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Theory of Electrical Machines EXIT Course Title: Theory of Electrical Machines Course Code: EE 359 Topic : D C Machines Branch: ME Semester: V Text Books:

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Presentation on theme: "Theory of Electrical Machines EXIT Course Title: Theory of Electrical Machines Course Code: EE 359 Topic : D C Machines Branch: ME Semester: V Text Books:"— Presentation transcript:

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2 Theory of Electrical Machines EXIT Course Title: Theory of Electrical Machines Course Code: EE 359 Topic : D C Machines Branch: ME Semester: V Text Books: 1.S. Chapman, Electric Machinery Fundamentals, 4th Ed., McGraw- Hill, R. K. Rajput, Electrical Machines, 3rd Ed., Laxmi Publications (P) Ltd., 2003

3 Theory of Electrical Machines EXIT D.C Machine 1.Introduction 2.Construction 3.Principle of Operation 4.Types of DC Machines 5.Characteristics 6.Applications

4 Theory of Electrical Machines EXIT DC MACHINES INTODUCTION A DC machine is an electro-mechanical energy conversion device. It can convert Mechanical power into Electrical Power. When output electrical power is DC, it is called DC Generator. When it converts DC electrical power into mechanical power, it is known as DC Motor.

5 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES 3. FIELD or EXCITING COILS 1. BODY OR MAGNETIC FRAME OR YOKE 2. POLE CORE AND POLE SHOES 4. ARMATURE CORE 5. ARMATURE WINDING 6. COMMUTATOR MAIN CONSTRUCTIONAL FEATURES

6 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES 9. BEARINGS 7. BRUSHES 8. END HOUSINGS 10. SHAFT

7 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES Body / Yoke Field Winding Shaft Commutator Armature Pulley Brush holder Field Core Bearing End Housing

8 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES + - YOKE ARMATURE COMMUTATOR SHAFT BRUSH FIELD POLE & COIL

9 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES The outer cylindrical frame to which main poles and inter poles are fixed and by means of the machine is fixed to the foundation is called YOKE. 1. MAGNETIC FRAME or YOKE :

10 Theory of Electrical Machines EXIT DC MACHINES MAIN CONSTRUCTIONAL FEATURES It serves two purposes: a) It provides mechanical protection to the inner parts of the machines. 1. MAGNETIC FRAME or YOKE :

11 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES b) It provides a low reluctance path for the magnetic flux. The yoke is made of cast iron for smaller … 1. MAGNETIC FRAME or YOKE :

12 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES machines and cast steel or fabricated rolled steel for larger machines. 1. MAGNETIC FRAME or YOKE :

13 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES The pole core and pole shoes are fixed to the yoke by bolts. They serves the following purpose : a) They support the field or exciting coils. 2. POLE CORE AND POLE SHOES : POLE CORE

14 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES b) They distribute the magnetic flux on the armature periphery more uniformly. 2. POLE CORE AND POLE SHOES :

15 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES c) The pole shoes have larger X- section, so, the reluctance of the magnetic path is reduced. The pole core 2. POLE CORE AND POLE SHOES :

16 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES and pole shoes are made of laminated steel assembled by riveting together under hydraulic pressure. 2. POLE CORE AND POLE SHOES :

17 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES Field coils or exciting coils are used to magnetise the pole core. Enameled copper wire is used for the construction of these coils.When direct 3. FIELD or EXCITING COILS :

18 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES current is passed through these coils/ winding, it sets up the magnetic field which magnetise the pole core to the reqd. flux. 3. FIELD or EXCITING COILS :

19 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES DC MACHINES MAIN CONSTRUCTIONAL FEATURES Armature is a rotating part of the DC machine, reversal of flux takes place, so hysteresis losses are produced. To minimise this loss, silicon steel is used for the construction. 4. ARMATURE CORE:

