Ev Motor Prius Rfl Comparison Paper Rfl Vs Prius Final 020510
Radial Flux L A B O R A T O R I E SElectric Vehicle Comparison Analysis Toyota Prius II vs. Radial Flux Laboratories
BackgroundRadial Flux Labs has a 15 year track record of electric car motor development. Starting with thesecond generation RFL design, the T-Flux, RFL developed a number of different standard and wheelmotors for a range of clients during the mid 1990’s. The T-Flux won over 40 awards and was heavilyused by organisations and universities involved in the 90’s electric and solar car industries.This design heavily relied on processes which were not established, requiring the development ofbespoke machines and tooling to allow for large scale production of the T-Flux design. To solve thisissue the next generation RFL design was developed focusing on manufacturability, but still deliveringthe benefits of the extremely successful T-Flux motor.The new design showed the same core benefits of the T-Flux, being less than half the weight andsize for the same output, having exceptional delivery of startup overload torque (300%+) andefficiencies in the mid to high 90% range.ComparisonTo highlight the characteristics of the RFL motor we have undertaken a comparison analysis of theToyota Prius Motor, the class leader. The data used in this comparison was obtained from 4 USDepartment of Energy reports into the Prius motor undertaken by the Oak Ridge National Laboratory.(Report references: ORNL/TM2004-185 , ORNL/TM-2004-247, ORNL/TM-2005/33,ORNL/TM-2004/137). This data was compared to results of testing of the RFL prototype undertakenin-house using NATA calibrated equipment.Motor Specifications Toyota Prius Gen II RFLMax Torque 339 Nm @ 250 Amps 340 Nm @ 250 AmpsNominal Max Power 50 Kw @ 1200 Rpm 45 Kw @ 1500 RpmTested continuous 21 Kw @ 1200 Rpm 29 Kw @ 3000 RpmPowerStator resistance & Poles 0.155 Ohms 8 Poles 16 Magnets 0.095 Ohms 18 Poles 18 MagnetsStator Length 83.56 mm 50 mmStator OD 270.0 mm 230 mmActive Material weight 79.5 Lbs( 36 Kg) 22.49 Lbs (10.2 Kg)Cooling Liquid Air or liquidTable 1
AnalysisThe RFL design for a comparable torque utilizes less than a third of the active material than the PriusTo give a clearer indication of the difference between the two designs we have undertaken thefollowing scaling exercise to indicate the output of the RFL design if it was of the same size as thatused in the PriusThe Prius II Motor has a 1.174 bigger Outside Diameter and is 1.671 longer than the RFL design.Output increases to the square of the Diameter x length, therefore the power increase =(1.174*1.174*1.671 ) = 2.303. This gives a RFL motor of the same size as a Prius ll a power output of(45*2.303 =) 104.5 Kw and a max torque of 783 Nm.In the US reports the ANL Chassis Dynamometer Power Flow Test showed that the Toyota Priusneeded 30 Kw from the engine and 10 Kw from the Electric Motor on acceleration. This is 40 Kw ofrequired power, most of which was required to be drawn from the combustion engine.When we look at the latest Toyota Prius Specification we find that the electric motor has gone up inpower output to 60 Kw while retaining the same torque. This has been undertaken to reduce the loadwhich the Prius II motor placed on the petrol engine under acceleration and improve the fuel efficiency(therefore reducing emissions).This highlights some of the current constraints in the industry, the inability to fit motors small andpowerful enough to hybrid and electric cars to free them from the real-time assistance of combustionengines which are required to run at variable speed thus significantly reducing their efficiency andtherefore increasing fuel consumption.The RFL design can achieve this with its exceptional torque, high overload start up and a lightweightand compact design. A car with a 104 Kw /780 Nm Motor could run without the real-time assistance ofthe combustion engine. The RFL design would allow a configuration which would enable thedevelopment of a Hybrid Car with a separate engine Generator and a separate drive Motor with nomechanical connection needed as in the Toyota Prius. This is the configuration that would deliver thegreatest fuel efficiency.The RFL design could achieve this increase in power for both the Motor and Generator in the samepackage and weight of the current designs. It would allow the RFL design to be used in cars thatfunctioned as plug in Hybrid’s, or battery only electric cars without much modification. The engine andgenerator could easily be replaced by batteries or a fuel cell, as these technologies become available.Alternatively as there is only a requirement for the combustion engine to charge the batteries and notprovide supplemental drive a much smaller high speed engine or small gas turbine could replace thecurrent engines to further reduce the weight.
