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Electromechanical Electromechanical Systems & ActuatorsSystems & Actuators
DC MACHINESDC MACHINES
Dr. Adel Gastli
These slides are the contributions of: Dr. A. Gastli, Dr. A. Al-Badi, and Dr. Amer Al-Hinai
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 2
DC Machines
LEARNING GOALS IntroductionApplication of DC MachineAdvantages & Disadvantages of DC Machine
Construction of DC MachineField SystemArmatureCommentatorBrush
Principle of OperationFaraday’s LawArmature Voltage & Developed Torque
Classification of DC MachinePermanent MagnetSelf-ExcitedSeparately-Excited
DC Machine Representation
Magnetization Curve (Saturation)
DC Motor & Generator Equations
Power Flow & Efficiency
Torque-Speed Characteristics
Starting of DC Machine
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 3
Introduction
Most of the electrical machine in service are AC type.
DC machine are of considerable industrial importance.
DC machine mainly used as DC motors and the DC
generators are rarely used.
DC motors provides a fine control of the speed which
can not be attained by AC motors.
DC motors can developed rated torque at all speeds
from standstill to rated speed.
Developed torque at standstill is many times greater
than the torque developed by an AC motor of equal
power and speed rating.
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 4
The d.c. machine can operate as either a motor or a
generator, at present its use as a generator is limited because
of the widespread use of ac power.
Large d.c. motors are used in machine tools, printing
presses, fans, pumps, cranes, paper mill, traction, textile mills
and so forth.
Small d.c. machines (fractional horsepower rating) are
used primarily as control device-such as tachogenerators for
speed sensing and servomotors for position and tracking.
Application of DC Machines
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 5
DC Motor
Paper Mills
Oil Rigs
MiningMachine Tools Petrochemical
RobotsSteel Mills
Application of DC Machines
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 6
Advantages
• High starting torque
• Rapid acceleration and deceleration.
• Speed can be easily controlled over wide speed range.
• Used in tough gobs (traction motors, electric trains,
electric cars,….)
• Built in wide range of sizes.
Disadvantages
• Needs regular maintenance
• Cannot be used in explosive area
• High cost
Advantages & Disadvantages Of D.C. Motors
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 7
GeneratorMechanical
InputElectricalOutput
MotorElectricalInput
MechanicalOutput
Energy FlowMotor
Generator
Electromechanical Energy Conversion
T Mechanical systemElectrical system v
ωi
+
_
Ideal Electric Machine
Introduction
Electric Machine
v i=T ω
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 8
Shaft
Field yoke
Trailing pole tip
Leading pole tip
Pole core
Field coil
Armature winding
Pole face
Pole axisRotation
Armature coreParts of a DC MachineParts of a DC Machine
Construction of DC Machine
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 92 Pole DC Machine
Stator
Armature
CommutatorCommutator
Shaft
Field coil
Construction of DC Machine
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 10
2000HP DC Motor field System
The field system is to produce uniform magnetic field within which the armature rotates. This consists of Yoke or frame: Acts as a mechanical support of the machine
Construction of DC Machine: Field System
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 11
Cooling ducts for air circulation
TeethSlots
Slots for wedges
Portion of an armature lamination of a dc machine showing slots and teeth
The rotor or the armature core, which carries the rotor or armature winding, is made of sheet-steel laminations. The laminations are stacked together to form a cylindrical structure
The armature coils that make the armature winding are located in the slots
Non-conducting slot liners are wedged in between the coil and the slot walls for protection from abrasion, electrical insulation and mechanical support
Construction of DC Machine: Armature
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 12
Construction of DC Machine: Armature
Armature of a DC Machine
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 13
Commutator
Construction of DC Machine: Commutator
Commutator: is a mechanical rectifier, which converts the alternating voltage generated in the armature winding into direct voltage across the brush. It is made of copper segments insulated from each other by mica and mounted on the shaft of the machine. The armature windings are connected to the commutator segments.
