What is a Switched Reluctance Motor & Its Working

The SRM or switched reluctance motor is not a new technology; however, their commercial versions have appeared simply from the past few years. The designers of this motor declared that they provide high efficiency, better reliability, less price as well performance as compared with other types of motors. The concept of SRM has been recognized in the year 1838. The main benefits of switched reluctance motor mainly include reliability, increased performance, high motor-drive efficiency, the overall system cost is less, etc. This article discusses an overview of the switched reluctance motor.

What is a Switched Reluctance Motor?

An electric motor like SRM (switched reluctance motor) runs through reluctance torque. Different from the types of common brushed DC motor, power can be transmitted to windings within the stator instead of the rotor. An alternate name of this motor is VRM (Variable Reluctance Motor). For a better operation of this motor, it uses a switching inverter. The control characteristics of this motor are the same as dc motors which electronically commutated. These motors are applicable where sizing, as well as horsepower (hp) to weight, is critical.

This motor simplifies its mechanical design to restrict the flow of current toward a rotary part; however, it complicates the design because some kind of switching system must be employed to transmit the power toward the different windings. This mechanical design can also be used for a generator. The load can be switched toward the coils within the sequence to coordinate the flow of current through the rotation. So these generators can also run at high speed as compared with conventional types of motors because the armature is made like a single piece of magnetizable material like a slotted cylinder.

Working Principle

The working principle of the switched reluctance motor is, it works on the principle of variable reluctance that means, the rotor of this motor constantly tries to align through the lowest reluctance lane.

The formation of the rotary magnetic field can be done using the circuit of power electronics switching. In this, the magnetic circuit’s reluctance can mainly depend on the air gap. Therefore, by modifying the air gap among the rotor as well as a stator, we can also modify the reluctance of this motor.
Here, reluctance can be defined as resistance toward the magnetic flux. For Electrical circuits, reluctance is the combination of resistance as well as the magnetic circuit.

Switched Reluctance Motor Construction

The construction of the switched reluctance motor is shown below. This motor includes 6- stator poles as well as 4 rotor poles. The design of the stator can be done using silicon steel stampings inside projected poles. The poles in the stator are either an odd number or even number. Most of the electric motors have an even number of poles within the stator which have field coils.

When the poles are opposite then the field coils will be connected in series. So their magnetomotive forces are additive which are called phase windings. A set of coils or a single-coil can comprise phase windings. Each winding can be connected to the motor terminal and these are properly connected toward the o/p terminals of a switching circuitry of power semiconductor. The input of this is a DC supply.

The designing of the rotor can be done with Si steel stampings through externally projected poles. The rotor poles are dissimilar as compared with the stator poles.  In most of the existing motors, the rotor poles are 4 otherwise 6 based on the number of stator poles like 6 8. The shaft of the rotor holds a position sensor. So the operating of various devices in the power semiconductor circuitry is mainly controlled through the signals attained from this sensor.

In this motor, both the stator as well as rotor includes a projected pole that is designed with a soft iron as well as silicon stampings which are used for reducing hysteresis losses. The stator of the motor includes a field winding whereas the rotor doesn’t. In the stator, each winding can be connected within the series through the opposite poles for increasing the magnetomotive force of the circuit.

Types of Switched Reluctance Motor

The categorization of switched reluctance motor can be done based on the construction like linear SRM as well as rotary SRM.

Linear SRM

The linear SRM or linear switched reluctance motors are known as servos in the market. It includes a single-step stator as well as the rotor.

Rotary SRM

The rotary SRM or rotary switched reluctance motors are available in two types like radial field as well as the axial field. Axial field SRMs are classified into two types like single stack and multi-stack. This rotary SRM includes more than one rotor and stator.

Switched Reluctance Motor Working

The operating of SRM (switched reluctance motor) can be done through switching currents within the stator windings of the motor by making changes within the magnetic circuit. This circuit can be formed through the stator as well as the rotor of the motor.

The stator of this motor includes windings that are related to a BLDC motor; however, the design of the rotor can be done with steel that is turned into salient poles without magnets or windings. Once the poles of the stator & the rotor are out of position, then the magnetic circuit among them includes a high reluctance.

