Unit-II PPT basics for engineering students

Basics

1. BE 8255 -BASIC ELECTRICAL, ELECTRONICS AND MEASUREMENT ENGINEERING Semester-II BE Computer Science and Engineering/AML/ADS

2. UNIT II ELECTRICAL MACHINES DC and AC ROTATING MACHINES: Types, Construction, principle, emf and torque equation, application, Speed Control - Basics of Stepper Motor – Brushless DC motors- Transformers-Introduction- types and construction, working principle of Ideal transformer- emf equation- All day efficiency calculation.

3. Introduction Machine- simplify work Machine energised by electricity- electrical machine Brushless DC motor Stepper motor Generator (Mechanical energy to electrical energy) Motor (Electrical energy to mechanical energy) Synchronous machine Induction motor 3

4. ELECTRICAL MACHINE An electrical machine deals with the energy transfer either from mechanical to electrical form or from electrical to mechanical form. This process is called electromechanical Energy Conversion Electrical machine which converts mechanical energy in to electrical energy is called an electric generator. Electrical machine which Converts Electrical Energy in to mechanical energy is called motor. Electrical machines are broadly classified as, AC machines DC machines

5. Construction of DC Machine

6. Parts of Construction of DC Machine 1 Yoke a) Functions : Outermost cover of the DC machine which protects the machine from harmful atmospheric elements like moisture, dust and various gases like SO2, acidic fumes etc. It provides mechanical support to the poles. It forms a part of the magnetic circuit and provides a path of low reluctance for magnetic flux. b) Choice of Material  : Cast iron, rolled steel, cast steel, silicon steel are used which provides high permeability i.e. low reluctance and gives good mechanical strength.

7. Construction of DC Machine 2 Poles  Pole is divided into two parts namely, i ) Pole core and ii) Pole shoe a) Functions of pole core and pole shoe: Pole core carries field winding which is necessary to produce the flux. It directs the flux produced through air gap to armature core. Pole shoe enlarges the area of armature core for maximizing the flux in the airgap, which is necessary to produce large induced emf . b) Choice of Material :  It is made up of magnetic material like cast iron or cast steel. As it requires a definite shape and size, laminated construction is used. The laminations of required size and shape are stamped together to get a pole which is then bolted to the yoke.

8. Construction of DC Machine 3 Field Winding (F1-F2)    The field winding is wound on the pole core with a definite direction.   a) Functions :  To carry current due to which pole core, on which the field winding is placed, behaves as an electromagnet, producing necessary flux. it is called Field winding or Exciting winding. b) Choice of material :  A luminium or copper is the choice. But field coils are required to take any type of shape and bend about pole core and copper has good pliability i.e. it can bend easily. So copper is the proper choice. 4 Armature  It is divided into two parts namely, Armature core and Armature winding  

9. Construction of DC Machine i ) Armature core  : Armature core is cylindrical in shape mounted on the shaft. It consists of slots on its periphery and the air ducts to permit the air flow through armature for cooling. a) Functions : Armature core provides house for armature winding i.e. armature conductors. To provide a path of low reluctance to the magnetic flux produced by the field winding. b) Choice of Material :  As it has to provide a low reluctance path to the flux, it is made up of magnetic material like cast iron or cast steel.   It is made up of laminated construction to keep eddy current loss as low as possible.

10. Construction of DC Machine ii) Armature winding  : The interconnection of the armature conductors, placed in the slots provided on the armature core periphery. When the armature is rotated, in case of generator, magnetic flux gets cut by armature conductors and e.m.f . gets induced in them. a) Functions : Generation of e.m.f takes place in the armature winding in case of generators. To carry the current supplied in case of d.c. motors. To do the useful work in the external circuit.  b) Choice of material :  it has to be made up of conducting material, carries large current. Made of copper. Armature winding is generally former wound. The conductors are placed in the armature slots which are lined with tough insulating material.

11. Principle of Operation of a DC machine as Generator Faraday’s law of Electromagnetic Induction: Whenever the flux linking with a conductor or a coil changes, an electromotive force is set up in that conductor. The change in flux can exist only when there is a relative motion between the conductor and the flux. The relative motion can be achieved by rotating the conductor w.r.t flux or by rotating flux w.r.t conductor. Voltage gets generated in a conductor as long as there exist a relative motion between the conductor and flux.

12. Fleming’s Right Hand Rule If three fingers of right hand, namely thumb, index finger and middle finger are outstretched so that everyone of them is at right angles with the remaining two. If index finger is made to point in the direction of lines of flux, thumb in the direction of the relative motion of the conductor then middle finger gives the direction of emf induced in the conductor . Magnitude of emf is , E= Blv

13. Types of DC Generator Separately Excited DC Generator If the field winding is excited by separate DC supply, then the generator is called separately excited DC generator. Armature current Ia = Load current IL Terminal Voltage ,V = Eg – Ia Ra –V brush Generated Voltage Eg = V+IaRa+Vbrush Electric Power developed = Eg * Ia Power Delivered to load = VIa

14. Types of DC Generator Self Excited DC Generator Field winding is supplied from the armature of the generator itself. Residual flux is present in the poles. When armature is rotated, a small emf is produced in the armature winding because of residual flux. This emf produces a small current in the field winding and hence flux pole increases. Increased flux increases the induced emf , which further increases the flux. Process is cumulative and generator reaches its rated voltage

15. Self excited generators can be classified as, Series Generator Shunt Generators Compound Generator : Long shunt Compound Generator Short shunt Compound Generator

16. Series Generator Fig.

17. Shunt Generator Fig.

18. Long shunt Compound motor .

19. Short Shunt Compound motor Fig.

20. Characteristics-Separately Excited DC Generator Fig. Open Circuit Characteristics Internal ( Eg Vs Ia )& External Characteristics (V Vs IL)

21. DC shunt Generator Characteristics .

22. .

