Assertion (A): Switched Reluctance Motors (SRMs) exhibit high torque ripple. Reason (R): SRMs operate based on constant and steady excitation of stator phases.

Which of the following is correct?

Explanation:

Switched Reluctance Motors (SRMs) and Their Characteristics

Assertion (A): Switched Reluctance Motors (SRMs) exhibit high torque ripple.

Reason (R): SRMs operate based on constant and steady excitation of stator phases.

To analyze the correctness of the given assertion and reason, let us delve into the working principle of Switched Reluctance Motors (SRMs), their operational characteristics, and the factors influencing torque ripple.

Working Principle of Switched Reluctance Motors:

Switched Reluctance Motors (SRMs) operate based on the principle of variable reluctance. The rotor of an SRM is designed without windings or permanent magnets and is made of ferromagnetic material. The stator, on the other hand, contains salient poles with windings. When a stator phase is excited, it creates a magnetic field that attracts the nearest rotor pole, causing the rotor to align with the stator pole and minimizing the reluctance in the magnetic circuit. By sequentially energizing the stator phases, the rotor experiences a continuous torque and rotates accordingly.

Torque Ripple in SRMs:

Torque ripple refers to the periodic variation in the torque output of a motor during its operation. In SRMs, high torque ripple is a common characteristic due to the following reasons:

• The torque is generated only when the rotor poles are aligning with the stator poles. This results in non-uniform torque production over a rotation cycle.
• The excitation of stator phases is not constant; rather, it is pulsed or switched sequentially, leading to intervals where no active torque is generated.
• The non-linear magnetic characteristics of the SRM, including saturation effects, further contribute to torque ripple.

Hence, the assertion that SRMs exhibit high torque ripple is true.

Excitation of Stator Phases in SRMs:

The reason (R) suggests that SRMs operate based on constant and steady excitation of stator phases. This statement is false. In reality, the excitation in SRMs is not constant but rather switched or pulsed to sequentially energize the stator phases. This switching is essential to create the magnetic field necessary for rotor movement. The pulsed nature of excitation is a primary factor contributing to the torque ripple in SRMs.

Conclusion:

The assertion (A) is true, but the reason (R) is false. The high torque ripple in SRMs is not due to constant and steady excitation of stator phases, but rather due to the switched nature of excitation and the inherent design and operational characteristics of the motor. Therefore, the correct option is:

Option 1: A is true, but R is false.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Both A and R are true, but R is not the correct explanation of A.

This option is incorrect because the reason (R) is false. The statement that SRMs operate based on constant and steady excitation of stator phases does not align with their actual working principle.

Option 3: A is false, but R is true.

This option is incorrect because the assertion (A) is true. SRMs do exhibit high torque ripple due to their operational characteristics. Additionally, the reason (R) is false.

Option 4: Both A and R are true, and R is the correct explanation of A.

This option is incorrect because, although the assertion (A) is true, the reason (R) is false. The high torque ripple in SRMs is not due to constant and steady excitation, but rather due to the switched nature of excitation and the design of the motor.

Conclusion:

Switched Reluctance Motors (SRMs) are known for their robustness, simplicity, and ability to operate without permanent magnets or rotor windings. However, their high torque ripple is a significant drawback that arises due to the sequential switching of stator phases and the non-linear magnetic properties of the motor. Understanding these operational characteristics is crucial for optimizing SRM performance and mitigating torque ripple through advanced control strategies, such as torque ripple minimization techniques or improved motor design.