Ch-3 Induction Motor-1.pdfvvvvvvvvvvv
- 1. 1 CHAPTER 3 INDUNCTION MACHINES By: Semret Minda (Msc) Menschen für Menschen Foundation Agro –Technical and Technology College Electrical and Electronics Technology Department Electrical Machine (EET4323)
- 2. Outlines: ▪ Introduction to Induction Motor(IM) ▪ Equivalent Circuit of 3-Phase Induction Motor ▪ Circle Diagram ▪ AC Machines Armature Winding ▪ Methods of Starting 3-phase Induction Motors ▪ Speed Control of Induction Motors ▪ Fractional Horsepower Motors 2 4/23/2024
- 3. Introduction to Induction Motor(IM) 3 4/23/2024 • An induction motor is an asynchronous AC electric motor which converts AC electricity into the mechanical energy. • It is called an induction motor because the electric current in the rotor circuit required to produce the deriving torque is obtained through electromagnetic induction from the rotating magnetic field of the stator winding. • IM are the most widely used electric motors in industrial applications. • Almost all IM run at essentially constant speed from no-load to full-load conditions.
- 4. Working Principle of IM 4 •Rotating Magnetic Field: The motor has two main components – the stationary part called the stator and the rotating part called the rotor. When AC (alternating current) electricity flows through the stator windings, it creates a rotating magnetic field around the stator. •Induced Current in Rotor: As the rotating magnetic field cuts across the rotor, it induces a flow of electric current in the rotor’s conductors. •Interaction of Magnetic Fields: The induced current in the rotor creates its own magnetic field. This induced magnetic field interacts with the rotating magnetic field of the stator. •Torque and Rotation: The interaction between the stator’s rotating magnetic field and the rotor’s induced magnetic field results in a force, known as torque. This torque causes the rotor to start rotating and follow the rotation of the stator’s magnetic field. •Continuous Rotation: As long as the stator is supplied with AC power, the rotating magnetic field will persist, continually inducing currents in the rotor and keeping the motor rotating.
- 5. CONSTRUCTION FEATURES 5 4/23/2024 ➢ 3 AC IM are commonly used in industrial applications. ➢ This type of motor has 3 main parts, rotor, stator, and enclosure. The stator and rotor do the work, and the enclosure protects the stator and rotor. A. Stator • The stator is composed of laminations of high-grade sheet steel and is built up of sheet steel lamination of 0.4 to 0.5mm thickness. • Laminations are insulated from each other by means of varnish coating or oxide. • At 3 winding is put in slots punched out on the inner surface of the stator frame. It is made up of a number of stampings which are slotted to receive the windings, (Figure 1(a)).
- 6. CONSTRUCTION FEATURES 6 4/23/2024 • It is wound for a definite no of poles, the no of poles being determined by the requirements of speed. Greater the number of poles, lesser the speed and vice versa. • synchronous speed is given by Fig1: (a) Induction machine laminations Stator
- 7. CONSTRUCTION FEATURES 7 4/23/2024 b) Rotor • The rotor also consists of laminated ferromagnetic material, with slots punched out on the outer surface (Figure 1 (b)). • The frequency of the rotor flux is very low; as a result thicker laminations can be used without excessive iron losses. Fig1: (b) Induction machine laminations rotor
- 8. Types of Induction Motors 8 4/23/2024 Depending on the type of input supply, induction motors are classified into the following 2 types: • 1 Induction Motors− An induction motor that works on single-phase AC supply is called as a single-phase induction motor. • 3 Induction Motors− An induction motor which requires three-phase AC supply to operate is called a three-phase induction motor.
- 9. Types of Induction Motors 9 4/23/2024 • 2 types of rotor construction is normally used for three phase induction motor: i. Squirrel-cage roto ii. wound rotor i. Squirrel –Cage Rotor • Almost 90% of IM are squirrel-cage type. • It consists of a cylinder of steel laminations, with aluminum or copper conductors embedded in its surface. • In operation, the non-rotating stator winding is connected to an AC power source; the AC in the stator produces a rotating magnetic field.
- 10. Types of Induction Motors 10 4/23/2024 ii. wound rotor • A wound-rotor motor, also known as slip ring-rotor motor, is a type of IM where the rotor windings are connected through slip rings to external resistance. • The speed and torque characteristics of a wound-rotor motor can be adjusted by changing the external resistance.