23. .

24. .

25. DC MOTORS

26. DC MOTOR- Principle of Operation Converts Electrical Energy in to mechanical energy. When a current carrying conductor is placed in a magnetics field, it experiences a mechanical force. Field winding produces the required magnetic field and armature conductor plays a role of current carrying conductor. As the conductors are placed in the periphery of the armature, Individual force experienced by the conductor acts as a twisting and turning force on the armature , called torque. Torque is a product of force and the radius at which the force acts . So armature experiences a torque and starts rotating.

27. DC Motor- Principle of Operation Contd.

28. Right Hand Thumb Rule & Cork Screw Rule

29. DC Motor- Principle of Operation Contd.

30. Magnitude of Force &Direction of Rotation of Motor Magnitude of force experienced by the conductor is, F=BIL Newton Where, B-Flux Density L-Active Length I-Magnitude of current passing Direction of rotation can be obtained by Flemings Left Hand Rule

31. Fleming’s Left Hand Rule

32. Back EMF When the armature of a d.c motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence an e.m.f . is induced in them. The induced e.m.f . acts in opposite direction to the applied voltage V(Lenz’s law) and is known as back or counter e.m.f . E b . E b = (ΦZN/60)(P/A)

33. Voltage Equation of a DC Motor In a d.c. motor, Let, V = applied voltage E b = back e.m.f . R a = armature resistance I a = armature current Since back e.m.f . Eb acts in opposition to the applied voltage V, the net voltage across the armature circuit is V- E b . The armature current I a is given by, This is known as voltage equation of the d.c. motor V = E b + I a R a

34. Power Equation of a Motor Voltage Equation of a motor is, V = E b + I a R a Multiplying by I a on both sides, V I a = E b I a + I a 2 R a V I a : Net Electrical Power input E b I a : Electrical Equivalent of gross mech. power I a 2 R a :Power loss due to armature resistance.

35. Torque Equation of a DC motor Torque is the twisting or turning force about an axis and is measured by the product of force (F) and radius (R) at right angle to which the force acts T=F*R

36. Torque Equation of a DC motor Contd. Consider a wheel of radius ‘R’ meters acted up on by a circumferential force F Newton. The wheel is rotated at a speed of N-rpm Angular speed, ω =2πN/60 rad/sec Work done in one revolution is , W=F*Distance travelled in one revolution =F*2πR Power developed, P= Work Done/Time =(F*2πR)/time for one revolution =(F*2πR)/ (60/N) =(F*R)*( 2πN/60 ) P = T* ω ; where T-Torque in N-m Let T a be the armature torque and the gross mechanical power developed in the armature is E b I a. If the speed of the motor is N rpm, then Power in armature = Armature toque * ω E b I a = T a *(2πN/60 ) I a (ΦZNP)/60A = T a *(2πN/60 ) T a = 0.159* I a ΦZP /A Nm

37. Problem A 4 pole dc motor takes 50A armature current. The armature has lap connected conductors. The flux per pole is 20mWb. Calculate the gross torque developed by the armature of the motor. P=4,A=P=4, Z=480 Φ = 20mWb, Ia =50A T a = 0.159* I a ΦZP /A Nm =0.159*50*20*10 -3 *4*480/4 =76.394Nm. EE8353 Electrical Drives and Controls 1

38. A 4 pole dc motor takes 50A armature current. The armature has lap connected conductors. The flux per pole is 20mWb. Calculate the gross torque developed by the armature of the motor. EE8353 Electrical Drives and Controls 1

39. Types of DC motor Separately Excited motor Self Excited DC motor Series motor Shunt motor Compound motor Long shunt compound motor Short shunt compound motor EE8353 Electrical Drives and Controls 1

40. Separately Excited DC motor Field winding and armature windings are physically separated. Field winding is excited by a separate DC source V = E b + I a R a + V brush E b = V - I a R a – V brush EE8353 Electrical Drives and Controls 1

41. DC Shunt Motor In a shunt wound motor the field winding is connected in parallel with armature and this combination is connected across the supply. Field winding should have more number of turns with less cross sectional area. Ra<< Rsh Flux produced by the field winding is proportional to the current passing through R sh . Φ α el I sh Total Current Drawn from the supply is, I L = I a + I sh I sh = V/ R sh V = E b + I a R a + V brush V = I sh R sh

42. DC Series Motor In series wound motor the field winding is connected in series with the armature. Field winding should have less number of turns of thick wire. Resistance of series winding will be small. Flux produced by the field winding is proportional to the current passing through R se Φ α el I se I L = I se = I a V = E b + I a ( R a + R se ) + V brush The entire armature current flows through series field winding . Hence, flux Φ α el I se α el I a

43. DC compound Motor-Long Shunt When the shunt winding is so connected that it shunts the series combination of armature and series field, it is called long-shunt connection. I L = I se + I sh I se = I a I sh = V/ R sh V = E b + I a R a + I a R se + V brush V = I sh R sh

44. DC compound Motor-Short Shunt When the shunt field winding is connected across the armature terminals and series field winding is connected in series with this combination, it is called short-shunt connection. I L = I se I L = I a + I sh V = E b + I a R a + I se R se + V brush I sh = (V - I se R se )/ R sh = ( E b + I a R a + V brush )/ R sh    

45. Compound Motor -Classification Compound motors can be classified in to two types based on the winding construction, Cumulative Compound motor Differential compound motor Cumulative Compound motor In this type of motor, the two winding fluxes aids each other Flux due to the series field winding strengthens the flux due to the shunt field winding. EE8353 Electrical Drives and Controls 1

46. Differential Compound motor In this type of motor, the two winding fluxes aids each other Flux due to the series field winding strengthens the flux due to the shunt field winding. EE8353 Electrical Drives and Controls 1

