So What is a Switched Reluctance Motor?

It is a type of electric motor induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor lacks any windings like we think of on a regular electric motor. The Torque is generated through the phenomenon of magnetic reluctance.

These are different types of reluctance motors:

  • Synchronous reluctance motor
  • Variable reluctance motor
  • Switched reluctance motor
  • Variable reluctance stepping motor.

Reluctance motors can deliver very high power density at a low cost, which in turn makes them ideal for many applications.

  By Majo statt Senf (Own work based on: Reluktanzmotor.JPG) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons


By Majo statt Senf (Own work based on: Reluktanzmotor.JPG) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

 

 

Switched Reluctance Motor

The Switched Reluctance Motor (SRM), a stepper motor powered by reluctance power, is a motor which runs on electricity. SRM differs from DC motor in that mechanical motion is produced by stator windings instead of the rotor, as in the case of DC motors. Though having a simple mechanical design with power being delivered to a static part, the SRM has a complex electrical design because the motor needs a switching system to efficiently deliver power to the windings of the stator. Unlike other stepper motors, the SRM isn’t affected by inaccurate timing, making it a better option for most electronic devices. However, they are adversely affected by torque ripple effect.

The Switch Reluctance Motor has similar mechanical design as a generator, in that loads are switched in a pattern synchronous to the flow of current in the switch. A generator with this type of design can run at a speed, higher than that of other conventional generators, having an armature with strong magnetic property. SRM can sometimes be used to mean Switched Reluctance Machine, though can be used interchangeably with SRG (Switch Reluctance Generator).

Method of Operation

Similar to a DC motor, the SRM has stator winding which consists of wound electric coils. However, neither coils nor magnets are attached to its rotor. The rotor of the SRM are made of laminated steel, a soft magnetic material which gives the rotor projecting magnetic poles, hence described as “salient-pole” rotor. Force is produced when the stator winding receives power, and the rotor creates a magnetic reluctance which forces the rotor pole to align to a stator pole nearest to it. Control switches mounted on the stator windings are used to maintain the rotation. It achieves this by making the magnetic field around the stator lead that in the rotor pole, thus creating a forward movement. Unlike some other conventional motor that use a difficult to maintain commutator for switching current in the stator windings, the SRM makes use of electronic sensors, capable of detecting the angle at which the rotor shaft is aligned. It also has a solid state sensor, responsible for switching the stator windings, and in so doing creates dynamic control which takes the form of shaping and pulse timing.

Although having similar design as the induction motor with windings powered at rotating phase, the rotor of the SRM differs significantly from that of an induction motor in that its magnetization is static. Static magnetization in the sense that a “North” salient pole for instance, will remain in the same state while the motor rotates. On the other hand, the rotor of an induction motor is made of slip which makes it at an almost synchronous speed. Without the slip, it will be near impossible to accurately tell the position of the motor. The slip also allows for the slow stepping of the motor.

 

Simple Switching

Switched Reluctance Motor Simple Switching

Switched Reluctance Motor Simple Switching

As can be seen from the diagram above, the rotor of the SRM will try to align itself with poles A0 and A1, once the two poles have been energized. After this, the two stator poles will lose their energy, which will then be used to energize poles B0 and B1. This now sends the rotor to position b. The process will continue until it reaches c before starting all over again. The rotor can be made to move the opposite direction by reversing this sequence. While in operation, the entire sequence can become unstable, primarily due to high load or acceleration/deceleration. This can also cause the motor to miss a step, with the rotor taking the wrong angle. In the end, the motor will end up taking one step backwards rather than three steps forward.

Improved Sequence

Switched Reluctance Motor

Switched Reluctance Motor

The “quadrature” sequence is often used in stabilizing a system. To start with, poles A0 and A1 will first have to be energized. After this, poles B0 and B1 will be energized, creating a pulling effect on the rotor, causing it to align with poles A and B. For the rotor to be aligned with poles of B, poles A will first have to be de-energize, with the entire sequence continuing to BC, C and CA. At this point, a rotor has completed a full rotation cycle. The direction of the rotor can be reversed by reversing the sequence. Missteps tend to occur more when the rotor moves at higher speed, or large load is placed on it. This is because when two coils are energized concurrently, the number of steps between their positions increases with increasing magnetization.

This operation, being stable often leads to what is called “duty cycle”. A duty cycle is one with ½ phase instead of 1/3 phase common with other simple sequence.

Control

The SRM’s control system provides the electrical pulse which the motor’s circuit needs to activate a phase when it is needed. Although electro-mechanical systems like analog timing circuits and commutators can be used to achieve control for the SRM, advanced electronic systems offer greater control.

Most controllers installed in the SRM are fitted with Programmable Logic Controller (PLC), in lieu of electromechanical controllers. Some SRMs achieve control through the use of microcontrollers because they can accurately time and control phase activation. Microcontrollers make it possible for soft start function control software to be installed on Switch Reluctance Motor, thus minimizing the need for hardware.

Powered circuitry

Asymmetric Bridge Converter

Asymmetric Bridge Converter

Asymmetric Bridge Converter is most commonly used for supplying power to the Switch Reluctance Motor. Asymmetric Bridge Converter comprises of 3 phases, with each phase corresponding to a phase of a reluctance motor. For example, a phase on the SRM will become actuated when a corresponding phase on the bridge converter is switched on. The switch shall then be switched off the moment the current rises above the threshold value. The motor winding will now become energized, causing the current to be maintained in the same position. This will continue until the stator winding becomes de-energized again.

Capacitors are used to eliminate acoustic and electrical noise, a problem common to Switch Reluctance Motor. It does this by reducing, if not eliminating voltage fluctuation.

The circuitry of Switch Reluctance Motor have been designed in such a way that it will continue to function in the same way even after it has been altered, with some components removed.

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