- 11. Comparison of squirrel cage and wound rotors. 11 Parameter of Comparison Squirrel-Cage Induction Motor Slip-Ring Induction Motor Construction Simple Complicated External Rotor Resistance NO external resistance External resistance is to be added during starting. Starting Torque Low High Brushes Not used Used Maintenance Less maintenance is required. Frequent maintenance is required. Copper Loss Low High Efficiency High Low Power Factor High Low Speed Control Not possible Possible Starting Current High Low Cost Cheap Expensive Application In lath machine, fans, pumps, blowers, etc. In cranes, hoists, elevators and loads which require high starting torque, etc.
- 12. ROTATING MAGNETIC FIELD 12 • when 3 windings displaced in space by 1200 are fed by 3 current displaced in time by 1200 they produce a resultant magnetic flux which rotated in space as if actual magnetic poles were being rotated mechanically. • Let a 3 , 2-pole stator having 3 identical winding placed 1200 space apart and the flux ( assumed sinusoidal) due to 3 windings as shown in Figure 2(a). • The assumed positive directions of the fluxes are shown Figure 2(b).
- 13. Rotating Magnetic Field Derivation 13
- 14. PRINCIPLE OPERATION of 3 IM 14 • The difference between the synchronous speed Ns and the actual speed N of the rotor is known as slip, • Sometimes, Ns – N is called the slip speed. Obviously, rotor speed is N = Ns (1-S) • Let at any slip speed & the frequency of the rotor current be fr . Then, • Rotor frequency, 𝒇𝟐=S𝒇𝟏 • Speed of stator field with respect to stator structure:
- 15. Example 15 Example 1 A 3-phase, 50 Hz induction motor has a full-load speed of 1440 r.p.m. For this motor, calculate the following: (a) number of poles ; (b) full-load slip and rotor frequency ; (c) speed of stator field with respect to i) stator structure and ii) rotor structure
- 16. Example 16 a) number of poles ; Solution. (a) The use of full-load speed of 1440 rpm as synchronous speed gives Since the number of poles must be even and a whole number, the induction motor must have 4 poles. b) full-load slip and rotor frequency ; Synchronous speed, c) speed of stator field with respect to i) stator structure and ii) rotor structure
- 17. Advantages of Induction Motors 17 4/23/2024 The following are some major advantages of induction motors: • IM have simple and rugged construction. • IM are relatively less expensive. • IM have relatively high efficiency. • IM can be designed to have characteristics to meet the industrial requirements. • IM need little maintenance.
- 18. Disadvantages of Induction Motors 18 4/23/2024 The main disadvantages of induction motors are as follows: • The speed of IM cannot be changed without reducing its • IM draw a high inrush current at starting. • IM always operate at lagging power factor. • Single-phase IM are not self-starting; hence we need provide some extra starting mechanism.
- 19. Equivalent Circuit of 3 IM 19 • The general form of the equivalent circuit for a 3 IM can be derived from the equivalent circuit of a 3 transformer(Tf). • The IM can be considered a 3 Tf whose 20 or the rotor, is short-circuited and revolves at the motor speed. • It is similar to Tf, that’s why IM called as rotating transformer.
