Newly Developed Motor Cooling Method Using Refrigerant

Article Newly Developed Motor Cooling Method Using Refrigerant

Hidemasa Fujita *, Atsushi Itoh and Tohru Urano

Mitsubishi Motors Corporation, Tokyo 108-8410, Japan; [email protected] (A.I.); [email protected] (T.U.) * Correspondence: [email protected]; Tel.: +81-80-8650-4865

Received: 9 April 2019; Accepted: 1 June 2019; Published: 4 June 2019

Abstract: One of the greatest issues for electric vehicles such as an electric vehicle (EV), a hybrid vehicle (HV), a plug-in hybrid electric vehicle (PHEV) and a fuel cell vehicle (FCV) is further improvement of eﬀective motor cooling, since higher rated torque is achieved with higher cooling performance. In this paper, we introduce and propose a newly developed motor cooling method we tested using refrigerant, comparing with conventional water cooling. Test results show higher cooling performance of refrigerant cooling, which achieved the rated torque 60% higher than that of water cooling.

Keywords: refrigerant; motor cooling; electric vehicle

1. Introduction The experience of rally competitions with Outlander PHEV has made us aware that the enhancement of motor cooling performance is essential to win the race. Rallies require driving of high speed, high torque, long period and long range. Such driving generates serious motor heating. To protect the motor materials, such as magnets or the insulation of copper coil windings, torque limitation is required at the upper limit of coil temperature. Once coil temperature exceeds the upper limit, torque is inhibited at limitation level, which results in deterioration of acceleration performance. This is why higher motor cooling performance plays the key role for the faster driving. Beyond rally cars, higher motor cooling performance contributes greatly to mass-produced vehicles by the following two aspects:

Rated torque increase • Downsizing • First contribution is rated torque, which should be increased without negotiating the motor size. The rated torque is generally deﬁned as the torque where motor cooling maintains thermal equilibrium so that a motor can operate continuously. Higher cooling performance enhances motor’s rated torque. Second is downsizing of a motor with realizing higher rated torque. Diameter of copper coil windings deﬁnes a motor size. Thinner windings increase its resistance that generates more heat. Enhanced cooling performance should absorb increased heat generated by thinner windings, which should realize downsizing of a motor. Table1 shows three methods of conventional motor cooling:

Air cooling • Oil cooling • Water cooling •

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TableTableTable 1.1. 1. MotorMotor Motor coolingcooling cooling methods.methods. methods. Table 1. Motor cooling methods. RefrigerantRefrigerantRefrigerant Air Air Air CoolingCooling Cooling Oil Oil Oil CoolingCooling Cooling Water Water Water CoolingCooling Cooling Air Cooling Oil Cooling Water Cooling RefrigerantCoolingCoolingCooling Cooling

Schematic SchematicSchematicSchematic Illustration IllustrationIllustrationIllustration

Heat exchange by Heat exchange by Heat exchange by Heat exchange by oil directly flowing flowing water in a flowingHeatHeatHeat exchange refrigerantexchange exchange byby by and wind HeatHeatHeat exchangeexchange exchange byby by HeatHeatHeat exchangeexchange exchange byby by HeatHeatHeat exchangeexchange exchange byby by into heat source water jacket flowingflowingvaporizationflowing refrigerantrefrigerant refrigerant heat oiloiloil directlydirectly directly flowingflowing flowing flowingflowingflowing waterwater water inin in aa a Cooling windwindwind andandand vaporizationvaporization vaporization Lowintointointo heatheat heat High sourcesource source waterwater Averagewater jacketjacket jacket High Performance heatheatheat CoolingCoolingCooling CoolingCooling LowLowLow HighHighHigh AverageAverageAverage HighHighHigh PerformancePerformance UniformLow Non-uniformHigh UniformAverage UniformHigh PerformanceUniformity CoolingCoolingCooling UniformUniformUniform Non-uniformNon-uniformNon-uniform i-MiEV, Uniform Uniform Uniform Uniform Uniform Uniform UniformityUniformityApplicationUniformity Uniform Non-uniformOutlander Uniform Uniform Uniformity Mainly Train Outlander HERE PROPOSED ApplicationApplicationApplicationExample PHEV(front)OutlanderOutlanderOutlander i-MiEV,i-MiEV,i-MiEV, MainlyMainlyMainly TrainTrain Train PHEV(Rear) HEREHEREHERE PROPOSEDPROPOSED PROPOSED ExampleExampleExample PHEV(front)PHEV(front)PHEV(front) OutlanderOutlanderOutlander PHEV(Rear)PHEV(Rear) PHEV(Rear)