20 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES The rotating armature cuts the main magnetic field, therefore an e.m.f is induced in the armature core.This e.m.f circulates eddy currents in the core which results in eddy current loss in it. 4. ARMATURE CORE:

21 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES The armature core is laminated to reduce the eddy current loss. Armature core serves the following purposes: a) It houses the conductors in the slots. b) It provides an easy path for magnetic flux 4. ARMATURE CORE:

22 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES The no. of conductors in form of coils placed in the slots of the armature and suitably inter connected are called winding. 5. ARMATURE WINDING ; ARMATURE WINDING

23 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 5. ARMATURE WINDING ; This is the armature winding where conversion of power takes place i.e. in case of generator,

24 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 5. ARMATURE WINDING ; mechanical power is converted into electrical power and in case of a motor, electrical

25 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 5. ARMATURE WINDING ; power is converted into mechanical power.

26 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 5. ARMATURE WINDING ; Depending upon the types of inter connection. of coils, the winding can be classified into two types; i) Lap Winding; The conductors/coils are connected in such a way that no of parallel paths are equal to no. of poles.

27 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 5. ARMATURE WINDING ; If machine has ‘P’ no. of poles and ‘Z’ no. of conductors, then there will be ‘P’ no. of parallel paths.And each path will have ‘Z/P’ no of conductors in series. Also the no. of brushes are equal to no. of parallel paths.

28 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 5. ARMATURE WINDING ; Out of which half of the brushes will be positive and remaining will be negative. ii) Wave Winding; The conductors are so connected that they are divided into two parallel paths only, irrespective of the no. of poles. If machines has ‘Z’ no. of …

29 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 5. ARMATURE WINDING ; conductors, there will be only two parallel paths and each will be having ‘Z/2’ no. of conductors connected in series with only two brushes. Click here to study detailed contents of winding

30 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 6. COMMUTATOR It is the most important part of a DC machine and serves the following purpose :- i) It connects …

31 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 6. COMMUTATOR the rotating armature conductors to the stationary external circuit through the brushes.

32 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES ii) It converts altering current induced in the armature conductors into unidirectional ….. COMMUTATOR 6. COMMUTATOR

33 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 6. COMMUTATOR current in the external load circuit in generating action and it converts alternating torque into unidirectional COMMUTATOR

34 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 6. COMMUTATOR COPPER SEGMENT RISER END RING ADJUSTING NUT METAL SLEEVE SHAFT MICA INSULATION

35 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 6. COMMUTATOR torque produced in the armature in motoring action. The commutator is of cylindrical shape and is made of wedge shaped hard drawn copper segments.The segments are insulated from each ….

36 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 6. COMMUTATOR other by a thin sheet of mica.The segments are held together by means of two V-shaped rings that fit into the V-grooves cut into the segments. Each armature coil is connected to the commutator segment through riser.

37 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 7. BRUSHES Brushes are made of high grade carbon.They form the connecting link between armature winding and the external circuit. The brushes are held in particular position around the commutator by brush holders.

38 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 8. END HOUSINGS They are attached to the ends of main frame and support bearing. The front housing supports ….. END HOUSING

39 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 8. END HOUSINGS the bearing and the brush assembly whereas rear housing supports the bearing only. END HOUSING

40 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 9. BEARINGS The function of the bearing is to reduce friction between the rotating and stationary parts of the machines.These are fitted in the end housings. Generally, high carbon steel is used for the construction of the bearings.

41 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES 10. SHAFT The function of shaft is to transfer mechanical power to the machine or from the machine. SHAFT

42 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES Shaft is made of mild steel with maximum breaking strength. All the rotating parts like SHAFT 10. SHAFT

43 Theory of Electrical Machines EXIT MAIN CONSTRUCTIONAL FEATURES LECTURE 7 OF 40 DC MACHINES MAIN CONSTRUCTIONAL FEATURES Armature core,,commutator, cooling fan etc. are keyed to the shaft. SHAFT 10. SHAFT

44 Theory of Electrical Machines EXIT Principle of Operation

45 Theory of Electrical Machines EXIT Fig. 1 DC MACHINES FIELD POLES N S FIELD WINDING ARMATURE CONDUCTORS ARMATURE YOKE BRUSH MAIN FIELD AXIS BRUSH AXIS + _

46 Theory of Electrical Machines EXIT DC MACHINES The field windings are shown as excited from external source. The polarity of electro-magnetic field will depend upon the direction of field current as shown in the fig.