- 20. Equivalent Circuit of 3 IM 20
- 21. Equivalent Circuit of 3 IM 21
- 22. Equivalent Circuit of 3 IM 22
- 23. Per Phase Total Equivalent Circuit of 3 IM 23
- 24. Power Flow Diagram of IM 24
- 25. Important points in torque-slip characteristics 25 ▪ Torque is directly proportional in low slip region of IM ▪ Torque is inversely proportional in high slip region of IM ▪Low slip region called stable region ▪Maximum torque obtained is called pull out torque in IM ▪The slip at which the pullout torque occur is called critical slip ▪Critical slip = 𝑅2/𝑋2 ▪By increasing 𝑹𝟐 in rotor it is possible to increase the starting torque (this is possible only slip ring IM)
- 26. Important Points in Equivalent Circuit of an IM 26
- 28. Circle Diagram of IM . 28 4/23/2024 • Circle Diagram: It is the diagrammatic representation of the performance of the IM. It provides information about the power output, losses and the efficiency of the IM. We cannot determine the frequency by using the circle diagram. Circle Diagram of IM Numerically asked
- 29. . 29 4/23/2024
- 30. . 30 4/23/2024 • Short circuit current at normal voltage ✓ 𝐼𝑆𝐶=38 A • Short circuit Power at normal voltage ✓ 4000W power taken on blocked rotor test with 150V supply voltage & 400V rated voltage
- 31. . 31 4/23/2024
- 32. Steps for Drawing the Circle Diagram for IM • Draw perpendicular bisect NC of line O′A from the points we get from arcs. Point C is where the line intersects horizontal line O′D 32
- 33. Steps for Drawing the Circle Diagram for IM • Draw a vertical line from A to the horizonal axis. Mark F as point where it intersects O′D & • Mark G as point where the line intersects OX, the horizontal axis. 33 4/23/2024
- 34. Steps for Drawing the Circle Diagram for IM • Vetical distance from 𝑂′ to horizontal axis represents the no load i/p, which supply core, friction & windage losses. 34 4/23/2024
- 35. Method to Read Operating Parameter from Circle Diagram 35 4/23/2024
- 36. Method to Read Operating Parameter from Circle Diagram 36 4/23/2024
- 37. Taking Data from the Line LK 37 4/23/2024 • Zoom at line LK & understand easily
- 38. Taking Data from the Line LK 38 4/23/2024 • Zoom at line LK & understand easily
- 40. Important Terms Related to Armature Winding . 40 4/23/2024 ➢ Winding: is the arrangement of conductors in the slots to produce emf in relative motion in a magnetic field. ➢ The stator and rotor cores are wound with conductors which are insulated from each other & from their cores. ➢ The conductors are arranged in series & in parallel a) In series, to increase the voltage rating b) In parallel, to increase the current rating
- 41. CONT… . 41 4/23/2024 ▪ Conductor: the active length of a wire or strip in the slots. ▪ Turn: consists of 2 conductors which separate each other by pole pitch/coil pitch. ▪ coil: A coil has several turns in the series. One turn is attached to the beginning of the next turn, forming a continuous path for the flow of electrical current. The symbols (S)-Start and (F)- Finish, respectively. • Each coil has active and inactive sides • Acoil have any number of turns • single turn/coil has 2 active sides or conductors
- 42. CONT… . 42 4/23/2024 • Active side of a coil: part of the which lies in the slots under a magnetic pole & emf induced in this part only. • Inactive side of a coil: back side & front end side. • Front side: used to connect other coil. • Back end side: joints of two active side. • Coil group: one or more coils connected in series. Coil group = number of poles × the number of phases • Pole pitch Is defined as the angular distance between the 2 opposite poles. A pole pitch is always 180°. • Coil span or coil pitch Is defined as the distance between the two active sides of the same coil adjust under opposite poles. It expressed in terms of slots/pole.
- 43. FULL PITCH COIL: 1 2 3 4 5 6 7 slots N S One side Second side Pole pitch Coil pitch 4/23/2024 Full Pitch Coil: • If the coil pitch is equal to pole pitch then the coil is called as full pitch coil. • In such cases, coil pitch = pole pitch = 1800 electrical.
- 44. SHORT PITCH OR FRACTIONAL PITCH COIL: Short pitch coil: the coil pitch is less than the pole pitch (i.e., less than 1800 electrical) For example, as shown above, pole pitch = 6 slots and the coil pitch = 5 slots. Thus the coil is short by one slot. Here, 6 slots = 1800 electrical 5 slots = (1800 /6) x 5 = 1500 electrical Thus, coil pitch = 5 slots or 1500 electrical or the coil is short by 1800 –1500 = 300 electrical 1 2 3 4 5 6 7 slots N S One side Short pitch coil Full pitch coil
- 45. Types of AC Machine Winding . 45 4/23/2024 A. Single layer winding ( has only one coil side/slot) B. Double layer( 2 layer) (two coil sides/ slot) A. Single Layer Winding ❖ The three most common types of single layer windings • Concentric windings ( Unequal coil span) • Chain windings (equal coil span) • Mush windings (equal coil span)
- 47. Concentric winding: . 47 4/23/2024 ➢ Three phase concentrating winding consists of coil groups laid in the slots so that all the coils of each group are concentric. i.e. The coil with smallest pitch is surrounded by the coil with the next largest slot pitch and so on to make up the coil group.
