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DC Machines
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Introduction: What are DC Machines?
Are DC generators that convert mechanical energy to DC electric energy.
Are DC motors that convert DC electric energy to mechanical energy.
Chapman S.J., “Electric Machinery Fundamentals”
3
Introduction
DC machine can be used as a motor or as a generator.
DC Machine is most often used for a motor.
Cutaway view of a dc motor
DC motors are found in many special industrial environments Motors drive many types of loads from fans and pumps to presses and conveyors
The major advantages of dc machines over generators are easy to control 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.
4
Types of DC Motors
DC motors are classified according to electrical connections of
armature windings and field windings.
Armature windings: a winding which a voltage is induced
Field windings: a winding that produces the main flux in machines
Five major types of DC motors:-
Separately excited DC motor
Shunt DC motor
Permanent Magnet DC motor
Series DC motor
Compounded DC motor
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DC Machines Construction
DC motor stator with poles visible
Rotor of a dc motor
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DC Machines Construction
DC machines, like other
electromechanical energy conversion devices have
two sets of electrical
windings
field windings - on stator
amarture windings - on the rotor.
.
7
DC Machines Construction
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.
8
DC Machines Construction
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.
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DC Machines Construction
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.
10
DC Machines Construction
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.
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DC Motor Operation
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Current in DC Motor
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Magnetic Field in DC Motor
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Force in DC Motor
15
Basic Principle of Operation
The generated voltage of a DC machines having (p) poles and (Z) conductors on the armature with (a) parallel path between brushes as below :
K
a
pZEA
2where K = pZ /(2πa) = machine constant
The mechanical torque which also equal to electromagnetic torque, is found as follows:
AAA
me IKIE
In the case of a generator, m is the input mechanical torque, which is converted to electrical power. For the motor, e is developed electromagnetic torque, which used to drive the mechanical load.
16
ARMATURE winding are defined as the winding which a voltage is induced.
FIELD windings are defined as the windings that produce the main flux in the machines.
The magnetic field of the field winding is approximately sinusoidal, thus AC voltage is induced in the armature winding as the rotor turns under the magnetic field of stator.
The COMMUTATOR and BRUSH combination converts the AC generated voltages to DC.
Basic Principle of Operation
17
The induced or generated DC voltage (EA) appearing between the brushes is a function of the field current (IF) and the speed of rotation () of the machine. This generated voltage is :
FA IKE '
Where K’ = voltage constant = rotation per min
If the losses of the DC machine are neglected, the electrical power is equal to the mechanical power
mAAIE
Basic Principle of Operation
18
Generation of Unidirectional Voltage
As the rotor is rotated at an angular velocity (), the armature flux linkage () change and a voltage eaa’ is induced between terminal a and a’. The expression for the voltage induced is given by Faraday’s Law
dt
deaa
'
Two pole DC generator
a) Flux linkage of coil aa’; b) induced voltage;
c) rectified voltage
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DC Motor Equivalent Circuit
Note: Because a dc motor is the same physical machine as a dc generator, its equivalent circuit is exactly the same as generator except for the direction of current flow.
RA
Armature circuit (entire rotor structure)
The brush voltage drop
Field Coils
External variable resistor used to control the amount of current in the field circuit
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Simplified Equivalent Circuit
The brush drop voltage (Vbrush ) is often only a very tiny fraction of the generated voltage in the machine – Neglected or included in RA.
Internal resistance of the field coils is sometimes lumped together with the variable resistor and called RF - Combining Radj with field resistance (RF).
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The internal generated voltage in the motor KEA
From the equation,
EA is directly proportional to the flux () in the motor and speed of the motor ().
The field current (IF) in dc machines produces a field magnetomotive force (mmf)
This magnetomotive force (mmf) produces a flux () in the motor in accordance with its magnetization curve.
