Radial FluxL A B O R A T O R I E S

Electric Vehicle Comparison Analysis

Toyota Prius II vs. Radial Flux Laboratories

Background

Radial Flux Labs has a 15 year track record of electric car motor development. Starting with the second generation RFL design, the T-Flux, RFL developed a number of different standard and wheel motors for a range of clients during the mid 1990’s. The T-Flux won over 40 awards and was heavily used 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 of bespoke machines and tooling to allow for large scale production of the T-Flux design. To solve this issue the next generation RFL design was developed focusing on manufacturability, but still delivering the 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 and size for the same output, having exceptional delivery of startup overload torque (300%+) and efficiencies in the mid to high 90% range.

Comparison

To highlight the characteristics of the RFL motor we have undertaken a comparison analysis of the Toyota Prius Motor, the class leader. The data used in this comparison was obtained from 4 US Department 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 undertaken in-house using NATA calibrated equipment.

Motor Specifications

Toyota Prius Gen II RFL

Max Torque 339 Nm @ 250 Amps 340 Nm @ 250 Amps

Nominal Max Power 50 Kw @ 1200 Rpm 45 Kw @ 1500 Rpm

Tested continuous Power

21 Kw @ 1200 Rpm 29 Kw @ 3000 Rpm

Stator resistance & Poles 0.155 Ohms 8 Poles 16 Magnets 0.095 Ohms 18 Poles 18 Magnets

Stator Length 83.56 mm 50 mm

Stator OD 270.0 mm 230 mm

Active Material weight 79.5 Lbs( 36 Kg) 22.49 Lbs (10.2 Kg)

Cooling Liquid Air or liquid

Table 1

Analysis

The RFL design for a comparable torque utilizes less than a third of the active material than the Prius To give a clearer indication of the difference between the two designs we have undertaken the following scaling exercise to indicate the output of the RFL design if it was of the same size as that used in the Prius

The 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 Prius needed 30 Kw from the engine and 10 Kw from the Electric Motor on acceleration. This is 40 Kw of required 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 in power output to 60 Kw while retaining the same torque. This has been undertaken to reduce the load which 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 and powerful enough to hybrid and electric cars to free them from the real-time assistance of combustion engines which are required to run at variable speed thus significantly reducing their efficiency and therefore increasing fuel consumption.

The RFL design can achieve this with its exceptional torque, high overload start up and a lightweight and compact design. A car with a 104 Kw /780 Nm Motor could run without the real-time assistance of the combustion engine. The RFL design would allow a configuration which would enable the development of a Hybrid Car with a separate engine Generator and a separate drive Motor with no mechanical connection needed as in the Toyota Prius. This is the configuration that would deliver the greatest fuel efficiency.

The RFL design could achieve this increase in power for both the Motor and Generator in the same package and weight of the current designs. It would allow the RFL design to be used in cars that functioned as plug in Hybrid’s, or battery only electric cars without much modification. The engine and generator 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 not provide supplemental drive a much smaller high speed engine or small gas turbine could replace the current engines to further reduce the weight.

Electric and Hybrid Car Designs

Figure 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 delivering 104 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 drive cars, or where the drive motor needs to be separated from the engine by some distance.

Electric Car Hybrid Car Add-on

Fig 1

0 – 5000rpm70-320Nm

Batt

Inverter

RFL Generator

Combustion Eng.

Controller

1200 Nm at the wheel

RFL motor

3.8:1 Differential Ratio

Vehicle Characteristic

0 – 1300 RPM Motor Speed

Speed 0 – 130 Kph

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.

