1. EMT 113/4 ELECTRICAL ENGINEERING TECHNOLOGY
Chapter 2 : DC Machines
Chap 2: DC Machines

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 is DC Machines?
EMT 113 ELECTRICAL ENGINEERING TECHNOLOGY
Introduction: What is 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
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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 are the easy speed and torque regulation.
However, their application is limited to mills, mines and trains. As examples, trolleys and underground subway cars may use dc motors.
In the past, automobiles were equipped with dc dynamos to charge their batteries.
Chap 2: DC Machines

5. EMT 113 ELECTRICAL ENGINEERING TECHNOLOGY
Types of DC motors
DC motors are classified according to electrical connections of armature windings and field windings.
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
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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
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

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 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 Principles 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 Principles 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
= fluks
= 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 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. 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. Torque-Speed Characteristic
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. Torque-Speed Characteristic
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. Torque-Speed Characteristic
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.
Deceasing IF decreases .
Decreasing  lowers EA
Decreasing EA increases IA
Increasing IA increases
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 increases IA [= (VA  – EA)/RA]
Increasing IA increases
Increasing τind makes
increasing ω.
Increasing ω increases EA (=Kω  )
Increasing EA decreases IA [ = (VA – EA)/RA]
at a higher ω.
Decreasing IA decreases τind until
Chap 2: DC Machines

39. 2: Changing The Armature Voltage
The speed control is shiftted 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 113 ELECTRICAL ENGINEERING TECHNOLOGY
The Terminal Characteristic of a Series DC Motor.
Also, EA = K, substituting these expression yields:
We know ;
We know ;
Substituting the equations so the induced torque equation can written as
Therefore, the flux in the series motor can be written as :
Chap 2: DC Machines
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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