The magnetization curve of a ferromagnetic material ( vs F)
The Magnetization Curve of a DC machine
IF mmf flux
22
Since the field current (IF) is directly proportional to magnetomotive force (mmf) and…….
EA is directly proportional to the flux, the magnetization curve is presented as a plot EA versus field current for a given speed.
The Magnetization Curve of a DC machine
AE
The magnetization curve of a dc machine expresses as a plot of EA versus IF, for a fixed speed ω0
Note: To get the maximum possible power, the motors and generators are designed to operate near the saturation point on the magnetization curve (at the knee of the curve).
23
The Magnetization Curve
AE
The magnetization curve of a dc machine expresses as a plot of EA versus IF, for a fixed speed ω0
The induced torque developed by
the motor is given as
Aind IK
24
The equivalent circuit of Separately Excited DC Motor
F
FF
R
VI
AAAT RIEV
AL II
Separately excited motor is a motor whose field current is supplied from a separate constant-voltage power supply.
25
The equivalent circuit of a Shunt DC Motor
F
TF
R
VI
AAAT RIEV
FAL III
A shunt dc motor is a motor whose field circuit get its power directly across the armature terminals of the motor.
26
How Shunt response to load? - Speed-Torque Characteristics
KEA
AAT RIKV
AAT RIKV
Consider the DC shunt motor. From the Kirchoff’s Law
Induced Voltage
Substituting the expression for induced voltage between VT and EA.
AAAT RIEV
Since then, current IA can be expressed as
KI ind
A
indAT
K
R
K
V
2)(
Finally, solving for the motor's speed yield
Aind
T RK
KV
27
Torque-speed characteristic of a shunt or separately excited dc motor
ind then , with constant VT,
otherwise it affect the torque-speed curve
This equation is a straight line with a negative slope. The graph shows the torque-speed characteristics of a shunt dc motor.
indAT
K
R
K
V
2)(
Speed-Torque Characteristics
28
indAT
K
R
K
V
2)(
Torque-speed characteristic of a motor with armature reaction present.
Affect of Armature Reaction (AR) will reduce flux as the load increase (ind also increase), so it will increase motor speed (). =>
If the motor has compensating winding, the flux () will be constant.
KEA
Speed-Torque Characteristics
29
In order for the motor speed to vary linearly with torque, the other term in this expression must be constant as the load changes.
The terminal supplied by the dc power source is assumed to be constant – if not, then the voltage variations will effect the shape of the torque-speed curve.
However, in actual machine, as the load increase, the flux is reduced because of the armature reaction. Since the denominator terms decrease, there is less reduction in speed and speed regulation is improved (as shown in previous slide).
If a motor has compensating windings, of course there will be no flux-weakening problem in the machines, and the flux in the machine will be constant
Speed-Torque Characteristics
30
Speed Control of Shunt DC Motor
Two common ways in which the speed () of a shunt dc machine can be controlled. • Adjusting the field resistance RF (and thus the field flux) • Adjusting the terminal voltage applied to the armature. The less common method of speed control is by • Inserting a resistor in series with armature circuit.
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1 : Changing The Field Resistance
F
T
R
V
K
A
AT
R
EV
loadind
1. Increasing RF causes IF
6. Increasing τind makes
to decrease.
2. Decreasing IF decreases .
3. Decreasing lowers EA
4. Decreasing EA by increasing IA
5. Increase IA by increasing )( Aind IK
with the change in IA dominant over the change in flux ().
and the speed ω increases.
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1: Changing The Field Resistance
7. Increasing speed to increases EA = K again.
8. Increasing EA decreases IA.
loadind 9. Decreasing IA decreases until ind at a higher speed ω
Decreasing RF would reverse the whole process, and the speed of the motor would drop.
