Electrical power drivers et3026 wb lecture5

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Electrical power drivers et3026 wb lecture5

  1. 1. February 19, 2009 1 Elektrische Aandrijvingen WTB Lokatie/evenement P.BAUER
  2. 2. February 19, 2009 2 Three phase induction machines
  3. 3. February 19, 2009 3 • Stator • Rotor – Squirrel Cage rotor - Wound rotor Three phase induction machines
  4. 4. February 19, 2009 4 FIGURE 13-1 Super-E, premium efficiency induction motor rated 10 hp, 1760 r/min, 460 V, 3-phase, 60 Hz. This totally-enclosed fan-cooled motor has a full-load current of 12.7 A, efficiency of 91.7%, and power factor of 81%. Other characteristics: no-load current: 5 A; locked rotor current: 85 A; locked rotor torque: 2.2 pu; breakdown torque: 3.3 pu; service factor 1.15; total weight: 90 kg; overall length including shaft: 491 mm; overall height: 279 mm. (Courtesy of Baldor Electric Company) FIGURE 13-2 Exploded view of the cage motor of Fig. 13.1, showing the stator, rotor, end-bells, cooling fan, ball bearings, and terminal box. The fan blows air over the stator frame, which is ribbed to improve heat transfer. (Courtesy of Baldor Electric Company) FIGURE 13-3a Die-cast aluminum squirrel-cage rotor with integral cooling fan. (Courtesy of Lab-Volt) Three phase induction machines
  5. 5. February 19, 2009 5 Principle of operation • Voltage E=B l v • Induced voltage = current I • Current + magnetic field = force
  6. 6. February 19, 2009 6 FIGURE 13-5b Ladder bent upon itself to form a squirrel cage. FIGURE 13-6 Elementary stator having terminals A, B, C connected to a 3-phase source (not shown). Currents flowing from line to neutral are considered to be positive. Principle of operation
  7. 7. February 19, 2009 7 FIGURE 13-7 Instantaneous values of currents and position of the flux in Fig. 13.6.
  8. 8. February 19, 2009 8 FIGURE 13-9a Phase group 1 is composed of a single coil lodged in two slots. Phase group 2 is identical to phase group 1. The two coils are connected in series. In practice, a phase group usually consists of two or more staggered coils.
  9. 9. February 19, 2009 9 FIGURE 13-9b Two-pole, full-pitch, lap-wound stator and resulting magnetic field when the current in phase A = +10 A and Ib = Ic = -5 A. FIGURE 13-10a The four phase groups of phase A produce a 4-pole magnetic field. FIGURE 13-10b Four-pole, full-pitch, lap-wound stator and resulting magnetic field when Ia = +10 A and Ib = Ic = -5 A. FIGURE 13-11 Eight-pole, full-pitch, lap-wound stator and resulting magnetic field when Ia = +10 A and Ib = Ic = -5 A. Number of poles
  10. 10. February 19, 2009 10 FIGURE 13-12b Flux pattern when the current in phase A is at its maximum negative value. The pattern is the same as in Fig. 13.12a but it has advanced by one pole pitch.
  11. 11. February 19, 2009 11 • Starting characteristics of a squirrel cage motor • Acceleration of the rotor slip • Motor under load • Slip and slip speed • S= (ns - n) /ns • Voltage and frequency induced in the rotor • f2 = s f • E2 = s Eoc
  12. 12. February 19, 2009 12 FIGURE 13-13 At no-load the flux in the motor is mainly the mutual flux Φm . To create this flux, considerable reactive power is needed. Characteristics of squirel cage IM FIGURE 13-14 At full-load the mutual flux decreases, but stator and rotor leakage fluxes are crea The reactive power needed is slightly greater than in Fig. 13.13. • No load under load
  13. 13. February 19, 2009 13 FIGURE 13-15 Active power flow in a 3-phase induction motor. Active power flow
  14. 14. February 19, 2009 14 FIGURE 13-16a See Example 13-7.
  15. 15. February 19, 2009 15 FIGURE 13-16b Power flow in Example 13-7.
  16. 16. February 19, 2009 16 FIGURE 13-17 Typical torque-speed curve of a 3-phase squirrel-cage induction motor. Torque vs speed
  17. 17. February 19, 2009 17 FIGURE 13-18 Rotor resistance affects the motor characteristics. Effect of rotor resistance
  18. 18. February 19, 2009 18 FIGURE 13-19 External resistors connected to the three slip-rings of a wound-rotor induction motor. Wound rotor
  19. 19. February 19, 2009 19 FIGURE 13-29 Current and magnetic poles at low speed.
  20. 20. February 19, 2009 20 FIGURE 13-30 Currents and magnetic poles at high speed.

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