*2.1. Pole Selection*

The number of stator poles *Ps* and the number of rotor poles *Pr* are usually selected based on previous experience with the application requirements and converter configuration to be used. The combinations of stator and rotor poles are of few choices for good overall design of SRM to be used in general; however, special applications may lead the designer to explore more of less frequent combinations to achieve the application requirements. This paper primarily focuses on the popular combination of 6/4 and 8/6 machines. The 6/4 machine has the advantage of using less switches in the converter, two less terminals and less core losses because of less switching losses than 8/6 machine; however, it has the disadvantage of higher torque ripple than the other common combination of (8/6 machine).

**Figure 1.** Design process flow chart.

*2.2. Rotor and Stator Poles Arcs Selection*

> Referring to [2], the minimum stator pole arc to achieve self starting :

$$
\min[\beta\_s] = \frac{4\pi}{P\_s P\_r},
\\
\text{rad.}\tag{1}
$$

The angle between the corners of adjacent rotor poles must be greater than the stator pole arc or there will be an overlap between the stator and rotor poles in the unaligned position. This condition is represented as:

$$
\beta\_s + \beta\_r \le \frac{2\pi}{P\_r}.\tag{2}
$$

The implication of this condition not being followed is that the machine will start having a positive inductance rate of change before reaching the minimum value. This causes the unaligned inductance value to be higher and leads to a lower torque generation.

## *2.3. Main Dimensions*

For the dimensions shown in Figure 2 and Table 1, the outer diameter (*Do*) is determined by the available space in the application. The shaft diameter (*Dsh*) is obtained from the shaft's standard sizes. The outer diameter(*Do*) and (*Dsh*) are fixed and they are not changed while searching for the suitable design since they are space constraints. However, the axial length (*L*) and the bore diameter (*D*) can be changed during the design process (Note that the axial length increase is limited by the maximum axial length available in

the application). The air gap length(*g*) can be changed too but here it is fixed of 0.5 mm. The remaining dimensions are to be changed in order to reach the desired value of torque, efficiency ··· and so forth.

**Table 1.** SRM dimensions.


**Figure 2.** Lamination dimensions considered in optimization process.

## *2.4. Limits of Variables*

The limits of variables depend on the application and the available space. Here, the frame size, shaft diameter, air gap length and axial length are kept constant by making their limits at the same value. The rest of the limits are set by the previous experience. Table 2 shows the maximum and minimum values of all variables.


**Table 2.** Limits of variables.