47 Theory of Electrical Machines EXIT Fig. 2 DC MACHINES N S  TeTe A B _ +            S N TLTL MECHANICAL LOAD ( MOTOR ) v DC SUPPLY

48 Theory of Electrical Machines EXIT APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS DC MACHINES The armature carries conductors in side the slots.Two brushes are placed at the right angle to the main field axis. The brushes are stationary whereas armature is free to rotate.

49 Theory of Electrical Machines EXIT APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS DC MACHINES When the armature is rotated in the magnetic field, an e.m.f will be induced in the armature conductors.The direction of the induced e.m.f can be found by applying Fleming’s Right Hand Rule.

50 Theory of Electrical Machines EXIT DC MACHINES The direction of induced e.m.f will depend upon the direction of rotation of armature, if polarity of field poles to be kept unchanged.When load is connected across the armature terminals, the current will flow through the armature circuit.

51 Theory of Electrical Machines EXIT DC MACHINES The direction of current will be same as that of induced e.m.f. The armature will now be considered as electro-magnet and its polarity is shown in the fig. 2.The electro-magnetic torque T e will be developed in the anti-clock wise direction as shown in the fig. no. 1 and 2.

52 Theory of Electrical Machines EXIT DC MACHINES The magnitude of T e will depend on the strength of the field poles and armature field which further depends upon the currents flowing through the respective windings. As the external load on the generator is increased, the magnitude of T e increases.

53 Theory of Electrical Machines EXIT DC MACHINES As T e acts in the opposite direction to the applied mechanical torque, more torque will be required through the prime mover to maintain the speed of armature.

54 Theory of Electrical Machines EXIT DC MACHINES The direction of currents in the upper conductors in the armature are indicated by ‘dots’ and conductors in lower half of armature are indicated by ‘cross’.

55 Theory of Electrical Machines EXIT DC MACHINES The brush ‘B’ will collect dot currents and brush ‘A’ will collect cross currents as the armature continues to rotate in clockwise direction. In the out put circuit, across terminals ‘A’ and ‘B’, current will flow in one direction.

56 Theory of Electrical Machines EXIT DC MACHINES The dc machine shown in fig. 2 is working as Generator. The same machine will work as motor, if the armature is provided with electric supply as shown in fig. 3.

57 Theory of Electrical Machines EXIT Fig 3 DC MACHINES N S  TeTe A B _ +             S N ( GENERATOR ) ELECTRICAL LOAD TmTm MECHANICAL INPUT

58 Theory of Electrical Machines EXIT DC MACHINES The armature is connected across a supply voltage ‘V’ and the field windings are excited from the same supply or from any external dc source. The magnetic polarities due to the current in armature winding will be as shown in fig.3.

59 Theory of Electrical Machines EXIT DC MACHINES The electro-magnetic torque T e will be developed in the anti-clockwise direction as opposite poles of armature field and main field will attract each other. The armature will rotate in anti- clockwise direction due to T e.

60 Theory of Electrical Machines EXIT DC MACHINES To reverse the direction of rotation of armature, either the direction of current in the field winding or armature winding will have to be reversed.If the direction of currents in both the windings are reversed, direction of rotation of armature will be unchanged.

61 Theory of Electrical Machines EXIT DC MACHINES As the mechanical load on the armature i.e. rotor shaft represented by load torque T L is increased, more and more electro-magnetic torque will be developed by the armature to balance the mechanical torque requirements for which the armature will draw more current from the supply mains.

