Synchronous Reluctance Motor

A ferromagnetic rotor, like those found in reluctance motors, induces non-permanent magnetic poles despite the absence of windings. This rotor uses magnetic reluctance to produce torque. This type of motor is one that is individually excited, and the rotor it uses is not symmetrical. Different types of reluctance motors are available, including synchronous, variable, switched, and variable stepping reluctance motors. Due to the difficulty in designing and controlling them, this motor had a limited use at the beginning of the twenty-first century. Therefore, this can be avoided by developing design tools, theory, and embedded systems. An overview of the synchronous reluctance motor is covered in this article.

What is Synchronous Reluctance Motor?

One type of synchronous electric motor is the synchronous reluctance motor, which lacks field windings or permanent magnets and generates torque due to the difference in magnetic conductivities along the direct and quadrature axes of the rotor. Due to its simple & robust construction, this type of motor is currently becoming very popular as a choice for hybrid and electric vehicles. The primary advantage of this motor is the absence of rotor cage losses, which allows for a permanent torque that is higher than that of an IM (Induction Motor) of the same size.

The main features of synchronous reluctance motor mainly include the following.

• The exact torque can be suitable that doesn’t affect the temperature of the rotor.
• As compared to induction motor drives, the control algorithm based on the field is simple
• Compared to other motors like induction and permanent magnet, the rotor of this motor is less expensive.

Synchronous Reluctance Motor Construction

This motor’s design is comparable to that of the salient pole synchronous motor. This motor’s stator has three phase symmetrical winding, but the rotor lacks any field winding. This winding will produce a sinusoidal rotating magnetic field within the air gap, and the induced magnetic field within the rotor can result in the development of a reluctance torque. This rotor has a tendency to connect to the stator field with the least amount of resistance possible.

Using iron laminations in the axial direction that are divided by non-magnetic material, the rotor of the current reluctance motor can be designed. This motor’s performance is comparable to that of an induction machine, but because it is simpler, less expensive, and has a sturdy construction, it may have a higher efficiency than an induction motor.

This motor’s primary application is in high power applications. This can be categorised using the methods listed below.

• Axially Laminated
• Radially Laminated

Reluctance motors are ideal for a variety of applications because they can deliver extremely high power density at a lower cost. The main drawbacks are high torque ripple when used at lower speeds, which also produces noise. This motor’s stator primarily consists of salient electromagnet poles that are comparable to those in a BLDC motor.

A soft magnetic material, such as laminated Si steel, is used in the rotor. Several projections in this material function like salient magnetic poles with magnetic reluctance. The rotor poles in the motor have fewer than the stator poles, which lessens torque ripple and prevents the poles from connecting altogether.

A rotor pole is fully in the unaligned position when it is located in the middle of two adjacent stator poles. This is the highest point of magnetic reluctance for the rotor pole. Many rotor poles are completely connected to many stator poles and are in a less reluctant position when they are aligned.

Once a stator pole is turned on, the rotor’s torque will change in a way that reduces resistance. In order to connect through the stator field, the adjacent rotor pole can be pulled from its unconnected position. By consistently pulling the rotor, the stator field should rotate before the rotor poles in order to maintain revolution.

Certain motor types’ substitutes will run on three-phase AC power. The majority of the designs that are currently on the market are of the switched reluctance type because electronic commutation offers significant control advantages for smooth operation, motor speed control, and starting.

High efficiency at synchronous speed without the use of rare earth permanent magnets is one of synchronous reluctance motor’s key characteristics. The distributed sinusoidal AC stator windings that are connected via a specific rotor lamination design are primarily supported by these motors. These motors’ simple, low-inertia revolving assembly design enables synchronous speed operation. Fewer torque applications that call for efficient operation can use synchronous reluctance motors.

Synchronous Reluctance Motor Working Principle

This motor’s stator has just one winding, which is referred to as the main winding. This winding, however, is unable to produce a rotary magnetic field. Therefore, a minimum of two windings that are divided by the particular phase angle are required for the production of rotary magnetic fields. As a result, the stator of the motor has an additional winding known as an auxiliary winding. A capacitor is a part of this winding and is linked in series with it.

As a result, the current-carrying windings have a phase difference and experience equivalent fluxes. The revolving magnetic field is produced by the interaction of these two fluxes, which is known as the split-phase method for producing the rotary magnetic field.

The synchronous speed is the same as the magnetic field speed. The number of poles on which the stator winding is wound determines this speed. The rotor functions like the squirrel-cage rotor of an induction motor and holds the short-circuited copper or aluminium bars.

When an iron component is placed within a magnetic field, it connects at the location with the least resistance, allowing it to become magnetically locked. Similar to this, the rotor in a reluctance motor tries to connect to the axis of the rotating magnetic field in the area with the least amount of reluctance. However, when the rotor is at a standstill, it is not possible due to its inactivity.

