Beie unit-3
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This document provides information about basic electrical and instrumentation engineering. It discusses Faraday's law of electromagnetic induction, which states that a changing magnetic field induces an electromotive force (emf) in a conductor. It also discusses Lenz's law, which describes the direction of induced current. The document then covers three-phase circuits, DC machines including their construction and operation principles, and DC motors including their characteristics and speed control methods.
Beie unit-3
- 1. EVEN SEMESTER 2017-2018 P. MARIA SHEEBA ASSISTANT PROFESSOR /ECE MOUNT ZION COLLEGE OF ENGINEERING AND TECHNOLOGY PUDUKKOTTAI 1 BE8254- BASIC ELECTRICAL AND INSTRUMENTATION ENGINEERING
- 2. md d dx Blx Bl dt dt dt dx Blv Bl dt E m Therefore, d dt E CONCLUSION: to produce emf one should make ANY change in a magnetic flux with time! Faraday’s Law 2
- 3. Changing magnetic flux produces an emf (or changing B- Field produces E-Field) The rate of change of magnetic flux is required Faraday’s Law 3
- 4. The direction of the emf induced by changing flux will produce a current that generates a magnetic field opposing the flux change that produced it. Lenz’s Law 4
- 5. Lenz’s Law: emf appears and current flows that creates a magnetic field that opposes the change – in this case an decrease – hence the negative sign in Faraday’s Law. B, H N S V+, V- Lenz’s Law 5
- 6. Lenz’s Law: emf appears and current flows that creates a magnetic field that opposes the change – in this case an increase – hence the negative sign in Faraday’s Law. B, H N S V-, V+ Lenz’s Law 6
- 7. Faraday’s Law for a Single Loop dt d E 7
- 8. Faraday’s Law for a coil having N turns dt d NE 8
- 9. Claim: Direction of induced current must be so as to oppose the change; otherwise conservation of energy would be violated. • Why??? – If current reinforced the change, then the change would get bigger and that would in turn induce a larger current which would increase the change, etc.. – No perpetual motion machine! Conclusion: Lenz’s law results from energy conservation principle. Lenz’s Law 9
- 10. Induced Current – quantitative Suppose we pull with velocity v a coil of resistance R through a region of constant magnetic field v w x x x x x x x x x x x x x x x x x x x x x x x x x We must supply energy to produce the current and to move the loop (until it is completely out of the B-field region). The work we do is exactly equal to the energy dissipated in the resistor, i.e. W=I2Rt 10
- 11. Generators 11
- 12. Applications of Magnetic Induction • AC Generator Water turns wheel rotates magnet changes flux induces emf drives current 12
- 13. ))(240cos()( ))(120cos()( ))(cos()( VtVtv VtVtv VtVtv mc mbn man VoltagesPhaseousInstantane 2120mV Vcn Vbn Van 0 120 240 3 phase voltage Three phase circuits 13
- 16. Y-connected Source D-connected Source Three phase circuits 16
- 17. • Balanced phase voltages are equal in magnitude and are out of phase with one another by 120 degrees. • Phase voltages sum up to zero. • Two possible combinations: abc or (+) sequence acb or () sequence Three phase circuits 17
- 18. Y-connected Load D-connected Load A balanced load is one in which the phase impedances are equal in magnitude and in phase. Three phase circuits 18
- 19. Construction of DC machines An electrical machine deals with the energy transfer either from mechanical to electrical or electrical to mechanical form. This process is called electromechanically energy conversion. The device which is used for electromechanical energy conversion is known as machine. Machine Generator - mechanical to electrical Motor- electrical to mechanical 19
- 20. Construction of DC machines • The machine is DC generator or motor the constructions are remain same. • The main parts of de machine are:- Yoke Pole Field winding Armature Commutator Brushes 20
- 21. Commutator Construction of DC machines 21
- 22. 22
- 23. Yoke • Functions :- It serves the purpose of outermost cover of D.C. machine. It provide mechanical support to the pole. It provide low reluctance path. 23
- 24. Yoke Choice of material:- To provide low reluctance path, it must be made up of some magnetic material. It is prepared by cost iron because it is cheapest. For large machine rolled steel, cast steel, silicon steel is used. 24
- 26. Functions :- Poles Choices of material:- Cost iron. Pole core basically carries a field winding. It directs the flux. Pole shoe enlarge the area of armature core to come across the flux. 26
- 27. • The field winding is wound on the pole core with a definite direction. Functions:- • To carry the current due to pole core, on which the field winding is placed. Choice of material:- • It has carry the current hence its made up of copper. Field Winding 27
- 28. It is further divided in to two parts namely, 1. Armature core 2. Armature winding. Armature core : Armature core is cylindrical in shape. It consists of slots on its periphery. It has air ducts to permit the air flow through armature for cooling purpose. Armature 28
- 29. Functions: • Armature core provides house for armature winding. Choice of material: • Cost iron or cost steel. Armature core 29
- 30. Functions: Generation of EMF take place in the armature winding in case of generator. To carry the current supply in case of d.c. motors. Choice of material: Copper. Armature winding 30
- 31. Functions: Collection of current from armature conductors. To convert alternating EMF to Unidirectional EMF. Commutator 31
