1. EMT 462 ELECTRICAL SYSTEM TECHNOLOGY
Chapter 2 : DC Machines
By:
En. Muhammad Mahyiddin Ramli
MMR

2. Contents Introduction DC Machines Construction
DC motors : Principles of Operation, Equivalent circuit & Characteristics
DC generators : Principles of Operation, Equivalent circuit & Characteristics
Review
Chap 2: DC Machines

3. Introduction: What are DC Machines?
EMT 462 ELECTRICAL SYSTEM TECHNOLOGY
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”
Chap 2: DC Machines
MMR

4. 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.
Chap 2: DC Machines

5. EMT 462 ELECTRICAL SYSTEM TECHNOLOGY
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
Chap 2: DC Machines
MMR

6. DC Machines Construction
DC motor stator with poles visible
Rotor of a dc motor
Chap 2: DC Machines

7. 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. .
Chap 2: DC Machines

8. 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.
Chap 2: DC Machines

9. 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.
Chap 2: DC Machines

10. DC Machines Construction
EMT 462 ELECTRICAL SYSTEM TECHNOLOGY
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.
Chap 2: DC Machines
MMR

11. 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.
Chap 2: DC Machines

12. Review of magnetism
Lines of flux define the magnetic field and are in the form of concentric circles around the wire.
The magnetic lines around a current carrying conductor leave from the N-pole and re-enter at the S-pole.
"Left Hand Rule" states that if you point the thumb of your left hand in the direction of the current, your fingers will point in the direction of the magnetic field.
The flow of electrical current in a conductor sets up concentric lines of magnetic flux around the conductor.
Chap 2: DC Machines

13. Review of magnetism
The poles of an electro-magnetic coil change when the direction of current flow changes.
Chap 2: DC Machines

14. Review of magnetism The thumb will indicate the direction of motion
The motor has a definite relationship between the direction of the magnetic flux, the direction of motion of the conductor or force, and the direction of the applied voltage or current.
Fleming's left hand rule can be used.
The thumb will indicate the direction of motion
The forefinger will indicate the direction of the magnetic field
The middle finger will indicate the direction of current.
In either the motor or generator, if the directions of any two factors are known, the third can be easily determined.
Chap 2: DC Machines

15. DC Motor Operation
Chap 2: DC Machines

16. Current in DC Motor
Chap 2: DC Machines

17. Magnetic Field in DC Motor
Chap 2: DC Machines

18. Force in DC Motor
Chap 2: DC Machines

19. 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 :
where K = pZ /(2πa) = machine constant
The mechanical torque which also equal to electromagnetic torque, is found as follows:
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.
Chap 2: DC Machines

20. 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.
Chap 2: DC Machines

21. 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 :
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
Chap 2: DC Machines

22. 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
Flux linkage of coil aa’; b) induced voltage;
c) rectified voltage
Chap 2: DC Machines
Two pole DC generator

23. Generation of Unidirectional Voltage
The internal generated voltage in the DC machines defined as:
Where EA = armature voltage
K = motor constant
= flux
= rotation per min
Chap 2: DC Machines

24. DC Motor Equivalent Circuit
The brush voltage drop RA
External variable resistor used to control the amount of current in the field circuit
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.
Chap 2: DC Machines

25. 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).
Chap 2: DC Machines

26. The Magnetization Curve of a DC machine
The internal generated voltage in the motor
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)
IF  mmf  flux
Chap 2: DC Machines

27. The Magnetization Curve of a DC machine
The magnetization curve of a dc machine expresses as a plot of EA versus IF, for a fixed speed ω0
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.
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).
Chap 2: DC Machines

28. The Magnetization Curve
The induced torque developed by
the motor is given as
The magnetization curve of a dc machine expresses as a plot of EA versus IF, for a fixed speed ω0
Chap 2: DC Machines

29. 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.
Chap 2: DC Machines

30. The equivalent circuit of a Shunt DC Motor
A shunt dc motor is a motor whose field circuit get its power directly across the armature terminals of the motor.
Chap 2: DC Machines