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2.1.2.1.2.1.2.1. Concept ConceptConcept Concept ToToTo enhanceenhance enhance thethe the conventionalconventional conventional cooling coolingcooling performance,performance, performance, wewe we havehave have developeddeveloped developed aa a newnew new coolingcooling cooling method:method: method: ToTo enhance enhance the the conventional conventional cooling cooling performance,performance, we have have developed developed a anew new cooling cooling method: method: refrigerantrefrigerantrefrigerant cooling. cooling.cooling. The TheThe refrigerant refrigerantrefrigerant cooling coolingcooling system systemsystem is isis connected connectedconnected with withwith a aa car carcar air airair conditioning conditioningconditioning (A/C) (A/C)(A/C) refrigerantrefrigerant cooling. cooling. The The refrigerantrefrigerant coolingcooling system is is connected connected with with a acar car air air conditioning conditioning (A/C) (A/ C) systemsystemsystem inin in parallelparallel parallel asas as shownshown shown inin in FigureFigure Figure 1.1. 1. ItIt It usesuses uses aa a jacketjacket jacket forfor for waterwater water coolingcooling cooling asas as refrigerantrefrigerant refrigerant pathpath path thatthat that isis is systemsystem in in parallel parallel as as shown shown in in Figure Figure1 .1. It It uses uses aa jacketjacket forfor waterwater cooling as as refrigerant refrigerant path path that that is is installedinstalledinstalled in inin a aa motor motormotor housing. housing.housing. A AA thermal thermalthermal expansion expansionexpansion valve valvevalve (TXV) (TXV)(TXV) is isis attached attachedattached at atat an anan inlet inletinlet of ofof the thethe motor motormotor installed in a motor housing. A thermal expansion valve (TXV) is attached at an inlet of the motor housing,housing,housing, atatat whichwhichwhich liquidliquidliquid refrigerantrefrigerantrefrigerant evaporatesevaporatesevaporates tototo somesomesome extent.extent.extent. TheTheThe restrestrest ofofof liquidliquidliquid refrigerantrefrigerantrefrigerant housing, at which liquid refrigerant evaporates to some extent. The rest of liquid refrigerant evaporates evaporatesevaporatesevaporates inin in thethe the pathpath path absorbingabsorbing absorbing heatheat heat fromfrom from the thethe momo motor.tor.tor. AA A compressorcompressor compressor regulateregulate regulatesss itsits its operationoperation operation raterate rate toto to in the path absorbing heat from the motor. A compressor regulates its operation rate to control the controlcontrolcontrol thethe the coolingcooling cooling performance.performance. performance. cooling performance. 2.2.2.2.2.2. ExpectedExpected Expected BenefitsBenefits Benefits 2.2. Expected Beneﬁts ThreeThreeThree expectedexpected expected benefitsbenefits benefits fromfrom from replacingreplacing replacing waterwater water coolingcooling cooling systemsystem system byby by refrigerantrefrigerant refrigerant coolingcooling cooling are:are: are: Three expected beneﬁts from replacing water cooling system by refrigerant cooling are: •• • FewerFewerFewer componentscomponents components forfor for PHEVPHEV PHEV •• Fewer• GreaterGreaterGreater components coolingcooling cooling foruniformityuniformity uniformity PHEV • •• Greater• HigherHigherHigher cooling coolingcooling cooling uniformity performanceperformance performance • Higher cooling performance • FirstFirstFirst of ofof all, all,all, integration integrationintegration of ofof c ccoolingoolingooling systems systemssystems with withwith the thethe A/C A/CA/C system systemsystem should shouldshould reduce reducereduce components. components.components. FigureFigureFigure 2a 2a2a shows showsshows the thethe current currentcurrent cooling coolingcooling system systemsystem of ofof Ou OuOutlandertlandertlander PHEV. PHEV.PHEV. Four FourFour heat heatheat exchangers exchangersexchangers are areare located locatedlocated