62 Theory of Electrical Machines EXIT B Q LOAD A B A A P  MAGNETIC FIELD 0o0o

63 Theory of Electrical Machines EXIT B Q LOAD A B A P  MAGNETIC FIELD _+ e 30 o tt

64 Theory of Electrical Machines EXIT B Q LOAD A B A P  MAGNETIC FIELD +_ e 60 o tt

65 Theory of Electrical Machines EXIT Q LOAD A B A  MAGNETIC FIELD B P +_ e 90 o tt

66 Theory of Electrical Machines EXIT Q LOAD BA A A P  MAGNETIC FIELD B +_ e 120 o tt

67 Theory of Electrical Machines EXIT Q LOAD A B A A P  MAGNETIC FIELD B +_ e 150 o tt

68 Theory of Electrical Machines EXIT A Q LOAD A B A B P  MAGNETIC FIELD + e 180 o tt

69 Theory of Electrical Machines EXIT A Q LOAD A B B P  MAGNETIC FIELD +_ e 210 o tt

70 Theory of Electrical Machines EXIT A Q LOAD B A B P  MAGNETIC FIELD +_ e 240 o tt

71 Theory of Electrical Machines EXIT Q LOAD B A B  MAGNETIC FIELD A P +_ e 270 o tt

72 Theory of Electrical Machines EXIT B Q LOAD A B A A P  MAGNETIC FIELD +_ e 300 o tt

73 Theory of Electrical Machines EXIT B Q LOAD B A A A P  MAGNETIC FIELD +_ e 330 o tt

74 Theory of Electrical Machines EXIT B Q LOAD B A A A P  MAGNETIC FIELD e 360 o tt

75 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) TYPES OF DC Machines Depending upon the type of excitation to the field winding. The dc machine can be classified into three categories: 1.Machines with permanent field, 2.Separately excited 3.Self excited type

76 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) TYPES OF EXCITATIONS ( DC MOTORS ) Dc motors with permanent magnetic field, are manufactured for small rating applications such as toys, cassette tape recorders etc.large rating dc motors …

77 Theory of Electrical Machines EXIT TYPES OF EXCITATIONS ( DC MOTORS ) are constructed with electro-magnetic field i.e field winding is placed on the field core and this winding is supplied with dc current called excitation. Depending upon the type of connections to the field winding for ….

78 Theory of Electrical Machines EXIT TYPES OF DC MOTORS The dc motors can be classified into two categories ; 1) Separately excited dc motors 2) Self excited dc motors. DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS )

79 Theory of Electrical Machines EXIT SEPARATELY EXCITED DC MOTORS A AA E RaRa IaIa V + _ V DC IfIf + _ F FF + _ M Supply RfRf DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS )

80 Theory of Electrical Machines EXIT The field winding is excited from a supply which is not connected to the armature winding. It may be noted that current flowing through the field winding is independent of load and is equal to V / R f, where R f is the…. SEPARATELY EXCITED DC MOTORS DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS )

81 Theory of Electrical Machines EXIT field circuit resistance. The flux produced is proportional to the field current i.e. Ø  I f SEPARATELY EXCITED DC MOTORS

82 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SUPPLY E RaRa ILIL V FF IfIf IaIa AA + _ F M A SELF EXCITED DC MOTORS ( DC SHUNT MOTORS )

83 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SELF EXCITED DC MOTORS ( DC SHUNT MOTORS ) In this type of excitation, armature and field windings are connected across a constant source of supply. The field current I f is drawn from the same source as that of armature current. As shown in fig. shown in fig.

