DC Machines
1
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”
2
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.
3
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
4
DC Machines Construction
DC motor stator with poles visible
Rotor of a dc motor
5
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.
6
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.
7
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.
8
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.
9
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.
10
DC Motor Operation
11
Current in DC Motor
12
Magnetic Field in DC Motor
13
Force in DC Motor
14
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 :
pZ
EA 
 K
2a
where K = pZ /(2πa) = machine constant
The mechanical torque which also equal to electromagnetic torque, is found
as follows:
e m 
EA I A

 KI A
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.
15
Basic Principle of Operation
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.
16
Basic Principle of Operation
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 :
EA  K ' I F
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
E A I A   m
17
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
eaa'
d

dt
a) Flux linkage of coil aa’; b) induced voltage;
c) rectified voltage
Two pole DC generator
18
DC Motor Equivalent Circuit
The brush
voltage
drop
External variable resistor
used to control the
amount of current in the
field circuit
RA
Armature circuit
(entire rotor structure)
Field Coils
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.
19
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).
20
The Magnetization Curve of a DC machine
The internal generated voltage in the motor E A  K
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.
IF  mmf  flux
The magnetization curve of a ferromagnetic material ( vs F)
21
The Magnetization Curve of a DC machine
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.
EA  
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).
22
The Magnetization Curve
The induced torque developed by
the motor is given as
EA  
 ind  KI A
The magnetization curve of a dc
machine expresses as a plot of EA
versus IF, for a fixed speed ω0
23
The equivalent circuit of Separately Excited DC Motor
Separately excited motor is a motor whose field current is supplied from a
separate constant-voltage power supply.
IF 
VF
RF
IL  IA
VT  E A  I A RA
24
The equivalent circuit of a Shunt DC Motor
VT
RF
VT  E A  I A RA
IF 
A shunt dc motor is a motor whose
field circuit get its power directly
across the armature terminals of the
motor.
IL  IA  IF
25
How Shunt response to load? - Speed-Torque
Characteristics
Consider the DC shunt motor. From the Kirchoff’s Law
VT  E A  I A RA
Induced Voltage
VT  K  I A RA
E A  K
Substituting the expression for induced
voltage between VT and EA.
VT  K  I A RA
Since then, current IA can be expressed as
IA 
 ind
K
VT  K 
 ind
K
RA
Finally, solving for the motor's speed yield
VT
RA



2 ind
K ( K)
26
Speed-Torque Characteristics
This equation is a straight line with a negative slope. The graph shows the
torque-speed characteristics of a shunt dc motor.
VT
RA



2 ind
K ( K)
ind  then , with constant VT,
otherwise it affect the torque-speed curve
Torque-speed characteristic of a shunt or separately excited dc motor
27
Speed-Torque Characteristics
Affect of Armature Reaction (AR) will reduce flux as the load increase (ind
also increase), so it will increase motor speed (). => E  K
A
If the motor has compensating winding, the flux () will be constant.
VT
RA



2 ind
K ( K)
Torque-speed characteristic of a motor with armature reaction present.
28
Speed-Torque Characteristics
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 fluxweakening problem in the machines, and the flux in the machine will be
constant
29
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.
30
1 : Changing The Field Resistance
 VT  to decrease.

1. Increasing RF causes IF  
 RF  
2. Decreasing IF decreases .
3. Decreasing  lowers EA
 K   
 VT  E A  

RA


4. Decreasing EA by increasing IA  

5. Increase IA by increasing  ind  ( K  I A )
with the change in IA dominant over the change in flux ().
6. Increasing τind makes
 ind  load
and the speed ω increases.
31
1: Changing The Field Resistance
7. Increasing speed to increases EA = K again.
8. Increasing EA decreases IA.
9. Decreasing IA decreases  ind until
 ind   load
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
32
2: Changing The Armature Voltage
Armature voltage control of a shunt (or
separately excited) dc motor.
1. An increase in VA by increasing IA = [ (VA  – EA)/RA]
2. Increasing IA increases
3. Increasing τind makes
 ind  ( KI A )
 ind  load
increasing ω.
4. Increasing ω increases EA =(Kω  )
5. Increasing EA by decreasing IA = [(VA – EA)/RA]
6. Decreasing IA decreases τind until
 ind   load
at a higher ω.
33
2: Changing The Armature Voltage
The speed control is shifted by this
method, but the slope of the curve
remains constant
The effect of armature voltage speed control on a shunt motor’s
torque speed characteristic
34
3 : Inserting Resistor in Series with Armature Circuit
Add resistor in series
with RA
Equivalent circuit of DC shunt
motor
The effect of armature resistance speed
control on a shunt motor’s torque – speed
characteristic
Additional resistor in series will drastically increase the slope of the
motor’s characteristic, making it operate more slowly if loaded
35
3 : Inserting Resistor in Series with Armature Circuit
Add resistor in series
with RA
VT
RA



