? To instill an understanding of the
underlying electromagnetic effects
permitting electric machine operation
and introduce basic DC machine
types
• To describe the construction of these
machines
• To examine the main types of DC
machine
By Sintayehu Challa
ECEg439:Electrical Machine II
DC Machines
? The direct current (dc) machine can be
used as a motor or as a generator.
? DC Generator convert Mechanical
Energy Input at their shafts into to
Electrical Energy in the form of
Voltage or Current.
? A DC motor convert Electrical energy
into rotary (or linear) mechanical
energy at the output shaft.
? The major advantages of dc machines
are the easy speed and torque
regulation.
By Sintayehu Challa
ECEg439:Electrical Machine II
3
Principle of DC machines
? The working principle of the DC
generator is Faraday’s Law, which states
that emf and electric current if the circuit
is closed, is produced when a conductor
cuts through magnetic force lines.
? The opposite of the law applies for the
DC motor. Motion is produced when a
current carrying wire is put in a magnetic
field.
By Sintayehu Challa
ECEg439:Electrical Machine II
Conversation of Energy
Electrical Energy ? Mechanical
E . I .t ? F . d
Energy
( BlV ). I .t ? ( BIl ). d
( B . I .l ).( V .t ) ? ( BIl ). d ?
( BIl ). d ? ( BIl ). d
d ? (V .t )
1. A Generator converts Mechanical Energy into Electrical Energy, Where as
a Motor converts Electrical Energy into Mechanical Energy.
2. The Energy conversion from Electrical to mechanical or vice-versa take
place via the magnetic field provided by the field system.
3. A single rotating dc machine can either be operated as a generator or as
a motor.
By Sintayehu Challa
ECEg439:Electrical Machine II
commutation
?
In DC machines the current in
each wire of the armature is
actually alternating, and hence a
device is required to convert the
alternating generated current
into the DC by a mechanical
device is called a commutator.
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
Fig. 1( a) DC generator:
Induced AC emf is converted to
DC voltage using commutator ;
Fig. 1(b) DC motor: input direct
current is converted to alternating
current in the armature at to
produce a unidirectional torque.
By Sintayehu Challa
ECEg439:Electrical Machine II
DC Generator
Operation
By Sintayehu Challa
ECEg439:Electrical Machine II
8
DC Generator Operation
? The N-S poles of a dc machine produces constant
magnetic field and the rotor coil turns in this field.
? The conductors in the rotor slots cut the magnetic
flux lines, which induce voltage in the rotor coils.
? The coil has two sides: one is placed in slot a, the
other in slot b.
v
B
a
N
1
S
30
Vdc
2
b
v
Ir_dc
(a) Rotor current flow from segment 1 to 2 (slot a to b)
By Sintayehu Challa
ECEg439:Electrical Machine II
9
DC Generator Operation
?
?
?
In Figure below , the conductors in slot a are cutting the field
lines entering into the rotor from the north pole,
The conductors in slot b are cutting the field lines exiting
from the rotor to the south pole.
The cutting of the field lines generates voltage in the
conductors.
1. The induced voltage is connected to the
generator terminals through the
commutators (1 & 2) and brushes.
S
N
30
V dc
2
3. The positive terminal is connected to
commutator segment 2 and to the
conductors in slot b.
4. The negative terminal is connected to
segment 1 and to the conductors in slot a.
B
a
1
2. The induced voltage in b is positive, and in
a is negative.
v
b
v
Ir_dc
(a) Rotor current flow from segment 1 to 2 (slot a to b)
By Sintayehu Challa
ECEg439:Electrical Machine II
DC Generator Operation
In Figure B
? the positive terminal is
connected to commutator
segment 1 and to the
conductors in slot a.
? The negative terminal is
connected to segment 2
and to the conductors in
slot b.
B
a
S
2
?
?
?
When the coil passes the neutral zone:
? Conductors in slot a are then moving toward the south pole
and cut flux lines exiting from the rotor
? Conductors in slot b cut the flux lines entering the in slot b.
This changes the polarity of the induced voltage in the coil.
The voltage induced in a is now positive, and in b is negative.
The simultaneously the commutator reverses its terminals, which
assures that the output voltage (Vdc) polarity is unchanged.
30
v
v
N
V dc
1
?
b
Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
By Sintayehu Challa
ECEg439:Electrical Machine II
11
DC Motor
Operation
By Sintayehu Challa
ECEg439:Electrical Machine II
12
DC Motor Operation
? In a dc motor, the stator poles are
supplied by dc excitation current,
which produces a dc magnetic field.
? The rotor is supplied by dc current
through the brushes, commutator
and coils.
