What is Synchronous Reluctance Motor & Its Working

A type of electric motor like reluctance motor includes a ferromagnetic rotor which doesn’t include any windings but it will induce non-permanent magnetic poles. This rotor generates torque using magnetic reluctance. This kind of motor is an individually excited one and the rotor used in this motor is a non-symmetrical rotor. Reluctance motor is available in different types like synchronous reluctance motor, variable reluctance motor, switched reluctance & variable stepping reluctance motor. In the early 21st century, this motor was used limitedly through the difficulty of designing as well as controlling them. So this can be overcome by improving the design tools, theory as well as embedded systems. This article discusses an overview of the synchronous reluctance motor.


What is Synchronous Reluctance Motor?

Synchronous reluctance motor is one kind of synchronous electric motor, where the torque of this motor is because of the disparity of magnetic conductivities through the rotor’s direct axes as well as quadrature, which doesn’t have permanent magnets otherwise field windings. At present, this type of motor is becoming very popular by becoming an option for electric as well as hybrid vehicles because of its easy & strong construction. The main benefit of this motor mainly depends on the nonexistence of the losses of rotor cage by allowing a permanent torque that is higher as compared to the torque of an IM (Induction Motor) with the same size.

Synchronous Reluctance Motor
Synchronous Reluctance Motor

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


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

Synchronous Reluctance Motor Construction

The construction of this motor is similar to the salient pole synchronous motor. The rotor in this motor does not include any field winding but the stator includes 3- phase symmetrical winding. This winding will create the sinusoidal turning magnetic field within the air gap & the reluctance torque can be developed when the induced magnetic field within the rotor. This rotor comprises a tendency to make the rotor connect through the stator field at the least reluctance location.

In the current reluctance motor, the designing of the rotor can be done using iron laminations within the axial way which are divided through non-magnetic material. This motor performance is similar to an induction machine but its efficiency may be high as compared with an induction motor as there is no copper loss within the rotor, simplicity, less cost, and strong construction.

Synchronous Motor Construction
Synchronous Motor Construction

This motor is mainly designed to use in high power applications. The classification of this can be done like the following.

  • Axially Laminated
  • Radially Laminated

Reluctance motors are mainly used to deliver extremely high power density at less cost to make them perfect for several applications. The main drawbacks are high torque ripple once operated at less speed & causes noise through torque ripple. The stator of this motor mainly includes salient electromagnet poles which are equal to a BLDC motor.

The rotor includes a soft magnetic material like laminated Si steel. This material includes several projections that work like salient magnetic poles with magnetic reluctance. As compared to the stator poles in the motor, the rotor poles are less which reduces torque ripple & stops the poles by connecting altogether.

Once a rotor pole is in the middle from the two contiguous stator poles, then the rotor pole is completely in the position of unaligned. For the rotor pole, this is the utmost position of magnetic reluctance. In the position of aligned, many rotor poles are completely connected with several stator poles & are in a less reluctance position.

Once s stator pole is activated then the torque of the rotor will be in a way that will decrease reluctance. Therefore, the adjacent rotor pole can be pulled from the not connected position to connect through the stator field. To maintain revolution, the stator field should turn before the rotor poles by continually pulling the rotor.

The alternatives of some kinds of motor will function using three-phase AC power. Most of the current designs available are the switched reluctance type as electronic commutation provides important control benefits for starting the motor, motor speed control & smooth operation.

The main characteristics of synchronous reluctance motor are high efficiency at synchronous speed without using rare earth permanent magnets. These motors mainly allow the distributed sinusoidal AC stator windings which are connected through a particular rotor lamination design. These motors provide synchronous speed operation through an easy, less inertia revolving assembly design. The synchronous reluctance motor is applicable for fewer torque applications that require an efficient operation.

Synchronous Reluctance Motor Working Principle

The stator of this motor includes a single winding that is known as main winding. However, this winding cannot generate a rotary magnetic field. Thus for rotary magnetic field production, there should be a minimum of two windings that are divided through the specific phase angle. Therefore, the motor’s stator includes an extra winding known as auxiliary winding. This winding includes a capacitor that is connected with it in series.

