The document discusses DC motors and their construction and operation. It provides three key points: 1) It describes the basic construction of a DC motor, including its stator, rotor, commutator, and brushes. The stator contains poles that generate a magnetic field, while the rotor contains coils connected by the commutator. 2) It explains how a DC motor operates, with the interaction between the magnetic field and current in the rotor coils generating a torque that causes rotation. The commutator switches the current direction to maintain rotation. 3) It presents the equivalent circuit of a DC motor/generator, modeling the magnetic flux generated by the field coils, the back EMF induced in

1. EE331
Electromechanical Energy
Conversion II
Dr. Ali M. Eltamaly
DC Motors
1

2. 8.1 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 the
easy 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.
2

3. 8.1 DC Motor
• Even today the starter is a series dc motor
• However, the recent development of power
electronics has reduced the use of dc motors
and generators.
• The electronically controlled ac drives are
gradually replacing the dc motor drives in
factories.
• Nevertheless, a large number of dc motors are
still used by industry and several thousand are
sold annually.
3

4. 8.1 Construction
4

5. DC Machine Construction
Figure 8.1 General arrangement of a dc machine
5

6. 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. .
6

7. 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.
7

8. 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 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.
8

9. DC Machines
• The rotor has a ring-shaped
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.
9

10. DC Machines
• The commutator switches the
current from one rotor coil to
the adjacent coil,
• The switching requires the
interruption of the coil
current.
• The sudden interruption of
an inductive current
generates high voltages .
• The high voltage produces
flashover and arcing between
the commutator segment and
the brush.
10

11. DC Machine Construction
Rotation
Ir_dc/2 Ir_dc Ir_dc/2
Brush Pole
winding
Shaft
|
1
2
8
N 7 3
S
6 4
5
Insulation Copper
Rotor Ir_dc segment
Winding
Figure 8.2 Commutator with the rotor coils connections.
11

12. DC Machine Construction
Figure 8.3 Details of the commutator of a dc motor.
12

13. DC Machine Construction
Figure 8.4 DC motor stator with poles visible.
13

14. DC Machine Construction
Figure 8.5 Rotor of a dc motor.
14

15. DC Machine Construction
Figure 8.6 Cutaway view of a dc motor.
15

16. 8.2.1 DC Motor
Operation
16

17. DC Motor Operation
• In a dc motor, the stator
poles are supplied by dc Rotation
Ir_dc/2
Ir_dc/2
excitation current, which Brush
Ir_dc Pole
produces a dc magnetic Shaft
winding
field. |
• The rotor is supplied by
1
2
8
dc current through the N 7 3
S
brushes, commutator
6 4
5
and coils.
• The interaction of the Insulation
Rotor Ir_dc
Copper
segment
magnetic field and rotor Winding
current generates a force
that drives the motor
17

18. 8.2.1 DC Motor Operation
v B
• The magnetic field lines a
enter into the rotor from the
S N
north pole (N) and exit Vdc
1
30
toward the south pole (S).
2
b
• The poles generate a v
magnetic field that is Ir_dc
perpendicular to the current (a) Rotor current flow from segment 1 to 2 (slot a to b)
carrying conductors.
• The interaction between the B
field and the current a
produces a Lorentz force, S N Vdc
2
v 30 v
• The force is perpendicular to
1
both the magnetic field and
b
conductor Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
18

19. 8.2.1 DC Motor Operation
v B
• The generated force turns the rotor a
until the coil reaches the neutral
S N
point between the poles. Vdc
1
30
2
• At this point, the magnetic field b
becomes practically zero together v
with the force. Ir_dc
• However, inertia drives the motor
(a) Rotor current flow from segment 1 to 2 (slot a to b)
beyond the neutral zone where the
direction of the magnetic field B
reverses. a
• To avoid the reversal of the force
S N Vdc
direction, the commutator changes
2
v 30 v
1
the current direction, which b
maintains the counterclockwise
rotation. . Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
19

20. 8.2.1 DC Motor Operation
• Before reaching the neutral zone, v
a
B
the current enters in segment 1 and
exits from segment 2, S N Vdc
1
30
• Therefore, current enters the coil
2
end at slot a and exits from slot b b
during this stage. v
Ir_dc
• After passing the neutral zone, the
current enters segment 2 and exits (a) Rotor current flow from segment 1 to 2 (slot a to b)
from segment 1, B
• This reverses the current direction a
through the rotor coil, when the coil S N
passes the neutral zone. Vdc
2
v 30 v
1
• The result of this current reversal is b
the maintenance of the rotation. Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
20

21. 8.2.2 DC Generator
Operation
21

22. 8.2.2 DC Generator Operation
v B
• The N-S poles produce a a
dc magnetic field and the S N
rotor coil turns in this Vdc
1
30
2
field. b
• A turbine or other v
Ir_dc
machine drives the rotor.
(a) Rotor current flow from segment 1 to 2 (slot a to b)
• The conductors in the
slots cut the magnetic flux B
lines, which induce a
voltage in the rotor coils. S N Vdc
2
v 30
v
• The coil has two sides:
1
one is placed in slot a, the
b
other in slot b. Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
22

