Three Phase Induction Motors

Lecture 2 Introduction Effect of Poles Synchronous Speed Dept of E & E, MIT Manipal L2 – 01 Introduction Effect of Poles Synchronous Speed Working Principle Slip Speed Lecture 2 Exercise Summary

Synchronous Speed For a 2 pole machine, 1o mechanical = 1o electrical Dept of E & E, MIT Manipal L2 – 03 For a 2 pole machine, 1o mechanical = 1o electrical For a ‘P’ pole machine, 1o mechanical = (P/2)o electrical NS = Speed of RMF, rpm f = Frequency of ac supply, Hz P = No. of poles Speed of rotation of rotating magnetic field is called Synchronous Speed

Slip ‘s’ NS = Synchronous Speed, rpm N = rotor speed, rpm If N = NS, Dept of E & E, MIT Manipal L2 – 06 NS = Synchronous Speed, rpm N = rotor speed, rpm If N = NS, No flux cut by rotor conductors No emf induced across rotor conductors No current flow, no torque Hence N < Ns must for rotor rotation Slip speed = (NS – N), rpm

Lecture 2 Exercise Dept of E & E, MIT Manipal L2 – 07 [1] A 3 induction motor is wound for 4 poles and is connected across a 50 Hz supply system. Determine the (a) synchronous speed (b) speed of the motor when slip is 4 % (c) new operating speed if the slip is now increased to 6 % [2] A 3, 50 Hz, 4 pole induction motor has a full load speed of 1460 rpm. Determine (a) synchronous speed (b) full load slip (c) Speed of stator field with respect to stator structure and rotor structure (d) Speed of rotor field with respect to rotor structure, stator structure and stator field

Lecture 2 Summary Dept of E & E, MIT Manipal L2 – 08 Poles : No physical presence, only effect of poles produced Synchronous Speed: Speed of rotating magnetic field Working Principle : Rotor always follows the rotating magnetic field Slip NS > N necessary for rotor movement Slip speed expressed as fraction of synchronous speed Always expressed in % Sub synchronous speed operation - Asynchronous Motor