Torque of Induction Motor

Torque of Induction Motor

Torque is turning or twisting moment about the axis that causes rotation. There are some torque is require to run the motor to do useful work. Following are the brief explanation of motor torque :

§  Starting Torque : Starting Torque means the torque produce by motor when it starts. It is also define as torque which is require to overcome the motor from standstill to running condition. At the start rotor speed is zero. To determine the starting torque & starting current of motor lock rotor or short circuit test is perform. Also by performing this test we know the soundness of rotor. For lock rotor test motor is lock by rod or prime mover. This test is perform on low voltage. This is already explain in motor testing blog.

§  Pull Up Torque : Pull Up Torque is minimum torque develop by motor from when it run from zero to full load speed. To measure pull up torque mount the motor with suitable loading arrangement & rotor is fully lock condition. Then rated voltage & rated frequency is applied to motor. Loading of motor is reduce slowly when the rotor stars rotation & pick up the speed. That time the value of torque at which the motor break away from locked rotor condition & attains the speed corresponding to pull out torque condition shall be noted & reported as pull up torque.

§  Pull out or Break Down Torque : To measure the pull out torque same loading arrangement is require as pull up torque measurement. Only difference is that motor for measurement of pull out torque is no load condition. Rated voltage & rated frequency is applied to motor under no load condition. Then load on motor increase gradually & the maximum load at which motor stalls may be noted. The torque calculated at this point is Pull Out or break down torque.

Following are the Torque-Speed characteristics of three phase induction motor : 

 

   Induction motor Torque-Speed characteristics give the information of relationship between mechanical torque and speed of motor. It’s also call as Torque-Slip characteristics. Torque-speed curve is shown in above figure give the information about operation or practical working of squirrel- cage induction motor. The induce torque of the motor is zero at synchronous speed .In that condition motor slip is also zero. The curve is almost linear in normal operating condition which are between synchronous speed & pull out speed. Maximum possible torque in induce is called as pull out or break down torque and it limit the short- time overload capability of motor. It is usually two to three times of the rated full load torque of motor. The starting torque of induction motor is slightly greater than its full-load torque of motor. Therefore induction motor carrying any load that it can supply at full power. Torque-speed characteristics is require to predict behavior of induction motor & it is useful to selection of load & staring method for motor.