EducationWhat is reluctance motor - Switch reluctance motor, Theory, Equation

What is reluctance motor – Switch reluctance motor, Theory, Equation

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What is reluctance motor

In recent times, small synchronous motors that have a specific design that do not have excitation windings have been extensively used. The most prominent members of this category comprise reluctance motors, motors with permanent magnets in the rotor, and the hysteresis motors. The reluctance motors can be described as synchronous motors designed as open-pole motors with no excitation windings. The only torque that they produce can be described as that of the reluctance torque, which is for larger machines as defined using the following formula

The torque is induced despite the absence of excitation because of the dynamic effect that the field exerts the magnetically asymmetrical component made from ferromagnetic material. Because of this force it is the intention of the rotor to be in a location in relation to the stator at where there is a reluctance to flux becomes the lowest. If the rotor is the magnetically symmetrical element in relation to any axis the reluctance moment will not occur (in any location of the rotor relation to the stator that is because the reluctance in the flux route is identical).

The existence of the existence of a reluctance torque allows to create an electric machine that is a single winding capable of turning electric energy to mechanical power and in reverse. For instance, if an 3-phase winding that has the number of poles p is inserted into the stator and is supplied with 3-phase current and the rotor composed of open poles having the same number of pairs teeth (poles) and a torque will develop in the synchronous speed of the rotating. A stator’s field can “pull” the rotor. It will remain synchronous for as long as the load torque is lower than the maximum electromagnetic torque that the motor can produce at a particular voltage.

A reluctance motor as one that is a synchronous motor with excitation does not produce a start-up torque, so the rotor of the motor is required to be brought into synchronization. In addition to the established ways of startingmanually, manual start-up is not unusual.

Reluctance motors feature three-phase windings in the stator that is supplied by the three-phase network. Or an alternating winding, with one capacitor that is supplied by one-phase networks. The rotors are constructed so they have a longitudinal reluctance is distinct than the transverse.

Reluctance motors are also reduced and impulse motors.

reluctance motor

Switch reluctance motor

Moving reluctance motors (SRMs) are a part to the category of electronically computable motors which operate which relies on the principle of variable reluctance that is visible through the winding band , and dependent on the location of the motor’s rotor. Because of the absence of windings within the rotor as well as due to the relatively straightforward arrangement of the armature winding strands (in the stator) SRMs are one of the motors that have the lowest manufacturing cost. In addition they offer long life spans, like induction machines.

The mechanical properties of SRMs are like the characteristics that of DC motors. The working characteristics of SRMs are based on an electronic system, also known as an electronic converter. It is controlled by an electronic controller which implements an advanced and preset control algorithm. Utilizing a suitable control algorithm allows to adjust the amount of electromagnetic torque generated from motor to the load torque. motor in relation to load force at certain speed.

The amount of equipment powered by reluctance motors is growing every year, yet the majority of drives use DC motors as well as induction motors. Customers (manufacturing firms) can modify their drive systems, subject to certain economic consequences. Any upgrade of equipment required to produce results in a expense in terms of money and a decline in profit in the period of payback for the investment. It is possible to use an entirely new drive system should result in an income, such as an increase in the cost of electricity or an improvement in the performance of the drive with the same amount of energy consumed.

Switch reluctance motor operating principle

The magnetic flux produced by the current that flows through the winding creates an electromagnetic torque, which turns the rotor. The decrease in the air gap between metal piece that is the pole, and rotor’s tooth increase the inductance in the phase band. This results in increasing the time between rise and fall of the current within the motor winding. The amount of motor’s driving torque created through the motor is determined by the angles where the current in the phase band is turned on αon and off αoff.

switch reluctance motor

By altering the values the angles are adjusted, which can alter the amount of electromagnetic torque produced by the motor and consequently the amount of drive speed. Reluctance motors are reluctance motor is a machine that requires the control of phases of the band in relation to the location of the rotor. These can be switched off and on during its motion. For the correct running in the reluctance motor drive there is a need for feedback that is accomplished through the rotor’s position recognition system. This could be transducers with rotary pulses, slot optocouplers , or any other less or more advanced methods for detecting position.