The effect of field resistance speed control on a shunt motor’s torque speed characteristic: over the motor’s normal operating range
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1. An increase in VA by increasing IA = [ (VA – EA)/RA]
4. Increasing ω increases EA =(Kω )
2: Changing The Armature Voltage
2. Increasing IA increases )( Aind IK
3. Increasing τind makes loadind increasing ω.
5. Increasing EA by decreasing IA = [(VA – EA)/RA]
6. Decreasing IA decreases τind until loadind at a higher ω.
Armature voltage control of a shunt (or separately excited) dc motor.
34
2: Changing The Armature Voltage
The effect of armature voltage speed control on a shunt motor’s torque speed characteristic
The speed control is shifted by this method, but the slope of the curve remains constant
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3 : Inserting Resistor in Series with Armature Circuit Add resistor in series with RA
The effect of armature resistance speed control on a shunt motor’s torque – speed
characteristic
Equivalent circuit of DC shunt motor
Additional resistor in series will drastically increase the slope of the motor’s characteristic, making it operate more slowly if loaded
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3 : Inserting Resistor in Series with Armature Circuit Add resistor in series with RA
Equivalent circuit of DC shunt motor
This method is very wasteful method of speed control, since the losses in the inserted resistor is very large. For this it is rarely used.
indAT
K
R
K
V
2)(
The above equation shows if RA
increase, speed will decrease
37
The Series DC Motor
Equivalent circuit of a series DC motor.
The Kirchhoff’s voltage law equation for this motor
)( SAAAT RRIEV
38
Induced Torque in a Series DC Motor
The induced or developed torque is given by Aind IK
The flux in this motor is directly proportional to its armature current. Therefore, the flux in the motor can be given by
AcI
where c is a constant of proportionality. The induced torque in this machine is thus given by
2
AAind KcIIK
This equation shows that a series motor give more torque per ampere than any other dc motor, therefore it is used in applications requiring very high torque, example starter motors in cars, elevator motors, and tractor motors in locomotives.
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To determine the terminal characteristic of a series dc motor, an analysis will be based on the assumption of a linear magnetization curve, and the effects of saturation will be considered in a graphical analysis The assumption of a linear magnetization curve implies that the flux in the motor given by :
The Terminal Characteristic of a Series DC Motor.
AcI
)( SAAAT RRIEV
The derivation of a series motor’s torque-speed characteristic starts with Kirchhoff’s voltage law:
KcI ind
A
From the equation; the armature current can be expressed as:
2
AAind KcIIK
40
The Terminal Characteristic of a Series DC Motor.
Also, EA = K, substituting these expression yields:
)( SAind
T RRKc
KV
2c
Kind
Substituting the equations so the induced torque equation can written as
indK
c
Therefore, the flux in the series motor can be written as :
We know ; c
I A
41
The Terminal Characteristic of a Series DC Motor.
Substituting the previous equation for VT yields:
)( SAind
indT RRKcK
cKV
The resulting torque – speed relationship is
Kc
RR
Kc
V SA
ind
T
1
One disadvantage of series motor can be seen immediately from this equation. When the torque on this motor goes to zero, its speed goes to infinity.
In practice, the torque can never go entirely to zero, because of the mechanical, core and stray losses that must be overcome.
42
The Terminal Characteristic of a Series DC Motor.
However, if no other load is connected to the motor, it can turn fast enough to seriously damage itself.
NEVER completely unload a series motor, and never connect one to a load by a belt or other mechanism that could break.
Fig : The ideal torque- speed characteristic of a series dc motor
43
Speed Control of Series DC Motor
Method of controlling the speed in series motor.
1. Change the terminal voltage of the motor. If the terminal voltage is increased, the speed also increased, resulting in a higher speed for any given torque. This is only one efficient way to change the speed of a series motor.
Kc
RR
Kc
V SA
ind
T
1
2. By the insertion of a series resistor into the motor circuit, but this technique is very wasteful of power and is used only for intermittent period during the start-up of some motor.
44
The Compounded DC Motor.
A compound DC motor is a motor with both a shunt and a series field
Two field windings : One is connected in series with armature (series field) and the other is connected in parallel with the armature (shunt field).
