Single Phase Reluctance Motor

It is also known as single-phase synchronous motor but it does not require any excitation like the three-phase synchronous motor for rotation.

A reluctance motor is an electric motor that operates based on the principle of magnetic reluctance. It achieves rotation by exploiting the tendency of the rotor to align itself with the least magnetic reluctance path within a stator's magnetic field. This alignment generates torque and powers the motor. Reluctance motors are known for their simple structure, absence of brushes or permanent magnets, and reliability in various industrial applications. 

Construction of Reluctance Motor

• A reluctance motor consists of stator and rotor.
• The stator of Reluctance Motor consists of the single phase winding with auxiliary winding to the produced synchronous revolving magnetic field.
• Squirrel-cage rotor does not have any windings. It has unsymmetrical magnetic construction, which is achieved by symmetrically removing some of the teeth from the squirrel-cage rotor to produce salient poles on the rotor.

Working and Operation of Reluctance Motor

• When the stator is energized, a synchronously-revolving field is produced. Due to the synchronously-revolving field, the motor started as a standard squirrel-cage induction motor and then it accelerated to near its synchronous speed.
• When the rotor reaches to synchronous speed, then the rotating stator flux will exert reluctance torque on the rotor poles tending to separate the salient-pole axis with the axis of the rotating field. Consequently, the motor will continue to run at the speed of the synchronous speed.
• When a mechanical load is applied to the motor, the rotor poles fall slightly behind the stator poles, while continuing to turn at synchronous speed. As the load on the motor is increased, the mechanical angle between the poles increases progressively. If the load is increased too much or increased beyond the amount then the motor may pull out of synchronism and stop.

Characteristics of Reluctance Motor

• The starting torque of Reluctance Motor depends upon the position of a rotor. The value of starting torque varies to 3 to 4 times of its full load torque. The motor operates at a constant speed over approximately 2 times of its full load torque.

Advantages of Reluctance Motor:

• Simple construction and there is no slip ring.
• Cost is low
• Easy to maintain.

Disadvantages of Reluctance Motor:

• Its power factor is poor because the rotor of Reluctance Motor is unexcited which also cause poor efficiency.
• Size per output of the motor is large as compared to the synchronous motor because it has no DC field excitation.
• The pull-in and pull-out torques of such motors are weak.

Application of Reluctance Motor: Electric clock timer, signaling device, recording instrument etc.

Features of Reluctance Motor:

Simple Construction:

• Reluctance motors have a straightforward design without the need for brushes, commutators, or permanent magnets. The rotor is typically made of laminated steel to minimize eddy current losses and improve performance.

Low Starting Torque:

• These motors usually have lower starting torque compared to induction motors. This can be improved with proper motor design but remains a key feature to consider in applications where high starting torque is not essential.

Synchronous Speed Operation:

• Reluctance motors operate at synchronous speed when under load. This is because the rotor aligns with the rotating magnetic field created by the stator, producing torque through reluctance alignment.

High Reliability and Low Maintenance:

• The absence of brushes or slip rings leads to fewer mechanical wear components, which enhances durability and reduces maintenance needs. This makes reluctance motors ideal for industrial applications requiring dependable performance.

Energy Efficiency:

• Modern reluctance motors, especially synchronous reluctance motors, can achieve high efficiency due to minimal rotor losses. They are increasingly used in energy-conscious applications.

• The performance of reluctance motors can vary based on their design. Switched reluctance motors (SRMs), for instance, have better control over torque and speed due to their electronic control systems, while simpler designs may have limited capabilities.

Torque Ripple:

• One disadvantage is that reluctance motors, particularly switched reluctance motors, can exhibit torque ripple, which can lead to vibrations and noise. Advanced control techniques are often employed to mitigate these effects.

Good Efficiency Under Load:

• Reluctance motors perform efficiently under full load, with energy loss primarily limited to the stator and minimal rotor heating, which contributes to their overall energy-saving features.

Compatibility with Variable Frequency Drives (VFDs):

• Reluctance motors, especially modern designs, are often compatible with VFDs, enabling adjustable speed control, which adds versatility to their application.

Reluctance Motor Important Questions for SSC JE, RRB JE Exam:

Question: Which of the following is an unexcited single-phase synchronous motor?

1. Repulsion motor
2. Reluctance motor
3. AC series motor
4. Universal motor

Correct Option: 2 Solution: The reluctance motor operates without an excitation source for the rotor and runs synchronously, making it an unexcited single-phase synchronous motor.

Question: A switched reluctance motor can produce torque at a speed __________.

1. Equal to synchronous speed
2. Triple the synchronous speed
3. Double the synchronous speed
4. Less than synchronous speed

Correct Option: 1 Solution: Switched reluctance motors can produce torque when operating at synchronous speed due to the alignment of magnetic fields between the stator and rotor.

Question: Select the incorrect statement about characteristics of reluctance motor.

1. a only
2. a, b, and c
3. a and c
4. b and c

Correct Option: 3 Solution: Reluctance motors can accelerate high inertia loads and exhibit both variable and constant speed characteristics, making the selection of incorrect statements crucial for understanding their limits.