- 48. Data must be known to construct Concentric AC Windings . 48 4/23/2024 In order to Construct Concentric AC Windings the following data must be known:
- 50. PhaseA ,B and C Coil groups interconnection and end terminal leads . 50 4/23/2024
- 52. Mush winding . 52 4/23/2024 • This winding is very common used for small induction motor having circuit conductors. ▪ This is a single layer winding where all the coils have the same span /pitch. ❖ Each coil is wound on a former, making one coil side shorter than the other. ▪ It will be observed that the ends of coil situated in adjacent slots cross each other. ✓ i.e. proceed to left & right alternatively. ▪ That is why sometimes it is known as a basket winding.
- 55. Data must be known to construct/designing a mush winding . 55 4/23/2024 ▪ There is only one coil side per slot and therefore: ✓ the number of coil sides = the number of slots. ▪ There is only one coil group per phase per pole pair and therefore: ✓ the maximum number of parallel paths per phase = the pole pair. ▪ The coil span should be odd.
- 56. PhaseA ,B and C Coil group interconnections and Terminals . 56 4/23/2024
- 58. Chain winding . 58 4/23/2024 ❖ In all aspect this winding is similar to that of mush winding except that a) It is diamond shape b) The coil span should not be odd
- 59. Connection of phases A, B and C and End terminals . 59 4/23/2024
- 60. . 60 4/23/2024 Draw winding diagram for 3 phase single-layer winding
- 61. . 61 4/23/2024 Example: draw a winding diagram for 3 phase, 50 Hz, 24 slot and 1500 r.p.m single layer induction motor stator Winding design. Calculate Required Data • Pole • Coil Pitch • Total coil • Coil per Phase Note: 1 coil side / slot Number of coils = Number of Slots/2
- 62. . 62 4/23/2024
- 63. . 63 4/23/2024
- 64. . 64 4/23/2024
- 65. . 65
- 66. . 66 4/23/2024
- 67. . 67 4/23/2024
- 68. . 68 4/23/2024
- 69. . 69
- 70. . 70 4/23/2024
- 71. . 71 4/23/2024 Double layer (2 layer) stator winding
- 72. Double layer (2 layer) stator winding . 72 4/23/2024 ❖Two layer windings differ from single layer winding mainly: ➢ There are two coil sides per slot. ✓ One coil side on top layer & the other is on the bottom ➢ A majority stator winding uses a shortened pitch because: ✓ To reduce the amount of copper used in the end windings.
- 73. Double layer (2 layer) stator winding . 73 4/23/2024 ▪ In AC machine winding, if the number of slots per pole per phase q=S/mp is an integer, then the winding is called integral slot winding. ▪ In case the number of slots per pole per phase is not an integer, the winding is called fractional slot winding. ▪ The coils are wound with continuous length of wire to reduce the number of connection to be made between coils of a coil group.
- 74. Rules for double layerwindings . 74 4/23/2024 ▪ The coil groups should be connected each other by joining the leads of like polarity ✓ that is the finish of one group to the finish of the next group & the start to the start ❖ Next slide shows winding diagram for 3 phase double-layer winding.
- 75. . 75 Example: Develop/draw Winding diagram of 3-phase, 4 -pole, 24- slots, double layer induction motor stator Winding design. Calculate Required Data • Coil Pitch • Coil per phase • Slot/pole/phase • Phase angle Note: 2 coil side / slot Number of coils = Number of Slots
- 76. Draw winding diagram for 3 phase double-layer winding . 76 4/23/2024 ❖ Calculate Required Data: 1. Coil pitch = Slot/pole ------------------------------------------- 24/4=6 2. Coil per phase = total coil/phase ------------------------------ 24/3=8 3. Slot/pole/phase = coil per phase/pole ----------------------- 8/4=2 4. Phase angle = 120*coil pitch/180 ------------------------------120*6/180=4
- 77. . 77
- 78. Draw winding diagram for 3 phase double-layer winding (R-phase) . 78 4/23/2024
- 79. Draw winding diagram for 3 phase double-layer winding (R-phase) . 79
- 80. Draw winding diagram for 3 phase double-layer winding (R-phase) . 80
- 81. Draw winding diagram for 3 phase double-layer winding (Y-phase) . 81
- 82. Draw winding diagram for 3 phase double-layer winding (B-phase) . 82
- 83. Advantages and Disadvantages of Single Layer Windings . 83 Advantages • Individual conductors of the coil are introduced one by one into the slots & when the winding is over, it is impregnated. ▪ Thus very narrow slot openings are sufficient, ▪ Easier to repair and to remove a defective coil and ▪ The winding is cheaper because fewer coils have to be made. Disadvantages ▪ The greatest disadvantage the coils cannot be short pitched. ▪ Thus harmonics' cannot be suppressed because of short pitching. ▪ Therefore restricted to small sized machines only.