47. A 4 pole 250V series motor has a wave connected armature with 1254 conductors. The flux per pole is 22mWb. The motor takes an armature current of 50A. Armature and field resistances are 0.2 ohm and 0.2 ohm respectively. Calculate its speed. EE8353 Electrical Drives and Controls 1

48. A 4 pole 250V series motor has a wave connected armature with 1254 conductors. The flux per pole is 22mWb. The motor takes an armature current of 50A. Armature and field resistances are 0.2 ohm and 0.2 ohm respectively. Calculate its speed. P=4, V-250V,Z=1254 , flux per pole is 22mWb, Ia =50A, Ra =0.2 ohm, Rse =0.2, Wave connected, A=2 V = E b + I a ( R a + R se ) + V brush E b =250- 50(0.2+0.2)= 250-20=230 E b = (ΦZN/60)(P/A) N= (230*60*2)/(22*10 -3 *4*1254) =250 rpm EE8353 Electrical Drives and Controls 1

49. A 10KW, 250V DC shunt machine has an armature and field resistances of 0.2 ohm and 125 ohms respectively. Calculate the total armature power developed when running as a motor taking 10KW input. The back emf of a shunt motor is 230V, the field resistance is 160ohm, and field current is 1.5A. If the line current is 37A, find the armature resistance. Also find the armature current when the motor is stationary. EE8353 Electrical Drives and Controls 1

50. Characteristics of DC motor Various characteristics of a motor are, Speed –Armature current characteristics (N/ Ia ) Torque-armature current characteristics (Ta/ Ia ) or (Electrical characteristics) Speed-torque characteristics (N/Ta) or (Mechanical characteristics) We know that, T a = 0.159* I a Φ Z P /A Nm Therefore, Therefore,   EE8353 Electrical Drives and Controls 1

51. Characteristics of DC shunt motor Field winding is connected across the armature as well as the supply voltage , Since the supply voltage is constant, the field current and hence the flux is constant. Hence they are called the constant flux motors. Speed –Armature current characteristics where, k = (Z, N,P and A are constant) ( ) since E b = V - I a R a Since and are constant, N=K(V- ) This implies that, the speed is nearly constant except for a small drop.   EE8353 Electrical Drives and Controls 1

52. Characteristics of DC shunt motor contd. Due to this characteristics, DC shunt motor acts as a constant speed motor. Applications: Machine tools, lathes, wood working machines In general, for applications where constant speed is required. EE8353 Electrical Drives and Controls 1

53. Characteristics of DC shunt motor contd. b) Torque-armature current characteristics (T/ Ia ) or (Electrical characteristics) The DC shunt motor is torque is directly proportional to flux and armature current Here, flux is constant, hence, So, when armature current increases, the torque also increases. The dotted line indicates shaft torque.   EE8353 Electrical Drives and Controls 1

54. Characteristics of DC shunt motor contd. c) Speed-torque characteristics (N/Ta) It is also called mechanical characteristics. It is obtained from the Speed –Armature current characteristics and Torque-armature current characteristics When the load torque increases, the speed slightly decreases. EE8353 Electrical Drives and Controls 1

55. DC series motor characteristics Field winding is connected in series with the armature winding I L = I se = I a V = E b + I a ( R a + R se ) ( V brush is neglected) a) Speed-Armature current Characteristics ( ) N (Flux proportional to ) From this equation, we can conclude that, speed decreases by by increasing the current. This is the reason why, dc series motor should never start on no-load. Otherwise, the speed will rise to dangerously high value and get damaged.   EE8353 Electrical Drives and Controls 1

56. b) Torque-armature current characteristics (N/ Ia ) –series motor In dc series motor, flux is directly proportional to armature current. before saturation after saturation At light load, armature current and flux is small. But as increases, increases as square of the current. Hence this characteristics is a parabola. After saturation the flux is constant and is independent of . Hence, .   EE8353 Electrical Drives and Controls 1

57. c) Speed-torque characteristics: It is also called mechanical characteristics. It is obtained from the Speed –Armature current characteristics and Torque-armature current characteristics In DC series motor, when the speed is high, torque is low and vice-versa. EE8353 Electrical Drives and Controls 1

58. Compound Motor characteristics Characteristics of compound motor depend on whether the series and shunt field are assisting each other or opposing each other. In the cumulative connections, the characteristics will be between those of shunt and series motor. In the differentially compound motor the characteristics will tend towards those of the series motor. Curve (2) is cumulative and Curve (3) differential connections EE8353 Electrical Drives and Controls 1

59. EE8353 Electrical Drives and Controls 1

60. Types of Losses in motor Copper Loss : Armature Copper loss Field copper loss(Shunt field and series field) Brush contact resistance. Magnetic Losses: Hysteresis Loss and Eddy Current losses Mechanical Losses : Frictional Loss and windage loss Mechanical Loss+ Magnetic Loss : Stray losses The efficiency of a dc motor is the ratio of output power to the input power

61. STEPPER MOTORS A stepper motor is a brushless DC motor whose rotor rotates in a discrete angular displacements when its stator windings are energized in a programmed manner. Rotation occurs due to the rotor poles and poles of sequentially energized stator winding. Rotor has no electrical winding but has salient and magnetized poles. Stepper motor is a digital actuator whose input is in the form of discrete angular rotation. Applications: Printers, graph plotters, disk drives, Robotics, X-Y recorders, electric watches etc.

