1. Three Phase Induction Motors

2. Lecture 4 Introduction Power Stages Additional Exercises
Induction Motor Starting Considerations
Starting Methods
Summary
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3. Power Stages Stator Losses Rotor Losses Stator Copper Loss Core Loss
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Stator
Losses
Stator Copper Loss
Core Loss
Rotor Copper Loss
Rotor
Losses
Stator
Input
Stator Output
(Rotor Input)
Gross Power
Output
Other
Losses
Friction & Windage
Loss
Net
Output
Stator Input = Motor Input (PIN)
Stator Losses = Stator Copper Loss (PSCU) + Core Loss (PCO)
Stator Output = Stator Input - Stator Losses
Rotor Input = Stator Output
Rotor Losses = Rotor Copper Loss (PRCU)
Gross Power Output (Pg) = Rotor Input - Rotor Losses
Net Power Output (PO) = Gross Power Output (Pg) – Friction & Windage Losses (PFWL)

4. Relationship between Rotor Quantities
Pg = Gross output, Prcu = Rotor copper loss, P2 = Rotor input
Power transferred from stator to rotor
Power developed by the rotor
Rotor copper loss, Prcu = P2 - Pg
Rotor input (P2) = Rotor copper loss (Prcu) + Gross output (Pg)
From Prcu = sP2 & Pg = (1-s)P2
Dept of E & E, MIT Manipal
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5. 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
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6. Induction Motor Starting Considerations
Dept of E & E, MIT Manipal
L4 – 05 X2 E2
I2r
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

7. Starting Methods Direct Online Starter (DOL) Star-Delta Starter
Auto transformer starter
Rotor resistance starter (Slip ring Type only)
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8. Star - Delta Starter Run Position A2 A1 B1 B2 C2 C1 A2 A1 B1 B2 C2 C1
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Run Position A2 A1 B1 B2 C2 C1 A2 A1 B1 B2 C2 C1
Run Position
Delta Connection A2 A1 B1 B2 C2 C1
Start Position
Star Connection A2 A1 B1 B2 C2 C1

9. Star - Delta Starter : Salient Points
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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

10. Lecture 4 Summary Relationship between rotor quantities 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