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LECTURE ON INDUCTION MACHINE

LECTURE ON INDUCTION MACHINE. LECT. RAKESH KUMAR DEPTT. OF EE BHSBIET. The following points are to be explained in this lecture:-. Induction Motor Construction Basic Induction Motor Concepts The Equivalent Circuit of an Induction Motor. Power and Torque in Induction Motor.

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LECTURE ON INDUCTION MACHINE

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  1. LECTURE ON INDUCTION MACHINE • LECT. RAKESH KUMAR DEPTT. OF EE BHSBIET BHSBIET LEHRAGAGA

  2. The following points are to be explained in this lecture:- • Induction Motor Construction • Basic Induction Motor Concepts • The Equivalent Circuit of an Induction Motor. • Power and Torque in Induction Motor. • Induction Motor Torque-Speed Characteristics • Variations in Induction Motor Toque-Speed Characteristics • Starting of Induction Motors • Speed Control of Induction Motor • Some important features of IM BHSBIET LEHRAGAGA

  3. 3 PHASE INDUCTION MACHINE PART-I BHSBIET LEHRAGAGA

  4. INTRODUCTION BHSBIET LEHRAGAGA

  5. Overview of Three-Phase Induction Motor • Induction motors are used worldwide in many residential, commercial, industrial, and utility applications. • Induction Motors transform electrical energy into mechanical energy. • It can be part of a pump or fan, or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer.

  6. Cutaway view of a modern induction motor BHSBIET LEHRAGAGA

  7. The induction machines may be classified in many ways. BHSBIET LEHRAGAGA

  8. Construction • The three basic parts of an AC motor are the rotor, stator, and enclosure. • The stator and the rotor are electrical circuits that perform as electromagnets.

  9. Construction • An induction motor is composed of a rotor, known as an armature, and a stator containing windings connected to a poly-phase energy source. BHSBIET LEHRAGAGA

  10. Construction (Rotor construction) • The rotor is the rotating part of the electromagnetic circuit. • It can be found in two types: • Squirrel cage • Wound rotor • However, the most common type of rotor is the “squirrel cage” rotor.

  11. Construction (Rotor construction) • Induction motor types: • Squirrel cage type: • Rotor winding is composed of copper bars embedded in the rotor slots and shorted at both end by end rings • Simple, low cost, robust, low maintenance • Wound rotor type: • Rotor winding is wound by wires. The winding terminals can be connected to external circuits through slip rings and brushes. • Easy to control speed, more expensive.

  12. The main parts of any IM are:- BHSBIET LEHRAGAGA

  13. STATOR OF IM BHSBIET LEHRAGAGA

  14. COILS IN THE ROTOR BHSBIET LEHRAGAGA

  15. SQUIRREL CAGE ROTOR BHSBIET LEHRAGAGA

  16. A WOUND ROTOR WITH SLIP RINGS BHSBIET LEHRAGAGA

  17. SLIP RINGS BHSBIET LEHRAGAGA

  18. Constructional features of squirrel cage and wound type motor • Stator: The stator consists of stator frame, core and stator winding. • Stator frame:- It is the outer body of the motor used to support stator core and windings and also to protect the inner parts of the machine. The frame may be die-cast or fabricated. BHSBIET LEHRAGAGA

  19. Stator core :- The stator core is assembled of high grade, low electrical loss, silicon steel punching. • The thickness varies from 0.35 mm to 0.7 mm. • The laminations are used to reduce eddy current loss. • The laminations are slotted on the inner periphery and are insulated from each other. The insulated stator conductors are placed in these slots. BHSBIET LEHRAGAGA

  20. Stator or field winding:- The stators conductors are connected to form a three phase winding. The three phases of the winding can be connected in either star or delta. BHSBIET LEHRAGAGA

  21. ROTOR:- • The rotor comprises of a cylindrical laminated iron core with slots on outer periphery. • The rotor conductor are placed in these slots. • The laminated cylindrical core is mounted directly on the shaft or a spider carried by the shaft. BHSBIET LEHRAGAGA

  22. BHSBIET LEHRAGAGA

  23. SQUIRREL CAGE ROTOR:- • In cage construction, rotor conductors, in the form of bars made of copper, or aluminium are placed in rotor slots parallel to the rotor shaft. • The rotor bars are short circuited by end rings of same material at each end. • The rotor slots are not parallel to the motor shaft but are skewed to reduce magnetic locking of stator and rotor and also to reduce humming noise while running. BHSBIET LEHRAGAGA

