3phase induction motor


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3phase induction motor

  2. 2. Contents1 History2 Principle of operation and comparison to synchronous motors3 Formulas4 Construction5 Speed control6 Starting of induction motor7 Types of starters
  3. 3. INTRODUCTIONAn induction motor(IM) is a type ofasynchronous ACmotor where power is supplied to therotating device bymeans ofelectromagneticinduction Three-phase induction motors
  4. 4. HistoryThe induction motor with a wrapped rotor wasinvented by Nikola Tesla in 1888 in the UnitedStates. In his scientific work, Tesla laid thefoundations for understanding the way the motoroperates.The induction motor with a cage was invented byMikhail Dolivo-Dobrovolsky about a year later inEurope. Technological development in the fieldhas improved to where a 100 hp (73.6 kW) enginefrom 1976 takes the same volume as a 7.5 hp (5.5kW) engine did in 1897. Currently, the mostcommon induction motor, is the cage rotor motor.
  5. 5. Principle of operationThe basic difference between an induction motor anda synchronous AC motor is that in the latter a currentis supplied onto the rotor. This then creates amagnetic field which, through magnetic interaction,links to the rotating magnetic field in the statorwhich in turn causes the rotor to turn. It is calledsynchronous because at steady state the speed of therotor is the same as the speed of the rotatingmagnetic field in the stator.
  6. 6. The induction motor does not have any directsupply onto the rotor; instead, a secondarycurrent is induced in the rotor. To achievethis, stator windings are arranged around therotor so that when energised with a polyphasesupply they create a rotating magnetic fieldpattern which sweeps past the rotor
  7. 7. At the moment illustrated, the current in the statorcoil is in the direction shown and increasing. Theinduced voltage in the coil shown drives current andresults in a clockwise torque. Note that this simplified motor will turn once it isstarted in motion, but has no starting torque.Various techniques are used to produce someasymmetry in the fields to give the motor a startingtorque.
  8. 8. In a three phase induction motor, the induced emf in the rotorcircuit depends on the slip of the induction motor and themagnitude of the rotor current depends upon this induced emf(electromotive force). When the motor is started, the slip is equalto 1 as the rotor speed is zero, so the induced emf in the rotor islarge. As a result, a very high current flows through the rotor. Thisis similar to a transformer with the secondary coil short circuited,which causes the primary coil to draw a high current from themains. Similarly, when an induction motor starts, a very highcurrent is drawn by the stator, on the order of 5 to 9 times the fullload current. This high current can damage the motor windingsand because it causes heavy line voltage drop, other appliancesconnected to the same line may be affected by the voltagefluctuation. To avoid such effects, the starting current should belimited
  9. 9. The rotational speed of the rotor is controlled by the number ofpole pairs (number of windings in the stator) and by the frequencyof the supply voltage. Before the development of cheap powerelectronics, it was difficult to vary the frequency to the motor andtherefore the uses for the induction motor were limited. There arevarious techniques to produce a desired speed. The mostcommonly used technique is PWM(Pulse Width Modulation), inwhich a DC signal is switched on and off very rapidly, producing asequence of electrical pulses to the inductor windings. Thedutycycle of the pulses, also known as the mark-space ratio,determines the average power input to the motor.
  10. 10. TYPES OF MOTORS Based on type of phase supply ○ Three phase induction motor (self starting in nature)  single phase induction motor (not self starting) Other  Squirrel cage induction motor Slip ring induction motor
  11. 11. The rotor bars in squirrel-cage induction motorsare not straight, but have some skew to reducenoise and harmonics.The motors phase type is one of two types1. Single-phase induction motor2. 3-phase induction motor
  12. 12. In a DC machine, the stator winding is excited by DCcurrent and hence the field produced by thiswinding is time invariant in nature. In this machine theconversion of energy from electrical to mechanical form orvice versa is possible by one of the following ways:1.rotating the rotor in the field produced by the stator2.feeding external dc current through carbon brushes tothe rotor
  13. 13. Electric motors convert electrical power to mechanicalpower in its rotor (rotating part).There are several ways to supply power to the rotor.AC motor this power is induced in the rotating device. An induction motor can be called a rotatingtransformer because the stator(stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side.Induction motors are widely used, especially polyphaseinduction motors, which are frequently used in industrialdrives.
  14. 14. Construction The stator consists of wound poles that carry thesupply current that induces a magnetic field in theconductor. The number of poles can vary betweenmotor types but the poles are always in (i.e. 2,4,6 etc)pairs. There are two types of rotor 1.Squirrel-cage rotor 2.Slip ring rotorThe most common rotor is a squirrel-cage rotor. It ismade up of bars of either solid copper (most common)or aluminum that span the length of the rotor, and areconnected through a ring at each end
  15. 15. Slip Ring RotorResistance-start ac induction motor.
  16. 16. FormulasThe relationship between the supply frequency, f, thenumber of pole pairs, p, and the synchronous speed, n,is given by f = p*n.From this relationship:Speed of rotating field (n) = f/P (revs.s-1)Speed of rotor = n(1-S) (rev.s-1)where S is the slip.Slip is calculated using:% slip = (n - r) / n * 100where r is the rotor speedIn contrast, a synchronous motor always runs at either aconstant speed N=(120f)/P or zero.