Fig 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

Phase A-B Phase A

Technical Paper on RFL EV Motor Design Concepts

Optimized Motor and Controller Design for Electric Vehicle Application

The design criteria influencing Electric Vehicle performance outcomes include the following

1. High Electrical efficiency across the desired rev range determines the distance travelled on a given charge.2. High starting torque determines the start up acceleration capability and hill start and climbing capability 3. High Energy Density enables a smaller and lighter wheel hub motor design4. High Regeneration energy recovery for battery recharge under braking deceleration 1. Electrical EfficiencyThe RFL design is unique in having an efficiency curve (see Figure1.) that maintains high efficiency across a broad range of speeds and loads. This means the battery life for 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 understanding how the RFL reluctance –assisted design achieves the optimal battery life

“3.2 OPTIMIZATION CONSIDERING ACTUAL LIFETIME OPERATING CYCLES

Selection of the optimal reluctance-assisted PM motor configuration should not be based only on the steady-state performance curves. The anticipated lifetime operating cycle should also be considered. For hybrid electric vehicles (HEVs), examples of two such standard operating 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 Driving Schedule (FHDS) there are no intermediate stops and the speed is seldom

below 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 power maps. The overall efficiency thus depends on the driving cycle. In addition, consideration of regeneration during braking is important especially when the cycle includes frequent decelerations.”

2. Starting Torque

The RFL patented dual stator design achieves a unique combination of high starting torque and high efficiency as seen in the test results stemming from its low rotor and magnet losses. The startup capability, especially when loaded with passengers, is a function 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 to their absence of reluctance torque which is in addition to the base permanent magnet torque. Their design is typical of many motors for electric vehicles. This high reluctance torque is a unique feature of the RFL design

3. High Energy Density

The RFL design provides benefits of high energy density giving smaller weight and size for a given output. The Radial Flux dual stator configuration is superior to the fractional slotted arrangement in this respect with typically 50% greater active material packing.

4. Power Regeneration

The RFL Dual Stator design provide a higher efficiency conversion of braking power regeneration due to lower no load losses. This provides greater battery recharge particular in the FUDS cycle which is expected to be the typical vehicle environment

Technical Analysis

This section reviews the RFL Dual Stator PM Motor by comparing it with 3 alternative current 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 Overview

The RFL Dual Stator is an IPM design with buried magnets which have their magnetic orientation tangential to the flux in the stator windings. It is wound with overlapping, concentrated windings with a patented Wave winding methodology to give maximum slot fill and very short end winding.

The Dual Stator design effectively reduces the stator length by half giving very high energy density. The rotor design is very robust with very high reluctance torque, no demagnetization under high current loads and excellent containment of flux and good thermal properties.

Rotor and Magnet Losses

The RFL design has a conventional overlapping- concentrated winding arrangement. As a result the rotor is not subject to large rotor eddy current losses resulting from the 7th harmonics like the fractional slot machines. The result is a motor with lower losses at high current loads, such as starting torque and hill climbing.

Iron Losses

The RFL design has very high energy density yet has lower pole numbers than conventional motor design and fit into will fit into the same space as the fractional slot designs without their high iron losses.

The RFL’s lower running frequency from the lower pole number results in lower stator core losses. This is especially evident at lower loads and at no load conditions such as downhill running when there is typically still battery drain. The RFL design reduces this level of battery drain.

The flux switching machines have their Permanent Magnets in the stator which is orientated in the same plane as the RFL design, thus also having no demagnetization under high loads,. However the magnets are subjected to very high eddy 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 energy density which have low internal resistance. The eddy current losses in the magnets can exceed the iron losses by a factor of 4. The fractional slot machines have not only a higher running frequency but are subjected to losses associated with the high 7th harmonics at higher loads. See Fig 1 for efficiency comparison

Reluctance 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, giving high efficiency under heavy overloads. The Fraction slot machines, and the Flux Switching Machines generally have no reluctance torque and suffer from loss of efficiency under high current loads. This effect is clearly evident in Fig 1 at torques over the rated torque of 40 Nm. It can be seen that at torques over 40 Nm that the efficiencies start to diverge with the RFL design staying up and the Kelly efficiency dropping 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, but they have one area which a potential EV problem. The design requires that the magnets are bonded to the inside of the rotor. These magnets are small and thin and requiring careful handling. They need to be bonded in place. This is a messy manufacturing process and can be unreliable. There is also a potential for them to become loose from high vibration loads especially when used as wheel motors over a long period or in rough conditions such as potholed roads

The Flux switching machines have a complex stator construction with lamination magnet sandwich segments, which need to be assembled and contained. As most of the heat is in the stator, which in a wheel motor is on the inside, cooling will potentially be a problem.