Construction of reluctance motor

An adjustable reluctance motor has a stator and rotor made from a bundle of electromagnetic sheets that have prominent poles that are distributed across the perimeter. At the top of each stator pole, are simple concentric windings, coils that, if located on opposite poles are connected in parallel or series to form independent circuits, known as phase bands.

In more contemporary designs, it is possible to find motors that have SRM Phase windings have been mounted around four stator teeth that are symmetrical to one another. One exception is the single-band reluctance motor that is the number of rotors teeth is the same as the stator poles.

This is why the motors will be more frequently employed for high-speed operations, and the ability to overcome dead zones can be achieved by the inertia generated by the moving masses. For a secure start-up the motor, a permanent magnet is used to permit the an appropriate positioning of the motor’s rotor “parking” after it stops.

So prepared, the starting position permits the rotor’s start-up to be reliable. The use of a permanent magnet can hamper the production process of motors motor and also increases the parasitic torque. Single-band SRMs using the amount of rotor/stator teeth 2/2, 4/4, 6/6, 8/8 are common.

They are distinguished by their easy design, low cost of manufacturing and a simple power supply systems. But their primary drawback is the existence of a large dead zone, where the electromagnetic torque is not generated and there is the possibility of extremely high torque pulses.

Synchronous reluctance motor

This synchronous reluctance motor with asynchronous starting, also known as an the asynchronous synchronous reluctance torque (ASMR) motor or reluctance motor for short and is like an induction squirrel motor. The most significant difference between it from a normal squirrel cage motor is the shape of its rotor. Due to the distinctive an asymmetrical arrangement of the wide and narrow bars of the cage, it is possible to create a reluctance torque is generated, which , at speeds that are close to synchronous speed causes the motor to move into synchronism and run at a synchronous speed (which is dependent upon the amount of pair of motor poles and also the frequency of the voltage supply) insofar that the load torque isn’t greater than what is considered to be the highest synchronous torque (Mmax).

switch reluctance motor
Cross-section of the electromagnetic circuit of the reluctance motor: 1 – rotor plate, 2 – stator plate, 3 and 4 – narrow and wide rods of rotor winding, 7 – stator winding

This type of motor is like an ordinary induction motor during initial startup and when it is operating with a load force higher than Mmax. Another benefit to such motors is that they’re able to get synchronization again after overload when the load is to below the Mmax.

switch reluctance motor
Mechanical characteristics of a synchronous reluctance motor with asynchronous starting

Reluctance motor torque equation

With a known field distribution in the motor area, the torque can be calculated by integrating Maxwell stresses. For a two-dimensional magnetic field distribution determined at a given flow rate θ of the fed band and at a given angular position ϑ of the rotor relative to the stator, the expression for torque is of the form:

Reluctance motor torque equation

The radius is r of the circle inside the air gap in the path to integration angle density of the tangential force longitude of motor (a, r), Bn ( a R) Bt ( a, the r) elements that are magnetically inductive, both normal and intangent towards the direction of the arc in the region ( a and the r).

Based on the precise determination of the components of induction and in accordance with the suggestion to combine the values of the moments measured on various circles with different radius, equally distributed throughout between the gaps in the air, the expression above will look like:

Reluctance motor torque equation

where k is the amount of evenly dispersed points over an arc with radius r. In which the induction components have been calculated. of circles with radii evenly distributed within the gap between air and.

What is variable reluctance stepper motor

Another kind of stepper motor that has less complicated design as than permanent magnet-based motors and is one that is a variable reluctance stepper motor. These motors have rotating rotors that are made of soft magnetic materials and diamagnets (materials which do not have magnetic properties) However, they, like permanently magnet motors they have motors with powered windings. Since the rotor itself isn’t magnetized , or is only weakly magnetized and there is no connection between the stator and it since there is no current flowing through the windings of the stator – thus stepper motors that are of the reluctance type don’t provide brake torque. Though they are relatively weak in torque when compared to permanent magnet models however, they are an ideal choice in the upper and middle speed ranges because of their lower torque drop and are also distinguished by their quiet operation, making them an excellent choice for those working in areas that have more strict noise emissions standards.

Michal Pukala
Electronics and Telecommunications engineer with Electro-energetics Master degree graduation. Lightning designer experienced engineer. Currently working in IT industry.