- 84. Advantages and Disadvantages of Double LayerWinding . 84 Advantages • The windings are suitable for short pitching them, undesirable harmonics can be eliminated. ▪ Thus All modern machines have double layer windings ▪ Using a fractional slot winding a double layer winding, the emf harmonics reduced. ▪ Lower leakage reactance and therefore, better performance of the machine, ▪ Better emf wave form in case of generators. Disadvantages ▪ Difficult to repair ▪ Cost of insulation is greater
- 85. . 85 4/23/2024 METHODS OF STARTING 3-PHASE INDUCTION MOTORS
- 86. Starting Torque (Ts) . 86 4/23/2024 ➢ The torque developed by the motor at the instant of starting is called starting torque. In some cases, it is greater than the normal running torque. The torque T developed by the rotor is directly proportional to: i. Rotor current ii. Rotor e.m.f iii. Power factor(Pf) of the rotor circuit
- 87. Starting Torque (Ts) . 87 4/23/2024 ❖ The starting torque is very sensitive the changes in the value of supply voltage.
- 88. METHODS OF STARTING 3-PHASE IM . 88 4/23/2024 ❖ The method to be employed in starting a given IM depends upon the size of the motor & the type of the motor. The common methods used to start induction motors are: 1. Direct-on-line (DOL) starting 2. Stator resistance starting 3. Autotransformer starting 4. Star-delta starting 5. Rotor resistance starting
- 89. METHODS OF STARTING 3-PHASE IM . 89 ➢ Methods (1) to (4) are applicable to both squirrel-cage and slip-ring motors. ➢ However, method (5) is applicable only to slip-ring motors. ➢ But slip-ring motors are always started by rotor resistance starting(5). ▪Methods (2) to (4) are reduced voltage starting method. ✓ Except direct-on-line (DOL) starting, all other methods of starting squirrel-cage motors employ reduced voltage across motor terminals at starting. ▪ Methods (1) full voltage starting method
- 90. Slip-Ring Motors Versus Squirrel Cage Motors: . 90 4/23/2024 ➢ The slip-ring IM have the following advantages over the squirrel cage motors: (i) High starting torque with low starting current. (ii) Smooth acceleration under heavy loads. (iii) No heating during starting. (iv) Good running characteristics after external rotor resistances are cut out. (v) Adjustable speed. ➢ The disadvantages of slip-ring motors are: (i) The initial and maintenance costs are greater than those of squirrel cage motors. (ii) The speed regulation is poor when run with resistance in the rotor circuit
- 91. . 91 4/23/2024 SPEED CONTROL OF INDUCTION MOTORS
- 92. SPEED CONTROL OF 3-PHASE IM . 92 4/23/2024 ❖ Speed N of an induction motor can be varied (controlled) by changing: (i) Supply frequency f (ii) Number of poles P on the stator & (iii) Slip s
- 93. SPEED CONTROL OF 3-PHASE IM . 93 ➢ The change of frequency is generally not possible because the commercial supplies have constant frequency. ➢ Therefore, the practical methods of speed control are either to change the number of stator poles or the motor slip. ➢ The speed of a squirrel cage motor is changed by changing the number of stator poles. The speed of wound (Slip-ring) rotor motors is changed by changing the motor slip. This can be achieved by; i. Varying the stator line voltage. ii. Varying the resistance of the rotor circuit. iii. Inserting and varying a foreign voltage in the rotor circuit.
- 94. SPEED CONTROL OF 3-PHASE IM . 94 4/23/2024 ➢ In Slip ring (wound-rotor ) IM , it is possible to change the shape of the torque- speed curve by inserting extra resistances into the rotor circuit of the machine. ➢ However, inserting extra resistances into the rotor circuit of an IM seriously reduces the efficiency of the machine. ➢ So it is normally used only for short periods because of this efficiency problem.
- 95. Fractional Horsepower Motors: Group Presentation Contents / Out-Lines of Presentation • Introduction • Types (if any) • Working Principle • Method of Starting • Speed Control • Application 95 4/23/2024 Group 1 Single phase Induction Motor Group 2 Shaded-Pole Motor Group 3 Universal Motor Group 4 Reluctance Motor Group 5 Hysteresis Motor