Development and performance analysis of switched reluctance motor for E-rickshaw

Objectives: The main purpose of this study is to investigate switched reluctance motor for E-rickshaw propulsion system. Methods: The paper gives details of Literature review of current system incorporated in electric three wheeled propulsion, selection process of SRM, the design procedure of 8/6, 48V, 0.5 kW, 2000 rpm Switched reluctance motor for E-Rickshaw. Design validation and performance estimation is done by virtue of ﬁnite element analysis (FEA) tool using Ansys Maxwell Software. Findings: In this present work we found that it is possible to deploy SRM instead of current existing brushless direct current motors (BLDC) and PMSM (Permanent magnet synchronous motors) which are used in power train of electric three wheelers. Incorporating SRM would boost to promote “MAKE IN INDIA” and thus the country would become independent in electric vehicle market, hardware of the proposed SRM is also developed and tested. Novelty: The work done with investigation of SRM for E-rickshaw has not been studied until now although it possesses various advantages over existing system like high reliability and wide torque-speed range which is the basic requirement of propulsion motor in electric vehicles. SRM also provides several beneﬁts like simple construction, high speed, resilience and good fault tolerance capability as all phases function independently.


Introduction
Now a days there is a proliferation in the sector of electrification of vehicles especially 3-Wheelers as it is the most efficient mode of transport for "Last-Mile connectivity". Selection of electric motor is a very important parameter in an E-Rickshaw. Key E-Rickshaw variants of various regional and international players like Bajaj, Mahindra, Kinetic Green, KAL etc. are using BLDC (Brushless direct current) and PMSM (Permanent magnets synchronous motors) motors ranging from 1kW to 5kW for its propulsion (1) . These motors are having high efficiency, low torque ripple, less noise and vibrations (2) . However, less availability and high cost of rare-earthed magnets used in them make them an unsuitable long term option for E-Rickshaws. Considering the long term productivity and mass manufacturing of E-Rickshaws it is necessary to find a low cost, highly reliable and high power density motor. Currently SRM is looking promising as an alternative to BLDC and PMSM as it offers various advantages like small size, low cost, low inertia rotor, high reliability, No rotor loss, robust, easy cooling, high pickup and high operating speed etc. (3) . SRM is also eco-friendly as it involves no use of rare-earthed magnets and it reduces the dependency on the nations producing them. (4) . After studying various kind of literature on E-Rickshaw, one major research gap was recognized. E-rickshaws employing switched reluctance motor are not seen in the market. It is perceived that E-Rickshaw application incorporating a SRM will be seen commonly in the near future. There is a dire need in the market to progress in electrification of three-wheeler segments. Thus, 500 Watt switched reluctance motor prototype analysis is carried out to propose an ever important alternative to the existing motors for E-Rickshaw application. This prototype is developed and studied by taking into account of a L5 category E-rickshaw.
The principle of operation of switched reluctance motor is to produce electromagnetic torque by varying magnetic reluctance. SRM is a doubly salient machine. It is also a single excited machine (5) . The switched reluctance motor works best when saliency ratio (L d /L q ) is maximum. L d = inductance in d-axis (here rotor is aligned with the poles) and L q = inductance in q-axis (here rotor is aligned with the gaps). In SRM, the stator poles carry the winding wherein the rotor is constructed of steel stampings. Thus, there is no rotor loss due to absence of winding. (6) It is an electronically switched motor i.e. it requires a SRD (switched reluctance drive) for its operation. When one of the stator poles (N s ) is excited, nearby rotor pole (N r ) tries to achieve the least reluctance position i.e. the output torque because of current flow in phase winding tends to shift the rotor in such a way so that it guides to curtailment in reluctance and eventually rise in inductance. (7) Torque produced by machine when magnetic saturation neglected is given by: From above equation (1) it can be understood that torque is independent on polarity of stator current and can only be generated when inductance changes. Also, if N s and N r are aligned then starting torque could not be produced. Therefore, they have dissimilar number of stator, rotor poles. In SRM, the phase winding is connected in series with the switch. Thus after any occurrence of fault in converter circuit, the current rise can be restricted by an inductance of winding. Therefore, short time is required to operate protective relaying to detach the circuit. And in comparison with other motors, SRM has independent phases so, if a fault occurs to any of its phases, the motor and its drive are capable to carry on with the operation but with decreased power output. (8) But still SRM has got some disadvantages like high torque ripple, high noise, vibrations and necessity of position sensor. Hence, it requires better, modern control strategy than AC and DC motor drives. (9) . Detailed analysis of SRM design procedure is carried out and research gap is addressed in this paper. This paper is arranged in following way: section II focuses on modeling of SRM, section III discusses the selection of switched reluctance motor, section IV deals with design approach of SRM for E-rickshaw, section V includes simulation results and section VI has concluding remarks.

