Reluctance Motor MCQ Quiz in తెలుగు - Objective Question with Answer for Reluctance Motor - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

• Usually, unexcited single-phase synchronous motor runs at a constant speed equal to the synchronous speed of revolving flux
• They do not need a dc excitation for their rotors
• That’s why they are called as unexcited single-phase synchronous motors
• These motors are divided into two types, one is reluctance motor and another one is hysteresis motor

• A single-phase reluctance motor is basically the same as the single-phase salient pole synchronous cage type induction motor, hence it is also called ​self-starting type synchronous motor
• The stator of the motor has the main and auxiliary winding
• The stator of the single-phase reluctance and induction motor is the same
• The rotor of a reluctance motor is a squirrel cage with some rotor teeth removed in certain places to provide the desired number of salient rotor poles
• The rotor is of unsymmetrical magnetic construction in order to vary reluctance path between stator and rotor
• It requires no D.C. field excitation for its operation
• The direction of rotation can be reversed ordinarily.

Reluctance motor is used in signaling and timing devices, analog electric meters, some washing machine designs, control rod drive mechanisms of nuclear reactors and hard disk drive motor.

Reluctance motor:

• The stator of the reluctance motor has the main and auxiliary winding
• The stator of the single-phase reluctance and induction motor are the same
• The rotor of a reluctance motor is a squirrel cage with some rotor teeth removed in certain places to provide the desired number of salient rotor poles
• The rotor needs no dc excitation
• It is self-starting machine and it rotates with constant speed
• It can operate on ac supply only

Important Points

Reluctance motors can deliver very high-power density at low cost, making them ideal for timing devices and control apparatus. The other applications of reluctance motor are

• Signaling and timing devices
• Phonographs
• Analog electric meters

Mistake Points

Hysteresis motor produced very less noise and is used in recording instruments.

The segmental rotors are used in the Flux switching machines (FSM’s).

Mostly, segmental rotors are used in the Reluctance motors.

A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. Torque is generated through the phenomenon of magnetic reluctance.

Reluctance motors can deliver very high-power density at low cost, making them ideal for timing devices and control apparatus. The other applications of reluctance motor are,

• Signaling devices
• Recording instruments
• Phonographs
• Analog electric meters

Synchronous Reluctance Motor

Definition: A Synchronous Reluctance Motor (SynRM) is a type of electric motor that operates using the principle of reluctance torque. It does not require windings or permanent magnets on the rotor, making it a simple and cost-effective motor design. The rotor in a synchronous reluctance motor is specifically designed to have a high degree of anisotropy, meaning it has different magnetic reluctance in different directions.

Flux Barriers and Their Purpose:

In the rotor of a synchronous reluctance motor, "flux barriers" are strategically placed to guide the magnetic flux. These barriers are essentially non-magnetic regions that prevent the magnetic flux from passing through certain parts of the rotor, thereby creating a preferred path for the flux. This anisotropy is key to the operation of the motor, as it enables the generation of reluctance torque.

The flux barriers in a synchronous reluctance motor are typically filled with air or non-magnetic material (Option 4). These materials have very low magnetic permeability, which ensures that magnetic flux is restricted from passing through them. By doing so, the rotor creates distinct magnetic paths with varying reluctances, thereby improving the motor's efficiency and torque production.

Advantages of Filling Flux Barriers with Air or Non-Magnetic Material:

• Cost-Effectiveness: Air or non-magnetic materials are inexpensive compared to other materials like ferrite or copper, reducing the overall cost of the motor.
• Lightweight: Using air or lightweight non-magnetic materials helps keep the rotor's weight low, improving dynamic performance and reducing inertia.
• Improved Torque Characteristics: The anisotropic design created by these barriers enhances the motor's ability to generate reluctance torque efficiently.
• Thermal Stability: Non-magnetic materials generally exhibit good thermal properties, ensuring stable operation over a wide range of temperatures.

Applications: Synchronous reluctance motors are widely used in applications requiring high efficiency and cost-effectiveness, such as in industrial drives, pumps, and fans.

Analysis of Other Options

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

Option 1: Ferrite

Ferrite is a magnetic material commonly used in transformers, inductors, and magnetic cores. However, it is not suitable for filling flux barriers in a synchronous reluctance motor. The purpose of the flux barriers is to create regions of high reluctance (low permeability), which is not achievable with ferrite since it is a magnetic material. Using ferrite would defeat the purpose of creating anisotropy in the rotor, thereby reducing the motor's efficiency and torque production.

Option 2: Copper

Copper is an excellent conductor of electricity and is commonly used in windings and electrical connections. However, it is not appropriate for filling flux barriers in a synchronous reluctance motor. Copper does not have the non-magnetic properties required for creating high reluctance regions. Additionally, using copper would increase the motor's cost and weight unnecessarily, without providing any benefits to the flux barrier design.