84 Theory of Electrical Machines EXIT SELF EXCITED DC MOTORS ( DC SERIES MOTORS ) A E RaRa ILIL V + _ IaIa AA I SE Y YY + _ M SUPPLY

85 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) The field winding is connected in series with the armature so that I f = I a = I L. Therefore field winding is made up of thick winding wire of less no. of turns as compared to that of shunt field winding so that armature current can... SELF EXCITED DC MOTORS ( DC SERIES MOTORS )

86 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) flow through it without overheating. In case of dc series machine, Ø  I f  I a.. The relationship between induced e.m.f. and terminal voltage is as follows ; ….. See Fig. SELF EXCITED DC MOTORS ( DC SERIES MOTORS )

87 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) V = E + I a R a + I a R se or E = V - Ia ( Ra + Rse ) I a = I se = I L and SELF EXCITED DC MOTORS ( DC SERIES MOTORS )

88 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS ) A E RaRa ILIL V IaIa AA I SE Y YY + _ I Sh Z ZZ R sh SUPPLY M

89 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS ) There are two field windings, namely a shunt field winding and a series field winding. The shunt field winding is connected in parallel with the armature and series field winding is connected in series with the combination.

90 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS ) Series field winding will carry a large armature current I a or I L and therefore it is made of wire of large cross section and has a few turns only. The resistance of series field winding is very small.

91 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS ) The shunt field winding is made up of wires of small cross section and has high resistance. Since the resistance of shunt field winding is high, the current flowing through it is very small as compared to that of series field winding

92 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS ) or armature current I a. The main magnetic field flux is produced by the shunt field current / winding but it is modified by the field of series winding. A compound machine therefore combines the best features of dc shunt machines and dc series machines.

93 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS ) Depending up on the connections of shunt field winding in the combination of armature and series field winding, dc compound generators can be named as i) Short shunt compound generators. ii) Long shunt compound generators.

94 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SHORT SHUNT TYPE ( DC COMPOUND MOTORS ) A E RaRa ILIL V IaIa AA I SE Y YY + _ I Sh Z ZZ R sh SUPPLY M

95 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SHORT SHUNT TYPE ( DC COMPOUND MOTORS ) In this case the shunt field winding is connected across the armature winding only as shown in the fig of slide no. i ) SHORT SHUNT DC COMPOUND MOTORS I se = I L = I a + I sh V = E + I a R a + I se R se = E + I a R a + ( I a + I sh ) R se

96 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) LONG SHUNT TYPE ( DC COMPOUND MOTORS ) A E RaRa ILIL V IaIa AA I SE Y YY + _ I Sh Z ZZ R sh SUPPLY M

97 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) LONG SHUNT TYPE ( DC COMPOUND MOTORS ) In this case the shunt field winding is connected across the combination of armature and series field winding as shown in the fig. I se = I a and I L = I a + I sh V = E + I a R a + I se R se = I sh R sh ii ) LONG SHUNT DC COMPOUND MOTORS

98 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS ) Depending upon the direction of flow of current through series field, we can classify dc compound motors into two categories namely ; I) Cumulative compound dc motors II) differential compound dc motors

99 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) CUMULATIVE TYPE ( DC COMPOUND MOTORS ) A E RaRa ILIL V IaIa AA I SE Y YY + _ I Sh Z ZZ R sh SUPPLY M Ø = Ø sh + Ø se

100 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) CUMULATIVE TYPE ( DC COMPOUND MOTORS ) The direction of current in the series field winding is such that magnetic field produced by it is in the direction to that of shunt field. Total magnitude of the field is the sum of shunt field and series field so that Ø = Ø sh + Ø se..

101 Theory of Electrical Machines EXIT DIFFERENTIAL TYPE ( DC COMPOUND MOTORS ) A E RaRa ILIL V IaIa AA I SE YYY + _ I Sh Z ZZ R sh SUPPLY M Ø = Ø sh - Ø se

102 Theory of Electrical Machines EXIT DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) DIFFERENTIAL TYPE ( DC COMPOUND MOTORS ) The direction of current in the series field winding is such that magnetic field produced by it is in the opposite direction to that of shunt field. Total magnitude of the field is the difference of shunt field and series field so that Ø = Ø sh - Ø se.

