2015 Seventh International Conference on Computational Intelligence, Modelling and Simulation
Electric Propulsion Unit Powered by Switch Reluctance Machine SRM Farukh Abbas, Sun Yingyun, Usama Rehman North China Electric Power University, Beijing, China farukhabbas32@gmail.com,sunyingyun@gmail.com,usama.rehman21@gmail.com
For designing mechnical part the weight is taken into account for better performance by choosing light weight metal and structure and for machine the reference of a DC machine alread used for wheelchairs is taken with rated power of 1.2kw, rated torque 3.4Nm and rated speed of 3400r/min, having a gear mechanism increasing torque by ratio of (1:20), having 24vdc power supply and drain 80A from the converter.
Abstract— the paper deals with designing and fabrication of a light weight (one man) electric vehicle prototype powered by switched reluctance machine SRM and controlled by half bridge low-voltage high-current converter. The machine will provide propulsion to drive light weight electric vehicle. The idea is to utilize the vehicle for disable persons by utilizing the methodology with advance design. The paper reveals the mechanical design specifications, control mechanism for SRM, different signals of power converter is analyzed using digital oscilloscope and discussion on the weight, speed and price/distance of prototype is given. The aim at the moment is to design, construct and perform an experimental run of a prototype in order to get a more comprehensive idea of the mechanical design and the driving circuitry.
Inorder to test a prototype is made and tested on road and result for the speed, miliage,driving circuirtry is recorded. II. STRUCTURAL DESIGN The main challenge in fabricating an effective vehicle chassis is to increase the strength and safety, but the weight should be minimized. Every extra kilogram requires more energy to move on the road. This means we must struggle to minimize weight and a key area is the metallic chassis. However, safety is a main concern and the chassis must meet appropriate strength and safety requirements. Usually, there are three types of chassis structure used in electric vechicles: space frame, semi monocoque, monocoque[8].
Keywords-SRM, design, SRM converter, experimental run.
I. INTRODUCTION The transport needs of our ever growing and changing society is becoming increasingly complex and more demanding.The need is to make more efficient and low cost propulsion units. A large number of propulsion systems and renewable energy sources have undergone research studies to find potential commercial and industrial applications of electric vehicle. Some projects have shown to work successfully, while other technologies are still well in their initial stage of development.Permanent magnet(PM) based designs are popular but have some international market problems[1].Induction machine(IM) on the other hand have no slip rings and PM, but its control is sometimes difficult due to real time calculation of slips and IM has high manufacturing price and complex structure than switched reluctance machine(SRM).SRM is cheap,efficient , endure electrical and mechinical stress and have simple control mechanism.
A space frame uses a welded or attatched tube type structure to support both body and load. The body is a light weight part, bearing no load, shell composite that is attached to the chassis independently. The carbon beam or semi-monocoque type chassis uses bulkheads and composite beams to support the loads part and is integrated into a type(non load bearing) composite belly pan. The upper sections of the car are usually separate body parts that are bolted to the belly pan. A monocoque type chassis ususlly uses the body structure to support the loads part. Using these three types of chassis we can make strong lightweight vehicles. The design used is a collective combination of the chassis types mentioned here. The design specification is given below in Figure 1(A, B, C) and Figure 2.
The design process of entire vehicle consists of two parts: one contains the design specification of mechanical structure(Chassis, Outer shell), the other part consists of the design specification of electronic power converter. The goal is to fabricate a prototype and test the vechicle on the road ensuring that the controller perform requird control strategy. 2166-8523/15 $31.00 Š 2015 IEEE DOI 10.1109/CIMSim.2015.19
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A
A
B B
C
Figure 3. Showing a three dimensional model.
Outer Shell is made of carbon fiber or glass Fiber with the polycarbonate or acrylic windscreen. A threedimensional model is given in Figure 3(A, B). In our design we are used knuckles steering system. It is a type of steering system in where the axles themselves are use to control movement, which are attached on a knuckles, out and away from the go kart, and they are actually rotating around t pivots, and makes the wheels to turn. Braking system; the braking should be done through simple spring loaded disk brakes on both front and rear wheels. The overall weight of the vehicle consists of: chassis(8kg), carbon fibre outer shell(15kg), gear box-driving seatwheels-stearing-break mechanism(18.5kg), with motorbatteries and driving circuitry the overall weight of vehicle is approximatly 60Kg.
Figure 1. Shows in meters ,a(top view),b(front view),c(side view).