Modeling of SRM
The equivalent circuit of single phase switched reluctance motor is shown in Figure 1 Where, R m = Resistance per phase L = Inductance which is dependent on rotor position and phase current λ = Flux linkage λ = L (θ , i)i In above equation (3), the term on right side shows the induced emf & resistive, inductive voltage drop, 'e' is EMF induced, Input power is as follows, Here, the last term is physically not interpretable; to draw a meaningful inference, it may be cast in terms of known variables as in the following: From equation 4 and 5 we obtain, Where, P i is input instantaneous power The power of air gap is, The air gap power (P) is nothing but a product of torque (T) and rotor speed.
Therefore, the torque equation is expressed as, This is the method to develop switched reluctance motor.
https://www.indjst.org/  (5) In all topologies two opposite poles constitutes one phase. Eg.10/8 topology has 5 phases whereas 8/6 SRM has 4 phases and so on. For electric vehicle application mainly 10/8 and 8/6 topology is used instead of 6/4. (10) 10/8 topology is costly as it requires more number of switches in its drive wherein 8/6 has a problem of high torque ripple. This problem may eventually be eliminated with proper tuning of motor. SRM topology is selected on the basis of application. (11) Figure 3 shows the most popular switched reluctance drive which uses Conventional converter (also called classic or Asymmetric bridge converter). Other Power electronic converters (PEC) are n+1 PEC, C-dump PEC etc. (12) . SRM topology and drive system can not only be deployed in three wheeled vehicle but also two and four wheeler segment can use for its propulsion system.

Factors for selecting motor for E-rickshaw
1. Good power density 2. Fast torque response 3. Low weight 4. Reasonable cost 5. Fault tolerant Currently, SRM is being used the electric and hybrid four wheelers but implementation of SRM in electric three vehicle segment has not been tested as around 90 % of E-rickshaws of various regional and international manufacturers are deploying BLDC and PMSM in their variants. (13) The motor rating ranges from 1kW to 5kW for three wheeler traction. (14) Hence, this research gap of incorporating SRM in E-rickshaw has been addressed in this article. For analysis purpose, a prototype of 500 W switched reluctance motor is selected and designing procedure, its simulation and hardware results is discussed in upcoming sections.

Mathematical design procedure
After fixing the initial parameters the various motor variables which are an integral part in designing a motor can be found out as discussed below: • First step is to find Torque required by the motor, The machine will be designed for a standard Frame size = 63 mm given according to IEC recommendations.
• Outer diameter of stator can be given by, • Stator pole flux (φ) is specified as, • The flux in the yoke (Ø y ) is stated as, https://www.indjst.org/ • Thus, yoke area (A y ) can be noted as, • The back iron thickness is obtained as, • Height of stator pole (h s ) is estimated as, • Rotor pole area (A r ) is given by, • Flux density of rotor pole (Br) is given by, • Rotor core area (A rc ) is given by, • Rotor pole height (h r ) is obtained as, • Air gap area can be estimated as, • Air gap flux density is stated by, • Magnetic field intensity of air gap is, https://www.indjst.org/  Figure 6 shows the 8 pole stator schematic obtained from RMxprt tool of Ansys Maxwell software. Figure 7 shows the 6 pole rotor schematic obtained from RMxprt tool. Various important constructional parameters like stator yoke thickness, outer diameter of stator and rotor, position of shaft diameter etc. are visible.