So there exists a phase disparity among the current-carrying windings as well as equivalent fluxes. These two fluxes respond to generate the revolving magnetic field which is known as the split-phase method for the production of the rotary magnetic field.

The magnetic field speed is nothing but the synchronous speed. This speed can be decided through the number of poles where stator winding is wound. The rotor holds the copper otherwise aluminum bars which are short-circuited & it works like an induction motor’s squirrel-cage rotor.

If an iron part is arranged within a magnetic field, then it connects within the least reluctance place to get locked magnetically. Likewise, the rotor within the reluctance motor tries to connect itself through the axis of the rotary magnetic field within the least reluctance place. However, because of the inactivity of the rotor, it is not achievable once the rotor is standstill.

So rotor in the motor begins to rotate close to synchronous speed like a squirrel cage induction motor. Once the speed of the rotor is synchronous, then the magnetic field of the stator will pull the rotor into the least reluctance place to keep it locked magnetically. After that, the rotor continuously turns and the speed of this is equivalent to synchronous speed.

The reluctance torque is nothing but a torque exert. Therefore the reluctance motor finally runs like a synchronous motor. The rotor resistance should be less; its combined inertia, as well as the load, must be low to operate the motor similar to a synchronous motor.

Torque Equation of Synchronous Reluctance Motor

The working of both the permanent magnet synchronous motor and synchronous reluctance motors are similar if the magnets are demagnetized otherwise left. The torque equation for synchronous reluctance motor is shown below. This equation includes two components; the first component is because of the field. So this component must be left out to get the equation for torque. In the following equation, the next component can be defined as reluctance torque.

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

Torque Equation of Synchronous Reluctance Motor
Torque Equation of Synchronous Reluctance Motor

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 rugged, less cost & have high efficiency. These motors operate at extremely high speeds. The conventional motors give poor efficiency, low power factor & poor torque density because of the low saliency 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 phasor diagram of synchronous reluctance motor includes the following. The most important characteristic of this motor is its constant speed. In the beginning, if the rotor fails to connect through the magnetic field of the stator, then damper winding comes into the mind in that situation. These are also utilized in synchronous motors. The arrangement of these windings can be done within pole shoes that generate damping torque because of the disparity in the relative speed among the rotor as well as the stator magnetic field.

Phasor Diagram
Phasor Diagram

This occurs once the rotor not works to connect through the stator. The damping torque generates depending on Lenz Law that seeks to resist the reason for its construction, which is the speed disparity among the magnetic field of the rotor as well as stator. Therefore, the damping torque moves the winding of the rotor such that it is locked magnetically through the stator magnetic field. After that, the rotor works at synchronous speed for the rest of the time.

The phasor diagram of synchronous reluctance motor is illustrated above. The two axes in the above diagram like d-axis & q-axis are defined depending on the motor’s two-axis theory. Likewise, we can define Vd and Vq, which are the voltage across the d and q axis. Here, gamma is the angle among ‘Is’ (stator current) & d-axis. Like a rotor angle, this can also be defined and once the synchronous torque is generated then it is a role of the rotor angle.

Advantages

The advantages of synchronous reluctance motor include the following.

  • It has less torque ripple
  • The construction of rotors in this motor can be done by using low cost and high strength materials.
  • The operation of this motor can be done using standard PWM AC inverters
  • This motor can survive in extremely high temperatures.
  • The construction of this motor is rugged as well as simple
  • This motor has a high-speed capacity
  • In this motor, there is no necessary field excitation at zero torque so the losses of electromagnetic spinning are removed.
  • There is no worry through demagnetization; therefore these motors are more reliable as compared to permanent magnet motors.
  • 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.
  • This motor as less power factor as well as it is heavier as compared to induction motor
  • It works by using a variable frequency drive.

Applications

The applications of synchronous reluctance motor include the following.

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

Thus, this is all about an overview of the synchronous reluctance motor. This motor gives assurance by providing a sustainable environmental solution to reduce the whole environmental impact because of the nonexistence of magnets as well as increased efficiency. Its reduced operating costs will allow a quick return. This product helps from the reliability as well as simplicity of the induction motors & the synchronous motor’s high-efficiency. Here is a question for you, what are the different types of synchronous motors available in the market?

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