As a result, the motor’s rotor starts to spin almost synchronously, much like a squirrel cage induction motor. When the rotor’s speed reaches synchronous, the magnetic field of the stator will pull the rotor to the location with the least resistance to maintain the magnetic lock. The rotor then continues to rotate at a speed that is equal to synchronous speed.

All that the reluctance torque is is a torque exert. The reluctance motor consequently operates like a synchronous motor in the end. To operate the motor similarly to a synchronous motor, the rotor resistance, combined inertia, and load must all be low.

Torque Equation of Synchronous Reluctance Motor

If the magnets are demagnetized, the operation of synchronous reluctance motors and permanent magnet synchronous motors is similar. Below is a diagram displaying the torque equation for synchronous reluctance motors. There are two parts to this equation; the first part is caused by the field. For the torque equation to be obtained, this component must be eliminated. The next element in the following equation is known as the reluctance torque.

So the reluctance motor’s developed torque can be expressed like the following.

In the above equation, where

‘Te’ is the developed torque

‘P’ is the no. of poles

‘Ψ’ is the induced flux linkage through the field current

‘Lds’ is the direct axis inductance

‘Lqs’ is the Quadrature axis inductance.

‘δ’ is the Torque angle.

Synchronous reluctance motors are reliable, affordable, and highly effective. These motors run at very fast speeds. Due to their low saliency, conventional motors exhibit poor efficiency, a low power factor, and poor torque density. i.e, low ratio of Ldm/Lqm.

But, the current development of these motors through anisotropic design has a high Ldm/Lqm ratio, which has considerably enhanced power factor, efficiency & torque density.

Phasor Diagram

The following is shown in the synchronous reluctance motor’s phasor diagram. This motor’s constant speed is its most significant feature. Damper winding first enters one’s mind in the event that the rotor is unable to connect with the stator’s magnetic field. Also used in synchronous motors are these. Because of the difference in relative speeds between the rotor and the stator magnetic field, these windings can be arranged inside pole shoes that produce damping torque.

When the rotor is unable to connect with the stator, this happens. The Lenz Law-based damping torque is produced in an effort to combat the speed disparity between the magnetic fields of the rotor and stator, which is what led to the construction of the device. As a result, the rotor’s winding is moved by the damping torque in such a way that it becomes magnetically locked through the magnetic field of the stator. The rotor then continues to operate for the remainder of the time at synchronous speed.

Above is an illustration of the synchronous reluctance motor’s phasor diagram. According to the two-axis theory of the motor, the d-axis and q-axis in the above diagram are defined. The voltage across the d and q axes, respectively, is defined as Vd and Vq. Gamma here is the angle between the d-axis and the “Is” (stator current). This can also be specified, and once the synchronous torque is produced, the rotor angle plays a part.

Advantages

The advantages of synchronous reluctance motor include the following.

• It has less torque ripple
• Standard PWM AC inverters can be used to operate this motor.
• The construction of rotors in this motor can be done by using low cost and high strength materials.
• The construction of this motor is rugged as well as simple
• This motor can survive in extremely high temperatures.
• The losses of electromagnetic spinning are eliminated in this motor because there is no need for field excitation at zero torque.
• This motor has a high-speed capacity
• These motors are more dependable than permanent magnet motors because demagnetization is not a concern.
• Because of this motor’s simplicity, it can be used within the multi-motor drive to operate several motors synchronously through a common power

Disadvantages

The disadvantages of synchronous reluctance motor include the following.

• These motors are expensive as compared to induction Motor.
• It requires synchronization of speed toward the o/p frequency of an inverter through rotor position sensor as well as sensorless control.
• It works by using a variable frequency drive.
• This motor as less power factor as well as it is heavier as compared to induction motor

Applications

The applications of synchronous reluctance motor include the following.

• It is used in applications where constant speed is required like timing devices, phonograph, control devices, recording instruments, etc.
• It is applicable in low-power applications like fiber spinning mills due to low cost, construction is robust, inherent simplicity, etc.
• These motors are used in turntables, regulators, synchronized conveyors, metering pumps, manufacturing devices of synthetic fiber.
• It is used like proportioning devices within conveyors otherwise pumps.
• Used in folding, wrapping machines, and auxiliary time machine
• Used in the process of film material otherwise continuous sheet.

The synchronous reluctance motor overview is what this article is all about. Due to its higher efficiency and lack of magnets, this motor offers assurance by offering a sustainable environmental solution to lessen its overall environmental impact. Its lower operating expenses will enable a quick payback. This product benefits from the high efficiency and dependability of synchronous motors as well as induction motors.