Reluctance Motor MCQ Quiz - Objective Question with Answer for Reluctance Motor - Download Free PDF

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.

Explanation:

The correct answer is 2) Reluctance motor.

Repulsion motor:

This is a type of single-phase induction motor that does not use a capacitor and relies on repulsion forces to start and run the motor. It requires DC excitation for the rotor.
Reluctance motor:

Also known as a single-phase synchronous motor, this type of motor operates without DC excitation. It uses the principle of reluctance torque to achieve synchronous speed.
AC series motor:

This type of motor has a series-connected stator and rotor winding and requires AC supply for both. It is not a synchronous motor.
Universal motor:

• This type of motor can operate on both AC and DC supply.
• It uses a series-connected stator and rotor winding and is not a synchronous motor.
• Therefore, only the reluctance motor fits the description of an unexcited single-phase synchronous motor.

Switched reluctance motor (SRM):

• The switched reluctance motor is a doubly salient, singly excited motor that produces torque at synchronous speed.
• It has salient poles on both the rotor and the stator, but only one member (usually the stator) carries windings.
• The rotor has no windings; magnet or cage windings but is built up from stacks of salient pole laminations.
• It is a stepper motor with closed-loop control and with rotor position sensor.
• It is designed for high speed and the closed loop system is necessary.
• It is normally necessary to use a rotor position sensor for communication and speed feedback. The turning ON and OFF operation of the various devices of the power semiconductor switching circuit are influenced by signals obtained from the rotor position sensor.

Advantages:

• Construction is simple and robust
• It requires less maintenance
• Its overall efficiency is better
• It is a flexible control driving motor as motoring mode and generating mode of operations of the machine can be easily achieved.

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 a self-starting machine and it rotates with constant speed.
• It can operate on ac supply only.
• Reluctance motors can deliver very high-power density at a low cost, making them ideal for timing devices and control apparatus.
• These motors cannot accelerate high-inertia loads to synchronous speed.
• The pull-in and pull-out torques of such motors are weak.
• Reluctance motors have poor efficiency and torque.
• Reluctance motors have a low power factor.
• Reluctance motors are cheaper than any other kind of synchronous motors.

Applications:

Applications of reluctance motors are Signalling devices

• Recording instruments
• Phonographs
• Analog electric meters

Concept:

Let N be the number of rotor poles (or teeth) and m is the number of stacks or phases. Then

Pole pitch = 360° / N

Step angle = 360° / (m × N)

Calculation:

In a three-stack 12/8 pole variable reluctance motor,

Number of rotor poles = 8

Pole pitch = 360/8 = 45°

 Mistake Points

Students usually get confused in pole pitch and step angle. Both are different terms with different meanings which are mentioned below.

Pole pitch: The distance measured in terms of armature slots or armature conductors between two adjacent pole centers.

Pole pitch = 360° / N

Step angle: The step angle of a stepper motor is defined as the angle by which the rotor of a stepper motor moves when one input pulse is applied to the stator of the motor.

The step angle for a stepper motor is given by:

\(β = \frac{360°}{m × N_r}\)

where, β = Step angle

m = No. of stator phases

Nr = No. of rotor teeth

Here in our question rotor pole pitch is to be calculated, so the calculations should be done accordingly.

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

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

Concept:

A reluctance motor is a constant speed motor like a synchronous motor.

Speed (N_s) = 120 f / P

f = frequency, P = number of poles

Angular speed (ω) = (2 π N_s) / 60

Calculation:

Given

P = 4, f = 50 Hz, power = 8000 W

N_s = 120 × 50 / 4 = 1500 RPM

ω = (2 × π × 1500) / 60

ω = 157 rad/s

Torque = Power / ω

Torque = 8000 / 157

Torque = 51 N-m

Concept

The load torque is given by:

\(T_L={P\over ω_s}\)

\(\omega_s={4π f\over P}\)= Motor speed in rad/sec

P = Load power

f = Frequency

P = No. of poles

Calculation

Given, P = 8 kW

f = 50 Hz

P = 4

\(\omega_s={4π \times 50\over 4}\) = 50 π 

\(T_L={8 \times 10^3\over 50 \pi}\)

T_L = 51 N-m

Reluctance 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.