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 14
Commutator and Brushes
Construction of DC Machine: Brush
The purpose of the brush is to ensure electrical connections between the rotating commutator and stationary external load circuit. It is made of carbon and rest on the commutator.
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 15
1 2 3
Brush
Bottomcoil sides
Topcoil sides
Commutator 1 2
Brush
Topcoil sides
Elements of Lap Winding Elements of Wave Winding
Construction of DC Machine: Armature Winding
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 16
Conductors
Turn Coil
End connection
Winding
Construction of DC Machine: Armature Winding
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 17
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
a b c d e
f g h
19 20 21
a b c d e
f g h
+ - + -
+-+
+
+
---
+ +
--
Ia
Icoil
+
-
N SSN
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
pba ==// paths brushes poles
Lap WindingLap Winding
Construction of DC Machine: Armature Winding
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
a b c d e
f g h
19 20 21
a b c d e
fgh
- + - +
-+
++ +
---
+
-
Ia
Icoil
+
-
N SSN
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
i j k1817
i j k
2=a
Wave WindingWave Winding
Nb. of // paths
Construction of DC Machine: Armature Winding
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 19
The Faraday Disk and FaradayThe Faraday Disk and Faraday’’s Laws Law
S
N
ω
φ
+
_
V
Copper disk
Magnet
Conducting shaft
Brush
An emf is induced in a circuit placed in a magnetic field if either:• the magnetic flux linking the circuit is time varying• or there is a relative motion between the circuit and the magnetic field such that the conductors comprising the circuit cut a cross the magnetic flux lines.
• 1st form of the law is the basis of transformers.• 2nd form is the basic principle of operation of electric generators.
Principle of Operation
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 20
B
u
V
Voltmeter
Conductorrails
Movingconductor
le
emf, e
Flux density, B
Velocity, u
The rightThe right--hand rule and generator actionhand rule and generator action
e=BluFaraday’s law or flux cutting rule
s.l.B
A.B
=Φ=Φ
Principle of Operation
dt
s.l.dB
dt
de =
Φ=
dt
dsu,
dt
ds.l.Be ==
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 21
t
v
External circuit
NS
ω
N-turn coil
brushes
Slip rings
v
φ
Field pole
l1
Principle of Operation
Without CommutatorWithout Commutator
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 22
t
v
NS
ab
ω
coil
brushesCommutator
segments
v
Principle of Operation
With CommutatorWith Commutator
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 23
Single-Phase Full wave Rectifier
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 24
p
oo md360ed180pitch pole One ==
Multi-Pole Machines
If p is the number of poles, then p/2 cycles of variation of the flux are encountered every complete mechanical rotation.
N
N
SS
θ
mded
p θθ2
=
θed : electrical degrees or angular measure in cyclesθmd : mechanical degrees or angular measure in space
N N
S S
θedθ3π2ππ 4π
Pole pitchB(θ)
2ππ
θmd
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 25
Principle of Operation: Armature Voltage
( ) 60
..
/60
.
.Re/
.Re/ m
mconductor
Np
N
p
vtime
vFluxEmf
Φ=
Φ==
pathconductorofNumber
EmfEmf conductor
Total /=
where
p = number of poles
Z = total number of armature conductors
a = number of parallel paths, 2 for wave and p for lab.
Φ = flux per pole (Weber)
Nm = speed of the motor in the revolutions per minute (rpm)
time of 1 revolution = 60/Nm (sec)
a
NZp
a
ZNpEmf mm
Total 60
.../
60
.. Φ=⎟
⎠⎞
⎜⎝⎛
⎟⎠⎞
⎜⎝⎛ Φ
=
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 26
Principle of Operation: Armature Voltage
Let π
ωπω.2
60.
60
..2 mm
mm N
N=⇒=
a
Zp
a
ZpEmf mm
Total ..2
...
.2
60..
60
..
πω
πω Φ
=Φ
=
ωm= speed of the motor in radians per second
maTotal KEmf ω..Φ= Ka: armature constanta
ZpK a ..2
.
