1. Torque
101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal
Nm
g
g
N2π
P60
TTorque,Gross =






−= 1
s
1
RI3P 2
2
2g
( )2
2
2
2
2
2
2
g
sXR
REsk
T
+
=
Nm
N2
P60
TTorque
π
=
Nm
sh
sh
N2
P60
TTorqueShaft
π
=
Nm
Fw
Fw
N2
P60
TTorqueWindage&Friction
π
=
TSh = Tg – TFw

2. Torque - Slip Characteristics
101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal
Torque
sM
Tst
Low to medium slip region sαTg
High slip region
s
1
αTg
1 0Slip
Speed0 NS
Tmax

3. Torque Slip Characteristics
101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal
Note: The external resistance can be varied in Slip Ring IM only.
sM
R2
Torque
SlipsM = 1
Tmax
0sM
R2+y
sM
R2+2y
R2+3y
sM

4. Induction Motor Starting Considerations
101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal 4
X2
E2 I2r
s
R2
2
2
2
2
2
r2
)sX(R
sE
I
+
=
At starting
• Higher magnitude of rotor induced emf
• Short circuited rotor conductors, Higher rotor current
magnitude
• Higher magnitude of Stator current, 5 to 8 times rated
value
• Damages the motor windings
• Large voltage drop in supply systemTo limit the larger starting current to a safe value, necessitates
a STARTER

5. Starting Methods
101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal 5
• Direct Online Starter (DOL)
• Star-Delta Starter
• Auto transformer starter
• Rotor resistance starter (Slip ring Type only)

6. Star - Delta Starter
101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal 6
Start Position
Star Connection 3
V
V L
Ph =
Run Position
Delta Connection LPh VV =
A2 A1
B1B2
C2 C1
R Y B

7. Star - Delta Starter
101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal 6
Start Position
Star Connection 3
V
V L
Ph =
Run Position
Delta Connection LPh VV =
A2 A1
B1B2
C2 C1
R Y B

8. Star - Delta Starter : Salient Points
101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal 7
• At Starting
 Star connected Stator windings
 Applied Phase voltage reduces by √3 times the line
voltage value
 Starting current reduces by √3 times the DOL current
value
 Starting Torque reduces by 3 times
Speed increases from zero to near about rated speed
• At near about rated speed
 Switch changed over to RUN position, delta connected
windings
 Full line voltage applied across all 3 phases