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The switched reluctance motor (SRM) is an
electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate f ...
that runs by reluctance torque. Unlike common
brushed DC motor A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source and utilizing an electric brush for contact. Brushed motors were the first commercially important application of electri ...
types, power is delivered to windings in the
stator The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric m ...
(case) rather than the
rotor Rotor may refer to: Science and technology Engineering * Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...
. This greatly simplifies mechanical design as power does not have to be delivered to a moving part which eliminates the need for a
commutator In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory. Group theory The commutator of two elements, ...
, but it complicates the electrical design as some sort of switching system needs to be used to deliver power to the different windings. Electronic devices can precisely time the switching of currents, facilitating SRM configurations. Its main drawback is torque ripple. Controller technology that limits torque ripple at low speeds has been demonstrated. Sources disagree on whether it is a type of
stepper motor A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can be commanded to move and hold at one of these steps without any posi ...
. The same electromechanical design can be used in a generator. The load is switched to the coils in sequence to synchronize the current flow with the rotation. Such generators can be run at much higher speeds than conventional types as the armature can be made as one piece of magnetisable material, as a slotted cylinder. In this case the abbreviation SRM is extended to mean Switched Reluctance Machine, (along with SRG, Switched Reluctance Generator). A topology that is both motor and generator is useful for starting the prime mover, as it saves a dedicated starter motor.


History

The first patent was by W. H. Taylor in 1838 in the United States. The principles for SR drives were described around 1970, and enhanced by Peter Lawrenson and others from 1980 onwards. At the time, some experts viewed the technology as unfeasible, and practical application has been limited, partly because of control issues and unsuitable applications, and because low production numbers result in higher cost.


Operating principle

The SRM has wound field coils as in a
DC motor A DC motor is any of a class of rotary electrical motors that converts direct current (DC) electrical energy into mechanical energy. The most common types rely on the forces produced by induced magnetic fields due to flowing current in the coil ...
for the stator windings. The rotor however has no magnets or coils attached. It is a solid salient-pole rotor (having projecting magnetic poles) made of soft magnetic material (often laminated steel). When power is applied to the stator windings, the rotor's
magnetic reluctance Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux. It represents the opposition to magnetic flux, and depends on the geo ...
creates a force that attempts to align the rotor pole with the nearest stator pole. In order to maintain rotation, an electronic control system switches on the windings of successive stator poles in sequence so that the magnetic field of the stator "leads" the rotor pole, pulling it forward. Rather than using a mechanical
commutator In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory. Group theory The commutator of two elements, ...
to switch the winding current as in traditional motors, the switched-reluctance motor uses an electronic position sensor to determine the angle of the rotor shaft and solid state electronics to switch the stator windings, which enables dynamic control of pulse timing and shaping. This differs from the apparently similar
induction motor An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor ...
that also energizes windings in a rotating phased sequence. In an SRM the rotor magnetization is static (a salient 'North' pole remains so as the motor rotates) while an induction motor has slip (rotates at slightly less than synchronous speed). SRM's absence of slip makes it possible to know the rotor position exactly, allowing the motor to be stepped arbitrarily slowly.


Simple switching

If the poles A0 and A1 are energised then the rotor will align itself with these poles. Once this has occurred it is possible for the stator poles to be de-energised before the stator poles of B0 and B1 are energized. The rotor is now positioned at the stator poles b. This sequence continues through c before arriving back at the start. This sequence can also be reversed to achieve motion in the opposite direction. High loads and/or high de/acceleration can destabilize this sequence, causing a step to be missed, such that the rotor jumps to wrong angle, perhaps going back one step instead of forward three.


Quadrature

A much more stable system can be found by using a "quadrature" sequence in which two coils are energised at any time. First, stator poles A0 and A1 are energized. Then stator poles B0 and B1 are energized which, pulls the rotor so that it is aligned in between A and B. Following this A's stator poles are de-energized and the rotor continues on to be aligned with B. The sequence continues through BC, C and CA to complete a full rotation. This sequence can be reversed to achieve motion in the opposite direction. More steps between positions with identical magnetisation, so the onset of missed steps occurs at higher speeds or loads. In addition to more stable operation, this approach leads to a
duty cycle A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a form ...
of each phase of 1/2, rather than 1/3 as in the simpler sequence.


Control

The control system is responsible for giving the required sequential pulses to the power circuitry. It is possible to do this using electro-mechanical means such as commutators or analog or digital timing circuits. Many controllers incorporate
programmable logic controller A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity t ...
s (PLCs) rather than electromechanical components. A microcontroller can enable precise phase activation timing. It also enables a soft start function in software form, in order to reduce the amount of required hardware. A
feedback loop Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled c ...
enhances the control system.


Power circuitry

The most common approach to powering an SRM is to use an asymmetric bridge converter. The switching frequency can be 10 times lower than for AC motors. The phases in an asymmetric bridge converter correspond to the motor phases. If both of the power switches on either side of the phase are turned on, then that corresponding phase is actuated. Once the current has risen above the set value, the switch turns off. The energy now stored within the winding maintains the current in the same direction, the so-called
back EMF Counter-electromotive force (counter EMF, CEMF, back EMF),Graf, "counterelectromotive force", Dictionary of Electronics is the electromotive force (EMF) manifesting as a voltage that opposes the change in current which induced it. CEMF is the EMF c ...
(BEMF). This BEMF is fed back through the diodes to the capacitor for re-use, thus improving efficiency. This basic circuitry may be altered so that fewer components are required although the circuit performs the same action. This efficient circuit is known as the (n+1) switch and diode configuration. A
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
, in either configuration, is used for storing BEMF for re-use and to suppress electrical and acoustic noise by limiting fluctuations in the supply voltage. If a phase is disconnected, an SR motor may continue to operate at lower torque, unlike an AC induction motor which turns off.


Applications

SRMs are used in some appliances.


References


External links


Switched Reluctance Motor Drives

Real-Time Simulation and Control of Reluctance Motor Drives for High Speed Operation with Reduced Torque Ripple

Torrey - Switched reluctance generators and their control DOI: 10.1109/41.982243

Asadi - Development and application of an advanced switched reluctance generator drive

SR database archive

Adam Biernat: Electrical Machines in the Power Engineering and Automatics (Warsaw Polytechnic)

Synchronous Reluctance Motor Introduction Concepts
{{Electric motor Electric motors