The equivalent compound DC motor (a) Long-shunt connection (cumulative compounding) (b) Short-shunt connection (differential compounding)
shu
nt
series
shu
nt
series
45
The equivalent compound DC motor (a) Long-shunt connection (b) Short-shunt connection
shu
nt
series
shu
nt
series
If the magnetic fluxes produced by both series field and shunt field windings are in same direction, that is, additive, the dc motor is cumulative compound. If the magnetic fluxes are in opposite, the dc motor is differential compound.
The Compounded DC Motor.
46
The equivalent compound DC motor (a) Long-shunt connection (b) Short-shunt connection
shu
nt
series
shu
nt
series
In long shunt compound dc motor, the series field is connected in series with armature and the combination is in parallel with the shunt field. In the short shunt field compound dc motor, the shunt field is in parallel with armature and the combination is connected in series with the series field.
The Compounded DC Motor.
47
The Kirchhoff’s voltage law equation for a compound dc motor is:
)( SAAAT RRIEV
The currents in the compounded motor are related by :
FLA III F
TF
R
VI
The net magnetomotive force given by
F net = F F ± FSE - FAR
FF = magnetmotive force (shunt field)
FSE = magnetomotive force (series field)
FAR = magnetomotive force (armature reaction)
The Compounded DC Motor.
48
The effective shunt field current in the compounded DC motor given by:
F
ARA
F
SE
FFN
FI
N
NII *
NSE = winding turn per pole on series winding
NF = winding turn per pole on shunt winding
The positive (+) sign is for cumulatively compound motor The negative (-) sign is for differentially compound motor
The Compounded DC Motor.
49
The Torque Speed Characteristic of a Cumulatively Compounded DC Motor
The cumulatively compounded motor has a higher starting torque than a shunt motor (whose flux is constant) but a lower starting torque than a series motor (whose entire flux is proportional to armature current). It combines the best features of both the shunt and the series motors. Like a series motor, it has extra torque for starting; like a shunt motor, it does not over speed at no load.
At light loads, the series field has a very small effect, so the motor behaves approximately as a shunt dc motor. As the load gets very large, the series flux becomes quite important and the torque speed curve begins to look like a series motor’s characteristic. A comparison of these torque speed characteristics of each types is shown in next slide.
50
The Torque Speed Characteristic of a Cumulatively Compounded DC Motor
Fig (a) The torque-speed characteristic of a cumulatively compounded dc motor compared to series and shunt motors with the same full-load rating.
Fig. (b) The torque-speed characteristic of a cumulatively compounded dc motor compared to a shunt motor with the same no-load speed.
51
The Torque Speed Characteristic of a Differently Compounded DC Motor
In a differentially compounded DC motor, the shunt magnetomotive force and series magnetomotive force subtract from each other. This means that as the load on the motor increase, IA increase and the flux in the motor decreased, (IA) As the flux decrease, the speed of the motor increase, () This speed increase causes an-other increase in load, which further increase IA, Further decreasing the flux, and increasing the speed again. All the phenomena resulting the differentially compounded motor is unstable and tends to run away. This instability is much worse than that of a shunt motor with armature reaction, and make it unsuitable for any application.
52
Speed Control in the Cumulatively Compounded DC
Motor
The techniques available for control of speed in a cumulatively compounded dc motor are the same as those available for a shunt motor: 1. Change the field resistance, RF
2. Change the armature voltage, VA
3. Change the armature resistance, RA
The arguments describing the effects of changing RF or VA are very similar to the arguments given earlier for the shunt motor.
53
DC Motor Starter
In order for a dc motor to function properly on the job, it must have some special control and protection equipment associated with it. The purposes of this equipment are: 1. To protect the motor against damage due to short circuits in the equipment
2. To protect the motor against damage from long term overloads
3. To protect the motor against damage from excessive starting currents
4. To provide a convenient manner in which to control the operating speed of the
motor
54
DC Motor Problem on Starting
DC motor must be protected from physical damage during the starting period.