? The interaction of the magnetic field
and rotor current generates a force
that drives the motor
By Sintayehu Challa
ECEg439:Electrical Machine II
13
DC Motor Operation
B
a
S
N
1
30
Vdc
2
b
v
Ir_dc
(a) Rotor current flow from segment 1 to 2 (slot a to b)
B
S
2
a
30
v
v
N
Vdc
1
? The magnetic field lines
enter into the rotor from
the north pole (N) and
exit toward the south
pole (S).
? The poles generate a
magnetic field that is
perpendicular to the
current carrying
conductors.
? The interaction between
the field and the current
produces a Lorentz force,
? The force is
perpendicular to both the
magnetic field and
conductor
v
b
Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
By Sintayehu Challa
ECEg439:Electrical Machine II
14
DC Motor Operation
v
B
a
N
30
Vdc
2
?
S
1
b
v
Ir_dc
(a) Rotor current flow from segment 1 to 2 (slot a to b)
B
a
S
2
?
The generated force turns the
rotor until the coil reaches the
neutral point between the
poles. At this point, the
magnetic field becomes
practically zero together with
the force.
However, inertia drives the
motor beyond the neutral zone
where the direction of the
magnetic field reverses.
To avoid the reversal of the
force direction, the commutator
changes the current direction,
which maintains the
counterclockwise rotation.
30
v
v
N
Vdc
1
?
b
Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
By Sintayehu Challa
ECEg439:Electrical Machine II
15
DC Motor Operation
Neutral
Zone
v
B
a
S
N
1
30
Vdc
2
b
v
Ir_dc
(a) Rotor current flow from segment 1 to 2 (slot a to b)
B
S
2
a
30
v
v
N
Vdc
1
? Before reaching the neutral
zone, the current enters in
segment 1 and exits from
segment 2,
? Therefore, current enters the
coil end at slot a and exits from
slot b during this stage.
? After passing the neutral zone,
the current enters segment 2
and exits from segment 1,
? This reverses the current
direction through the rotor coil,
when the coil passes the
neutral zone.
? The result of this current
reversal is the maintenance of
the rotation.
b
Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
By Sintayehu Challa
ECEg439:Electrical Machine II
16
Basic DC Motor Operation
? Consider the illustration below With the current flowing
the wire as shown, and the magnetic field in the
direction indicated, it is clear there is a force on the
conductors acting as shown below
• If the wire is free to rotate around the ends (terminals)
then the wire would rotate - beginnings of motor action.
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
? Consider the situation shown on the last
slide, The currents in the wire are, taking a
cross- section, in opposite directions
? The magnetic field across this crosssection clearly illustrates the areas where
the magnetic flux is increased and
decreased due to the magnetic flux from
the wires
? This illustration demonstrates the ‘elastic
band’ nature of lines of magnetic flux,
which will always act in a way to try to
shorten themselves
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
? Consider the situation if the wire rotates through 900
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
? The forces on the conductor remain acting in
the same directions , With the pivot point at
the wire ends (terminals) there is now zero
torque acting on the wire forcing it to rotate
(Neutral zone)
? Consider the situation after a further 900
rotation of the wire (assume the wire has
sufficient momentum to rotate)
? The torque acting on the conductors now
rotates the wire in the opposite sense.
? How can this be avoided?
The answer would be to reverse either the
magnetic field or the direction of current
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
• It is easier to reverse the current flow in the wire
this is managed by employing a commutator
• A commutator ensures that the current is
reversed in the armature (turning conductor)
every half rotation thereby allowing the forces on
the wire to aid rotation
• Take the previous example and add a commutator
to the supply end of the wires.
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
• By including a basic commutator it is possible
to obtain the forces on the conductors in the
same sense for a 3600 rotation.
• However there are still periods of zero torque
for the simple two piece commutator
considered
• This would lead to a very uneven drive and
could, dependent on the load, seriously effect
either the motor, the load or both
• To compensate for this effect utilize a
commutator with more segments
By Sintayehu Challa
ECEg439:Electrical Machine II
Contd.
By Sintayehu Challa
ECEg439:Electrical Machine II
Improved Commutator
? Having a commutator with more segments means that
there are no zero torque parts of the rotation cycle. This
significantly improves the drive of the motor
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
By Sintayehu Challa
ECEg439:Electrical Machine II
Ir_dc
Copper
segment
Example of many segment Commutator & Motor
? A common application of a DC motor is a battery
powered hand drill. The commutator has many
segments and delivers relatively smooth output torque
By Sintayehu Challa
ECEg439:Electrical Machine II
Commutator Action
? As the commutator passes a brush, the direction of
current flow reverses, ensuring constant drive
torque in the direction of rotation
By Sintayehu Challa
ECEg439:Electrical Machine II