π=
Generated voltage : generator operationBack emf : motor operation
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 27
Developed (or Electromagnetic) Torque
Consider the turn shown in the following Figure.
Area per pole A =p
rlπ2
The force on a conductor is a
IlBf a
c =
The torque developed by a conductor is rfT cc = a
Ipr
a
IlB aa
π2
Φ==
The total torque developed is
m
aaaa
ae
IEIK
a
IZpT
ωπ=Φ=
Φ=
2
lr
p
AB
π2
Φ=
Φ=Flux density
Current / conductor isa
II a
c =
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 28
Production of Unidirectional Torque and Operation of an Elementary
B
I
F
Left-hand rule
With this configuration the torque is unidirectional and independent of conductor position
Position of conductor a under N-pole
+a b
a
b
ω
I1 2
N S
F
F
Position of conductor a under S-pole
+a b
ba
ω
I1 2
N S
F
F
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 29
DC Machine
DifferentialCumulative
LongShunt
ShortShunt
Classification of DC Machine
Self-excited
Separately excited
Permanentmagnet
Shunt
Series
Compound
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 30
Field Armature
Separately excited
Field
Armature
Shunt
Field
Armature
Series
Short-shunt
φfφs
S1 S2F2F1
A2
A1
Long-shunt
φfφs
S1 S2F2F1
A2
A1
Classification of DC Machine
Motor operationGenerator operation
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 31
Cumulative compound
φfφs
S1 S2F2F1
A2
A1
Differential compound
φfφs
S1 S2F2F1
A2
A1
Classification of DC Machine
Motor operationGenerator operation
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 32
DC Machine Representation
Field
Armature
q-axis
d-axis
The mmf’s produced by the field circuit and the armature circuit are in quadrature.
Field mmf
d-axis
Arm
ature m
mf
q-a
xis
φf
φaArmature mmf
Field mmf
Φ
Fp
Flux-mmf relation in a dc machine
Saturation
Linear
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 33
Magnetization (or Saturation) Curve of a DC Machine
Ea
If
Magnetization curve
Speed ωm
0.5 ωm
The magnetizing curve is obtained experimentally by rotating the the dc machine at a given speed and measuring the open-circuit armature terminal voltage as the current in the field winding is changed.
MagnetizationCurve
Represents the saturation level in the magnetic system of the dcmachine for various values of excitation mmf .
If Nf
Φ
Flux-mmf relation in a dc machine
Saturation
Linear
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 34
Separately Excited DC MotorSeparately Excited DC Motor
Rfw
Rfc
If Vf+ −
Ra
Ia
It
Vt
+
−
ωm
aae
maa
aata
fff
IKT
KE
RIVE
IRV
Φ=Φ=−=
=
ω
Rfw: resistance of field winding.
Rfc: resistance of control rheostat used in field circuit.
Rf=Rfw+Rfc: total field resistance
Ra: resistance of armature circuit, including the effect of brushes. Sometimes Ra is shown as the resistance of armature winding alone; the brush-contact voltage drop is considered separately and is usually assumed to be about 2V.