23. 8.2.2 DC Generator Operation
• In Figure 8.11A, the v
a
B
conductors in slot a are
cutting the field lines S N Vdc
1
30
entering into the rotor
2
from the north pole, b
v
• The conductors in slot b Ir_dc
are cutting the field lines (a) Rotor current flow from segment 1 to 2 (slot a to b)
exiting from the rotor to
the south pole. B
• The cutting of the field a
lines generates voltage in S N Vdc
2
v
the conductors.
30
v
1
• The voltages generated in b
the two sides of the coil
Ir_dc
are added.
(b) Rotor current flow from segment 2 to 1 (slot b to a)
23

24. 8.2.2 DC Generator Operation
• The induced voltage is v
a
B
connected to the generator
terminals through the S N Vdc
1
30
commutator and brushes.
2
• In Figure 8.11A, the induced b
v
voltage in b is positive, and Ir_dc
in a is negative. (a) Rotor current flow from segment 1 to 2 (slot a to b)
• The positive terminal is
connected to commutator B
segment 2 and to the a
conductors in slot b. S N Vdc
2
v 30
v
• The negative terminal is
1
connected to segment 1 and b
to the conductors in slot a.
Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
24

25. 8.2.2 DC Generator Operation
• When the coil passes the v
a
B
neutral zone:
– Conductors in slot a are S N Vdc
1
30
then moving toward the
2
south pole and cut flux lines b
exiting from the rotor v
– Conductors in slot b cut the Ir_dc
flux lines entering the in (a) Rotor current flow from segment 1 to 2 (slot a to b)
slot b.
• This changes the polarity B
of the induced voltage in a
the coil. S N Vdc
2
v 30
v
• The voltage induced in a
1
is now positive, and in b is b
negative.
Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
25

26. 8.2.2 DC Generator Operation
v B
• The simultaneously the a
commutator reverses its S N
terminals, which assures Vdc
1
30
2
that the output voltage b
(Vdc) polarity is v
Ir_dc
unchanged.
(a) Rotor current flow from segment 1 to 2 (slot a to b)
• In Figure 8.11B
– the positive terminal is B
connected to commutator a
segment 1 and to the S N Vdc
2
conductors in slot a. v 30
v
1
– The negative terminal is b
connected to segment 2 and
to the conductors in slot b. Ir_dc
(b) Rotor current flow from segment 2 to 1 (slot b to a)
26

27. 8.2.3 DC Machine
Equivalent Circuit
27

28. Generator
28

29. 8.2.3 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
29

30. 8.2.3 DC Generator Equivalent
circuit
Vbrush
Rf Ra Load
Φmax
Iag
Vf If Eag Vdc
Mechanical Electrical
power in power out
• Figure 8.12Equivalent circuit of a separately excited dc
generator.
30

31. 8.2.3 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:
Φ ag = K 1 I f
31

32. 8.2.3 DC Generator Equivalent
circuit
• The rotor conductors cut the field lines
that generate voltage in the coils.
E ag = 2 N r B  g v
• The motor speed and flux equations are :
Dg
v =ω Φ ag = B  g D g
2
32

33. 8.2.3 DC Generator Equivalent
circuit
• The combination of the three equation
results the induced voltage equation:
 Dg 
E ag = 2 N r B  g v = 2 N r B  g ω
  = N r ( B  g D g ) ω = N r Φ ag ω
 2 

• The equation is simplified.
E ag = N r Φ ag ω = N r K 1 I f ω = K m I f ω
33

34. 8.2.3 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 1–3 V voltage
drop on the brushes.
• These two voltage drops reduce the terminal voltage of
the generator. The terminal voltage is;
E ag = Vdc + I ag Ra + Vbrush
34

35. Motor
35

36. 8.2.3 DC Motor Equivalent circuit
Vbrush Electrical
Rf Ra power in
Φmax
DC Power
V f If Iam Vdc
Eam supply
Mechanical
power out
• Figure 8.13 Equivalent circuit of a separately excited dc motor
• Equivalent circuit is similar to the generator only the current
directions are different
36

37. 8.2.3 DC Motor Equivalent circuit
• The operation equations are:
• Armature voltage equation
Vdc = E am + I am Ra + Vbrush
The induced voltage and motor speed vs angular
frequency
E am = K m I f ω ω = 2 π nm
37

38. 8.2.3 DC Motor Equivalent circuit
• The operation equations are:
• The combination of the equations results in
K m I f ω = E am = Vdc − I am Rm
The current is calculated from this equation. The output
power and torque are:
Pout
Pout = E am I am T= = K m I am I f
ω
38

39. 8.2.4 DC Machine
Excitation Methods
39

40. DC Motor Operation
• There are four different methods for
supplying the dc current to the motor
or generator poles:
– Separate excitation;
– Shunt connection
– Series connection
– Compound
40

41. 8.2.3 DC Motor Equivalent circuit
Vbrush
Ra
Iam Im DC Power
supply
Eam Φmax Rf
Vdc
If
Pout
• Figure 8.14 Equivalent circuit of a shunt dc motor
41

42. 8.2.3 DC Motor Equivalent circuit
Vbrush Ra
Im DC Power
Rf supply
Eam
Vdc
Φmax
Pout
• Figure 8.15 Equivalent circuit of a series dc motor
42

43. 8.2.3 DC Motor Equivalent circuit
Vbrush
Ra
Iam Im DC Power
Rfs supply
Eam
Φmax Vdc
Rfp Ifp
Pout
• Figure 8.16 Equivalent circuit of a compound dc motor
43