At starting conditions, the motor is not turning, and so EA = 0 V.
Since the internal resistance of a normal dc motor is very low, a very high current flows, hence the starting current will be dangerously high, could severely damage the motor, even if they last for only a moment.
Consider the dc shunt motor:
A
T
A
ATA
R
V
R
EVI
When EA = 0 and RA is very small, then the current IA will be very high. Two methods of limiting the starting current : • Insert a starting resistor in series with armature to limit the current flow
(until EA can build up to do the limiting). The resistor must be not permanently to avoid excessive losses and cause torque speed to drop excessively with increase of load.
• Manual DC motor starter, totally human dependant
55
Inserting a Starting Resistor in Series & Manual DC Motor
Fig : A shunt motor with a starting resistor in series with an armature. Contacts 1A, 2A and 3A short circuit portions of the starting resistor when they close
Fig : A Manual DC Motor
Human dependant: • Too quickly, the resulting current flow
would be too large. • Too slowly, the starting resistor could burn-
up
56
DC Motor Efficiency Calculations
To calculate the efficiency of a dc motor, the following losses must be determined : • Copper losses (I2R losses) • Brush drop losses • Mechanical losses • Core losses • Stray losses
Stray losses
Pout =out m
I2R losses Mechanical
losses Core loss
Pconv = Pdev = EAIA=indω
Pin =VTIL
57
DC Motor Efficiency Calculations
Electrical or Copper losses : Copper losses are the losses that occur in the Armature and field windings of the machine. The copper losses for the armature and field winding are given by : Armature Loss PA = IA
2RA
Field Loss PF = IF2RF
PA = Armature Losses PF = Field Circuit Losses The resistance used in these calculations is usually the winding resistance at normal operating temperature Brush Losses : The brush drop loss is the power loss across the contact potential at the brushes of the machines. It is given by the equation: PBD = VBDIA
Must consider RS for series and compound DC Motors
58
DC Motor Efficiency Calculations
Magnetic or core loss : These are the hysteresis and eddy current losses
occuring in the metal of the motor.
Mechanical loss : These are friction and windage losses.
• Friction losses include the losses caused by bearing friction and the friction
between the brushes andcommutator.
• Windage losses are caused by the friction between rotating parts and air
inside the DC machine’s casing.
Stray losses (or Miscellaneous losses) : These are other losses that cannot be
placed in one of the previous categories. (Is about 1% of full load-RULE OF
THUMB) [[pg 318,Electric Machinery and Transformers, BHAG S. GURU] and [pg
525, Electric Machinery Fundamentals, STEPHEN J. CHAPMAN]
59
DC Motor Efficiency Calculations
Rotational losses is when the mechanical losses, Core losses and Stray losses are lumped together. [pg. 193 Electromechanical Energy Devices and Power System, ZIA A. ZAMAYEE & JUAN L. BALA JR.] It also consider as combination between mechanical and core losses at no load and rated speed.[pg 317, Electric Machinery and Transformers, BHAG S. GURU] and [pg 593, Electric Machinery Fundamentals, STEPHEN J. CHAPMAN]
Motor efficiency :
%100
%100
XP
PP
XP
P
input
lossesinput
input
output
60
Speed Regulation
The speed regulation is a measure of the change speed from no-load to full load. The percent speed regulation is defined
Speed Regulation (SR):
%100
%100
X
or
X
fl
flnl
fl
flnl
+Ve SR means that the motor speed will decrease when the load on its shaft is increased.
-Ve SR means that the motor speed increases with increasing load.
61
DC Generators
DC generators are dc machines used as generator. There are five major types of dc generators, classified according to the manner in which their field flux is produced:
• Separately excited generator: In separately excited generator, the field flux is derived from a separately power source independent of the generator itself.
