1. Three Phase Induction Motors

2. 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

3. 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

4. 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

5. 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

6. 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