III. THE SRM AND ITS CONTROL CIRCUITRY As the interest is to develop a propulsion unit for light weight vehicle, the first step is to choose the electrical machine, the second step is the designing and building of the power converter to run the machine and have desired output power. This section will focus on some main design and control strategy related to the SRM[7]. Figure 2.Placement of equipment in structure.
A. SRM specifications The SRM for such propulsion system we chose SRM with respect to their stator to rotor number of poles ratio. A machine with 6/4 ratio is financially easy to build and control, but have high torque ripples. The chosen machine is an 8/6 ratio machine.
Chassis is made by Aluminum - Square (1’x1’) rods or rectangular (2’x1’) rods of 3mm (millimeter) gauge or with a combination of pipes and rods depending on the strength required at the particular support.
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The parameters of SRM are given in table I. The parameters consist of: the machine’s rated power (P2N), rated current (I), ), the rated speed (nN)and the rated torque (TN).
B
TABLE I. SRM SPECIFICATIONS
Chosen Parameters(SRM) Battery voltage Current imposed Rated torque Rated speed Output power Stator poles(numbers) Rotor poles (numbers)
Rated Value 24 80 3.4 3400 1.2 8 6
Unit specification [V] [A] [Nm] [r/min] [kW] -
Figure4. FEA simulated result on rated power of SRM
The DITC strategy directly controls the torque by using a double hysteresis band , computing the firing moments of the switches function of the imposed torque.It involves switching the supply voltage between positive DC, 0 and negative DC for each phase in order to shape the individual torque produced by each phase in such way that the overall torque is possibly close to straight line. In Figure 5 it can be seen that using this strategy the torque is smooth. This strategy is feasible for our desired process.
-
B. Control Srategy of SRM SRM has a disadvantage regarding the torque ripples as the current is switched from one phase to the other during operation [2]. This problem puts SRM in a position that is not suitable for propulsion system like electric vehicle, the other problem is related to mechanical stress of the transmissions.
A
Dicussing two methods to control SRM: the classical hysteresis current controller, direct instantaneous torque control(DITC) [3]. In classical hysteresis current controller further advance studies need to be done in order to have smooth torque. it can be seen in simulation in Figure 4(A, B) that using the classical hysteresis control of the currents the torque ripples are quite high, Simulation is done using finite element analysis (FEA) - using FLUX 2D software. This method is not feasible for our application.
B
A
C
Figure5. DITC topology, A (Instantaneous torque), B (Adjacent phase voltage), C (phase torque).
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IV. CONVERTER (2)
The required converter consists of following specifications: dc bus voltage of 24V, phase current of 80A and switching frequency of 30 to 60 kHz. Separate power stages is needed for each phase driven by independent driver.
Here, Crss is the reverse transfer capacitance and RdV is the established voltage rise slope.For external additional gate resistance, putting value from equation 2 to equation 3.
A. Components (3)
One half H-bridge configuration for each phase is used, power stage consist of twoIRFP4886PBF power switches on one diagonal of the bridge and the freewheeling diodes in pairs of two are in parallel for desired current achievement. The diodes type 60APU04PBF of 60A rating placed in parallel. For safe operation, the input of the driver is isolated using a high speed optocoupler. For a half H-bridge firing the power switches is complicated because of the floating side capacitor of driver (bootstrap capacitor), that is need to be charged through load. A direct connection between the Cboot (bootstrap capacitors) and the phase of the machine makes this a serious problem. Moreover highly variable inductance of machine, resonance can also damage the driver. So, in order to charge bootstrap capacitor independently from load MOSFET type transistors are used to make sure the charging of capacitor during off stage of switches.
Here, RDRV is driver’s internal resistance. From equation 3 the calculated value for on resistance is 32.5ohm, so a resistance of 33ohm is placed in series with the gate of the power switches. Using too high resistances in series with the gate would eliminate the voltage spikes when switching to transistor OFF state. However, the transient section is affected, having increased heat dissipation and reducing switching frequency range. Using a too low valued resistance will increase transient state voltage spikes (ON-OFF states). These spikes can destroy the power switches, reaching voltages over their absolute maximum values. B. Performance test Analyzing hysteresis current control strategy on data acquisition based oscilloscope(channel1-red,channel2purple,channel3-green,channel4-blue), few tests are done with detail as follows: Currents on the four phases of the machine are analyzed (during operation) at different rotational speeds and different amplitudes. Figure 6 shows all the four currents at switching frequency reaching about 40 kHz. Using four channels (four phases), Vertical 1V/div, Horizontal 500 s.
