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
- 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.
You may also like :
No comments:
Post a Comment