Electric and Hybrid Car DesignsFigure 1 below outlines one of the ways the RFL design could be integrated into an electric or Hybrid car. The compact nature of the RFL design delivering104 kw of power and 780 Nm of torque facilitates a simple and robust design, the motor could be built into a differential as a separate unit for 4 wheel drivecars, or where the drive motor needs to be separated from the engine by some distance. Electric Car Hybrid Car Add-on 0 – 5000rpm 70-320Nm Inverter RFL motor Combustion RFL Eng. Batt Generator Controller 3.8:1 Differential Ratio 1200 Nm at the wheel
EMF Waveforms Figure 2 and 3 below show the back EMF waveforms of the two designs. The A + B phases of the Prius design exhibit very sharp waveform peaks, while the RFL’s waveforms have significantly ‘flatter’ peaks. This is consistent with a lower peak voltage, resulting in lower voltage stress on the winding and improving efficiency. Conclusion: To achieve its rated levels of output the Prius design has used significant amounts of copper and other active materials (Fig 4.). Rather than just increase the amount of active materials used the innovation of the RFL design is that it makes much more effective use of a smaller amount of material (fig 5.). This results in a staggering reduction of active materials of over 70%. Therefore if fitted in place of the Prius Motor in the same space and casing size the RFL would provide an output of 104 Kw with 780 Nm of maximum torque, improving vehicle performance both in range and acceleration/top speed, facilitating the simplification of power source design and a reduction in cost of Hybrid and electric cars. In addition its attributes of light weight and high power fit well with the requirements of in-wheel motors for cars or bikes. Phase A-B Phase AFig 2. Toyota Prius Motor - Back EMF Waveform Fig 3. RFL - Back EMF Waveforms Fig 4. Wound Stator Prius Motor Fig 5. Wound Dual Stator RFL
Technical Paper on RFL EV Motor Design ConceptsOptimized Motor and Controller Design for Electric Vehicle ApplicationThe design criteria influencing Electric Vehicle performance outcomes include thefollowing1. High Electrical efficiency across the desired rev range determines the distancetravelled on a given charge.2. High starting torque determines the start up acceleration capability and hill startand climbing capability3. High Energy Density enables a smaller and lighter wheel hub motor design4. High Regeneration energy recovery for battery recharge under brakingdeceleration1. Electrical EfficiencyThe RFL design is unique in having an efficiency curve (see Figure1.) that maintainshigh efficiency across a broad range of speeds and loads. This means the battery lifefor a given load and travel cycle will give a longer distance between charging.To achieve the best design for a electric vehicle motor the following analysis by the U.S.Department of Energy Freedom CAR and Vehicle Technologies is relevant to understandinghow the RFL reluctance –assisted design achieves the optimal battery life“3.2 OPTIMIZATION CONSIDERING ACTUAL LIFETIME OPERATING CYCLESSelection of the optimal reluctance-assisted PM motor configuration should not be basedonly on the steady-state performance curves. The anticipated lifetime operating cycle shouldalso be considered. For hybrid electric vehicles (HEVs), examples of two such standardoperating cycles that represent urban and highway driving averages are shown in Figs.25(a) and (b). The speed versus time trace of the Federal Urban Driving Schedule (FUDS)includes frequent stops and limited operation above 40 mph. In the Federal Highway DrivingSchedule (FHDS) there are no intermediate stops and the speed is seldombelow 40 mph. The electric traction motor’s speed is directly related to the vehicle’s speed;thus, the driving cycle characterizes trajectories in the electric motor’s efficiency and powermaps. The overall efficiency thus depends on the driving cycle. In addition, consideration ofregeneration during braking is important especially when the cycle includes frequentdecelerations.”2. Starting TorqueThe RFL patented dual stator design achieves a unique combination of high startingtorque and high efficiency as seen in the test results stemming from its low rotor andmagnet losses. The startup capability, especially when loaded with passengers, is afunction of torque. Alternative motors designs sacrifice efficiency for torque as seen