• Shunt generator: In a shunt generator, the field flux is derived by connecting the field circuit directly across the terminals of the generators.
• Series generator: In a series generator, the field flux is produced by connecting the field circuit in series with the armature of the generator.
• Cumulatively compounded generator: In a cumulatively compounded generator, both a shunt and series field is present, and their effects are additive.
• Differentially compounded generator: In differentially compounded generator: In a differentially compounded generator, both a shunt and a series field are present, but their effects are subtractive.
62
DC Generators
These various types of dc generator differ in their terminal (voltage-current) characteristic, and the application is depending to which is suited.
DC generators are compared by their voltages, power ratings, efficiencies and voltage regulations:
%100
fl
flnl
V
VVVR
+VR = Dropping characteristics
-VR = Rising characteristic
63
Equivalent Circuit of DC Generators
The equivalent circuit of a DC generator
A simplified equivalent circuit of a DC generator, with RF combining
the resistances of the field coils and the variable control resistor
64
Separately Excited Generator
Fig : Separately excited DC generator
A separately excited DC generator is a generator whose field current is supplied by a separately external DC voltage source VT = Actual voltage measured at the terminals of the generator IL = current flowing in the lines connected to the terminals. EA = Internal generated voltage. IA = Armature current.
AL II
65
The Terminal Characteristic of A Separately Excited DC Generator
The terminal characteristic of a separately excited dc generator (a) with and (b) without compensating windings (EA = K)
For DC generator, the output quantities are its terminal voltage and line current. The terminal voltage is VT = EA – IARA (IA = IL)
Since the internal generated voltage EA is independent of IA, the terminal characteristic of the separately excited generator is a straight line.
Take note about the axes
between motors ( and
ind) and generators (VT
and IL)
66
The Terminal Characteristic of A Separately Excited DC Generator
When the load is supplied by the generator is increased, IL (and therefore IA) increase. As the armature current increase, the IARA drop increase, so the terminal voltage of the generator falls. (Figure (a) PREVIOUS SLIDE)
This terminal characteristic is not always entirely accurate. In the generators without compensating windings, an increase in IA causes an increase in the armature reaction, and armature reaction causes flux weakening. This flux weakening causes a decrease in EA = Kω which further decreases the terminal voltage of the generator. The resulting terminal characteristic is shown in Figure b (PREVIOUS SLIDE)
67
Control of Terminal Voltage
We control torque-speed in DC Motor, while in DC Generator we control VT
The terminal voltage of a separately excited DC generator can be controlled by changing the internal generated voltage EA of the machine. VT = EA – IARA
If EA increases, VT will increase, and if EA decreases, VT will decreases. Since the internal generated voltage, EA = KΦω, there are two possible ways to control the voltage of this generator: 1. Change the speed of rotation. If ω increases, then EA = KΦω increases, so VT
= EA - IARA increases too. 2. Change the field current. If RF is decreased, then the field current increases (IF =VF/RF ). Therefore, the flux Φ in the machine increases. As the flux rises,
EA= K ω must rise too, so VT = EA – IARA increases.
68
The Shunt DC Generator
A shunt DC generator is a DC generator that supplies its own field current by having its field connected directly across the terminals of the machine.
Figure : The equivalent circuit of a shunt DC generator.
