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Three Phase Induction Motors
Lecture 4
• Introduction
• Power Stages
• Additional Exercises
• Induction Motor Starting Considerations
• Starting Methods
• Summary
Dept of E & E, MIT Manipal
L4 – 01
Power Stages
Stator Copper Loss
Stator
Input
Rotor Copper Loss
Stator Stator Output
Losses (Rotor Input)
Rotor
Losses
Gross Power
Output
Core Loss
Other
Losses
Net
Output
Friction & Windage
Loss
Stator Input = Motor Input (PIN)
Stator Losses = Stator Copper Loss (PSCU) + Core Loss (PCO)
Stator Output = Stator Input - Stator Losses
Rotor Input = Stator Output
Efficiency η 
Rotor Losses = Rotor Copper Loss (PRCU)
Net Power Output
Motor Power Input
Gross Power Output (Pg) = Rotor Input - Rotor Losses
Net Power Output (PO) = Gross Power Output (Pg) – Friction & Windage Losses (PFWL)
Dept of E & E, MIT Manipal
L4 – 03
Relationship between Rotor Quantities
• Pg = Gross output, Prcu = Rotor copper loss, P2 = Rotor input
Power transferred
2π N s T
P 
from stator to rotor 2
60
Power developed
2π N T
P

g
by the rotor
60
Rotor copper loss, Prcu = P2 - Pg
Prcu  s P2
Rotor input (P2) = Rotor copper loss (Prcu) + Gross output (Pg)
Pg  (1 - s) P2
From Prcu = sP2 & Pg = (1-s)P2 Prcu  s Pg
1s
Dept of E & E, MIT Manipal
L4 – 02
Lecture 4 Exercise
[1] A A 3, 50 Hz, 4 pole star connected induction motor on full load develops a
useful torque of 300 Nm. If the rotor emf is 120 cycles per minute and torque lost in
friction is 50 Nm, determine (a) slip (b) operating speed (c) net power output (d)
gross torque (e) power lost due to friction & windage (f) gross power output (g) total
power input if total losses are 10 kW (h) efficiency
[2] A 3, 50 Hz, 36 kW, 4 pole induction motor has a full load efficiency of 84 %. The
friction & windage losses are one-third of no load losses and rotor copper losses
equal the iron loss at full load. Determine (a) Total Losses (b) Stator Core Loss (c)
Rotor Copper Loss (d) Friction & Windage Loss (e) Gross Power Output (f) Rotor
Input
Dept of E & E, MIT Manipal
L4 – 04
Induction Motor Starting Considerations
R2
s
E2
X2
I2r
I 2r 
sE2
R 22  (sX2 ) 2
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 system
To limit the larger starting current to a safe value, we need a STARTER
Dept of E & E, MIT Manipal
L4 – 05
Starting Methods
• Direct Online Starter (DOL)
• Star-Delta Starter
• Auto transformer starter
• Rotor resistance starter (Slip ring Type only)
Dept of E & E, MIT Manipal
L4 – 06
Star - Delta Starter
A2
A1
B2
B1
C2
C1
Start Position
Star Connection
VL
VPh 
3
Run Position
V
Delta Connection Ph
 VL
Run Position
Dept of E & E, MIT Manipal
L4 – 07
Star - Delta Starter : Salient Points
• At Starting
 Star connected Stator windings
 Applied Phase voltage reduced by 3 times the line voltage value
 Starting current reduced by 3 times the DOL current value
 Starting Torque reduced by 3 times
• At near about rated speed
 Switch changed over to RUN position, delta connected windings
 Full line voltage applied across all 3 phases
Dept of E & E, MIT Manipal
L4 – 08
Lecture 4 Summary
• Relationship between rotor quantities
P2 : Prcu : Pg  1 : s : (1  s)
• Power Stages
 Stator Input, Stator Losses (mainly core loss), Stator output
 Rotor Input, Rotor Losses (mainly copper loss), Rotor Output
• Necessity of Starter for starting 3 Induction Motors
 Higher current magnitude, Winding Damage, Supply System Drop
• Star Delta Starter
 Star Starting with reduced voltage
 Delta Running with full voltage
Dept of E & E, MIT Manipal
L4 – 09