in the comparison with a small scale lab test against a Kelly motor in Fig 1. due totheir absence of reluctance torque which is in addition to the base permanentmagnet torque. Their design is typical of many motors for electric vehicles. This highreluctance torque is a unique feature of the RFL design3. High Energy DensityThe RFL design provides benefits of high energy density giving smaller weight andsize for a given output. The Radial Flux dual stator configuration is superior to thefractional slotted arrangement in this respect with typically 50% greater activematerial packing.4. Power RegenerationThe RFL Dual Stator design provide a higher efficiency conversion of braking powerregeneration due to lower no load losses. This provides greater battery rechargeparticular in the FUDS cycle which is expected to be the typical vehicle environmentTechnical AnalysisThis section reviews the RFL Dual Stator PM Motor by comparing it with 3 alternativecurrent types of motor designs. 1. Conventional Outside magnet PM BLAC Motor With Winding :Overlapping, concentrated Winding 2. Fractional Slot, Outside Magnet PM BLAC Motor with Winding: Non- overlapping, all teeth wound (11) (The Kelly 4.5 Kw wheel motor was used for comparison purposes) 3. A New Outer-Rotor Permanent-Magnet Flux-Switching Machine, PMFS (10)Design OverviewThe RFL Dual Stator is an IPM design with buried magnets which have theirmagnetic orientation tangential to the flux in the stator windings. It is wound withoverlapping, concentrated windings with a patented Wave winding methodology togive maximum slot fill and very short end winding.The Dual Stator design effectively reduces the stator length by half giving very highenergy density. The rotor design is very robust with very high reluctance torque, nodemagnetization under high current loads and excellent containment of flux andgood thermal properties.
Rotor and Magnet LossesThe RFL design has a conventional overlapping- concentrated winding arrangement.As a result the rotor is not subject to large rotor eddy current losses resulting fromthe 7th harmonics like the fractional slot machines. The result is a motor with lowerlosses at high current loads, such as starting torque and hill climbing.Iron LossesThe RFL design has very high energy density yet has lower pole numbers thanconventional motor design and fit into will fit into the same space as the fractionalslot designs without their high iron losses.The RFL’s lower running frequency from the lower pole number results in lowerstator core losses. This is especially evident at lower loads and at no load conditionssuch as downhill running when there is typically still battery drain. The RFL designreduces this level of battery drain.The flux switching machines have their Permanent Magnets in the stator which isorientated in the same plane as the RFL design, thus also having nodemagnetization under high loads,. However the magnets are subjected to very higheddy currents. This was not such a problem when using traditional ferrite magnets,but with the newer NdFb magnets now more commonl due to their higher energydensity which have low internal resistance. The eddy current losses in the magnetscan exceed the iron losses by a factor of 4. The fractional slot machines have notonly a higher running frequency but are subjected to losses associated with the high7th harmonics at higher loads. See Fig 1 for efficiency comparisonReluctance Torque:The RFL design has embedded magnets and an overlapping concentrated winding.This arrangement has high reluctance torque. It has a salience factor o>3, givinghigh efficiency under heavy overloads. The Fraction slot machines, and the FluxSwitching Machines generally have no reluctance torque and suffer from loss ofefficiency under high current loads. This effect is clearly evident in Fig 1 at torquesover the rated torque of 40 Nm. It can be seen that at torques over 40 Nm that theefficiencies start to diverge with the RFL design staying up and the Kelly efficiencydropping away due to the high reluctance torque of the RFL design.Manufacturing Costs:The fraction slot machines generally have the lowest cost due to their simplicity, butthey have one area which a potential EV problem. The design requires that themagnets are bonded to the inside of the rotor. These magnets are small and thin andrequiring careful handling. They need to be bonded in place. This is a messymanufacturing process and can be unreliable. There is also a potential for them tobecome loose from high vibration loads especially when used as wheel motors overa long period or in rough conditions such as potholed roads
