1. Poles ?
ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal
A
A'
B
B'
C
C'
X
X
X
X
X
X
• No physical presence but only the effect of poles
A C
B

2. Working Principle
ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal
Stator
Rotor
Direction of rotation of magnetic field
Rotating magnetic field
Rotor
• Rotating magnetic field is cut by the
rotor conductor
• EMF is induced in rotor conductor
• Current I2 in the rotor conductor sets
up a magnetic field which opposes the
rotation of main field
• Main field is independent and hence
rotor field tries to catch up the speed
of main field to reduce the relative
speed
• Rotor rotates in the same direction as
that of rotating magnetic field

3. Synchronous Speed
ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal
• 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
P
f
120
NS 
Speed of rotation of rotating magnetic field is called Synchronous Speed

4. Slip ‘s’
ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal
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
%
100
x
N
N
N
s
%
S
S 


5. Rotor Frequency
ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal 1
120
N
P
f S

• For rotor speed N, relative speed = NS - N
120
)
N
N
(
P
f S
r


f
s
fr 
Note
At instant of starting, N = 0, s = 1, fr = f

6. Rotor Emf
ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal 2
• At standstill, s = 1, max relative speed, max rotor induced emf
• If E2 = per phase rotor induced emf at standstill,
E2  NS
• If E2r = per phase rotor induced emf under running conditions,
E2r  (NS - N)
• Thus for any slip,
2
2r E
s
E 

7. Rotor Resistance and Reactance
ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal 2
• Rotor Resistance:
R2r = R2
• Rotor Reactance:
At Standstill, rotor reactance = X2 = 2 π f XL
At running, rotor reactance = Xr = 2 π fr XL
Therefore,
2
r X
s
X 

8. Exercise
ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal
A 3, 50 Hz, 4 pole induction motor has a full load speed of
1440 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
Ans: 1500 rpm, 4%, 1500 rpm, 60 rpm, 0 rpm, 1440 rpm, 60
rpm