Dc Motors Equations
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 35
Shunt or SelfShunt or Self--Excited DC MotorExcited DC Motor
ftaLtt
aaemaa
aata
tfff
IIIRIV
IKTKE
RIVE
VIRV
−==Φ=Φ=
−=
==
,
,ω
Rfw
Rfc
If
+
−
Ra
Ia
It
Vt
+
−
ωm
Dc Motors Equations
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 36
Separately Excited DC GeneratorSeparately Excited DC Generator
La
LLt
maa
aata
ffffcfwf
II
RIV
KE
rIVE
IRIRRV
==
Φ=+=
=+=
ω
)(
Rfc
ra
RL
Ia IL
Vt
+
−
ωm
Rfw
If Vf+ −
Ea
+
−
Dc Generator Equations
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 37
SelfSelf--Excited DC GeneratorsExcited DC Generators
1. Shunt generator 1. Shunt generator
Rfc
RLRfw
If
+
−
ra
Ia
IL
Vt
+
−
ωm
−
Ea
fLa
LLt
maa
aata
tfff
III
RIV
KE
rIVE
VIRV
+==
Φ=+=
==
ω
Dc Generator Equations
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 38
2. Series Generator2. Series Generator
IL
RL+
−
ra
Rs
Ia
Vt
+
−
Ea
msaa
faL
saaat
KE
III
RrIEV
ωΦ=
==+−= )(
Dc Generator Equations
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 39
3. Compound3. Compound DC Generator DC Generator
Rfc
If
Rfw
+
−
Ra
Rs
Ia
IL
Vt
+
−
Ea
msshaa
fcfw
aaaf
faL
sLaaat
KE
RR
RIEI
III
RIRIEV
ω)( Φ±Φ=
+−
=
−=−−= ( )
msshaa
fcfw
tf
faL
saaat
KE
RR
VI
III
RRIEV
ω)( Φ±Φ=
+=
−=+−=
Rfw
Rfc
+
−
Ra
Rs
Ia
IL
Vt
+
−
Ea
If
Long ShuntShort Shunt
( ) msshaa KE ωΦ±Φ=
+ −
Cumulative Differential
Dc Generator Equations
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 40
Power Flow and Efficiency
Rfw
Rfc
If
+
−
Ra
Rs
Ia
IL
Vt
+
−
Ea
Poutput= Pelectrical
Pinput= Pmech = Pshaft
Rotational losses
a2a RI f
2f RI
aa IELaIVaa IV
sL RI 2
Lt IV
LossesRotationalIE
IV
LossesRotationalRIIV
IV
LossesP
P
P
P
aa
Lt
Lt
Lt
output
output
input
output
+=
++=
+==
∑
η
η
η
2
DC Generators
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 41
Power Flow and Efficiency
DC Motors
Rfw
Rfc
If
+
−
Ra
Rs
Ia
IL
Vt
+
−
Ea
Pinput = Pelectrical
Rotational lossesa
2a RIf
2f RI
aaIELaIV aaIV
sLRI 2
Poutput= Pmech= PshaftLtIV
Lt
aa
Lt
Lt
input
input
input
output
IV
LossesRotationalIE
IV
LossesRotationalRIIV
P
LossesP
P
P
−=
−−=
−==
∑
η
η
η
2
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 42
Ia
Torque-Speed Characteristics
Therefore , TK
r
K
V
a
a
a
tm 2)( Φ
−Φ
=ω
Φ−
=a
aatm K
rIVω
aa IKT Φ=
aaat rIEV +=maa KE ωΦ=
Separately excited & Shunt motors
(φ is independent of the load torque )
ωm
T
Slope 2)( Φa
a
K
rΦa
t
K
V
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 43
Torque-Speed Characteristics
Series motors
maa
saata
KE
RRIVE
ωφ=+−= )(
s
sa
s
tm
asaaaa
s
sa
as
tm
K
RR
TK
V
IKIKKIKTBut
K
RR
IK
V
+−=∴
===
+−=
ω
φ
ω
22
1
af IKIK 11 ==φNeglecting saturation
masmaaa IKIKKE ωω == 1
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 44
Torque-Speed Characteristics
Compound motors
seriesshuntt φφφ ±=Differential Compound
Cumulative Compound
seriesshuntt ATATAT ±=
TK
r
K
V
ta
a
ta
tm 2)( φφ
ω −=
Shunt motor
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 45
Starting of DC Machine
If a d.c. motor is directly connected to a d.c. power supply, the starting current will be dangerously high.
a
ata r
EVI
−= at starting 0E0 a =→=ω
a
tStartinga r
VI =∴
Since ra is small, the starting current is very large.The starting current can be limited by the following methods:1- Use a variable-voltage supply.2- Insert an external resistance at start, as shown in the Figure.