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World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 3 of 11 in the frontFirst of of a all, vehicle: integration Two radiators of cooling (one systems for an with engine the A not/C systemillustrated should in Figure reduce 2 components. and another for waterFigure cooling2a shows of EV the system); current cooling one oil system cooler of for Outlander oil cooling PHEV. of the Four front heat motor exchangers and arethe locatedgenerator; in and in the front of a vehicle: Two radiators (one for an engine not illustrated in Figure 2 and another for onethe condenser front of a vehicle:for the TwoA/C. radiators If inverters (one for and an engineconverter not illustrated are integrated in Figure into2 and the another refrigerant for water cooling water cooling of EV system); one oil cooler for oil cooling of the front motor and the generator; and systemcooling referring of EV system); to previous one oil researches cooler for oil[7,8], cooling refrigerant of the front cooling motor should and the reduce generator; these and four one heat onecondenser condenser for thefor Athe/C. A/C. If inverters If inverters and converter and conv areerter integrated are integrated into the refrigerantinto the refrigerant cooling system cooling exchangerssystem referring into two: to Aprevious radiator researches for the engine [7,8], co refrigerantoling and acooling condenser should for reducethe refrigerant these four cooling heat as shownreferring in Figure to previous 2b. The researches essentials [7 ,8to], realize refrigerant this cooling ideal system should are: reduce The these greater four cooling heat exchangers ability of the exchangersinto two: A into radiator two: A for radiator the engine for coolingthe engine and co aoling condenser and a for condenser the refrigerant for the cooling refrigerant as shown cooling in as compressor; the distribution control of refrigerant; and the compressor control to minimize the shownFigure in2b. Figure The essentials 2b. The toessentials realize this to idealrealize system this ideal are: The system greater are: cooling The greater ability ofcooling the compressor; ability of the power consumption. Second is that the refrigerant cooling achieves greater cooling uniformity than compressor;the distribution the controldistribution of refrigerant; control and of therefriger compressorant; and control the compressor to minimize thecontrol power to consumption. minimize the oilpower coolingSecond consumption. is by that letting the refrigerant refrigerantSecond is cooling that flow the achieves through refrigerant greater the cooling water cooling jacket.achieves uniformity The greater final than coolingbenefit oil cooling uniformityof the by lettingrefrigerant than coolingoilrefrigerant cooling is higher by flow letting cooling through refrigerant theperformance water flow jacket. throughthan The that final the of benefit water water ofjacket. thecooling. refrigerant The The final coolingstudy benefit in is of higherthis the paper refrigerant cooling aims to verifycoolingperformance the isfact higher that than thecooling that refrigerant of performance water cooling.cooling than perfor The that studymance of water in is this higher cooling. paper than aims The the tostudy verifywater in thecoolingthisfact paper thatthrough aims the to two approaches:verifyrefrigerant the fact simulation cooling that the performance refrigerantand experiment. is cooling higher than perfor themance water coolingis higher through than the two water approaches: cooling simulationthrough two approaches:and experiment. simulation and experiment.

FigureFigure 1. 1. Proposed refrigerant refrigerantrefrigerant cooling cooling cooling system. system. system.

(a()a )

Figure 2. Cont.

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(b)