The minimum capacitance is calculated as follows: equation 1 [4].
(1) where: Qg, gate charge of high side FET, I(qbs) quiescent current for high side driver circuitry, Q(ls) level shift charge required per cycle, I(cbs - leak) Bootstrap capacitor leakage current, fs is the minimum switching frequency, Vcc is the driver’s power supply voltage, VF the bias voltage of the internal diode, VLS the voltage drop on the drain to source circuit, Vmin is the minimum source voltage. From the equation 1 the bootstrap capacitance calculated is 1.2μf. The bootstrap capacitance consists of two capacitors as follows: equivalent system resistance (ESR) for fast recovery, electrolytic one in parallel. Protect circuit even in case of voltage spikes.
Figure6. Current (each of four phase)
The current and the voltage of one phase of the converter is shown in figure 7, having a fast current increase to achieve the desired result. In order to maintain the current (within hysteresis band), voltage switching is done between positive and negative DC values. Due to back EMF and selfinductance the current creates a negative voltage for small interval. In figure 7 channel1 (Vertical 2V/div) and channel 2 (Vertical 20V/div), Horizontal (200μs/div). All main signal are shown in figure 8. Channel 1 showing phase current, channel 2 showing output of
Another problem, when dealing with high currents at high frequencies switching, is having external gate resistor with correct value. It is calculated as [4], the total gate circuit resistance (internal and external) is given by equation 2.
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optocoupler, channel 3 showing gate to source voltage, channel 4 showing the phase voltage. When switching at high frequencies with high value of current, the problem of spikes occurs. These can be minimized by proper gate resistances and by using snubbers and proper circuit layout. In figure 8 Vertical divisions (channel (1) 1V/div, channel (2) 10V/div, channel (3) 10V/div, channel(4) 50V/div), Horizontal(10μs/div).
The voltage spike during switching off of transistor is shown in figure 9. A short interval voltage spike of 7V amplitude for DC link of 24V. In figure 9: channel1 (phase current) vertical (1V/div), channel 3(gate to source voltage) vertical (1V/div), channel 2(phase voltage) vertical (10V/div), horizontally 2μs/div. V. VEHICLE TEST The experimental run of a prototype is done in order to get a more comprehensive idea of the design performance of the chassis and the driving circuitry, this would enable us to point out the short-comings and the problems in the design and improve our design before constructing the final thing. Table II shows some parameters analyzed by running the prototype. TABLE II. ROAD TEST PARAMETERS
Parameters
Values
Units
Speed Vehicle(max)
of
34.4
Km/h
Weight vehicle
of
130
kg
24
V
118.6
km
Figure7. Current and voltage (converter phase)
Batteries Distance covered(one charge)
If batteries are charged by simple silicon controlled rectifier SCR of 10A rating and approximate ten hours required to completely charge two 12V-18Ah sealed rechargeable lead acid battery then power consumed in Kwh with a distribution system of 220V will be 22kWh given by equation 3[5].
Figure8. Main converter signals.
Figure10. Test vehicle
Energy (kWh) = wattage× hours of day
(3)
Figure9. Switching off transistor voltage spikes
For Pakistan the average price per kWh is 13.99[6].
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Total price for one charge will be Rs305.8 covering distance of 118.6 km. So per kilometer the price will be Rs2.58 ($0.026).The prototype constructed is shown in figure 10.
ACKNOWLEDGMENT This work is supported by grant from Pakistan Navy Engineering College. REFERENCES
VI. CONCLUSION [1]
The paper deal with the design, analysis and fabrication of light weight electric vehicle using low voltage, high current SRM drive, for controlling SRM we use hysteresis and direct instantaneous torque control and use half Hbridge configuration. In this paper details regarding the mechanical design and SRM are given and details regarding control strategy are given. The vehicle is fabricated and experimental run is done, the final prototype and price detail is given. The light vehicle with SRM machine is suitable for application in wheel chair technology or can be taken as commercial vehicle for short distance travel. Further with advance storage technology better performance can be achieved. Electric and hybrid powered vehicles are emerging as a popular transport mean.These types of vehicles emit less pollutants to the atmosphere and are more environment friendly than the single internal combustion engine, and have been proven to display cosiderable driving range.
[2]
[3]
[4] [5] [6]
[7]
[8]
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