- 32. Brushes Bearings are stationary and resting on the surface of the commutator. Functions: To collect the current from commutator. Choice of material: Carbon. 32
- 33. Bearing • Bearing :- For heavy duty machines roller bearings are preferred. • Types of armature winding:- Lap winding Wave winding 33
- 34. Mechanical energy is converted to electrical energy Three requirements are essential 1. Conductors 2. Magnetic field 3. Mechanical energy DC Generator 34
- 35. All generators work on a principle of dynamically induced EMF, this is also known as Faraday’s law of electromagnetic induction. • Whenever the number of flux linked with the coil changes , an electromotive force is set up in that coil. Generator action requires following basic components. The conductor The flux The relative motion between conductor and flux. Theory operation of DC generators 35
- 36. • Fleming’s right hand rule is also known as generator rule. Fleming’s right hand rule 36
- 37. Fleming’s right hand rule 37
- 38. • The magnitude of the induced EMF is given by, E =B*L*V L = length of conductor. V = relative velocity 38
- 39. EMF Equation of DC generator 39
- 40. 40
- 41. Types of DC generator 41
- 42. • In Separately Exited Generator, a separate d.c supply is used to provide exciting current through the field winding. • The d.c generator produces d.c voltage. If this generated voltage itself is used to excite the field winding of same d.c generator, it is called Self Excited Generator. 42
- 43. Separately Exited Generator • The field winding is supplied from external, separate d.c. supply. 43
- 44. Voltage and Current Relation 44
- 45. Self exited generator Based on how field winding is connected to the armature to derive its excitation, this is further divided in to following three types: 1. Shunt generator. 2. Series generator. 3. Compound generator. 45
- 46. Shunt Generator When the field winding is connected in parallel with the armature and the combination across the load then the generator is called shunt generator. 46
- 47. Voltage and Current Relation 47
- 48. Series generator • When the field winding is connected in series with the armature and the combination across the load then the generator is called series generator. 48
- 49. Voltage and Current Relation 49
- 50. A 250 V, 10 kW, separately exited generator has an induced e.m.f. of 250 V at full load. If the brush drop is 2 V per brush, calculate the armature resistance of the generator. Example 2 Solution 50
- 51. Note that 250 V, 10kW generator means the full load capacity of generator is to supply 10 kW load at a terminal voltage =250 V. 51
- 52. 52
- 53. • A short shunt compound d.c. generator supplies a current of 75 A at a voltage of 225 Calculate the generated voltage if the resistance of armature, shunt field and series field windings are 0.04 ohms , 0.90 ohms and 0.02 ohms respectively. Example 3 Solution Consider a short shunt generator 53
- 54. 54
- 55. • Drop across the armature terminal is, 55
- 56. 56
- 57. The principle of operation : • When a current carrying conductor is placed in a magnetic field it experiences a mechanical force. DC motors 57
- 58. Operating principle of DC motors 58
- 59. Operating principle of DC motors 59
- 60. Now there are two fluxes present, 1. The flux produced by the permanent magnet called main flux. 2. The flux produced by the current current carrying conductor. 60
- 61. 61
- 62. Force of DC motors 62
- 63. • In the practical d.c. motor the permanent magnet is replaced by a field winding which produces the required flux called main flux and all the armature conductors, mounted on the periphery of the armature drum, get subjected to the mechanical force. • Due to this , overall armature experiences a twisting, force called torque and armature of the motor starts rotating. 63
- 64. Direction of rotation of motor • The magnitude of the force experienced by the conductor in a motor is given by, B= flux density. L= length of conductor. I= Magnitude of current. 64
- 65. • In the figure it is shown that, a portion of a conductor of length L placed vertically in a uniform horizontal magnetic field strength H, produced by two magnetic poles N and S. If i is the current flowing through this conductor, Fleming Left Hand Rule 65
- 66. Two pole DC motors 66
- 67. Four pole DC motors 67
- 68. There are 2 types of winding - Lap and Wave winding Lap winding A = P The armature windings are divided into no. of sections equal to the no of poles Wave winding A = 2 It is used in low current output and high voltage. 2 brushes Armature windings DC motors 68
- 69. • It is seen in the generating action, that when a conductor cuts the lines of flux, e.m.f. gets induced in the conductor. After a motering action, armature starts rotating and armature conductors cut the main flux. So there is a generation action existing In a motor, the e.m.f induced due to generating action is known as back e.m.f Back EMF 69
- 70. Equivalent circuit of Motor 70
- 71. 71
- 72. • A 220 V, d.c. motor has an armature resistance of 0.75 ohms. It is drawing an armature current of 30 A, driving a certain load. Calculate the induced e.m.f. in the motor under this condition. Example 4 Solution 72
- 73. Power equation of DC motor • The voltage equation is; • Multiplying by Ia • The above equation is called power equation of DC motor 73
- 74. Condition for maximum power: • Pm = Gross mechanical power developed = Eb Ia 74
- 75. 75
