Torque - Slip Characteristic of the Induction Motor

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 .

torque - slip characteristic of three phase induction motor

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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