62. Classification of Stepper Motors 1 . Without permanent magnet Variable Reluctance motor Single stack Multi stack 2.With permanent magnet Claw pole motor Hybrid motor

63. What is a Stepper Motor? It is a brushless electromechanical device which converts the train of electric pulses applied at their excitation windings into precisely defined step-by-step mechanical shaft rotation. The shaft of the motor rotates through a fixed angle for each discrete pulse. This rotation can be linear or angular. It gets one step movement for a single pulse input. Applications Application of stepper motor in diverse areas ranging from a small wrist watch to artificial satellites. Power range 1W to 2.5KW Torque range 1µN to 40 Nm

64. Stepper Motor Working Principle A magnetic interaction takes place between the rotor and the stator, which make rotor move. Construction The stator has windings The rotor is of salient structure without any windings, and it may or may not have permanent magnets

65. Step Angle The angle through which the motor shaft rotates for each command pulse is called step angle. Step Angle β = 360/ mNr Step Angle β = (Ns-Nr/ NsNr )*360 m= No of phases Nr= No of rotor teeth Ns= No of stator poles

66. Resolution Resolution of stepper motor is defined as the number of steps needed to complete one revolution of the rotor shaft. Resolution = Number of steps/Number of revolution of the rotor Resolution = 360/ β Stepping Rate The number of steps per second is known as stepping rate

67. Types of Stepper Motor Variable Reluctance stepper motor Single stack Multi stack Permanent magnet stepper motor Hybrid stepper motor

68. Variable reluctance stepper motor Variable reluctance stepper motor works on the principle that a magnetic material placed in magnetic field experience a force to align minimum reluctance path

69. Single stack variable reluctance stepper motor Construction Stator The stator made up of silicon steel stampings. It has projecting poles, usually even no of poles. The pole carry concentric windings Rotor Usually made up of silicon steel. Solid silicon steel also used for core of rotor. The rotor has projecting teeth on its outer periphery The no of rotor teeth and stator pole should not be equal. This make motor self starting

70. 4-phase, 8-poles variable reluctance motor Step Angle β = 360/ mNr Step Angle β = (Ns-Nr/ NsNr )*360 β =15°

71. Modes of excitation Single phase ON mode or full step ON mode Two phase ON mode Half step mode Micro step mode

72. Single phases or full step ON mode

73. Variable reluctance stepper motor Variable reluctance stepper motor works on the principle that a magnetic material placed in magnetic field experience a force to align minimum reluctance path

74. Single stack variable reluctance stepper motor Construction Stator The stator made up of silicon steel stampings. It has projecting poles, usually even no of poles. The pole carry concentric windings Rotor Usually made up of silicon steel. Solid silicon steel also used for core of rotor. The rotor has projecting teeth on its outer periphery The no of rotor teeth and stator pole should not be equal. This make motor self starting

75. Single Stack Variable Reluctance Stepper Motor

76. Single Stack Variable Reluctance Stepper Motor There is no permanent magnet either on stator or rotor. Stator is made up of silicon steel stampings with inward projected poles or teeth. Each and every pole carries a field coil or exciting coil. Exciting coils of opposite poles are connected in series such that their mmf gets added. The combination of two coils is known as phase winding. Rotor is also made up of silicon steel stampings with outward projected poles and doesn’t have any electrical windings. Number of rotor poles should be different from that of stator in order to have self starting capability and bidirectional rotation. Stator and rotor materials should have high permeability and should be capable of allowing magnetic flux to pass through them even if a low magneto motive force is applied.

77. Single Stack Variable Reluctance Stepper Motor- Electrical Connection Phase ‘a’ consists of coil A and coil A’ Phase ‘b’ consists of coil B and coil B’ Phase ‘c’ consists of coil C and coil C’

78. Principle of Operation-Mode-I

79. Mode 1: 1-phase ON or Full-step Operation One phase is energized at a time If a current is applied to coils of phase ‘a’ , phase ‘a’ gets excited, the reluctance torque causes the rotor to turn until it aligns with the axis of ‘a’. The axis of rotor poles 1 and 3 are in alignment with the axis of stator poles A and A’. That is ∟ =0 The magnetic reluctance is minimized and this state provides a rest or equilibrium position to the rotor and rotor cannot move until phase ‘a’ is de –energized. Phase ‘b’ is energized by turning ON the semiconductor switch S2 and phase ‘a’ is de-energized by turning OFF the switch S1 Then the rotor poles 1,3 an d 2,4 experience torque in opposite direction. When the stator and rotor teeth are not aligned in the excited phase, the magnetic reluctance is large.  

80. Torque experienced in 1 and 3 are in clock wise direction and 2,4 are in anti clockwise direction. Later is more than the former. As a result, the rotor makes an angular displacement of 30⁰ in counter clockwise direction so that B,B’ and 2,4 are in alignment. Phases are excited in sequence a, b, c . The rotor turns with a step of 30⁰ in counter clockwise direction. The direction of rotation can be reversed by reversing the switching sequence of the phases. ie . The direction of rotation depends on the sequence in which phase windings energized and is independent of the direction of currents through the phase windings.

81. Variable reluctance stepper motor Variable reluctance stepper motor works on the principle that a magnetic material placed in magnetic field experience a force to align minimum reluctance path

82. Single stack variable reluctance stepper motor Construction Stator The stator made up of silicon steel stampings. It has projecting poles, usually even no of poles. The pole carry concentric windings Rotor Usually made up of silicon steel. Solid silicon steel also used for core of rotor. The rotor has projecting teeth on its outer periphery The no of rotor teeth and stator pole should not be equal. This make motor self starting

83. 4-phase, 8-poles variable reluctance motor Step Angle β = 360/ mNr Step Angle β = (Ns-Nr/ NsNr )*360 β =15°

84. Modes of excitation Single phase ON mode or full step ON mode Two phase ON mode Half step mode Micro step mode

85. Single phases or full step ON mode

86. Variable reluctance stepper motor Variable reluctance stepper motor works on the principle that a magnetic material placed in magnetic field experience a force to align minimum reluctance path

87. Single stack variable reluctance stepper motor Construction Stator The stator made up of silicon steel stampings. It has projecting poles, usually even no of poles. The pole carry concentric windings Rotor Usually made up of silicon steel. Solid silicon steel also used for core of rotor. The rotor has projecting teeth on its outer periphery The no of rotor teeth and stator pole should not be equal. This make motor self starting

88. Single Stack Variable Reluctance Stepper Motor There is no permanent magnet either on stator or rotor. Stator is made up of silicon steel stampings with inward projected poles or teeth. Each and every pole carries a field coil or exciting coil. Exciting coils of opposite poles are connected in series such that their mmf gets added. The combination of two coils is known as phase winding. Rotor is also made up of silicon steel stampings with outward projected poles and doesn’t have any electrical windings. Number of rotor poles should be different from that of stator in order to have self starting capability and bidirectional rotation. Stator and rotor materials should have high permeability and should be capable of allowing magnetic flux to pass through them even if a low magneto motive force is applied.