Concept:

Single phase synchronous motor:

A single phase synchronous motor is also called as reluctance motor.

Construction:

• The stator of the motor has the main and auxiliary winding. The stator of the single phase reluctance and induction motor are same.
• The rotor of a reluctance motor is a squirrel cage with some rotor teeth removed in the certain places to provide the desired number of salient rotor poles.

Working:

• Reluctance motor torque is produced due to the tendency of the rotor to align itself in the minimum reluctance position.
• Speed of the motor is close to the synchronous speed. The rotor pulls in synchronism.
• The load inertia should be within the limits, for proper effectiveness.
• At synchronism, the induction torque disappears, but the rotor remains in synchronism due to synchronous reluctance torque.

Advantages of reluctance motor:

• It doesn’t require DC supply.
• Stable characteristics
• Maintenance is less
• Less heat
• No magnets
• It is self-starting.

Repulsion Motor: It is a special kind of single-phase AC motor which works due to the repulsion of similar poles. The stator of this motor is supplied with a single-phase AC supply and the rotor circuit is shorted through a carbon brush.

The torque in the repulsion motor is given as

T_e = 0.5k I_s² N_s² sin 2α 

Where I_s and N_s are the stator filed-current and an effective number of stator turns.

α is the angle between the brush axis or rotor field axis to the stator field axis.

Since a constant stator field motor extract maximum torque when

sin 2α = 1 ⇒ 2α = 90°  ⇒ α = 45° 

Maximum torque in the repulsion motor is achieved when the stator and rotor field axis is 45° apart.

Important Points

Rotor current is maximum when the brush axis and direct axis coincide(α = 0°)	Rotor current is zero when the brush occupies a position in quadrature with the direct axis(α = 90°)

The correct answer is option 3):(Reluctance motor)

Single phase reluctance Motor:

• A single-phase reluctance motor is similar to the single-phase salient pole synchronous cage type induction motor,
•  it is also called ​a 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 the stator and rotor
• It requires no D.C. field excitation for its operation The direction of rotation can be reversed ordinarily.
• The torque of this motor can be generated because of the rotor tendency to connect itself in the least reluctance position, once the motor speed is nearer to the synchronous speed.
• Therefore, the rotor drags in synchronism.
• The inertia of load must be in the limits for suitable effectiveness.
• At synchronization, the torque of induction will disappear, except the rotor remains in synchronization because of the torque in synchronous reluctance.

The correct answer is option 2): (a, c and d are correct)

Concept:

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 a self-starting machine and it rotates with constant speed It can operate on ac supply only
• Reluctance motors can deliver very high-power density at a low cost, making them ideal for timing devices and control apparatus.
• The other applications of reluctance motors are Signalling devices Recording instruments Phonographs Analog electric meters
•  These motors cannot accelerate high-inertia loads to synchronous speed.The pull-in and pull-out torques of such motors are weak.
• Reluctance motors have poor efficiency and torque.
• Reluctance motors have low power factor.
• Reluctance motors are cheaper than any other kind of synchronous motors.

The correct answer is option1):(poor power factor and poor efficiency)

Concept:

• 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 ​a 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 motors have lower power factors than induction motors because the rotor requires a lot of magnetization. This requires a greater amount of reactive power (var). Since the power factor is the ratio between apparent power (VA) and real power (W), that number decreases.
• The power factor in the reluctance motor is a lagging power factor. The efficiency of the machine is about 55-75%.
• On the positive side, the rotor is “cold” (no current flows) and hence the overall power consumption of the reluctance motor is usually lower than that of an induction motor of similar size.

Additional Information