Figure 2. (a) Current cooling system of PHEV; (b) one example of ideal cooling systems using Figurerefrigerant 2. (a) cooling.Current cooling system of PHEV; (b) one example of ideal cooling systems using Figure 2. (a) Current cooling system of PHEV; (b) one example of ideal cooling systems using refrigerantrefrigerant cooling. cooling. 3. Simulation 3. SimulationSimulation3. Simulation shows that refrigerant cooling kept the coil temperature lower than water cooling. This simulationSimulation was performed shows that on refrigerant a one-dimensional cooling kept model the coil [9 temperature,10]. The simulation lower than used water the cooling. same model Simulation shows that refrigerant cooling kept the coil temperature lower than water cooling. for waterThis cooling simulation and refrigerantwas performed cooling. on a one-dimensio The model describesnal model a[9,10]. motor The cooling simulation structure used the without same an AC Thiscircuit. simulation Wemodel introduced for was water performed a cooling cylindrical-shaped and on refrigerant a one-dimensio water cooling. jacket The asnal model shown model describes in Figure[9,10]. a 3motor Theto simulate simulationcooling the structure complicated used the same model forwithout water ancooling AC circuit. and We refrigerant introduced a cooling.cylindrica l-shapedThe model water jacketdescribes as shown a motor in Figure cooling 3 to structure water/refrigerantsimulate the path. complicated Heat generated water/refrigerant by electrical path. current Heat generated through copperby electrical coil windings current through is transferred withoutto water an/copperrefrigerant AC coilcircuit. windings path We across isintroduced transferred the steal statorto a water/refrigerant cylindrica core and thel-shaped path aluminum across water the housing. stealjacket stator For as refrigerantcoreshown and thein cooling, Figure 3 to simulatea TXV wasthealuminum attachedcomplicated housing. at the For water/refrigerant inlet refrigerant of the cooling, refrigerant a TXVpath. path was Heat attached to let generated liquid at the inlet refrigerant ofby the refrigerantelectrical evaporate path current effectively. to through copperBased coil onlet the windingsliquid initial refrigerant experimental is transferred evaporate conditions effectively. to water/refrigerant Table Based2 shows, on the we initial calculatedpath experimental across the maximumthe conditions steal temperaturesstator Table 2core atand the aluminumthermalshows, equilibrium housing. we calculated For for water,refrigerant the refrigerantmaximum cooling, temperatures and coppera TXV at coilwas thermal windings. attached equilibrium As at athe for result, water,inlet the ofrefrigerant coil the temperature refrigerant and of path to copper coil windings. As a result, the coil temperature of refrigerant cooling was lower than that of refrigerant cooling was lower than that of water cooling by 48 C as shown in Table3. let liquid waterrefrigerant cooling by evaporate 48°C as shown effectively. in Table 3. Based on the◦ initial experimental conditions Table 2 shows, we calculated the maximum temperatures at thermal equilibrium for water, refrigerant and copper coil windings. As a result, the coil temperature of refrigerant cooling was lower than that of water cooling by 48°C as shown in Table 3.

FigureFigure 3. 3.Simulation Simulation model. model.

Table 2. Initial conditions.

Initial Motor Temperature ◦C 25 Maximum Motor Heat Loss kW 3 Water Temperature ◦C 40 Water Cooling Water Flow Rate L/min 10 Figure 3. Simulation model. Pressure at Outlet kPa 104 Refrigerant - HFC-134a Refrigerant Cooling Refrigerant Flow Rate kg/s 0.0024 Gas Phase Ratio at Inlet - 0.3 Pressure at Outlet MPa 0.3

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Table 4. Motor specification.

Type - PMSM

Max. output kW 47

Max. torque Nm 180

Max. speed rpm 8500 World Electric Vehicle Journal 2019, 10, 38 5 of 10 Max. system efficiency* % More than 90

MassTable 3. Simulationkg results. About 45

CoolingMax. Temperaturemethod of Water - or WaterMax. cooling Temperature of Copper Refrigerant [◦C] Coil Windings [◦C] * Total efficiency of motor and inverter. Water Cooling 62 81 Refrigerant Cooling Table 5. Experimental21 conditions. 33

4. Experiment Item Unit Exp.1 Exp.2 Exp.3

4.1. Procedure Room temperature °C 25

The cooling performanceMotor speed was compared between rpm water 1000 and refrigerant 1000 by using the 2500 same motor of the specification in Table4. All experiments started with water cooling as shown in Figure4a. A TXV was installed at the inlet of the water jacket as shown65 in Figure4b before experimenting180 Motor torque Nm 65~180 refrigerant cooling. The refrigerant cooling experiment used(Rated) the same motor as(Maximum) the water cooling experiment. The water jacket worked as the refrigerant path. In addition, the water circulation system Motor power kW 7 7~19 47 was replaced by an A/C system. Compressor regulated its operation rate to control the refrigerant cooling performance.Water temperature The compressor for inverter kept operating until the motor surface temperature declined °C 40 to a target temperature.cooling Note that the target temperatures for refrigerant cooling were set at 0, 10, and 20 ◦C. The compressor stopped its operation when the motor surface temperature was lowered below the target temperature. ThreeWater experiments were performed as shown in Table5. °C 20, 40, 60 temperature Table 4. Motor specification. Water cooling Water flow rate L/min 10 Type - PMSM Initial coil Max. output°C kWDependent 47 on water temperature temperature Max. torque Nm 180 RefrigerantMax. speed - rpm 8500 HFC-134a Max. system efficiency* % More than 90 Target Refrigerant °C 0, 10, 20 temperatureMass kg About 45 cooling Cooling method - Water cooling Initial coil * Total efficiency of° motorC and inverter. 25 temperature

(a)

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(b)

FigureFigure 4. Experimental 4. Experimental setup: setup: (a (a)) Water Water cooling;cooling; ( b(b) refrigerant) refrigerant cooling. cooling.