103 Theory of Electrical Machines EXIT FACTORS DETERMINING THE SPEED OF DC MOTOR The expression for back e.m.f. developed in the armature of a dc motor is given as follows : P Ø Z N 60 A E = …..(i) E = V - I a R a …..(ii)

104 Theory of Electrical Machines EXIT FACTORS DETERMINING THE SPEED OF DC MOTOR P Ø Z N 60 A = V - I a R a Comparing expressions (i) and (ii) K Ø N = V - I a R a OR N = V - I a R a K Ø

105 Theory of Electrical Machines EXIT FACTORS DETERMINING THE SPEED OF DC MOTOR Where K is the constant of proportionality and equal to PZ / 60 A Now in the above expression for speed, the speed can be varied by varying the applied voltage ‘V’, field flux Ø and resistance of the armature.

106 Theory of Electrical Machines EXIT FACTORS DETERMINING THE SPEED OF DC MOTOR It is clear that speed is directly proportional to the supply voltage ‘V’. So the speed increases with increase in voltage ‘V’ and vice versa. The speed is inversely proportional to the field flux Ø. So speed decreases as the Flux Ø increases and vice versa.

107 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS The important characteristics of dc motors are : 1) Speed - armature current ( Load ) characteristics 2) Torque - armature current ( Load ) characteristics 3) Speed - Torque characteristics CHARACTERISTICS OF DC MOTORS

108 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS It is very much important to know the characteristics mentioned above for different types of dc motors because it enables the selection of a specific type of dc motor for specific purpose. CHARACTERISTICS OF DC MOTORS

109 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SHUNT MOTORS SUPPLY E RaRa ILIL V FF IfIf IaIa AA + _ F M A

110 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SHUNT MOTORS N IaIa FULL LOAD 0 ( Amps)

111 Theory of Electrical Machines EXIT CHARACTERISTICS OF DC SHUNT MOTORS For a dc motor, we know that ; N = V - I a R a K Ø A dc shunt motor is connected across the mains having supply voltage ‘V’. N IaIa FULL LOAD 0 ( Amps) 1. Speed - Armature current (Load ) characteristics

112 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SHUNT MOTORS This supply voltage is assumed to be constant.The field winding is connected across the armature as shown in Fig. The magnetic flux Ø produced by field current I f will be constant as V remains constant. Speed - Armature current (Load ) characteristics

113 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SHUNT MOTORS 1. Speed - Armature current (Load ) characteristics But in actual practice, the air gap flux is slightly reduced due to the effect of armature reaction. From the expression for the speed mentioned earlier, it is evident that as the armature current I a …...

114 Theory of Electrical Machines EXIT CHARACTERISTICS OF DC SHUNT MOTORS increases, speed will decrease by a small amount due to an increase in I a R a drop is very small as compared to V. The speed verses armature current characteristics is shown in Fig. 1. Speed - Armature current (Load ) characteristics

115 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SHUNT MOTORS The shunt motor being thus more or less a constant speed motor, can be used in the applications such as driving of line shafts, lathes conveyors etc. 1. Speed - Armature current (Load ) characteristics

116 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SHUNT MOTORS T IaIa 0 ( Amps) 2. Torque - Armature current (Load ) characteristics

117 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SHUNT MOTORS 2. Torque - Armature current (Load ) characteristics The equation for torque can be written as follows ; T = k t Ø I a If flux Ø is taken as constant, the torque T becomes directly proportional to armature current (Load current) I a. It is a straight line passing through the origin.

118 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SHUNT MOTORS N T 0 ( N-m) 3. Speed - Torque characteristics

119 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SHUNT MOTORS 3. Speed - Torque characteristics And The relationship between speed and torque can be drawn as shown in Fig. N = V - I a R a K Ø The relation between T and I a and N and I a are as under ; T = k t Ø I a

120 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SERIES MOTORS A E RaRa ILIL V + _ IaIa AA I SE Y YY + _ M SUPPLY

121 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SERIES MOTORS 1. Speed - Load characteristics N 0 IaIa ( Amps)

122 Theory of Electrical Machines EXIT CHARACTERISTICS OF DC SERIES MOTORS 1. Speed - Load characteristics From the expression ; N = V - I a R a K Ø It is seen that the speed N is inversely proportional to flux Ø For a dc series motor,magnetic flux Ø is proportional to I a.