89. Principle of Operation-Mode-I

90. Mode 1: 1-phase ON or Full-step Operation One phase is energized at a time If a current is applied to coils of phase ‘a’ , phase ‘a’ gets excited, the reluctance torque causes the rotor to turn until it aligns with the axis of ‘a’. The axis of rotor poles 1 and 3 are in alignment with the axis of stator poles A and A’. That is ∟ =0 The magnetic reluctance is minimized and this state provides a rest or equilibrium position to the rotor and rotor cannot move until phase ‘a’ is de –energized. Phase ‘b’ is energized by turning ON the semiconductor switch S2 and phase ‘a’ is de-energized by turning OFF the switch S1 Then the rotor poles 1,3 an d 2,4 experience torque in opposite direction. When the stator and rotor teeth are not aligned in the excited phase, the magnetic reluctance is large.  

91. Torque experienced in 1 and 3 are in clock wise direction and 2,4 are in anti clockwise direction. Later is more than the former. As a result, the rotor makes an angular displacement of 30⁰ in counter clockwise direction so that B,B’ and 2,4 are in alignment. Phases are excited in sequence a, b, c . The rotor turns with a step of 30⁰ in counter clockwise direction. The direction of rotation can be reversed by reversing the switching sequence of the phases. ie . The direction of rotation depends on the sequence in which phase windings energized and is independent of the direction of currents through the phase windings.

92. Variable reluctance stepper motor Variable reluctance stepper motor works on the principle that a magnetic material placed in magnetic field experience a force to align minimum reluctance path

93. Single stack variable reluctance stepper motor Construction Stator The stator made up of silicon steel stampings. It has projecting poles, usually even no of poles. The pole carry concentric windings Rotor Usually made up of silicon steel. Solid silicon steel also used for core of rotor. The rotor has projecting teeth on its outer periphery The no of rotor teeth and stator pole should not be equal. This make motor self starting

94. 4-phase, 8-poles variable reluctance motor Step Angle β = 360/ mNr Step Angle β = (Ns-Nr/ NsNr )*360 β =15°

95. Modes of excitation Single phase ON mode or full step ON mode Two phase ON mode Half step mode Micro step mode

96. Single phases or full step ON mode

97. Single phases or full step ON mode

98. Two phase ON mode Phase S1 S2 S3 S4 Angle(Deg) AB 1 1 7.5 BC 1 1 22.5 CD 1 1 37.5 DA 1 1 52.5 AB 1 1 67.5

99. One phase ON Vs Two phase ON Phase S1 S2 S3 S4 Angle(Deg) AB 1 1 7.5 BC 1 1 22.5 CD 1 1 37.5 DA 1 1 52.5

100. Half step mode This mode operating on alternating one phase ON and two phase ON mode. This method is also known as wave excitation. The rotor can rotate each step angle half of the full step angle. Hence the name half step mode. Phase S1 S2 S3 S4 Angle A 1 AB 1 1 7.5 B 1 15 BC 1 1 22.5 C 1 30 CD 1 1 37.5 D 1 45 DA 1 1 52.5

101. Types of Motors World of Motors DC Motors AC Motors Brush DC Brushless DC Single Phase Poly-Phase (3 phase) Linear Stepper Universal Electric Motors Pneumatic Motors Hydraulic Motors Servo Motors

102. Brushless DC Motor Many of the limitations of the classic permanent magnet "brushed" DC motor are caused by the brushes pressing against the rotating commutator creating friction As the motor speed is increased, brushes may not remain in contact with the rotating commutator At higher speeds, brushes have increasing difficulty in maintaining contact Sparks and electric noise may be created as the brushes encounter flaws in the commutator surface or as the commutator is moving away from the just energized rotor segment Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance Brushless DC motors avoid these problems with a modified design, but require a more complex control system

103. Features Conventional DC motor PMBLDC Motor Maintenance High Low Commutation method Mechanical contact between brushes & commutator . Electronic switching using power semiconductor devices ie , transistor,MOSFET Detecting method Automatically detected by brushes Rotor position can be detected by using sensor ie , Hall sensor, optical sensor Control Speed controllable is difficult Speed can be easily controllable, so that it is possible to have very high speed

104. PMBLDC Motor

105. Construction of PMBLDC motor Rotor accommodates PM The rotor shaft carries a rotor position sensor. Sensor provides information about the position of the shaft. This shaft position signal send to electronic commutator .

106. Applications Automotive and aircraft auxiliaries. Textile and industries. Computer and robotics. Small appliances such as fans, mixers etc

107. PMBLDC

108. Construction and working of PMBLDC Stator: Made up of Silicon steel stampings with slots to accommodate either closed or opened distributed armature winding. This winding is connected to DC supply through a power electronic switching circuitry. Rotor: Made up of Forged steel. Rotor accommodate permanent magnet. Rotor shaft carries a position sensor, which provides information about the position of the shaft position at any instant to the controller. Controller sends suitable signals to the electronic commutator.

109. Commutation in DC motors Mechanical Commutator Electronic Commutator

110. Mechanical Commutator Commutator is made up of specially designed commutator segments, made of copper. These segments are insulated from each other by a thin layer of mica. It forms a cylindrical shape.

