By Mike Howell
EASA Technical Support Specialist

The switched reluctance motor (SRM), also known as the variable reluctance motor (VRM), originated in the mid-1830s. It was first used as a locomotive traction motor. However, the power electronics necessary for satisfactory control of SRMs were not patented until the early 1970s. This entailed electronic commutation synchronized with rotor position. Service centers are seeing an increase in the number of SRMs received for repair, and some of the technicians encountering them are unfamiliar with how they work. As with any other rotating machine, a basic understanding of operating principles can be useful in troubleshooting and repair. One of the most critical things for service center personnel to understand upfront is that these machines cannot be operated without a special drive, which typically would need to be supplied by the end-user or the manufacturer.

The most distinguishing mechanical feature of an SRM is a salient pole rotor with no windings or permanent magnets (see Figure 1). Stator windings for SRMs are concentrated coils, wound over one stator tooth, and are designed with either 3, 4 or 5 phases, making them robust and fault-tolerant. Single-phase and two-phase designs suffer from high torque ripple and starting issues. Additionally, unlike most rotating machines, the stator and rotor of an SRM will have a different number of poles. The pole counts are usually designated by stator poles/rotor poles. For example, a machine with 6 stator poles (N_s) and 4 rotor poles (N_r) would commonly be designated as a 6/4 machine.   

Two of the most important characteristics to understand for operation or control of any type of rotating machine are rotational speed and torque. For the SRM, the rotational speed (n) depends on the operating frequency (f) and the number of rotor poles (N_r). So, if we take an example of a 6/4 machine with a frequency of 133.33 Hz, the operating speed would be:

To understand how the SRM produces torque, let’s begin with a review of some basic circuits.

Basic Circuits Analogy
When introducing magnetic circuits, an analogy is often used with basic electric circuits (see Figure 2). In this electric circuit, the voltage, also called electromotive force (EMF), drives a current (I), which is limited by the total resistance (R₁ + R₂). In this magnetic circuit, the magnetomotive force (MMF) drives a magnetic flux (Φ), which is limited by the total reluctance (R_core + R_gap). 

For the electric circuit shown in Figure 2, observe that if the resistance R₁ is much greater than the resistance R₂, the voltage drop due to R₁ will also be much greater than the voltage drop due to R₂. In the magnetic circuit, the same relationship is true. If the reluctance R_gap is much greater than the reluctance R_core, the MMF drop due to R_gap will be much greater than the MMF drop due to R_core. Just as with an electric circuit, steel is a much better conductor of magnetic flux than air. Also, as shown in Table 1, note that the resistance of an electrical conductor and the reluctance of a magnetic path are both determined by the length (ℓ), cross-sectional area (A) and a factor based on the materials of construction (ƿ for electrical resistivity and σ for magnetic permeability).

Inductance 
An additional concept is needed to discuss torque production in the SRM conventionally. Assuming a magnetic circuit is not saturated, we can define an inductance (L) in terms of the number of turns (N) in a winding and the total reluctance (R_total) of the magnetic circuit associated with that winding. Inductance is measured in henries (H) and winding inductance is usually small (mH).

From this, one can see that high reluctance results in low inductance and vice versa. Additionally, the inductance changes with the number of turns squared: a small change in the number of turns results in a large change in the inductance.

Torque Production

Let’s look at a practical 6/4 machine that has three-phase windings, 6 stator poles and 4 rotor poles (see Figure 3). We will assume motoring operation in the counterclockwise (CCW) direction and isolate 1 phase winding, shown in red. When the stator pole is located directly between two rotor poles, this is called the unaligned position (see Figure 4.A). In this position, inductance is at a minimum, and energizing the phase winding shown will produce no torque. If the rotor is displaced from the unaligned position some CCW distance (see Figure 4.B), a torque will be produced tending to align the closest rotor pole with the energized stator pole. The blue shaded region shown here is called the torque zone, and this is the angle through which the phase winding shown will be energized.

The position shown in Figure 4.C is referred to as the aligned position. In the aligned position, inductance is maximum and there is no reluctance torque produced since the poles are aligned. Note that if this phase winding remained energized as the rotor passed through the aligned position as shown in Figure 4.D, the torque produced would act as a restoring torque, pulling the rotor backward in the clockwise direction until it returned to position Figure 4.C. So, just before the rotor reaches the aligned position, the phase winding shown would be de-energized and the next phase winding in the direction of rotation would be energized through its torque zone, continuing operation in the desired CCW direction. The average torque produced by the SRM through one torque zone depends on the winding current (i) and the change in the inductance (L_max - L_min) over the angle swept (ΔΘ). This average torque can be expressed as:

An important thing to note is that through the torque zone, the magnitude of inductance is not as important as the range of inductance. The range of inductance (L_max - L_min) is sometimes referred to as saliency. Additionally, the torque is proportional to the square of the current, if the core is not saturated. Unlike most stator windings, another interesting characteristic of the SRM is that the polarity of the winding does not affect the torque direction.

Torque ripple is a major issue in SRM design, as in control of acoustical noise. This is a significant problem with single- and two-phase designs, but it can be an issue with more phases as well. With three-phase machines, the pole widths must be chosen carefully to minimize this. Figure 5 demonstrates how torque ripple is reduced as the number of phases increased from three to five. This is very similar to adding more commutator segments per pole in a DC machine.

Aside from reduction of torque ripple, using four or five phases has another advantage in that multiple phases can be energized for short periods of time. However, as the number of phases increases, the cost and complexity of the drive also increase. The number of stator poles (N_s) must be a multiple of the number of phases. Some common configurations are shown in Table 2. Other options are possible, but these combinations are often found in practice.

To energize the appropriate phase winding while the rotor is within its torque zone as shown in Figure 5, the rotor position must be known with great accuracy. This is most often done by using rotor position sensors such as encoders or Hall effect sensors, which add cost and decrease reliability. Sensorless control is of great interest to SRM designers. A common approach is to use the inductance and stator current to estimate flux linkage but operating with the core near or in saturation makes this challenging. 

This material should be helpful for introducing switched reluctance motor basic operating principles, including speed and torque. Let’s review a few tips for dealing with these machines in the service center.

• Remember that the number of stator poles (slots) will be a multiple of the number of phases and that the number of stator poles and rotor poles will be different.
• Replace windings in-kind if a rewind is necessary. Winding resistance, insulation resistance, high-potential testing and impedance testing (or surge testing) are reasonable for winding assessments. 
• Absent manufacturer instructions, most typical mechanical procedures used for repair of induction motors are applicable to SRM, e.g., bearings, fits.
• An SRM cannot be operated without a special drive, which typically would need to be supplied by the end-user or the manufacturer.

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Categories: Technical topics, AC motors, Synchronous motors, Design/theory/application

Tags: Synchronous motors Reluctance motors Variable reluctance motors Switched reluctance motors

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