The RFL design has less active material (Magnets, Lamination and Copper) and therefore 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 easily automated. Given good automation process design it should have a lower manufacturing cost than more complex Fraction slot or Flux Switching designs while giving a superior performance.

Conclusion: It can be seen that the RFL design fits the requirements for an EV Motor in all aspects. High Energy Density, High Efficiency over a broad range of loads and speeds and high reluctance torque for low speed high torque running.

Abstract:“This report contains a derivation of the fundamental equations used to calculate the base speed, torque delivery, and power output of a reluctance-assisted PM motor which has a saliency 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 the computer 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) the attainable current in the voltage-limited region above base speed as a function of terminal voltage, speed, and current-phase angle; (3) the attainable current in the voltage-limited region 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 a function of speed; (5) the optimal voltage-phase angle in the voltage-limited region above base speed required to obtain maximum-power output; (6) the maximum-power speed curve which was linear from rest to base speed in the current limited region below base speed; (7) the current angle 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 region below base speed. The equations were applied to model Motor X using numerically-oriented methods with the computer tool LabVIEW. The equations were solved iteratively to find optimal current and voltage angles that yield maximum power and maximum efficiency from rest through the current-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 external loops 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 corresponds to a PM pitch angle of 130°, a value close to the maximum-saliency ratio in a plot of saliency ratio versus PM pitch angle. Further, the strength of Motor X magnets may be lowered to 80% of full strength without significantly impacting motor performance for PM pitch angles between the peak saliency (130°) and peak-characteristic current (160°).

It is recommended that future research involve maximizing a driving-cycle-weighted efficiency based on the Federal Urban Driving Cycle and the Federal Highway Driving Cycle as criteria for selecting the final optimal-PM fraction and magnet strength for this inset PM motor.

Results of this study indicate that the reduction in PM torque due to reduced-magnet fraction will be more than compensated by the reluctance torque resulting from the higher saliency 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 motors with 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.

Efficiency Against Output Torque

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Output Torque Nm

Effi

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ncy

RFL Eff

Kelly Eff

Figure 1

REFERENCES

1. N. Bianchi and T. Jahns, “Design, Analysis, and Control of Interior PM Synchronous Machines,” Chapter 6 in Tutorial Course Notes, IEEE Industry Applications Society Annual Meeting, Seattle, Washington, October 3, 2004.

2. M. Kamiya, “Development of Traction Drive Motors for the Toyota Hybrid System,” 2005 International Power Electronics Conference, Toki Messe in Niigata, Japan, April 4–8,

2005. 3. T. J. E. Miller, M. I. McGilp, and J. S. Hendershot, SPEED, software by SPEED

Software 4. Laboratory, University of Glasgow, distributed by MagSoft, received 2004. 4. O. I. Elgerd, Electric Energy Systems Theory: Introduction, McGraw-Hill Book

Company, 5. Chapter 4, 1971. 5. R. E. Doherty and C. A. Nickle, “Sunchronous Machines,” pp. 912–942 in AIEE

Trans., 45, 1926. 6. R. H. Park, “Two Reaction Theory of Synchronous Machines – Generalized Method

of Analysis,” 6. pp. 716–727 in AIEE Trans., 48, 1929. 7. G. R. Slemon and X. Liu, “Core Losses in Permanent Magnet Motors,” pp. 1653–

1655 in IEEE 7. Trans. on Magnetics, 16(5), September 1990.

8. Jian-Xin Shen, Yu Wang and Cai-Fei Wang. “ Design and Analysis of New Outer-Rotor Premanent-Magnet Flux-Switching Machine for Electric Vehicle Propulsion” March 2009

9. Z.Q Zhu “ Fractional Slot Permanent Magnet Brushless Machines and Drive for Electric and Hybrid Propulsion Systems” March 2009