2 ind
K ( K)
The above equation shows if RA
increase, speed will decrease
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.
36
The Series DC Motor
Equivalent circuit of a series
DC motor.
The Kirchhoff’s voltage law equation for this motor
VT  E A  I A ( RA  RS )
37
Induced Torque in a Series DC Motor
The induced or developed torque is given by
 ind  KI A
The flux in this motor is directly proportional to its armature current.
Therefore, the flux in the motor can be given by
  cI A
where c is a constant of proportionality. The induced torque in this machine
is thus given by
 ind  KI A  KcI A
2
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.
38
The Terminal Characteristic of a Series DC Motor.
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 :
  cI A
The derivation of a series motor’s torque-speed characteristic starts with
Kirchhoff’s voltage law:
VT  E A  I A ( RA  RS )
From the equation;
expressed as:
 ind  KI A  KcI A 2
IA 
the armature current can be
 ind
Kc
39
The Terminal Characteristic of a Series DC Motor.
Also, EA = K, substituting these expression yields:
 ind
VT  K 

We know I A 
c
Kc
( RA  RS )
;
Substituting the equations so the induced torque equation can written as
 ind
K 2
 
c
Therefore, the flux in the series
motor can be written as :

c
 ind
K
40
The Terminal Characteristic of a Series DC Motor.
Substituting the previous equation for VT yields:
 ind
c
VT  K
 ind  
( RA  RS )
K
Kc
The resulting torque – speed relationship is

VT
1
Kc  ind
R A  RS

Kc
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.
41
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
42
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.

VT
1
Kc  ind

R A  RS
Kc
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.
43
The Compounded DC Motor.
shunt
series
shunt
series
The equivalent compound DC motor (a) Long-shunt connection (cumulative
compounding) (b) Short-shunt connection (differential compounding)
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).
44
The Compounded DC Motor.
shunt
series
shunt
series
The equivalent compound DC motor (a) Long-shunt connection (b) Shortshunt connection
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.
45
The Compounded DC Motor.
shunt
series
shunt
series
The equivalent compound DC motor (a) Long-shunt connection (b) Shortshunt connection
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.
46
The Compounded DC Motor.
The Kirchhoff’s voltage law equation for a compound dc motor is:
VT  E A  I A ( R A  RS )
The currents in the compounded motor are related by :
IA  IL  IF
VT
IF 
RF
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)
47
The Compounded DC Motor.
The effective shunt field current in the compounded DC motor given by:
N SE
FAR
I  IF 
IA 
NF
NF
*
F
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
48
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.
49
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.
50
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.
51
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.
52
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
53
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:
IA 
VT  E A VT

RA
RA
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
54
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 burnup
55
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
Pconv = Pdev = EAIA=indω
Pout =out m
Pin =VTIL
I2R losses
Mechanical
losses
Core loss
Stray losses
56
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 = IA2RA
Must consider RS for series
2
Field Loss PF = IF RF
and compound DC Motors
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
57
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]
58
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 :


Poutput
Pinput
X 100%
Pinput  Plosses
Pinput
X 100%
59
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):
 nl   fl

X 100%
 fl
or
nl   fl

X 100%
 fl
+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.
60
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.
61
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:
VR 
Vnl  V fl
V fl
100%
+VR = Dropping characteristics
-VR = Rising characteristic
62
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
63
Separately Excited Generator
Fig : Separately excited DC
generator
IL  I A
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.
64
The Terminal Characteristic of A Separately Excited
DC Generator
Take note about the axes
between motors ( and
ind) and generators (VT
and IL)
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.
65
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 = Kω which further decreases the terminal
voltage of the generator. The resulting terminal characteristic is shown in Figure
b (PREVIOUS SLIDE)
66
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.
67
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.
I A  IF  IL
VT  E A  I A RA
V 
I F   T 
 RF 
Because of generator supply it own field
current, it required voltage buildup
Figure : The equivalent circuit of a
shunt DC generator.
68
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
E A  K res 
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
69
Voltage Buildup in A Shunt Generator
Voltage buildup
occurred in discrete
steps
EA may be a volt or
two appear at the
terminal during
start-up
70
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
71
Voltage Buildup in A Shunt Generator
• 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.
72
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 -IARA 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
73
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.
74
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 :
VT  E A  I A ( RA  RS )
75
Terminal Characteristic of a Series Generator
Figure : A series generator terminal
characteristic with large armature
reaction effects
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.
76
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.
77
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
I
*
F
N SE
FAR
 IF 
IA 
NF
NF
78
The Cumulatively Compounded DC
Generator
The other voltage and current relationships for this generator are
IA  IF  IL
VT  E A  I A ( RA  RS )
VT
IF 
RF
79
The Cumulatively Compounded DC
Generator
Another way to hook up a cumulatively compounded generator. It is the “shortshunt” 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
80
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.
81
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.
82
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
N SE
FAR
I eq 
IA 
NF
NF
The total effective shunt field current is
I F*  I F  I eq
where,
NSE = series field turns
NF = shunt field turns
FAR = armature force
IA = armature current
83
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
84
The Differentially Compounded DC Generator
IA  IL  IF
VT
IF 
RF
VT  E A  I A ( RA  RF )
The equivalent circuit of a differentially compounded
DC generator
A differentially compounded DC generator is a generator with both shunt and
series fields, but this time their magnetomotive forces subtract from each other.
85
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 :
N SE
FAR
I eq  
IA 
NF
NF
The total effective shunt field current in this machine is
I  I F  I eq
*
F
or
N SE
FAR
I  IF 
IA 
NF
NF
*
F
86
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
87
Voltage Control of Differentially Compounded
DC Generator
88
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.
89