F
TF
AAAT
LFA
R
VI
RIEV
III
Because of generator supply it own field current, it required voltage buildup
69
Voltage Buildup in A Shunt Generator
Assume the DC generator has no load connected to it and that the prime mover starts to turn the shaft of the generator. The voltage buildup in a DC generator depends on the presence of a residual flux in the poles of the generator. This voltage is given by
resA KE
This voltage, EA (a volt or two appears at terminal of generators), and it causes a current IF to flow in the field coils. This field current produces a magnetomotive force in the poles, which increases the flux in them. EA, then VT increase and cause further increase IF, which further increasing the flux and so on. The final operating voltage is determined by intersection of the field resistance line and saturation curve. This voltage buildup process is depicted in the next slide
70
EA may be a volt or two appear at the terminal during start-up
Voltage buildup occurred in discrete steps
Voltage Buildup in A Shunt Generator
71
Voltage Buildup in A Shunt Generator
Several causes for the voltage to fail to build up during starting which are : • Residual magnetism. If there is no residual flux in the poles, there is no
Internal generated voltage, EA = 0V and the voltage will never build up. • Critical resistance. Normally, the shunt generator builds up to a voltage
determined by the intersection of the field resistance line and the saturation curve. If the field resistance is greater than critical resistance, the generator fails to build up and the voltage remains at the residual level. To solve this problem, the field resistance is reduced to a value less than critical resistance.
Refer Figure 9-51 page 605 (Chapman)
Critical resistance
72
• The direction of rotation of the generator may have been reversed, or the connections of the field may have been reversed. In either case, the residual flux produces an internal generated voltage EA. The voltage EA produce a field current which produces a flux opposing the residual flux, instead of adding to it.
Under these conditions, the flux actually decreases below res and no voltage can ever build up.
Voltage Buildup in A Shunt Generator
73
The Terminal Characteristic of a Shunt DC Generator
Figure : The terminal characteristic of a shunt dc generator
As the load on the generator is increased, IL increases and so IA = IF + IL also increase. An increase in IA increases the armature resistance voltage drop IARA, causing VT = EA -IARA to decrease. However, when VT decreases, the field current IF in the machine decreases with it. This causes the flux in the machine to decrease; decreasing EA. Decreasing EA causes a further decrease in the terminal voltage, VT = EA - IARA
74
Voltage Control for Shunt DC Generator
There are two ways to control the voltage of a shunt generator: 1. Change the shaft speed, ωm of the generator. 2. Change the field resistor of the generator, thus changing the field current.
Changing the field resistor is the principal method used to control terminal voltage in real shunt generators. If the field resistor RF is decreased, then the field current IF = VT/RF increases. When IF , the machine’s flux , causing the internal generated voltage EA. EA causes the terminal voltage of the generator to increase as well.
75
The Series DC Generator
Figure : The equivalent circuit of a series dc generator
A series DC generator is a generator whose field is connected in series with its armature. Because the field winding has to carry the rated load current, it usually have few turns of heavy wire. Clear distinction, shunt generator tends to maintain a constant terminal voltage while the series generator has tendency to supply a constant load current. The Kirchhoff’s voltage law for this equation : )( SAAAT RRIEV
76
Terminal Characteristic of a Series Generator
The magnetization curve of a series DC generator looks very much like the magnetization curve of any other generator. At no load, however, there is no field current, so VT is reduced to a very small level given by the residual flux in the machine. As the load increases, the field current rises, so EA rises rapidly. The IA (RA + RS) drop goes up too, but at first the increase in EA goes up more rapidly than the IA(RA + RS) drop rises, so VT increases. After a while, the machine approaches saturation, and EA becomes almost constant. At that point, the resistive drop is the predominant effect, and VT starts to fall.
Figure : A series generator terminal characteristic with large armature reaction effects
77
The Cumulatively Compounded DC Generator
Figure : The equivalent circuit of a cumulatively compounded DC generator with a long shunt connection
A cumulatively compounded DC generator is a DC generator with both series and shunt fields, connected so that the magnetomotive forces from the two fields are additive.
78
The Cumulatively Compounded DC Generator
The total magnetomotive force on this machine is given by
Fnet = FF + FSE - FAR where FF = the shunt field magnetomotive force FSE = the series field magnetomotive force FAR = the armature reaction magnetomotive force
NFI*F = NFIF + NSEIA - FAR
F
ARA
F
SEFF
N
FI
N
NII *
79
The Cumulatively Compounded DC Generator
The other voltage and current relationships for this generator are
F
TF
SAAAT
LFA
R
VI
RRIEV
III
)(
80
Another way to hook up a cumulatively compounded generator. It is the “short-shunt” connection, where series field is outside the shunt field circuit and has current IL flowing through it instead of IA.
