 Hello, fellows, I hope all of you are having fun in your life. In today’s tutorial, we will discuss Variations in Induction Motor Torque-Speed Characteristics. In the last tutorial Induction Motor Torque-Speed Characteristics, we find the different parameters of the torque and speed of the induction motor. Also, find the induced torque equation by solving the equivalent circuit of the induction motor. The main important thing we find the maximum (pullout) torque of the motor.

In today’s post, we will discuss some factors that varied the torque-speed Characteristics of the motor. For these factors, we take different types of rotors like deep rotor or double cage rotor and find the effect of the torque and speed on them. So let’s get started with Variations in Induction Motor Torque-Speed Characteristics.

#### Variations in Induction Motor Torque-Speed Characteristics

• In a previous article for torque-speed characteristics, we studied the different parameters related to that curve. Suppose if we have such rotor that has a higher value of the rotor resistance Rr due to this higher value of the resistance the starting torque will also larger, as torque is higher so the slip will also higher.
• As we know that this equation.

Pconv = (1-s)PAG

• Due to the higher value of the slip, the power converted from the electrical to mechanical will have less value, due to less power conversion the efficiency of the motor will decreases.
• From that, we can conclude that such a motor that has higher rotor resistance, has larger starting torque but less efficiency.
• If the resistance of the rotor is less than the starting torque will also less but the efficiency of the motor will be larger than the motor that has a high value of the rotor’s resistance.
• The manufacturer of the motor always tries to cooperation among the differing requirements of higher initial torque and suitable efficiency.
• The solution for this is that if we put some additional resistance in the rotor of the wound motor during its starting for higher torque then during its normal working remove this resistance for good efficiency.
• But there is a problem for this condition that the wound rotor motor is costly, its repairing price is also larger, need a multifaceted automatic control circuitry than cage rotor motors.
• Similarly, it is sometimes significant to entirely cover a motor when it is sited in a risky or fiery atmosphere, and this is calmer to do with an entirely self-contained rotor.
• It would be pleasing to add additional rotor resistance at initial and to eliminate it during normal operation without automatic control circuitry.
• In the given figure you can see the 2 wound rotor motors characteristic curves, one motor has high resistance rotor and second has less resistance rotor.
• At higher slip value, the anticipated motor should act like the higher-resistance wound-rotor motor curve and at less slip, it should act like the less-resistance wound-rotor motor curve. #### Control of Induction Motor Characteristics by Cage Rotor Design

• The equivalent circuit of the induction motor the term X2 is leakage reactance. It is the reactance due to the flux of the rotor that does not interact with the stator of the motor.
• In a simple word, if the distance between the stator and the rotor bar is larger than the value of leaked reactance will be high, because less amount of the rotor flux approaches to the stator.
• So, if the bars of a cage rotor are located near the surface of the rotor, there will be less leakage flux due to less value of leakage flux the X2 will also lesser in the equivalent circuitry of the motor.

Smax = R2/R2th + (Xth+X2)2

• As we know that

Pconv = (1-s)PAG

• This motor is similar to the wound rotor motor that has additional resistance added to its rotor circuitry.
• Due to larger resistance at the rotor, the starting torque will also higher with larger value slip.
• A cage motor with this kind of rotor structure is named NEMA design class D.

#### Deep-Bar and Double-Cage Rotor Design

• Both of the earlier rotor designs A and D are principally alike to a wound-rotor motor with a rotor resistance.
• One question here arises.

How can an adjustable rotor resistance be shaped to combine the higher starting torque and less starting current of a class D design with the less normal working slip and higher efficiency of a class A design?

Double-Cage Rotor Design #### Disadvantage of Double cage Rotor

• There are some disadvantages to the double cage rotor that are described here.
• This type of rotor is expensive than other categories of the cage rotor.
• Their price is less than the wound rotors.
• They provide features like the wound rotor, such as higher initial torque with less starting current and good efficiency.
• The repairing cost of a deep bar rotor is less as compare to the wound rotor motor, as it does not have slip rings and brushes that wound rotor motor has.

So it is the complete tutorial on the Variations in Induction Motor Torque-Speed Characteristics if you have any query ask in comments. Take it until the next post.

You can also read some related articles to the induction motor. That is described here.