31. How Shunt response to load? - Speed-Torque Characteristics
Consider the DC shunt motor. From the Kirchoff’s Law
Induced Voltage
Substituting the expression for induced voltage between VT and EA.
Since then, current IA can be expressed as
Finally, solving for the motor's speed yield
Chap 2: DC Machines

32. 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.
ind  then , with constant VT,
otherwise it affect the torque-speed curve
Torque-speed characteristic of a shunt or separately excited dc motor
Chap 2: DC Machines

33. Speed-Torque Characteristics
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.
Torque-speed characteristic of a motor with armature reaction present.
Chap 2: DC Machines

34. 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 flux-weakening problem in the machines, and the flux in the machine will be constant
Chap 2: DC Machines

35. 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.
Chap 2: DC Machines

36. 1 : Changing The Field Resistance
Increasing RF causes IF
to decrease.
Decreasing IF decreases .
Decreasing  lowers EA
Decreasing EA by increasing IA
Increase IA by increasing
with the change in IA dominant over the change in flux ().
Increasing τind makes
and the speed ω increases.
Chap 2: DC Machines

37. 1: Changing The Field Resistance
Increasing speed to increases EA = K again.
Increasing EA decreases IA.
Decreasing IA decreases until
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
Chap 2: DC Machines

38. 2: Changing The Armature Voltage
Armature voltage control of a shunt (or separately excited) dc motor.
An increase in VA by increasing IA = [ (VA  – EA)/RA]
Increasing IA increases
Increasing τind makes
increasing ω.
Increasing ω increases EA =(Kω  )
Increasing EA by decreasing IA = [(VA – EA)/RA]
Decreasing IA decreases τind until
at a higher ω.
Chap 2: DC Machines

39. 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
Chap 2: DC Machines

40. 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
Chap 2: DC Machines

41. 3 : Inserting Resistor in Series with Armature Circuit
Add resistor in series with RA
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.
Chap 2: DC Machines

42. The Series DC Motor Equivalent circuit of a series DC motor.
The Kirchhoff’s voltage law equation for this motor
Chap 2: DC Machines

43. Induced Torque in a Series DC Motor
The induced or developed torque is given by
The flux in this motor is directly proportional to its armature current. Therefore, the flux in the motor can be given by
where c is a constant of proportionality. The induced torque in this machine is thus given by
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.
Chap 2: DC Machines

44. 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 :
The derivation of a series motor’s torque-speed characteristic starts with Kirchhoff’s voltage law:
From the equation; the armature current can be expressed as:
Chap 2: DC Machines

45. The Terminal Characteristic of a Series DC Motor.
EMT 462 ELECTRICAL SYSTEM TECHNOLOGY
The Terminal Characteristic of a Series DC Motor.
Also, EA = K, substituting these expression yields:
We know ;
Substituting the equations so the induced torque equation can written as
We know ;
Therefore, the flux in the series motor can be written as :
Chap 2: DC Machines
MMR

46. The Terminal Characteristic of a Series DC Motor.
Substituting the previous equation for VT yields:
The resulting torque – speed relationship is
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.
Chap 2: DC Machines

47. 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
Chap 2: DC Machines

48. Speed Control of Series DC Motor
Method of controlling the speed in series motor.
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.
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.
Chap 2: DC Machines

49. The Compounded DC Motor.
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).
Chap 2: DC Machines

50. The Compounded DC Motor.
series
series
shunt
shunt
The equivalent compound DC motor (a) Long-shunt connection (b) Short-shunt 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.
Chap 2: DC Machines

51. The Compounded DC Motor.
series
series
shunt
shunt
The equivalent compound DC motor (a) Long-shunt connection (b) Short-shunt 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.
Chap 2: DC Machines

52. The Compounded DC Motor.
The Kirchhoff’s voltage law equation for a compound dc motor is:
The currents in the compounded motor are related by :
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)
Chap 2: DC Machines