Question: An 8 kW, 4-pole, 220 V, 50 Hz reluctance motor has a torque angle of 30°. Calculate load torque.

1. TL = 71 N-m
2. TL = 51 N-m
3. TL = 41 N-m
4. TL = 61 N-m

Correct Option: 2 

Given:

• $P$ W,
• The motor is a 4-pole, 50 Hz system,
• So synchronous speed $N_{\mathrm{s }}=\; (\frac{120\mathrm{f)/}}{\mathrm{P }}=\; (\frac{120\times \mathrm{50)/}}{\mathrm{4 }}=\; 1500\; RPM$
  

Convert synchronous sepped​ to radians per second:

$${\mathrm{\omega }}_{}=\; (\frac{2\pi N_{\mathrm{s)/}}}{\mathrm{60 }}=\; (\frac{2\pi \times \mathrm{1500)/}}{\mathrm{60 }}= 157.08\text{ rad/s}$$

Substitute into the torque formula:

$$T_{\mathrm{L }}=\; P/$$ω = (8000)/157.08 ≈ 51 N-m

Conclusion: The correct load torque for the motor is approximately 51 N-m.

Question: In which application would a reluctance motor be the best choice?

1. Hoists and lifts
2. Signaling and timing device
3. Refrigerators
4. Electric shavers

Correct Option: 2 Solution: Reluctance motors are commonly used in signaling and timing devices due to their synchronous operation and reliability.

Question: In which motor is torque produced due to rotor alignment at minimum reluctance?

1. Hysteresis motor
2. Repulsion motor
3. Reluctance motor
4. Universal motor

Correct Option: 3 Solution: The torque production in a reluctance motor is driven by the rotor aligning with the least magnetic reluctance path near synchronous speed.

Question: The reluctance motor has:

1. Poor power factor and poor efficiency
2. High power factor and poor efficiency
3. High power factor and high efficiency
4. Poor power factor and high efficiency

Correct Option: 1 Solution: Reluctance motors are known for having a poor power factor and low efficiency, affecting their use in specific applications.

Question: At what speed can a reluctance motor produce torque?

1. Sub-synchronous speed
2. Standstill
3. Super-synchronous speed
4. Synchronous speed

Correct Option: 4 Solution: Reluctance motors produce torque at synchronous speed as the rotor aligns with the rotating magnetic field.

Question: Which motors are suitable for signaling and timing devices?

1. Reluctance motor
2. Shaded pole motor
3. Hysteresis motor
4. Two-value capacitor motor

Correct Option: 1 Solution: Reluctance motors are preferred for precise timing applications due to their synchronous operation.

Question: Which of the following is an unexcited single-phase synchronous motor?

1. Reluctance motor
2. Repulsion motor
3. Universal motor
4. AC series motor

Correct Option: 1 Solution: The reluctance motor fits the description of an unexcited single-phase synchronous motor.

Question: Which is not a type of reluctance motor?

1. Synchronous reluctance motor
2. Switched reluctance motor
3. UPF reluctance motor
4. None of these

Correct Option: 3 Solution: UPF reluctance motor is not a standard type; typical types include synchronous and switched reluctance motors.

Question: A reluctance motor is usually preferred in:

1. Electric shavers
2. Refrigerators
3. Lifts and hoists
4. Recording instruments

Correct Option: 4 Solution: Due to precision and stability, reluctance motors are used in recording instruments.

Question: Reluctance motor is:

1. A DC shunt motor
2. A servo motor
3. A DC series motor
4. A single-phase synchronous motor

Correct Option: 4 Solution: It operates as a single-phase synchronous motor under suitable conditions.

Question: The segmental rotor is used in:

1. DC motor
2. Universal motor
3. Split phase induction motor
4. Reluctance motor

Correct Option: 4 Solution: Segmental rotors are common in reluctance motors for optimal torque production.

Question: In a 12/8 pole variable reluctance motor, rotor pole pitch is:

1. 15°
2. 30°
3. 45°
4. 60°

Correct Option: 3 Solution: The rotor pole pitch for a 12/8 pole motor is 45°, important for design specifications.

Question: A reluctance motor is usually preferred in:

1. Electric shavers
2. Refrigerators
3. Lifts and hoists
4. Recording instruments

Correct Option: 4 Solution: Its reliable operation makes it suitable for precision instruments.

Question: Which is NOT a characteristic of an unexcited synchronous motor?

1. Operates from a single-phase AC supply
2. Requires DC excitation
3. Self-starter
4. Runs at a constant speed

Correct Option: 2 Solution: Unexcited synchronous motors do not need DC excitation.

Question: How to maximize torque in a repulsion type motor?

1. Brush-stator field axis angle = 180°
2. Brush-stator field axis angle = 0°
3. Brush-stator field axis angle = 90°
4. Brush-stator field axis angle = 45°

Correct Option: 4 Solution: Setting the angle to 45° maximizes torque output.