When the pairs of the pole in the stator are switched, the rotor switches to connect through the activated stator poles to reduce the reluctance of the circuit. When the stator poles are switched then they should be exactly timed to make sure that it happens because the rotor pole is moving toward to connect with the activated stator pole.

Not like stepper motors, these motors need position feedback using Hall Effect sensors otherwise an encoder to manage the stator currents commutation depending on the exact position of the rotor.

As compared with stepper motors, these motors include fewer poles as well as a larger stepping angle. Stepper motors are mainly used for placing applications, wherever high resolution, as well as step integrity, is significant. But SRMs are applicable where a primary concern is power density.
These motors include rotors without windings, magnets & lower inertia. Thus, they can attain higher speeds as well as accelerations as compared with stepper motors because they have permanent magnet rotors.

The main difference between SRMs (switched reluctance motors) & stepper motors is the construction of stator. In an SRM, the phases are autonomous with each other that means, if one otherwise more phases stop working, then the motor will operable even though by decreased torque output.

Switched reluctance motors generate more clear noise as compared with stepper motors. The main source of noise can be the distortion of the stator because of the radial forces that happen once the pairs of stator poles are activated. These pairs are attracted to cause radial forces to alter the stator.

Switched Reluctance Motor Characteristics

The characteristics of the switched reluctance motor include the following.

• This kind of reluctance motor is a 1-phase or 3-phase
• Speed control of this motor is simple.
• The triggering circuit can be changed to get high speed
• It operates with a DC supply once used with an inverter.
• Once the firing angle of any switching device can be changed then different speeds can be achieved.
• Control of one phase is independent of the other two phases.
• The unutilized energy fed to the motor can be retrieved by using the feedback diodes. This improves efficiency.

Advantages

The advantages of a switched reluctance motor include the following.

• These motors are very simple & the rotors in this motor are extremely strong
• These motors are applicable for high-speed applications.
• The VFDs (variable frequency drives) of this motor are somewhat simpler as compared with conventional VFDs.
• This motor doesn’t use any additional ventilation system when the stator, as well as rotor slots, is projected. So the airflow can be maintained among the slots.
• These are less expensive because of the nonexistence of permanent magnets.
• Fault tolerance is high
• This motor works with a simple two-phase or three-phase pulse generator.
• Phase losses do not change the operation of the motor.
• Once the phase sequence is changed then the motor direction will be changed.
• Inertia Ratio or High Torque
• Self-starting without using additional arrangements

Disadvantages

The disadvantages of a switched reluctance motor include the following.

• Switched reluctance motors have less torque capacity & normally these motors are noisy.
• While operating this motor at high speed, it creates a torque ripple.
• High noise level
• It uses an external rotor position sensor
• These are applicable for medium to high speed, low-cost applications wherever controllability & shaft or noise torque ripple are not dangerous.
• This motor generates harmonics when it operates at high speed, so to reduce this, large size capacitors need to install.
• Since the nonexistence of a permanent magnet, the SRM has to carry a high i/p current to increase the necessity of converter KVA.

Applications

The applications of switched reluctance motors include the following.

• These types of motors are used as an alternative for induction motors in different applications wherever the operating conditions of this motor do not suit them.
• In textile machinery like towel looms, rapier looms, etc
• Used in electric vehicles
• Oilfield machinery like beam pumps, vertical pumps, well testing machinery, etc.
• Mining machinery like conveyors, shearers, winches, ball mills, boring machines, coal crushers, etc.
• Used in all kinds of mechanical presses like screw presses.

These motors are used in miscellaneous applications which include the following.

• Machine tools like vertical lathes, planers, drilling machines, etc.
• Coil winding as well as unwinding equipment
• General machinery like pumps, fans, compressors, etc.
• Equipment used in paper mills
• Machinery used for food mixing
• Rolling mill for metals
• Lifting machines such as winches, lifts, conveyors, etc
• Manufacturing of plastic-like extrusion, injection molding devices
• Power generation device like load control using wind turbine rotor blade
• Used in domestic appliances like vacuum cleaners, washing machines, fans, etc.

Thus, this is all about an overview of switched reluctance motors (SRMs). These motors have many benefits in different applications. At present, the linear version of this motor has been implemented to process the same attributes as well as prospects by owing their design & high force density. Here is a question for you, what are the different types of reluctance motor?