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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 starting torque does not become zero at starting condition or at standstill condition (
S = 1 ) and it is known as starting torque.

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} .

_{2}R_{2}/ [ R_{2}^{2}+ ( SX_{2})^{2}]_{2}also becomes constant._{2})^{2}is very high as compared to R_{2}^{2}, the rotor resistance is neglected therefore the starting torque is directly proportional to_{s}a S / ( SX

_{2})

^{2}

_{s}a 1 / S ( As rotor reactance X

_{2}is constant )

_{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 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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