A Novel Concept of Short-Flux Path Switched Reluctance Motor for Electrical Vehicles

This paper deals with the design of a novel Switched Reluctance Motor (SRM) with short flux path for electrical vehicles. Design consists of the segmented water cooled stator with the toroidal winding and the rotor with salient poles also in a form of segments. The SRM dimensions have been calculated on the base of input requirements. The static and dynamic parameters of SRM are obtained from simulation models based on Finite element method and torque, power versus speed characteristics are presented.


Introduction
At the present time, the discussion about the electrical vehicles (EVs) development and improvement is very extensive. One of the main challenges to overcome is the price of this car. The EV consists of three basic parts: mechanical parts (similar with conventional car, also in the price range), fuel parts (in which the battery pack is the most expensive component of the car) and electrical equipment including electrical drive. The price of the drive depends on the electric motor used and its frequency converter and its control scheme.
As it is known, DC, AC and special electrical drives are used in EVs. Traditionally DC series motors were used because of their mechanical characteristics. These characteristics are suitable from the point of view trac-tion applications requirements. The development of power electronics caused the replacement of series DC machines by separately excited ones, whose field and armature windings are fed from two independent DC controlled sources and the mechanical characteristic can be adapted to traction ones [1]. This type of electrical drive is used in the first Slovak EV [2].
Nowadays, the best electrical drive solution seems to be PMSM because of very high power density to volume ratio. Still, the price of PMs and their limited operating temperature are the main disadvantages of this drive [3]. In many papers, Switched Reluctance Motor is mentioned as a promising drive alternative for EVs [4], [5], [6], [7]. SRM drive is becoming a strong competitor mainly to the conventional drives with IM. SRM has simpler construction, lower production costs, higher robustness and lower maintenance requirements as the IM drives. The SRM can be operated only from the frequency converter and it can operate in wider speed range than IM. The control of the SRM is also simpler as the vector control used for IM drives.
The profile of the SRM torque/speed characteristic looks similar to traction characteristic. This is an important reason for the use of SRM over IM drives. On the other hand, the drawbacks of SRM are its high torque ripple and the need for a rotor position sensor. As mentioned, the SRM construction is very simple. Stator and rotor have salient poles and only stator carries winding coils, which are connected to create a phase of the motor. The magnetic flux is provided by phase current to develop a reluctance torque [8], [9], [10], [11]. The cross-section of the proposed novel three-phase 12/8 SRM design is shown in the Fig. 1.
The aim of this paper is to point out the performance characteristics of this novel SRM design. The mathematical SRM model is described and solved to simulate different operation states to obtain output torque and output power/speed characteristics, which are compared and discussed. The input parameters are: rated power 50 kW, rated speed 5000 rpm, DC voltage 300 V, rated current 250 A and maximum current 400 A.
On the base of this analysis and the suitability of the SRM for EVs a prototype will be manufactured to replace existing DC drive in first Slovak EV.

Motor Dimensions
Design procedure of SRM is made with the required outer geometric dimensions of the existed DC motor. Maximal outer stator diameter d s and stack length l F e , were chosen: d s = 250 mm and l F e = 200 mm, for voltage level of 300 V. The number of stator and rotor poles is given as N s /N r = 12/8, the number of phases is m = 3. This topology was chosen due to lower torque ripple and motor ability to start up from every rotor position. A novel concept of SRM consists of segmented stator with concentrated windings and shorted magnetic flux paths. Dimensions of the motor are calculated on the known analytical equations used during common SRM design procedure. It is described in [3], [11] and [16].

Winding Coil Design
The calculation of the turns number N p can be made by assuming that at the specified speed the conduction angle of power transistors is equal to the stroke angle ε, which is defined as: If there is no current -chopping the peak flux linkage per phase is given by: where ω is angular velocity and V is DC supply voltage. At rated speed ψ peak occurs well before the aligned position, typically when the overlap between the stator and rotor poles is about 2/3 of the stator pole arc. At this moment, it can be assumed that the ampere-turns are sufficient to bring the stator pole to the flux density B s , then: Substituting of Eq. (2) to Eq. (3), where n c is number of coils per phase, l F e is stack length and t s is a thickness of stator pole. The flux density of stator pole can be assumed to be 1.6 T. The cross section of coil wire depends on normalized conductor wires available. The calculation is based on constant flux density (1.6 T).

Static Parameters Calculation
On the base of this design the static characteristics as phase inductance, magnetic flux linkage and torque versus phase current and rotor position have been calculated by means of FEM. The analytical approach is used only for geometrical dimension calculations. The results are in the Fig. 3, Fig. 4 and Fig. 5, respectively.

Dynamic Analysis of SRM
By using a mathematical model in the dynamic analysis we can calculate phase current, speed, voltage and dynamic torque of the SRM. The electromagnetic torque of SRM can be calculated from:   The voltage equation of one SRM phase is given as: where R is phase resistance, i is phase current and ψ is flux linkage. The flux linkage depends on both parameters: phase current and rotor position, ψ = f (i, Θ). Then: The phase current can be calculated by combining equations Eq. (6) and Eq. (7) as: The real angular speed is calculated from equation: where J is the moment of inertia and T load is the load torque. The SRM is controlled on the base of rotor position Θ, calculated as follows: On the base of this mathematical model a simulation model has been created to solve transients for different speeds, loads, switch ON, switch OFF angles to find the optimal dynamic operation of SRM. The dynamic simulation of currents and torque had been done for demanded speed 5000 rpm with the maximum current level of 400 A and 250 A for rated power of 50 kW. The current waveforms for all three phases for speed of 5000 rpm are shown in the Fig. 6 and Fig. 8. The total torque given by all three phase and the torque ripple detail is in the Fig. 7.
On this base of all required parameters, the output characteristics torque and power versus speed have been obtained from dynamic simulation. These characteristics are shown in the Fig. 10 and Fig. 11 for current up to 400 A.
The losses calculation can be carried out in accordance with [11] and [12]. As it is known, two dominant parts of losses are in electrical machines: winding losses and core losses. In this case both of them have been analyzed and calculated.   To calculate the efficiency, the input power is needed. It could be given from known equation for instantaneous power. On this base, the efficiency of designed SRM for various speeds from 1000 to 5000 rpm has been calculated. The results are from 89 % to 94 %, what are acceptable values for SRM.
This SRM design described above has been calculated on the base of analytical calculation and FEM without its optimization. The prototype model of this novel SRM design with segmented stator is shown in the Fig. 10 and Fig. 11. Table 1 shows the total mass of  the active parts of a novel SRM design. In the future, the thermal analysis will be carried out and some recommendations will be given for manufacturing process.
Tab. 1: Total mass of the active parts of a (the) novel SRM design.
Stator mass 21.5 kg Rotor mass 11.8 kg Winding mass 6.9 kg Total mass 40.2 kg

Conclusion
The paper deals with the SRM novel design and static and dynamic parameters investigation from the point of view of its application and using in electrical cars.
In the future, the optimization of this concept will be carried out to minimize materials and losses and to improve the efficiency of this motor. On this base a prototype of SRM will be manufactured and used in the real car.