123 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC SERIES MOTORS 1. Speed - Load characteristics Thus, if V is constant, N is inversely proportional to I a. The N verses I a characteristics is therefore a rectangular hyperbola as shown in Fig. It is seen from the characteristics that ….

124 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Speed - Load characteristics the speed decreases as the load on the motor increases. At a very low load, the speed is dangerously high. Thus if a dc series motor is allowed to run on very light load or at No- Load, its speed will become much higher than its …….. CHARACTERISTICS OF DC SERIES MOTORS

125 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Speed - Load characteristics normal speed which may cause damage to the motor. For this reason, dc series motors are never started on No- Load and are not used in the applications where there is a chance of Load being completely removed, when the motor... CHARACTERISTICS OF DC SERIES MOTORS

126 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Speed - Load characteristics remains connected to the supply. The load on the dc series motor is connected through the gears and not through the belt pulley arrangement.This is because, in case of failure of belt, the load will be removed from the motor and thereby the CHARACTERISTICS OF DC SERIES MOTORS

127 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Speed - Load characteristics motor will attain a dangerously high speed. In case of load connected through the gears, however in the event of an accidental release of load, gears will provide some load on account of the frictional resistance of the gear teeth. CHARACTERISTICS OF DC SERIES MOTORS

128 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Torque - Load characteristics T 0 IaIa ( Amps) SATURATION OF SERIES FIELD CORE CHARACTERISTICS OF DC SERIES MOTORS

129 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Torque - Load characteristics The equation for the torque for dc motor is given by ; T = k t Ø I a The magnetic flux for a dc series motor is proportional to armature current I a. Thus the torque T = k t I a I a. Or T  I a 2 CHARACTERISTICS OF DC SERIES MOTORS

130 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Torque - Load characteristics CHARACTERISTICS OF DC SERIES MOTORS The relationship between torque and armature current, is therefore of the form of a parabola. With increase in I a, the field flux increases linearly but due to saturation of the core, beyond a certain magnitude of I a the increase in flux is

131 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Torque - Load characteristics CHARACTERISTICS OF DC SERIES MOTORS negligible.. Thus T is proportional to the square of I a up to the saturation point beyond which T varies linearly with I a. From the torque load characteristics, it can be observed that a dc series motor..

132 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Torque - Load characteristics CHARACTERISTICS OF DC SERIES MOTORS started on-load, develops a very high starting torque. Hence dc series motors are used in applications where high starting torque is required such as in electric trains, hoists, trolleys etc.

133 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Torque - Speed characteristics CHARACTERISTICS OF DC SERIES MOTORS T 0 N

134 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Torque - Speed characteristics CHARACTERISTICS OF DC SERIES MOTORS From the characteristics shown in slide no., it can be seen that for low speeds, the torque is high and for high speeds the torque is very small. This is why dc series motor is widely used in the …..

135 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Torque - Speed characteristics CHARACTERISTICS OF DC SERIES MOTORS applications where motor is to be started on bulk loads such as electric loco- motive.

136 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC COMPOUND MOTORS A E RaRa ILIL V IaIa AA I SE Y YY + _ I Sh Z ZZ R sh SUPPLY M

137 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS 1. Speed - Load characteristics CHARACTERISTICS OF DC COMPOUND MOTORS 0 N IaIa ( Amps) DIFFERENTIAL COMPOUND SHUNT CUMULATIVE COMPOUND

138 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC COMPOUND MOTORS 1. Speed - Load characteristics In cumulative compound motors, series field winding is connected in such a way that magnetic flux produced by it helps the flux produced by shunt field winding. Series field is directly proportional to the load current, …... ….

