14 March 2018

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 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 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 running 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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