- The torque in the three phase induction motor is given by

T = KFSE

_{2}R_{2}/ [ R_{2}^{2}+ ( SX_{2})^{2}]### Torque at standstill condition ( S = 1 )

- The torque develop by the induction motor at starting condition is known as starting torque. There is always some starting torque in the three phase induction motor.

### Starting condition

- The slip of the three phase induction motor is very high particular at starting condition.
- If
the supply voltage is kept constant, the stator flux F and rotor induced emf E
_{2}also becomes constant. - As the value of ( SX
_{2})^{2}is very high as compared to R_{2}^{2}, the rotor resistance is neglected therefore the starting torque is directly proportional to

T

_{s}a S / ( SX_{2})^{2}
T

_{s}a 1 / S ( As rotor reactance X_{2}is constant )- Therefore the starting torque is inversely proportional during starting condition.
- The shape of the torque – slip curve is rectangular hyperbola.
- As the slip decreases the torque increases and vice versa.
- The torque increases up to its maximum limit or say maximum torque.
- The maximum torque occurs at a slip of S = R
_{2}/ X_{2}.

### Running condition

- The value of
slip becomes very low during running condition of induction motor therefore the
SX
_{2}is neglected as compared to rotor resistance R_{2}. - The running torque in the induction motor during this condition is directly proportional to

T a
S ( As F, E

_{2 }and R_{2}are constant )- Therefore the torque slip graph is straight line during running condition.
- As the slip increases, the torque increases and vice versa.

### Effect of rotor resistance

- As the rotor resistance increases, the starting torque also increases.
- The rotor resistance does not affect the maximum torque.
- The speed or slip at which maximum torque occurs is depends upon rotor resistance.
- The maximum torque remains same for difference value of rotor resistance but it occurs at different slip or speed as shown in the Figure A.

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