The Flux switching machines have a complex stator construction with laminationmagnet sandwich segments, which need to be assembled and contained. As mostof the heat is in the stator, which in a wheel motor is on the inside, cooling willpotentially be a problem.The RFL design has less active material (Magnets, Lamination and Copper) andtherefore the raw material costs are lower. All the assembly process are standard,including the rotor which is a rigid bolted assembly, using large robust magnets.Although it has 2 stators to wind the Wave Winding process is simple and easilyautomated. Given good automation process design it should have a lowermanufacturing cost than more complex Fraction slot or Flux Switching designs whilegiving a superior performance.Conclusion: It can be seen that the RFL design fits the requirements for an EV Motor in allaspects. High Energy Density, High Efficiency over a broad range of loads andspeeds and high reluctance torque for low speed high torque running. Abstract:“This report contains a derivation of the fundamental equations used to calculate the basespeed, torque delivery, and power output of a reluctance-assisted PM motor which has asaliency ratio greater than 1 as a function of its terminal voltage, current, voltage-phase angle,and current-phase angle.The equations are applied to model Motor X using symbolically-oriented methods with thecomputer tool Mathematica to determine: (1) the values of current-phase angle and voltage-phase angle that are uniquely determined once a base speed has been selected; (2) theattainable current in the voltage-limited region above base speed as a function of terminalvoltage, speed, and current-phase angle; (3) the attainable current in the voltage-limitedregion above base speed as a function of terminal voltage, speed, and voltage-phase angle; (4)the maximum-power output in the voltage-limited region above base speed as afunction of speed; (5) the optimal voltage-phase angle in the voltage-limited region abovebase speedrequired to obtain maximum-power output; (6) the maximum-power speed curve which waslinear from rest to base speed in the current limited region below base speed; (7) the currentangle as a function of saliency ratio in the current-limited region below base speed; and (8)the torque as a function of saliency ratio which is almost linear in the current-limited regionbelow base speed.The equations were applied to model Motor X using numerically-oriented methods with thecomputer tool LabVIEW. The equations were solved iteratively to find optimal current andvoltage angles that yield maximum power and maximum efficiency from rest through thecurrent-limited region to base speed and then through the voltage-limited region to high-rotational speeds. Currents, voltages, and reluctance factors were all calculated and externalloops were employed to perform additional optimization with respect to PM pitch angle(magnet fraction) and with respect to magnet strength.
The conclusion was that the optimal-magnet fraction for Motor X is 0.72 which correspondsto a PM pitch angle of 130°, a value close to the maximum-saliency ratio in a plot of saliencyratio versus PM pitch angle. Further, the strength of Motor X magnets may be lowered to80% of full strength without significantly impacting motor performance for PM pitch anglesbetween the peak saliency (130°) and peak-characteristic current (160°).It is recommended that future research involve maximizing a driving-cycle-weightedefficiency based on the Federal Urban Driving Cycle and the Federal Highway Driving Cycleas criteria for selecting the final optimal-PM fraction and magnet strength for this inset PMmotor.Results of this study indicate that the reduction in PM torque due to reduced-magnetfraction will be more than compensated by the reluctance torque resulting from the highersaliency ratio. It seems likely that the best overall performance will require saliency;consequently, we think the best motor will be a reluctance-assisted PM motor.This should be explored for use with other types of PM motors, such as fractional-slot motorswith concentrated windings.”Above Quote from Modeling Reluctance-Assisted PM Motors.Report to: US Department of Energy. Freedom CAR and Vehicle Technologies By:Oak Ridge National Laboratory : P.J Otaduy, J.W. McKeever , Jan 2006.
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