AC MACHINES-1 (66761) Theory: Describe the condition for maximum torque at running and starting condition.

11. Recognize the concept of development of rotating magnetic field and torque in rotor.

11.6. Describe the condition for maximum torque at running and starting condition.

What is the condition for maximum torque?

To develop maximum torque at the standstill condition, the rotor resistance must be high and should be equal to X₂₀. But to develop a torque which is maximum at the running condition the rotor resistance must be low.

Maximum Torque Condition of an Induction Motor

In the article named Torque Equation of an Induction motor, we have seen the developed torque and its equation. Here, the Maximum Torque Condition of an induction motor is discussed. The torque produced in the induction motor mainly depends on the following three factors. They are the strength of the rotor current; the flux interacts between the rotor of the motor and the power factor of the rotor.The value of torque when the motor is running is given by the equation shown below.

The total impedance of the RC network always lies between 0º and 90º. The impedance is the opposition offered by the electronic circuit element to the flow of current. If the impedance of the stator winding is assumed to be negligible. Thus, for the given supply voltage V₁, E₂₀ remains constant

The developed torque will be maximum when the right-hand side of the equation (4) will be maximum. This condition is possible when the value of the denominator shown below is equal to zero.

Let,

Hence. The developed torque is maximum when the rotor resistance per phase is equal to the rotor reactance per phase under running conditions. By putting the value of sX₂₀ = R₂ in the equation (1) we get the equation for maximum torque.

The above equation shows that the maximum torque is independent of the rotor resistance.

If s_M is the value of slip corresponding to the torque which is maximum then from the equation (5)

Therefore, The speed of the rotor at maximum torque is given by the equation shown below.

The following conclusion about the maximum torque can be drawn from the equation (7) are given below.

It is independent of the rotor circuit resistance.
Torque at the maximum condition varies inversely as the standstill reactance of the rotor. Hence, for maximum torque, X₂₀ and therefore, the inductance of the rotor should be kept as small as possible.
By varying the resistance in the rotor circuit, maximum torque can be obtained at any desired slip or the speed. It depends upon the rotor resistance at the slip (s_M = R₂/X₂₀).

To develop maximum torque at the standstill condition, the rotor resistance must be high and should be equal to X₂₀. But to develop a torque which is maximum at the running condition the rotor resistance must be low.

So, when slip s = R₂ / X₂, the torque will be maximum and this slip is called maximum slip Sm and it is defined as the ratio of rotor resistance to that of rotor reactance. NOTE: At starting S = 1, so the maximum starting torque occur when rotor resistance is equal to rotor reactance.

What is the condition for maximum starting torque?

Maximum starting torque is obtained when the slip is equal to the ratio between the rotor resistance (r2) and the rotor inductive reactance (x2). This slip is also known as slip at maximum torque, labelled as Smt.

Sixth Semester

Miscellaneous

Second Semester

Sixth Semester

11. Recognize the concept of development of rotating magnetic field and torque in rotor.

11.6. Describe the condition for maximum torque at running and starting condition.

Maximum Torque Condition of an Induction Motor

So, when slip s = R2 / X2, the torque will be maximum and this slip is called maximum slip Sm and it is defined as the ratio of rotor resistance to that of rotor reactance. NOTE: At starting S = 1, so the maximum starting torque occur when rotor resistance is equal to rotor reactance.

So, when slip s = R₂ / X₂, the torque will be maximum and this slip is called maximum slip Sm and it is defined as the ratio of rotor resistance to that of rotor reactance. NOTE: At starting S = 1, so the maximum starting torque occur when rotor resistance is equal to rotor reactance.