## 14/03/2018

### The torque in the three phase induction motor is given by T = KFSE2R2 / [ R22 + ( SX2 )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 E2 also becomes constant.  As the value of ( SX2 )2 is very high as compared to R22, the rotor resistance is neglected therefore the starting torque is directly proportional to         Ts a S / ( SX2 )2          Ts a 1 / S    ( As rotor reactance X2 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 = R2 / X2 .

Running condition
• The value of slip becomes very low during running condition of induction motor therefore the SX2 is neglected as compared to rotor resistance R2
• The torque in the induction motor during this condition is directly proportional to
T a S  ( As F, E2 and R2 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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