111. Mechanical Commutator

112. It consists of 2 pole machine with 12 commutator segments. Carbon brush A contacts with CS 1 and brush B contacts with CS 7. When a dc supply is connected across A & B, a dc current passes through A-CS1-tapping1- tapping 7-CS 7 and through B. The current has 2 parallel path in the armature winding. Parallel path 1= 1-2-3-4-5-6-7 Parallel path 2=1-12-11-10-9-8-7

113. The current crossing through the armature conductor setup an mmf along the axis A and B. The commutator rotates along the counter clockwise direction, now the brush A makes contact with CS2 and brush B with CS 8. Now there are two parallel path 1 is 2-8 & parallel path 2 is 2-1-12-11-10-9-8. Function of commutator and bush arrangement is to setup an armature mmf whose axis is always in quadrature with the main field mmf irrespective of the speed of rotation of the motor.

114. Electronic Commutator

115. Switching Circuits of Electronic Commutator

116. S1 & S1’ are closed.other switches are open current has 2 parallel path in armature winding. Parallel path 1 =1-2-3-4-5-6-7 Parallel path 2 =1-12-11-10-9-8-7 These current setup in the armature mmf . S1 & S1’ are opened,S2 & S2’ are closed Now the current passes through tapping 2-8. Thus operating switches in the sequential manner, we are getting a revolving magnetic field.

117. For normal electronic commutator , 6 switching devices are employed. Here the windings may be connected either star or delta connection. Therefore the winding should have 3 tappings .

118. Mechanical Commutator Electronic Commutator Commutator is made up of commutator segments and mica insulation. brushes are made up of carbon. Power electronics switching devices are used in the commutator . Shaft position sensing is inherent in the arrangements. It requires a separate rotor position sensor. Commutator arrangement is located in the rotor Commutator arrangement is located in the stator. Sliding contact between commutator and brushes. No sliding contacts. Sparking takes place. There is no sparking It requires a regular maintenance It requires less maintenance. Number of commutator segments are very high. Number of switching devices is limited to 6. Difficult to control the voltage available across tapping Voltage available across armature tappings can be controlled by PWM techniques. Highly reliable Reliability depends on the switching devices.

119. Working Principle A brushless DC motor uses electronic sensors to detect the position of the rotor without using a metallic contact Using the sensor's signals, the polarity of the electromagnets ’ is switched by the motor control drive circuitry The motor can be easily synchronized to a clock signal, providing precise speed control Brushless DC motors may have: An external PM rotor and internal electromagnet stator An internal PM rotor and external electromagnet stator 5/19/2022 121 Special Electrical Machines

120. This example brushless DC motor has: An internal, permanent magnet rotor Example Brushless DC Motor Operation 5/19/2022 122 Special Electrical Machines

121. This example brushless DC motor has: An external, electromagnet stator Example Brushless DC Motor Operation 5/19/2022 123 Special Electrical Machines

122. This example brushless DC motor has: An external, electromagnet stator, with magnetic field sensors Example Brushless DC Motor Operation 5/19/2022 124 Special Electrical Machines

123. Brushless DC Motor Construction A a a com com b b B com c c C A a b B c C com 5/19/2022 125 Special Electrical Machines

124. A a b B c C com A a a com com b b B com c c C 1 Brushless DC Motor Operation 5/19/2022 126 Special Electrical Machines

125. A a b B c C com A a a com com b b B com c c C 2 Brushless DC Motor Operation 5/19/2022 127 Special Electrical Machines

126. A a b B c C com A a a com com b b B com c c C 3 Brushless DC Motor Operation 5/19/2022 128 Special Electrical Machines

127. A a b B c C com A a a com com b b B com c c C 5 Brushless DC Motor Operation 5/19/2022 129 Special Electrical Machines

128. A a b B c C com A a a com com b b B com c c C 6 Brushless DC Motor Operation 5/19/2022 130 Special Electrical Machines

129. A a b B c C com A a a com com b b B com c c C 1 Brushless DC Motor Operation 5/19/2022 131 Special Electrical Machines

130. Merits BLDC No field winding . Therefore, no field copper loss Length of motor is less as there is no mechanical commutator Size of the motor become less Better ventilation as armature is accommodated in the stator Motor can be designed for higher voltage subjected to the constraints posed by the power semiconductor device It is possible to have very high speed Motor can be operated in hazardous atmospheric conditions Efficiency is better It is a self starting motor and speed can be controlled Regenerative braking is possible

131. Demerits- BLDC Field cannot be controlled Power rating is restricted because of the maximum available size of permanent magnet Rotor position sensor is required Power electronics switching circuitry is required

132. Construction of three phase Induction motor The Induction motor has two main parts.- Stator Rotor EE8353 Electrical Drives and Controls 1

133. Stator: Made up of a number of stampings with alternate slots and tooth. Stampings are insulated from each other Stampings are 0.4-0.5mm thick. Number of stampings are stamped together to build the stator core. Stator core is then fitted in a casted or fabricated steel frame. Slots houses the three phase winding called stator winding (connected either in star or delta) EE8353 Electrical Drives and Controls 1

134. Rotor: There are two types of rotors. Squirrel cage type Slip ring type Squirrel cage type: Made up of a cylindrical laminated core with slots to carry the rotor conductors. Rotor conductors are heavy bars of copper or Aluminum, short circuited at both ends by end rings. Hence this rotor is called a short circuited rotor. The entire rotor resistance is very small and external resistance cannot be added in the rotor circuit. Majority of IM are squirrel cage type EE8353 Electrical Drives and Controls 1