4.2. Results and Discussion

4.2.1. Experiment 1 Figure 5 shows that refrigerant cooling kept the coil temperature lower than water cooling. Comparing water cooling of 60 °C and refrigerant cooling of 0 °C, the difference of the coil temperatures extended up to 53 °C after 30 minutes of motor operation at rated torque. The water cooling of 60 °C was the highest water temperature allowed in the specification. The refrigerant cooling of 0 °C was the lowest refrigerant temperature, which was determined by compressor ability. When the water temperature was set at 20 °C to adjust the initial coil temperature, refrigerant cooling even showed the higher cooling performance by 12 °C than water cooling.

Figure 5. Operation time at rated torque vs coil temperature.

The coil temperature was proportional to the temperature of fluid through the water jacket as shown in Figure 6, which shows the higher cooling performance of the refrigerant cooling. The compressor achieved this result by keeping the refrigerant temperature below the ambient temperature for its phase transition causing an endothermic reaction through the TXV. Figure 7 shows higher cooling performance of refrigerant cooling. The picture was taken after 30 minutes of motor operation at rated torque. The motor surface was frosted since the refrigerant temperature was set at 0 °C.

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In experiment 1, the motor temperature, which was operated at rated torque for 30 min, was measured. In experiment 2, new rated torque was estimated when water cooling was replaced by refrigerant cooling. In experiment 3, operation time at the maximum torque was measured.

Table 5. Experimental conditions.

Item Unit Exp.1 Exp.2 Exp.3

Room temperature ◦C 25 Motor speed rpm 1000 1000 2500 World Electric Vehicle MotorJournal 2019 torque, 10, x FOR PEER REVIEW Nm 65 (Rated) 65~180 180 (Maximum) 7 of 11 Motor power kW 7 7~19 47

Water temperature for inverter cooling ◦C 40

Water temperature ◦C 20, 40, 60 Water cooling Water ﬂow rate L/min 10 (b) Initial coil temperature ◦C Dependent on water temperature Refrigerant - HFC-134a Refrigerant cooling Target temperature ◦C 0, 10, 20

FigureInitial 4. Experimental coil temperature setup: (a) Water◦C cooling; (b) refrigerant cooling. 25

4.2.4.2. Results andand DiscussionDiscussion

4.2.1.4.2.1. Experiment 11 FigureFigure5 5shows shows that that refrigerant refrigerant cooling cooling kept kept the the coil coil temperature temperature lower lower thanthan waterwater cooling.cooling. ComparingComparing water water cooling cooling of 60of◦ C60 and °C refrigerantand refrigeran coolingt cooling of 0 ◦C, theof di0 ﬀ°C,erence the ofdifference the coil temperatures of the coil extendedtemperatures up to extended 53 ◦C after up 30 to minutes 53 °C after of motor 30 minu operationtes of motor at rated operation torque. Theat rated water torque. cooling The of 60water◦C wascooling the highestof 60 °C water was temperaturethe highest water allowed temperatur in the speciﬁcation.e allowed in The the refrigerant specification. cooling The of refrigerant 0 ◦C was thecooling lowest of refrigerant0 °C was the temperature, lowest refrigerant which was temperature, determined which by compressor was determined ability. When by compressor the water temperatureability. When was the set water at 20 temperature◦C to adjust was the initialset at 20 coil °C temperature, to adjust the refrigerant initial coil coolingtemperature, even showed refrigerant the highercooling cooling even showed performance the higher by 12 cooling◦C than perf waterormance cooling. by 12 °C than water cooling.

FigureFigure 5.5. Operation timetime atat ratedrated torquetorque vsvs coilcoil temperature.temperature.

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The coil temperature was proportional to the temperature of ﬂuid through the water jacket as shown in Figure6, which shows the higher cooling performance of the refrigerant cooling. The compressor achieved this result by keeping the refrigerant temperature below the ambient temperature for its Worldphase Electric transition Vehicle causing Journal 2019 an endothermic, 10, x FOR PEER reaction REVIEW through the TXV. 8 of 11

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Figure 6. Coil temperature after 30 min operation at rated torque vs water temperature or Figurerefrigerant 6. temperature.Coil temperature after 30 min operation at rated torque vs water temperature or refrigerant temperature. Figure7 shows higher cooling performance of refrigerant cooling. The picture was taken after 30Figure min of6. motorCoil temperature operation at after rated 30 torque. min operation The motor at surfacerated torque was frosted vs water since temperature the refrigerant or temperaturerefrigerant temperature. was set at 0 ◦C.