Option 3: Laminated Steel

Laminated steel is used in the construction of motor cores to reduce eddy current losses. However, it is a magnetic material with high permeability, making it unsuitable for flux barriers in a synchronous reluctance motor. The purpose of the flux barriers is to restrict the magnetic flux, which cannot be achieved with laminated steel. Instead, laminated steel is typically used in the stator core of the motor, where magnetic flux needs to be guided effectively.

Conclusion:

The correct option for filling flux barriers in a synchronous reluctance motor is air or non-magnetic material (Option 4). This choice ensures the creation of high reluctance regions in the rotor, which is essential for efficient motor operation. The use of air or non-magnetic materials contributes to the motor's cost-effectiveness, lightweight design, and improved torque characteristics. Other options such as ferrite, copper, and laminated steel are not suitable for this purpose, as they do not meet the requirements for creating the necessary anisotropic properties in the rotor.

Torque Production in Reluctance Motors

Definition: Reluctance motors are a type of synchronous motor where the torque is produced due to the tendency of the rotor to align itself with the position of the minimum reluctance path in the magnetic field. These motors do not rely on permanent magnets or a DC excitation but instead utilize the inherent magnetic properties of the rotor material and its magnetic saliency.

Working Principle: The torque in reluctance motors is primarily generated due to magnetic saliency. Magnetic saliency refers to the difference in magnetic reluctance along different axes of the rotor. The rotor is designed with anisotropic properties, meaning it has different magnetic characteristics along different directions.

When a rotating magnetic field is produced by the stator, the rotor experiences a torque due to the alignment tendency with the axis of minimum reluctance. This alignment minimizes the reluctance of the magnetic circuit, and the rotor continues to rotate in sync with the stator field to maintain this alignment. This synchronous operation is the primary mechanism of torque generation in reluctance motors.

Correct Option Analysis:

The correct option is:

Option 3: Magnetic saliency

The torque in reluctance motors arises due to the difference in reluctance in the rotor's magnetic path. The rotor aligns itself to minimize the reluctance, and this alignment tendency is responsible for producing torque. This phenomenon, called magnetic saliency, is the defining characteristic of reluctance motors.

Advantages of Magnetic Saliency in Reluctance Motors:

• Eliminates the need for permanent magnets or external excitation, reducing manufacturing costs.
• Simple and robust construction of the rotor, with no windings or magnets.
• High efficiency due to minimal energy losses in the rotor.

Limitations:

• Requires precise rotor and stator design to achieve optimal performance.
• Lower power factor compared to permanent magnet synchronous motors (PMSMs).
• Higher torque ripple, which may require additional control strategies to minimize vibrations.

Important Information

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

Option 1: Permanent magnet alignment

This option is incorrect as reluctance motors do not use permanent magnets for torque generation. Unlike permanent magnet synchronous motors (PMSMs), the reluctance motor relies solely on the rotor's magnetic saliency and the stator's magnetic field for torque production. Permanent magnet alignment is not a factor in reluctance motors.

Option 2: Induction principles

This option is also incorrect. While induction motors use electromagnetic induction to produce torque, reluctance motors operate on a completely different principle. There is no induced current in the rotor of a reluctance motor; instead, the torque is generated due to the rotor's alignment with the stator's magnetic field based on magnetic saliency.

Option 4: Eddy current generation

This option is incorrect because eddy currents are not a significant source of torque in reluctance motors. Eddy currents are undesirable phenomena in most electrical machines as they lead to energy losses in the form of heat. Reluctance motors are designed to minimize eddy current losses, and torque generation is independent of eddy currents.

Option 5: No correct answer

This option is clearly incorrect as magnetic saliency (Option 3) is the definitive and scientifically accurate explanation for torque generation in reluctance motors.

Conclusion:

Torque in reluctance motors is primarily produced due to magnetic saliency, which is the tendency of the rotor to align itself with the minimum reluctance path in the magnetic field. This principle differentiates reluctance motors from other types of motors, such as permanent magnet synchronous motors and induction motors. Understanding the concept of magnetic saliency and its role in torque production is crucial for designing and utilizing reluctance motors effectively in various applications.

I load torque remains same, then

\(V_1^2\sin 2{\alpha _1} = V_2^2\sin 2{\alpha _2}\)

Where α₁ and α₂ are torque angles

\(\begin{array}{l} {240^2}\sin \left( {2 \times 15^\circ } \right) = {\left( {200} \right)^2}\sin 2{\alpha _2}\\ {\left( {\frac{6}{5}} \right)^2}\sin 30^\circ = \sin 2{\alpha _2}\\ \sin 2{\alpha _2} = \frac{{36}}{{50}}\\ 2{\alpha _2} = 46.05^\circ \\ {\alpha _2} = 23.025^\circ \approx 23^\circ \end{array}\)

If the induction are constant irrespective of rotor position that means reluctance is constant

∴ No reluctance torque can be produced