135. Slip Ring or Wound rotor Rotor winding are similar to stator winding. The windings are either star or delta connected, distributed winding, wound for as many number of poles as stator is wound for. Three phases are brought out and connected to slip rings. Variable external resistance can be connected in the rotor circuit with the help of brushes and slip ring arrangement. By varying the external resistance in the rotor circuit, the motor speed and torque can be controlled. EE8353 Electrical Drives and Controls 1

136. Types and Principle of operation –Three phase IM There are two types of three phase induction motor. Squirrel cage Induction motor Wound rotor or slip ring induction motor. Principle of operation: Three phase supply is given to the stator winding, and due to this a current called the stator current flows through the stator winding. It produces a rotating magnetic field in space between stator and rotor and this magnetic field rotates at a synchronous speed given by, Where, is the synchronous speed, f-frequency and P-no of poles   EE8353 Electrical Drives and Controls 1

137. Since the rotor conductors cuts the rotating magnetic field, an emf is induced in the rotor. If the rotor winding are shorted (Cage rotor they are already shorted, and in slip ring motor, they have to be externally shorted), then the induced emf produces current. This current produces a rotor field (rotor mmf ) The interaction of rotor field and stator field develops torque. Thus the rotor conductor rotates in the same direction as the rotating magnetic field. When the rotor is at standstill, the frequency of rotor emf is equal to the supply frequency. Rotors tries to catch up with the rotating magnetic field. However, it cannot catch up and rotate at synchronous speed. In case if it does so, the relative speed would become zero and then there is no rotor emf, no current and hence no torque . Hence rotor runs at a speed slightly less than the synchronous speed. Since the rotor speed is always less than synchronous speed, IM are called asynchronous motors. EE8353 Electrical Drives and Controls 1

138. The difference between synchronous speed and rotor speed is called slip speed. Slip speed = -N Slip, s = N= (1-S) %slip = *100 At no-load, the difference between synchronous speed and rotor speed is only about 1% At loaded condition, the rotor slows down, emf induced in the rotor and hence rotor current increases. Hence torque increases, The variation of speed from no-load to full load is very small. Thus three phase induction motor is called constant speed motor. Variation of load is possible, but efficiency reduces.   EE8353 Electrical Drives and Controls 1

139. Frequency of Rotor Current or emf When the rotor is stationary, the relative speed between the rotor winding and rotating magnetic field is .Hence, the frequency of emf induced and the resultant current is (same as supply frequency) As the rotor speeds up, the relative speed is ( -N) and hence, rotor frequency = - [1] Since slip , s = ( -N) =s =s* Substituting in [1], Rotor frequency, = s* * = sf . Thus, the frequency of rotor induced emf in an IM is equal to the product of slip and supply frequency (Slip frequency)   EE8353 Electrical Drives and Controls 1

140. Single phase Induction motor Single-phase induction motors are usually two-pole or four-pole, rated at 2 hp or less, while larger motor can be manufactured for special purposes. They are widely used in domestic appliances and for a very large number of low power drives in industry. The single phase induction motor resembles, three-phase, squirrel-cage motor except that, single phase induction motor has no starting torque and some special arrangement have to be made to make it self starting. They are simple in construction. EE8353 Electrical Drives and Controls 1

141. Construction of Single phase Induction motor Consists of stator and rotor and Construction is similar to that of three phase squirrel cage IM Rotor is same as three phase squirrel cage IM, but the stator has only a single phase distributed windings. Airgap between stator and rotor is uniform. Single phase induction motor has no self starting torque . It can be explained using the following two theory: Double Field revolving theory Cross field theory EE8353 Electrical Drives and Controls 1

142. Double field revolving theory Any Alternating quantity can be resolved in to two components, which rotate in opposite directions and have half of the maximum magnitude. Therefore, the alternating flux produced in the single phase Induction motor can be represented by two revolving fluxes, each equal to half the value of the alternating flux ( each rotating synchronously ( ) in opposite directions. Let is the component rotating in anticlockwise direction and be the component rotating in clockwise direction. The resultant of these two components gives the instantaneous value of the stator flux at that instant.   EE8353 Electrical Drives and Controls 1

143. At start, both forward and backward component is shown opposite to each other. Thus the resultant fux , =0 (Instantaneous flux at start). After 90 degrees, the two components rotates in such a way that both are pointing in the same direction. Hence is the algebraic sum of the magnitude of two components Therefore, = + = ( Instantaneous value of flux at 90degree.   EE8353 Electrical Drives and Controls 1

144. Both the components are rotating and hence cut by the rotor conductors. Due to cutting of flux, emf gets induced in the rotor , which circulates the rotor current, which produces the rotor flux. This flux interact with the forward component produces a torque in a direction say anti clockwise direction. Rotor flux interacts with the backward component produces a torque in clockwise direction. If torque in anticlockwise direction is positive and clockwise direction is taken as negative. At start, these two torques are equal in magnitude but opposite in direction and hence the net torque experienced by the conductor is zero. Hence single phase motors are not self starting.   EE8353 Electrical Drives and Controls 1

145. Operation of Single phase Induction motor The stator winding of a single phase IM is connected to single phase ac supply. Magnetic field is developed in the stator whose axis is always along the axis of stator winding. With alternating current in the fixed stator coil the mmf wave pulsates in magnitude and varies sinusoidally with time. Due to the transformer action, currents are induced in the rotor conductors, and rotor flux is produced. Interaction of stator flux and rotor flux, torque is produced. In the single phase IM the initial torque angle is zero and no starting torque is developed in the motor. If rotor given a starting torque, the motor will pick up the speed and continue to rotate in the same direction. Thus single phase IM is not a self starting motor. Starting torque can be produced by external means. EE8353 Electrical Drives and Controls 1

146. Starting of Single phase Induction motor. An auxiliary winding is provided in addition to the main winding. Induction motor works as a two phase motor. The main winding axis and auxiliary winding axis are displaced by 90 electrical degrees. The impedance of the windings differ and currents in the main an auxiliary windings are phase shifted from each other. As a result of this a rotating magnetic field is produced. EE8353 Electrical Drives and Controls 1

