L38 3ph im

Torque

ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal

Nmg

g N 2π

P 60T Torque, Gross

1

s

1R I 3P 2

22g

2222

222

gsXR

R E s kT

Nm N 2

P 60T Torque

Nmshsh

N 2

P 60T Torque Shaft

NmFwFw

N 2

P 60T Torque Windage& Friction

TSh = Tg – TFw

Torque - Slip Characteristics


Torque

sM

Tst

Low to medium slip region s α Tg

High slip regions

1 α Tg

1 0Slip

Speed0 NS

Tmax

Torque Slip Characteristics


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

Induction Motor Starting Considerations

ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal 4

X2

E2E2 I2r

s

R 2

22

22

2r2

)sX(R

sEI

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

Starting Methods


• Direct Online Starter (DOL)

• Star-Delta Starter

• Auto transformer starter

• Rotor resistance starter (Slip ring Type only)

Star - Delta Starter


Start Position Star Connection 3

VV L

Ph

Run PositionDelta Connection LPh VV

A2 A1

B1B2

C2 C1

R Y B

Star - Delta Starter : Salient Points


• 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

Speed increases to rated speed

L38 3ph im

Business

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