Figure 7. Frosted motor surface by refrigerant cooling.

4.2.2. Experiment 2 Figure 8 shows thatFigureFigure the 7. rated 7.FrostedFrosted torque motor motor of surface surface the refr bybyigerant refrigerantrefrigerant cooling cooling. cooling. increased up to 60% more than that4.2.2. of Experiment the water 2cooling. The new rated torque was estimated from the highest coil temperature of 4.2.2. Experiment 2 the water cooling. After the operation of the water cooling motor with the water temperature of 60 Figure8 shows that the rated torque of the refrigerant cooling increased up to 60% more than °CFigure operated 8 shows for 30 that min the at ratedthe rated torque torque of the of 65refr Nmigerant, the coolingcoil temperature increased reached up to 60% 92 °C.more This than result that of the water cooling. The new rated torque was estimated from the highest coil temperature that of the water cooling. The new rated torque was estimated from the highest coil temperature of indicatesof the water that cooling. the rated After torque the operation of the of therefrigeran water coolingt cooling motor is withallowed the water to increase temperature until of the coil the temperaturewater cooling. becomes After the 92 °C.operation In that ofcase, the the wate motor coolingr torque motor was with105 Nm the whilewater the temperature refrigerant of cooling 60 60 ◦C operated for 30 min at the rated torque of 65 Nm, the coil temperature reached 92 ◦C. This result °C operatedwasindicates 0 °C. for that That 30 the min ratedwas at torquea the 60% rated of increase the torque refrigerant of of the 65 cooling Nmrated, the is torque allowed coil temperature of to increasethe refrigerant until reached the coilcooling 92 temperature °C. Thisas a resultresult of indicatesreplacingbecomes that 92the the◦ C.cooling Inrated that substancetorque case, the of motor from the torquewaterrefrigeran to was refrigerant.t 105 cooling Nm while is allowed the refrigerant to increase cooling until was 0the◦C. coil temperatureThat was becomes a 60% increase 92 °C. of theIn that rated case, torque the of moto the refrigerantr torque coolingwas 105 as Nm a result while of replacing the refrigerant the cooling cooling was4.2.3. substance0 °C. Experiment That from was water 3a 60% to refrigerant. increase of the rated torque of the refrigerant cooling as a result of replacingExperiment the cooling 3 substance aims to clarify from water any impact to refrigerant. on the maximum torque of the refrigerant cooling. Note that the inverter performance restricts the maximum torque. What was focused on here was 4.2.3. Experiment 3 whether refrigerant cooling affected the operation time at maximum torque. The results showed two aspects.Experiment 3 aims to clarify any impact on the maximum torque of the refrigerant cooling. Note thatFirst, the coolinginverter methods performance did not restricts affect anythe maximumoperation time torque. if the What initial wa coils focused temperature on here was was same whetheras shown refrigerant in Figure cooling 9. The affected time range the operation of 150 s was time too at shortmaximum for refrigerant torque. The cooling results to showedshow its two effect. aspects. First, cooling methods did not affect any operation time if the initial coil temperature was same as shown in Figure 9. The time range of 150 s was too short for refrigerant cooling to show its effect.

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FigureFigure 8. Coil 8. Coil temperature temperature after after 30 30 min min operation vs vs motor motor torque. torque.

4.2.3. Experiment 3 Experiment 3 aims to clarify any impact on the maximum torque of the refrigerant cooling. Note that the inverter performance restricts the maximum torque. What was focused on here was whether refrigerant cooling affected the operation time at maximum torque. The results showed two aspects. First, cooling methods did not affect any operation time if the initial coil temperature was same as shown in Figure9Figure. The time 8. Coil range temperature of 150 s was after too 30 short min operation for refrigerant vs motor cooling torque. to show its e ffect.

Figure 9. Coil temperature vs operation time at maximum torque.