53. The Compounded DC Motor.
The effective shunt field current in the compounded DC motor given by:
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
Chap 2: DC Machines

54. 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.
Chap 2: DC Machines

55. 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.
Chap 2: DC Machines

56. 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.
Chap 2: DC Machines

57. 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:
Change the field resistance, RF
Change the armature voltage, VA
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.
Chap 2: DC Machines

58. 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:
To protect the motor against damage due to short circuits in the equipment
To protect the motor against damage from long term overloads
To protect the motor against damage from excessive starting currents
To provide a convenient manner in which to control the operating speed of the motor
Chap 2: DC Machines

59. 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:
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
Chap 2: DC Machines

60. 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
Chap 2: DC Machines

61. 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
Chap 2: DC Machines

62. 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
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
Chap 2: DC Machines

63. 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]
Chap 2: DC Machines

64. DC Motor Efficiency Calculations
Rotational losses is when the mechanical losses, Core losses and Stray losses
are lumped together. [pg 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 :
Chap 2: DC Machines

65. 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):
+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.
Chap 2: DC Machines

66. 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.
Chap 2: DC Machines

67. 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 = Dropping characteristics
-VR = Rising characteristic
Chap 2: DC Machines

68. 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
Chap 2: DC Machines

69. 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.
Chap 2: DC Machines

70. 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.
Chap 2: DC Machines

71. 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)
Chap 2: DC Machines

72. 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:
Change the speed of rotation. If ω increases, then EA = KΦω increases, so VT = EA - IARA increases too.
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.
Chap 2: DC Machines

73. 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.
Because of generator supply it own field current, it required voltage buildup
Figure : The equivalent circuit of a shunt DC generator.
Chap 2: DC Machines

74. 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
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
Chap 2: DC Machines

75. Voltage Buildup in A Shunt Generator
EA may be a volt or two appear at the terminal during start-up
Voltage buildup occurred in discrete steps
Chap 2: DC Machines

76. 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
Chap 2: DC Machines

77. 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.
Chap 2: DC Machines

78. 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
Chap 2: DC Machines

79. Voltage Control for Shunt DC Generator
There are two ways to control the voltage of a shunt generator:
Change the shaft speed, ωm of the generator.
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.
Chap 2: DC Machines

80. 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 :
Chap 2: DC Machines

81. 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.
Chap 2: DC Machines

82. 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.
Chap 2: DC Machines

83. 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
Chap 2: DC Machines

84. The Cumulatively Compounded DC Generator
The other voltage and current relationships for this generator are
Chap 2: DC Machines

85. The Cumulatively Compounded DC Generator
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
Chap 2: DC Machines

86. 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:
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).
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.
Chap 2: DC Machines

87. 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:
Change the speed of rotation. An increase in  causes EA = K to increase, increasing the terminal voltage VT = EA – IA (RA + RS).
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.
Chap 2: DC Machines

88. 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
The total effective shunt field current is
where,
NSE = series field turns
NF = shunt field turns
FAR = armature force
IA = armature current
Chap 2: DC Machines

89. The upper tip triangle represents the internal generated voltage EA.
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
Chap 2: DC Machines

90. The Differentially Compounded DC Generator
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.
Chap 2: DC Machines

91. 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 :
The total effective shunt field current in this machine is or
Chap 2: DC Machines

92. Voltage Control of Differentially Compounded DC Generator
Two effects occur in the terminal characteristic of a differentially compounded
DC generator are
As IA increases, the IA (RA + RS) voltage drop increases as well. This increase tends to cause the terminal voltage to decrease VT.
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
Chap 2: DC Machines

93. Voltage Control of Differentially Compounded DC Generator
Chap 2: DC Machines

94. 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:
Change the speed of rotation, m.
Change the field current, IF.
Chap 2: DC Machines

95. A long journey start with a single step. - Confucious
Chap 2: DC Machines