Torque Slip Characteristics of Three Phase Induction Motor

Torque Slip Characteristics of Induction Motor


The torque slip characteristics of three phase induction motor is the important characteristics that gives information about the torque generation at the different slip regions in an induction motor. Additionally, this characteristics help us to study certain aspects of three phase induction motors like Importance of slip in induction motor, speed Control of the Induction Motor, Braking of the induction motor, Operation of Induction Generator etc.

 


To ease the study of torque slip characteristics, the operation of the three-phase induction motor is majorly divided into two regions i.e. Low Slip Region and High Slip Region, ranging the value of slip between 0 to 1 (as discussed in slip in induction motor). Zero slip means the induction motor runs at synchronous speed and the slip = 1 is at the starting of three phase induction motor.



Additionally, the operation of induction machine is further divided into two more region i.e. Negative Slip Region and the region in which slip is greater than one this further division help us in the study of operation of induction generator and braking of induction motor respectively.



The torque equation of three phase induction motor is given by.


Torque equation of Induction motor

where,


T  = Torque developed in induction motor

K   = proportionality constant

Er = induced emf in rotor

Rr = rotor resistance

Xr = rotor reactance

S   = slip


 

Low Slip Region


For low value of slip (sXr)2 in the denominator of torque equation is very low as compared to rotor resistance and it can be neglected. So, for low slip region the toque equation becomes.



From above equation it is clearly seen that, for low slip region torque is directly proportional to slip and inversely proportional to rotor resistance.

 

Note :- From above statement we can understand the importance of slip i.e. if there is no slip then no torque is developed in three phase induction motor.

 



High Slip Region


For high slip region the term (sXr)2 is much greater than Rr in numerator of the torque equation so, it cannot be neglected. In that case Rr in numerator can be neglected.

 

So, for high slip region the torque equation of three phase induction motor becomes



From above equation, it is clearly shown that for high slip region torque is inversely proportional to slip (assuming that the voltage, rotor reactance, and rotor resistance is constant).

 


Negative Slip Region


Negative slip in induction motor means that the speed of the induction motor is greater than the synchronous speed. This is the case in which an induction machine works as an induction generator.

 


Region in which slip > 1


When slip is greater than one it means that the direction of the rotation of the motor is reversed. It is the case of plugging (Electrical Braking of Three Phase Induction Motor) in which the direction of rotation of the motor is reversed by reversing any two phase of the three phase induction motor.


In this case the maximum value of the slip goes up-to 2.

 

 


Torque Slip Characteristics of Induction Machine

 

So, after analyzing all the four cases we understand that an induction motor or induction machine can be operated in three modes i.e. Motoring Zone (0<s<1), Generating Zone (s<0), Braking Zone (1>s<2).


The overall torque slip characteristics of three phase induction motor or an induction machine represents as shown in figure.


Torque Slip Characteristics of Induction Motor


The region lies in which slip lies between 0 to 1 is the motoring zone. In this zone we see that torque slip characteristics is straight between s = 0 to s = sm (sm is the value of slip at which the maximum torque occurs). This is the stable operating region of induction motor. In that region torque is directly proportional to slip.


The region between s = sm to s = 1 is unstable region and it is not recommended that the motor to be operated in this region. In this region torque is inversely proportional to slip.


However, an induction motor can be operated in this region by adding the external resistance in rotor circuit.

 


Effect of adding rotor resistance induction motor

 

If we increase the rotor resistance than according to the torque equation in low slip region, the slip should increase to supply the constant torque. And, also the slip at which maximum torque occurs is also increased. It is clearly shown in the given figure.


Torque Slip Characteristics of three phase induction motor


In above figure we can see that sm1, sm2 and sm3 are the different values of slip at which maximum torque occurs after adding R1, R2, and R3. From discussion it is concluded that R1>R2>R3 and the region in which torque and slip have linear relationship is increases with the resistance.

 


So, we can say that torque generation is possible at higher slip region or lower speed i.e. during starting of induction motor.


So, adding external resistance in the rotor circuit is very helpful practice in the starting of induction motor and it can only be done in case of slip ring induction motor.

 


Conclusion


From this discussion it is concluded that operating region of the induction motor lies between 0 and sm (slip at which maximum torque occurs) values of slip. However, it can be operated in high slip region with the help of additional resistance that is added to the rotor circuit. 


Addition of external resistance to the rotor circuit has done to start the slip ring induction motor.


Addition of external resistance to the rotor circuit results increase in rotor losses hence motor efficiency decreases that’s why this practice has been done only in starting.




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