However, when the motor was cooled down to 0 °C in advance of motor operation, the operation time at the maximum torque of the refrigerant cooling was extended by 34% as Figure 10 shows. Figure 11 shows that the lower initial coil temperature led to the longer operation time. In other words, the refrigerant cooling had an advantage over the water cooling since the compressor decreased the initial coil temperature before the motor operation.

FigureFigure 9. Coil 9. Coil temperature temperature vs vs operatio operationn time atat maximum maximum torque. torque.

However, when the motor was cooled down to 0 ◦C in advance of motor operation, the operation However,time at the maximumwhen the torquemotor of was the refrigerantcooled down cooling to was0 °C extended in advance by 34% of asmotor Figure operation, 10 shows. the operationFigure time 11 shows at the that maximum the lower initialtorque coil of temperaturethe refrigerant led to cooling the longer was operation extended time. by In34% other as words,Figure 10 shows.the Figure refrigerant 11 shows cooling that had the an advantage lower initial over coil the watertemperature cooling sinceled to the the compressor longer operation decreased time. the In otherinitial words, coil the temperature refrigerant before cooling the motor had an operation. advantage over the water cooling since the compressor decreased the initial coil temperature before the motor operation.

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FigureFigure 10.10. CoilCoilCoil temperature temperaturetemperature vs vsvs operation operationoperation time timetime at the atat maximumthethe maximummaximum torque torquetorque in case inin that casecase refrigerant thatthat refrigerantrefrigerant cooling coolingcoolingcooled thecooledcooled motor thethe in motormotor advance. inin advance.advance.

Figure 11. Operation time at maximum torque vs initial coil temperature. FigureFigure 11.11. OperationOperation timetime atat maximummaximum torquetorque vsvs initialinitial coilcoil temperature.temperature. 5. Conclusions 5.5. ConclusionConclusion The superiority of the refrigerant cooling has been revealed comparing to the conventional water The superiority of the refrigerant cooling has been revealed comparing to the conventional cooling,The because:superiority of the refrigerant cooling has been revealed comparing to the conventional waterwater cooling,cooling, because:because: The refrigerant cooling has the coil temperature lower than ambient temperature by the • •• The refrigerant cooling has the coil temperature lower than ambient temperature by the compressorThe refrigerant control; cooling has the coil temperature lower than ambient temperature by the compressorcompressor control;control; The rated torque was increased by 60% when the water cooling was replaced by the refrigerant • •• TheThe ratedrated torquetorque waswas increasedincreased byby 60%60% whwhenen thethe waterwater coolingcooling waswas replacedreplaced byby thethe cooling; and refrigerantrefrigerant cooling;cooling; andand Cooling the motor in advance of motor operation contributed to extend operation time at the • •• CoolingCooling thethe motormotor inin advanceadvance ofof motormotor operationoperation contributedcontributed toto extendextend operationoperation timetime atat thethe maximum torque by 34%. maximummaximum torquetorque byby 34%.34%. Author Contributions: Conceptualization, H.F.; formal analysis, A.I.; investigation, H.F.; writing—original Author Contributions:Contributions: Conceptualization,Conceptualization, H.F.; H.F.; formal formal analysis, analysis, A.I.; A.I.; investigation, investigation, H.F.; H.F.; writing—original writing—original draft draftdraftpreparation, preparation,preparation, H.F.; H.F. supervision,H.F.;; supervision,supervision, T.U. T.U.T.U.

Funding:Funding: ThisThisThis researchresearch research receivedreceived nono externalexternal funding.funding. Acknowledgments: The authors would like to thank K. Sakai, K. Kusafuka, and K. Nasu for technical assistance Acknowledgments:Acknowledgments: TheThe authorsauthors wouldwould likelike toto thankthank K.K. Sakai,Sakai, K.K. Kusafuka,Kusafuka, andand K.K. NasuNasu forfor technicaltechnical assistanceassistance with the experiments. We also thank S. Kino, E. Ishimaru and H. Hayakawa for great technical advice and withwith thethe experiments.experiments. WeWe alsoalso thankthank S.S. Kino,Kino, E.E. IshimaruIshimaru andand H.H. HayakawaHayakawa forfor greatgreat technicaltechnical adviceadvice andand Y.Y.

World Electric Vehicle Journal 2019, 10, 38 10 of 10

Y. Nakajima and S. Kito for supporting paper construction. Finally, we are grateful to K. Handa for giving us the opportunity to continue this research. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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