tailieunhanh - INDUCTION MOTORS

The speed of the rotating magnetic field 2πf1 ωsyn = P/2 60f1 nsyn = P/2 (rad/s) (rev/min) or where f1 is the frequency of the stator current, and P the number of poles. 2. The rotor speed of an induction machine is generally different from the speed of the rotating magnetic field. The percentage difference of speed, known as slip, is defined as s= ωsyn-ωm nsyn-n = n ωsyn syn where ωm and n are the speed of the rotor in rad/s and rev/min, respectively. 3. The induced emf in one phase of the stator winding is E1 = and. | INDUCTION MOTORS 1. The speed of the rotating magnetic field 2nf syn p 2 rad s _60f tr . X or nsyn P 2 rev min where f1 is the frequency of the stator current and P the number of poles. 2. The rotor speed of an induction machine is generally different from the speed of the rotating magnetic field. The percentage difference of speed known as slip is defined as s _ syn- 1 m _ nsyn-n syn nsyn where Dm and n are the speed of the rotor in rad s and rev min respectively. 3. The induced emf in one phase of the stator winding is E1 and the emf in the rotor winding E2s m where fi and f2 are the frequencies of the emfs in the stator and rotor windings Nikwi and N2kw2 the effective number of turns of the stator and rotor windings and om the magnitude of the rotating magnetic flux. Induction Motors Since f2 sfi we have E2s sE2 where E2 m is the rotor emf when the rotor is standstill. 4. T equivalent circuit 5. The power flow of an induction motor is illustrated in the diagram of T equivalent circuit. Efficiency Output Power Input Power 6. Parameters in the T equivalent circuit can be determined by the no-load test and the locked-rotor test. By the no-load test the magnetisation reactance can be determined as Xm 3INLsin NL v 3VNL 0 m INLsinỘNL for Y connection for A connection 2 Induction Motors where Ộnl arccos v I . By the locked rotor test we can obtain -Ilk Rlk 3Ilk2 Plk for Y connection or Rlk 3Ilk2 Vlk for A connection and Zlk - Lk ặ 3Vlk for Y connection ol Zlk - Ilk Hence XLK - ZLK2 - RLK2 for A connection. for both Y and A connections. Normally it can be assumed that X 1 X 2 and R2 Rlk - R1 where R1 is the stator winding resistance measured immediately after the locked-rotor test. 7. The Torque Speed or Torque Slip curve can be expressed either in terms of the T equivalent circuit parameters as _3_V1e2R2 s T - wsyn R1e R2 s 2 X1e X 2 2 or in terms of the maximum torque and the corresponding slip as