147. S tarting winding When the motor speed is about 75% of synchronous speed, the auxiliary winding is disconnected from the circuit. This is done by connecting a centrifugal switch in the winding, which is used for starting purpose only. That is why its called starting winding. Under running condition, a single phase IM can develop torque only with main winding. That is why its called running winding. EE8353 Electrical Drives and Controls 1

148. Types of Single-Phase Motors Types of Single-Phase Motors are, ( i )Resistance start or split-phase type induction motor (ii) capacitor start induction motor (iii) capacitor start capacitor run type induction motor (iv) shaded-pole type induction motor EE8353 Electrical Drives and Controls 1

149. Resistance start or split-phase type induction motor Consists of running /main winding and auxiliary winding or starting winding. The auxiliary winding has high resistance and low reactance and main winding has low resistance and high reactance. I r is the current in running winding and I s is the current through starting winding. EE8353 Electrical Drives and Controls 1

150. As the main winding is inductive, current I r lags the voltage by a large angle while I s lags the voltage by a small angle since it is highly resistive. Thus there exists a phase difference of α between the two currents and hence between the two fluxes produced by the two currents. The resultant of these two flux is a rotating a magnetic field, due to which the starting torque, which acts in only one direction is produced.   EE8353 Electrical Drives and Controls 1

151. C entrifugal switch The auxiliary winding has a centrifugal switch in series with it. When the motor gathers the speed of about 75-80% of the synchronous speed, this switch gets opened and running winding gets disconnected. After that motor runs only with the main winding. Therefore, the auxiliary winding is designed for short term use, while the main winding is designed for continuous use.

152. Since the currents, I s and I r are splitted by each other by an angle ‘ α ‘ at start, the motor is also called as split phase motor. Torque speed characteristics is shown below. The starting torque, Tst is proportional to the split angle ‘ α ‘ EE8353 Electrical Drives and Controls 1

153. Split phase motor give a starting torque of 125%-150% full load torque. The direction of rotation can be reversed by reversing the terminal of either main or auxiliary winding. Applications: Since they have low starting current and moderate starting torque, they are used in easily started loads like Fans, blowers, grinders, centrifugal pumps, washing machines etc. EE8353 Electrical Drives and Controls 1

154. Capacitor start Induction motor Construction is similar to the split phase machine. A capacitor is connected in series with the auxiliary winding. Since capacitive circuit draws a leading current, capacitor is used to increase the split phase angle ‘ α ’ between the two currents Im and Is. Depending on whether the capacitor remains in the circuit permanently or not, these motors can be classified in to two types. Capacitor start motor Capacitor start and run motor. EE8353 Electrical Drives and Controls 1

155. Capacitor Start Induction motor Construction is as shown below. The current Ir lags the voltage by and angle , while due to the capacitor current I st leads the voltage by an angle . Hence there exists a large phase difference between the two currents, which is almost 90⁰ (ideal case) Starting torque is proportional to α and hence such motor produce very high starting torque. When speed approaches to 75%-80% of the synchronous speed, the starting winding gets disconnected, and hence the capacitor remains in the circuit only at starting.   EE8353 Electrical Drives and Controls 1

156. Capacitor Start Capacitor Run Induction motor EE8353 Electrical Drives and Controls 1 No centrifugal switch and Capacitor remains permanently in the circuit. Hence, Power factor is improved. Performance at start as well as during running condition depends on the capacitor, hence the value of C is to be designed so as to compromise between starting and best running condition. Starting torque available in such motors are about 50-100% of full load torque.

157. Shaded pole Induction motor They have squirrel cage rotor and stator consists of salient poles. The poles are shaded. Each pole carries a copper band on one of its unequally divided part called shading band. When single phase ac is given to the stator winding, due to the shading provided to the poles, a rotating magnetic field is generated. The current carried by the stator winding is alternating and produces alternating flux. The distribution of this flux in the pole is greatly influenced by the copper shading band. EE8353 Electrical Drives and Controls 1

158. Consider three instants t1,t2 and t3 during first half cycle of the flux. At t1, rate of rise of current , flux is very high. Due to the transformer action, large emf gets induced in the copper shading band. This circulates current through the shading band as it is short circuited, produces its own flux. According to Lenz’s law, the direction of this current is so as to oppose the rise in current. Hence shading flux opposes the main flux. Hence crowding of flux in non-shaded part while weakening of flux in the shaded part. Overall the magnetic axis shifts in the non-shaded part. EE8353 Electrical Drives and Controls 1

159. At the instant t2, rate of rise of current, and hence rate of change of flux is almost zero, since flux almost reaches its maxium value. So =0. Hence there is very little induced emf in the shading ring , hence shading ring flux is negligible, hardly effecting the distribution of main flux. Hence main flux distribution is uniform and magnetic axis lies at the center of the pole face.   EE8353 Electrical Drives and Controls 1

160. At t3,current and flux is decreasing, crowding of flux near the shading band, hence strengthen the flux at the shaded portion. Flux will be weak at the unshaded portion. Torque speed characteristics of shaded pole motor is as shown below. EE8353 Electrical Drives and Controls 1

161. Disadvantages Lack of starting torque Reduced power factor Low efficiency.

162. Comparison between Brushless DC motor & Induction motor drives In the same frame, with the same cooling, the brushless PM motor will have better efficiency and power factor, and therefore a greater output power. The power electronic converter required with the brushless motor is similar in topology to the PWM inverters used in induction motor drives. The device ratings maybe lower if only a 'constant torque' characteristic is required. Induction motor can be inexpensively controlled with Triacs or d.c. system in efficiency, stability, response, and controlled speed range. To obtain comparable performance in the control sense, the induction motor must be fed from a PWM inverter, which is arguably more complex than the brushless PM motor drive.