139 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC COMPOUND MOTORS 1. Speed - Load characteristics therefore total flux increases with increase in load current / armature current due to the series field in addition to the voltage drop in the armature winding.The speed of dc motor is inversely proportional to the......

140 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC COMPOUND MOTORS 1. Speed - Load characteristics total main flux Ø. Therefore speed drops more sharply as compared to dc shunt motor.Refer Fig. See Fig.

141 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC COMPOUND MOTORS 2. Torque - Load characteristics T 0 IaIa ( Amps) DIFFERENTIAL COMPOUND SHUNT CUMULATIVE COMPOUND

142 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC COMPOUND MOTORS 2. Torque - Load characteristics The torque developed by a cumulative compound motor increases with sudden increase in load and at no-load, it has a definite speed. Cumulative compound motors are therefore, suitable where there is sudden application

143 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC COMPOUND MOTORS 2. Torque - Load characteristics of heavy loads like sheers, punches, rolling mills etc. The speed of differential compound motors remains more or less constant. With increase in load but its torque decreases with load. Since the dc shunt motor develops a good torque and

144 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC COMPOUND MOTORS 2. Torque - Load characteristics its speed does not vary appreciably with increase in load, differential compound motors are not preferred over dc shunt motors and hence are rarely used.

145 Theory of Electrical Machines EXIT PERFORMANCE AND CHARACTERISTICS OF DC MOTORS CHARACTERISTICS OF DC COMPOUND MOTORS 3. Speed - Torque characteristics 0 N T (N-m) DIFFERENTIAL COMPOUND CUMULATIVE COMPOUND

146 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES (i) Shunt motors are used in situations, such as driving a line shafting etc. where the speed as to be maintained approximately constant between no-load and full-load. APPLICATIONS OF DC MOTORS (a) DC SHUNT MOTORS

147 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES (ii) In situations where variable load is to be driven at different speeds but at each load, the speed is to be kept constant. Such as driving a lathe. APPLICATIONS OF DC MOTORS (a) DC SHUNT MOTORS

148 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES DC Series Motors are used in applications such as driving hoists, cranes, trains, etc., as in these cases a large starting torque is required. They are also used where the motor can be permanently coupled to the load, such as APPLICATIONS OF DC MOTORS (b) DC SERIES MOTORS

149 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES Fans, whose torque increases with speed. Where constancy in speed is not essential, the decrease of speed with increase of load has the advantage that the power absorbed by the motor does not increase as rapidly as the torque. APPLICATIONS OF DC MOTORS (b) DC SERIES MOTORS

150 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES Series motors acquire very high speed at no-load or at very light load. That is why they should not be used for a belt drive where there is a possibility of the load decreasing to very small value. APPLICATIONS OF DC MOTORS (b) DC SERIES MOTORS

151 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES DC Compound Motors are used in application where large starting torque are required but where the load may fall to such a small value that a series motor would reach a dangerously high speed. APPLICATIONS OF DC MOTORS (c) DC COMPOUND MOTORS

152 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES Where the supply voltage may fluctuate, for instance on a traction system, the series winding reduces the fluctuation of armature current partly by its inductance and partly by its influence on the value of APPLICATIONS OF DC MOTORS (c) DC COMPOUND MOTORS

153 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES flux and therefore on that of the induced e.m.f. When the load is of a fluctuating nature, e.g. for driving stamping processes, etc. the shunt excitation prevents the speed … APPLICATIONS OF DC MOTORS (c) DC COMPOUND MOTORS

154 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES Becoming excessive on light load, and the decrease of speed with increase of load enables the flywheel, usually fitted to such a machine, to assist the motor in in dealing with the peak load by giving APPLICATIONS OF DC MOTORS (c) DC COMPOUND MOTORS

155 Theory of Electrical Machines EXIT APPLICATION OF DC MACHINES up some of its kinetic energy. APPLICATIONS OF DC MOTORS (c) DC COMPOUND MOTORS * THANKS *


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