- 76. • N= speed, then angular speed is • Work done in one revolution is • W= F * distance traveled in one revelation • W= F * 2 𝜋R Torque Equation of DC Motor 76
- 77. 77
- 78. • Power in armature = Armature torque * 78
- 79. • DC shunt motor • DC series motor • DC compound motor Types of DC Motor 79
- 80. Shunt Motor • The parallel combination of two windings is connected across a common dc power supply. • The resistance of shunt field winding is always higher than the armature winding. 80
- 81. 81 Voltage and current relationship of DC Shunt Motor
- 82. Series Motor • The field winding is connected in series with the armature. • The current passing through the series winding is same as the armature current. 82
- 83. Series Motor 83
- 84. 84
- 85. Long Shunt compound motor • In this the series winding in series with the armature winding and the shunt winding is connected in parallel with the armature connection. 85
- 86. Short Shunt compound motor • In this the series winding is series with the parallel combination of armature winding and the shunt winding. • It has good starting torque and constant speed characteristic. 86
- 87. Cumulative compound DC motors • If the two field windings i.e. series and shunt are wounded in such a way that the fluxes produced by them add each other. Differential compound DC motors • If the two field windings i.e. series and shunt are wounded in such a way that the fluxes produced by them airways try to cancel each other. 87
- 88. Voltage and current relationship of DC series Motor 88
- 89. Voltage and current relationship of DC Shunt Motor 89
- 90. Voltage and current relationship of DC Long Shunt Compound Motor 90
- 91. Voltage and current relationship of DC Short Shunt Compound Motor 91
- 92. Flux is proportional to current 92
- 93. 93
- 94. 94
- 95. • The speed regulation for a d.c. motor is defined as the ratio of change in speed corresponding to no load and full load condition to speed corresponding to full load. Speed Regulation 95
- 97. DC Shunt Motor Characteristics Torque (vs) armature current characteristics 97
- 98. • From the speed equation we get, • When the load increases the armature current increases and hence drop IR also increases. DC Shunt Motor Characteristics Speed – Armature current characteristics 98
- 99. • This curve shows that the speed remains constant when the torque from no load to full load. DC Shunt Motor Characteristics Speed – torque characteristics 99
- 100. DC Series Motor Characteristics 100
- 101. DC Series Motor Characteristics 101
- 102. DC Shunt Motor Characteristics Speed – torque characteristics 102
- 103. DC Shunt Motor Characteristics Speed – torque characteristics 103
- 104. • Flux Control Method • Armature Control Method • Voltage Control Method Speed DC Motor 104
- 105. • It is seen that speed of the motor is inversely proportional to flux. Thus by decreasing flux speed can be increased and vice versa. • To control the flux, a rheostat is added in series with the field winding, as shown in the circuit diagram. Adding more resistance in series with field winding will increase the speed, as it will decrease the flux. Flux Control Method 105
- 106. • Field current is relatively small and hence I2R loss is small, hence this method is quiet efficient. • Though speed can be increased by reducing flux with this method, it puts a limit to maximum speed as weakening of flux beyond the limit will adversely affect the commutation. Flux Control Method 106
- 107. • Speed of the motor is directly proportional to the back emf Eb and Eb = V- IaRa. • That is when supply voltage V and armature resistance Ra are kept constant, speed is directly proportional to armature current Ia. • Thus if we add resistance in series with armature, Ia decreases and hence speed decreases. Armature Control Method 107
- 108. • A) Multiple voltage control: • B) Ward-Leonard System Multiple voltage control: • In this method the, shunt filed is connected to a fixed exciting voltage, and armature is supplied with different voltages. • Voltage across armature is changed with the help of a suitable switchgear. The speed is approximately proportional to the voltage across the armature. Voltage Control Method 108
- 109. This system is used where very sensitive speed control of motor is required (e.g electric excavators, elevators etc.) The arrangement of this system is as required in the figure beside. Ward-Leonard System M2 is the motor whose speed control is required. M1 may be any AC motor or DC motor with constant speed. G is the generator directly coupled to M1. 109
- 110. . Ward-Leonard System In this method the output from the generator G is fed to the armature of the motor M2 whose speed is to be controlled. The output voltage of the generator G can be varied from zero to its maximum value, and hence the armature voltage of the motor M2 is varied very smoothly. Hence very smooth speed control of motor can be obtained by this method 110
- 111. Type of Motor Applications shunt 1. Blowers 2. Fans 3. Lath machines 4. Drilling machines series 1. Cranes 2. Trolleys 3. Conveyers Cumulative compound 1. Rolling mills 2. elevators Differential compound Not suitable for any practical applications Applications of DC Motor 111
- 112. Type of Generator Applications Separately exited Generator 1. Electro plating 2. Electro refining Shunt Generator 1. Battery charging 2. Ordinary lighting purpose Series Generator 1. Welding 2. Arc lamp Cumulative compound Generator 1. Domestic lamp Differential compound Generator 1. Electric arc welding Applications of Generator 112