Simulation results of SRM
The same prototype motor (500 W, 48V) is simulated with Ansys Maxwell RMxprt tool and results are discussed below.

Machine specification
The specification of selected machine is given in Table 1 https://www.indjst.org/   Effect of different stator and pole angles: ßs and ßr should be propely selected in order to get desired output torque,efficiency and minimal losses. For proposed machine, it should be noted that ßs = 22 0 and ßr = 24 0 , the important parameters like output torque,efficiency,losses and speed are obtained as per our requirement. (Table 2 ) Effect of stack length on efficiency, torque,speed and losses. From the analysis an observation is made that for stack length 100 mm we get maximum efficiency, minimum losses and a very close value to the average torque. (Table 3 ) Effect of stacking factor on SR motor paramerts. Stacking factor should not exceed 1 so different stacking factor is taken and system is analysed .and thus, observed that stacking factor = 0.95 should be taken in order to get desired efficiency,less losses https://www.indjst.org/ Ansys Electronics desktop is used for finite element analysis and general data, No-load and full load operation data of 500 W SRM is obtained from the RMxprt module of the software. This designed sheet gives information of motor operation like No-load,Full load operation of motor,total consumption of material and copper consumption of winding can also be obtained. In general data,above specified parameters are to be keyed in by the designer while initializing the simulation. With the motor stator and rotor data, the material consumption is calculated and the results are as given in Figure 12 . In Figure 13 various important parameters like output power, efficiency, rated torque etc. can be obtained.

Simulation results
Some important graphs from our proposed SRM motor (500W,48V) are obtained and discussed below.
https://www.indjst.org/     Figure 17 represents the hardware of SRM developed and it is tested with variation of load and observations are presented in Table 5 .

Developed hardware model details
Hardware model is developed and its important parameters like torque, speed and efficiency are compared.
https://www.indjst.org/  Figure 19 , the obtained efficiency at rated speed 2000 rpm is 60%. After examining all the parameters some important parameter which are obtained from simulation, mathematical calculation and actual results got from motor after performing tests is compared and concluded with Table 6 . Simulated efficiency is 76.97% but it is likely to be 10 % less due to presence of gear box system. After performing routine test it is observed that the efficiency at rated speed 2000 rpm is around 60 % as there is a mechanical system present between motor and wheels.

Conclusion
In this study, switched reluctance motor for low powered electric three wheelers is presented and analyzed. Mathematical model has been developed and it is compared with the simulation results carried out from Ansys Maxwell software which is a FEA tool. With help of these results SRM hardware has been also developed and results are presented. The proposed system has advantages like small size, low cost, low inertia rotor, high reliability, No rotor loss, robust and most importantly no use of rare earthed magnets. As per the finite element analysis and design calculations, the proposed motor is 0.5 kW, 2000 rpm,48 V,8/6 pole construction which develops average torque up to 2.35 Nm and is able to operate at efficiency of 60-65%. Although SRM for E-Rickshaw possesses certain limitations like torque ripple and acoustic noise, they can be removed by proper tuning. In the coming future SRM incorporated E-Rickshaws would be seen commonly as it fits the bill in terms of efficient, reliable and ecofriendly execution of electric three wheeler segment. Implementation of switched reluctance motor would be a great alternative to current BLDC and PMSM motors for E-Rickshaw application. Also it would save millions in terms of CAD (Current account deficit). Switched reluctance motor could pave the way in intensifying the electrification in 3-Wheeler sector and achieving the targets under Go Electric campaign sooner than later. Use of such ecofriendly vehicles will eventually reap benefits for our diversity in future. Also, this designing approach made in this paper to determine the various important parameters of SRM prototype can further be helpful in designing the actual rating motor required for E-Rickshaw application.