Figure : The equivalent circuit of a cumulatively DC generator with a short shunt connection
The Cumulatively Compounded DC Generator
81
The Terminal Characteristic of a Cumulatively Compounded DC Generator
When the load on the generator is increased, the load current IL also increases. Since IA = IF + IL, the armature current IA increases too. At this point two effects occur in the generator: 1. As IA increases, the IA (RA + RS) voltage drop increases as well. This tends to
cause a decrease in the terminal voltage, VT = EA –IA (RA + RS).
2. As IA increases, the series field magnetomotive force FSE = NSEIA increases too. This increases the total magnetomotive force Ftot = NFIF + NSEIA which increases the flux in the generator. The increased flux in the generator increases EA, which in turn tends to make VT = EA – IA (RA + RS) rise.
82
Voltage Control of Cumulatively Compounded DC Generator
The techniques available for controlling the terminal voltage of a cumulatively compounded DC generator are exactly the same as the technique for controlling the voltage of a shunt DC generator: 1. Change the speed of rotation. An increase in causes EA = K to increase,
increasing the terminal voltage VT = EA – IA (RA + RS).
2. Change the field current. A decrease in RF causes IF = VT/RF to increase, which increase the total magnetomotive force in the generator. As Ftot increases, the flux in the machine increases, and EA = K increases. Finally, an increase in EA raises VT.
83
Analysis of Cumulatively Compounded DC Generators
The equivalent shunt field current Ieq due to the effects of the series field and armature reaction is given by
F
ARA
F
SEeq
NI
N
NI
F
The total effective shunt field current is eqFF III *
NSE = series field turns
NF = shunt field turns
FAR = armature force
IA = armature current
where,
84
Field Resistance
IA (RA + RS)
VT at no load condition will be the point at which the resistor line and magnetization curve intersect.
As load is added, mmf increased thus increasing the field current Ieq and the resistive voltage drop [IA(RA + RF)].
The upper tip triangle represents the internal generated voltage EA.
The lower line represents the terminal voltage VT
85
The Differentially Compounded DC Generator
)( FAAAT
F
TF
FLA
RRIEV
R
VI
III
A differentially compounded DC generator is a generator with both shunt and series fields, but this time their magnetomotive forces subtract from each other.
The equivalent circuit of a differentially compounded DC generator
86
The Differentially Compounded DC Generator
The net magnetomotive force is
Fnet = FF – FSE – FAR Fnet = NFIF – NSEIA - FAR And the equivalent shunt field current due to the series field and armature reaction is given by :
F
ARA
F
SEeq
NI
N
NI
F
The total effective shunt field current in this machine is
eqFF III *
or
F
ARA
F
SEFF
NI
N
NII
F*
87
Voltage Control of Differentially Compounded DC Generator
Two effects occur in the terminal characteristic of a differentially compounded DC generator are 1. As IA increases, the IA (RA + RS) voltage drop increases as well. This increase
tends to cause the terminal voltage to decrease VT.
2. As IA increases, the series field magnetomotive FSE = NSEIA increases too. This increases in series field magnetomotive force reduces the net magnetomotive force on the generator, (Ftot = NFIF – NSEIA), which in turn reduces the net flux in the generator. A decrease in flux decreases EA, which in turn decreases VT.
Since both effects tend to decrease VT, the voltage drop drastically as the load is increased on the generator as shown in next slide
88
Voltage Control of Differentially Compounded DC Generator
89
Voltage Control of Differentially Compounded DC Generator
The techniques available for adjusting terminal voltage are exactly the same as those for shunt and cumulatively compounded DC generator: 1. Change the speed of rotation, m. 2. Change the field current, IF.