SRM Motor Explanation

 SRM Motor:

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SRM motor also called a variable reluctance motor. Click the link to download the book.

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Switched Reluctance Motor (SRM)

The switched reluctance motor (SRM) is a type of motor doubly salient with phase coils mounted around diametrically opposite stator poles. There are no windings or permanent magnets on the rotor. The rotor is basically a piece of (laminated) steel and its shape forms salient poles. The stator has concentrated coils.
Switched reluctance motors (SRM) have a simple and robust structure, thus they are generally suitable for high-speed applications. High-speed motors have the advantage of high power density, which is an important issue of traction motors in electric vehicles (EV). Therefore, high-speed SRM seems to be promising candidates for this application.

Solidworks model of a Switched Reluctance Motor:

Solidworks model of a Switched Reluctance Motor

The designed switched reluctance motor is defined as a three-phase machine, which has six inner stator poles, eight outer rotor poles and a shaft, as shown in Figure 1.

EMS Simulation of the In-Wheel Switched Reluctance Motor

In EMS, these types of motors are studied using Transient Magnetic simulation coupled to motion.
EMS computes the time-domain magnetic fields. The quantities for which the transient magnetic solves are the magnetic flux density, B, the magnetic field, H, and the cur­rent distribution, J; Derived quantities such as forces, torques, energy, winding loss, solid loss, flux linkage, inductance, resistance, and induced voltage may be calculated from these basic field quantities.
In this simulation, Start Time, End Time, and Time Increment were set to be 0 s, 0.24 s and 0.0025 s respectively.

How do switched reluctance motors differ from stepper motors?

Switched reluctance motors operate by switching currents in the stator windings in response to changes in the magnetic circuit formed by the rotor and stator. The stator of a switched reluctance motor contains windings, similar to a brushless DC motor, but the rotor is simply made of steel that is shaped into salient poles, with no windings or magnets. To avoid a situation where all the poles of the rotor and the stator line up simultaneously (and no torque is produced), switched reluctance motors have fewer poles on the rotor than on the stator.

When the rotor and stator poles are out of alignment, the magnetic circuit between them has a high reluctance. As the stator pole pairs are energized, the rotor turns to align with the energized stator poles, which minimizes the reluctance of the magnetic circuit. This tendency of the rotor to move to a point of minimum reluctance produces what is referred to as reluctance torque.

Energizing of the stator poles must be precisely timed to ensure that it occurs as the rotor pole is approaching alignment with the energized stator pole. Unlike stepper motors, which can, and for most purposes do, operate in open-loop mode, switched reluctance motors require position feedback from an encoder or Hall effect sensors, to control commutation of the stator currents based on the precise rotor position.

Switched reluctance motors have fewer poles and a larger stepping angle than stepper motors. While stepper motors are typically chosen for positioning applications, where step integrity and high resolution are important, switched reluctance motors are used in applications where power density is a primary concern.

Because switched reluctance motors have rotors with no magnets or windings, they have lower inertia and can, therefore, achieve higher accelerations and speeds than motors with permanent magnet rotors, such as stepper motors. The lack of magnets on the rotor provides other benefits as well – including the ability to withstand higher temperatures (less cooling required) and simple, lower-cost construction than permanent magnet motors.

Another difference between switched reluctance motors and stepper motors lies in the stator construction. In a switched reluctance motor, there is no overlap of coils between successive phases – in other words, the phases are independent of one another. This means that if one or more phases fail, the motor will still be operable, although with reduced torque output.

The fact that both the stator and rotor have salient poles (referred to as a doubly salient design) means switched reluctance motors produce more audible noise than stepper motors. The primary source of noise is the distortion of the stator due to radial forces that occur when the stator pole pairs are energized. The energized pole pairs are attracted to one another, causing radial forces strong enough to distort the stator.