  24. SQUIRREL CAGE ROTOR OF IM BHSBIET LEHRAGAGA

  25. Squirrel cage rotor — a schematic BHSBIET LEHRAGAGA

  26. Wound rotor:- • The rotor is wound with an insulated winding similar to that of the stator. • The rotor winding is always 3 phase winding. The winding may be star or delta connected, but star connections are usually preferred. • The three terminals of star connections are brought outside the rotor and connected to three slip rings. • The carbon brushes are pressed on the slip rings. External resistors can be inserted in series with the rotor winding for speed and starting torque control. BHSBIET LEHRAGAGA

  27. WOUND ROTOR OF IM BHSBIET LEHRAGAGA

  28. Differences between squirrel cage IM and slip ring IM BHSBIET LEHRAGAGA

  29. PRINCIPLE OF OPERATION • Operation of 3-phase induction motors is based upon the application of Faraday’s Law and the Lorentz Force on a conductor. BHSBIET LEHRAGAGA

  30. If a short circuited conductor is placed within a rotating magnetic field, an emf is induced in the conductor due to EMI. • Due to this emf, current starts flowing in the conductor and sets up its own magnetic field. • Due to the interaction of these two field, a torque is produced and conductor tends to move BHSBIET LEHRAGAGA

  31. How rotor rotates? • A three phase IM stator have a three phase distributed winding. • When we give supply to stator then a rotating magnetic field produces which rotates at synchronous speed. • The rotating flux cuts the rotor conductors and emf produced in them. • Because these conductors are short circuited so current is produced in conductors so rotor m.m.f is produced which produces synchronously rotating rotor poles. BHSBIET LEHRAGAGA

  32. PRODUCTION OF TORQUE BHSBIET LEHRAGAGA

  33. Stator equivalent circuit BHSBIET LEHRAGAGA

  34. BHSBIET LEHRAGAGA

  35. Final equivalent circuit of IM BHSBIET LEHRAGAGA

  36. SLIP • The difference between the synchronous speed and rotor speed can be expressed as a percentage of synchronous speed, known as the slip s = slip, Ns = synchronous speed (rpm), N = rotor speed (rpm) BHSBIET LEHRAGAGA

  37. SLIP RINGS AND BRUSHES IN IM BHSBIET LEHRAGAGA

  38. • At no-load, the slip is nearly zero (<0.1%). • At full load, the slip for large motors rarely exceeds 0.5%. For small motors at full load, it rarely exceeds 5%. • The slip is 100% for locked rotor. BHSBIET LEHRAGAGA

  39. Frequency induced in the rotor • The frequency induced in the rotor depends on the slip: fR= frequency of voltage and current in the rotor f = frequency of the supply and stator field s = slip BHSBIET LEHRAGAGA

  40. Torque- slip curve BHSBIET LEHRAGAGA

  41. Harmonics in Induction Machines • Time Harmonics: • These affects torque and cause considerable heating in the machine and are hence a cause for concern. • These harmonics are called time harmonics since they are generated by a source that varies non sinusoidally in time. BHSBIET LEHRAGAGA

  42. Space Harmonics: • The space harmonics, are a result of non-sinusoidal distribution of the coils in the machine and slotting. • These have their effects on the speed, torque and current of the machine. BHSBIET LEHRAGAGA

  43. Effects of harmonics on loaded machine BHSBIET LEHRAGAGA

  44. Active Power Flow BHSBIET LEHRAGAGA

  45. AIR GAP POWER:- Air gap power is the power transferred from stator to rotor across the air gap. Pg = 3I22 (r2 / s) • Shaft power :- Shaft power is the output power i.e. available at the shaft. Psh = Pg – rotor ohmic loss – friction and windage loss BHSBIET LEHRAGAGA

  46. OPERATING TORQUE:- Torque from light load to full load conditions. • STARTING TORQUE:- Torque at start when slip = 1 • BREAKDOWN TORQUE:- Maximum torque that motor can develop. If motor is loaded beyond this torque, the motor will decelerate and come to stand still. BHSBIET LEHRAGAGA

  47. SPEED-TORQUE CHARACTERISTIC BHSBIET LEHRAGAGA

  48. Comments on the IM Torque Speed Curve • Induced Torque is zero at synchronous speed. • The graph is nearly linear between no load and full load (at near synchronous speeds). • Max torque is known as pull out torque or breakdown torque • Starting torque is very large. • Torque for a given slip value would change to the square of the applied voltage. • If the rotor were driven faster than synchronous speed, the motor would then become a generator. BHSBIET LEHRAGAGA

  49. Braking an Induction Motor(Plugging) • Plugging is a braking action to bring the rotor to a quick stop. Plugging is obtained by interchanging any two stator leads. • With this the phase sequence is reversed and the direction of rotating magnetic field becomes opposite to that of the rotor rotation. • The electromagnetic torque now acts opposite to rotor rotation and produces braking action BHSBIET LEHRAGAGA

  50. Variations in Induction Motor Torque-Speed Characterictics BHSBIET LEHRAGAGA

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