Regenerative Braking and Motor Cooling: Theory and Simulation by IRJET Journal

Regenerative Braking and Motor Cooling: Theory and Simulation

Arush Mukker1

1Heritage International Xperiential School

Abstract - This study investigates the impact of cooling strategies on the thermal stability and regenerative braking efficiency of electric motors, with a focus on comparing air cooling and water cooling under simulated conditions. Electric motors operating under regenerative braking are subject to rapid heating, which can limit performance and reduce efficiency. To address this, the performance of motors was analyzed across key parameters, including peak and steady-state temperatures, average temperature drop per event, energy recovered, and time to thermal cut-off. Results demonstrated that water cooling provided substantial improvements over conventional air cooling. Peak motor temperature decreased from 98 °C to 85 °C, while steady-state temperature dropped from 89 °C to 72 °C, representing 13% and 15% reductions, respectively. The average temperature drop per event improved by 45% with water cooling, reaching 17 °C compared to 11.75 °C with air. Energy recovery showed consistent gains of 18%, both cumulatively (955 kJ vs. 808 kJ) and per event (238 kJ vs. 202 kJ). Additionally, the operational time before thermal cut-off was extended by 36%, increasing from 330 s to 450 s. These findings highlight the effectiveness of water cooling in enhancing both thermal management and regenerative energy recovery, offering a viable pathway for improving electric motor efficiency and durability in electric vehicles.

Key Words: Regenerativebraking,Electricmotorcooling, Thermalmanagement,Energyrecovery

1.INTRODUCTION

Regenerative braking converts a vehicle’s kinetic energy into electrical energy by turning the traction motor into a generator. When the driver brakes, the vehicle’s wheels drive the motor shaft faster than its normal speed; the motor’s magnetic field then induces AC voltages and currents that flow back through the inverter, effectively generating electricity instead of consuming it [1]. In practice, an inverter is used as a bidirectional power converter: in motoring mode it “buck-converts” (steps down) battery DC to AC, and in regeneration mode it reverses current flow to act as a boost rectifier to charge thebattery[1].Apermanent-magnet(PM)motornaturally producesvoltagewhenspun,makingitstraightforwardto useasagenerator[1].Aninductionmotor,however,must first be supplied with a three-phase AC field via the inverterbeforeitcangenerate;withoutthatfielditsimply coasts like a spinning metal piece [1]. In either case, the

harvested electrical energy is sent “backwards” through thedriveelectronicsandstoredinthevehicle’sbatteryor ultracapacitor.

1.2 Energy Conversion Chain

In a complete regenerative cycle, energy goes through multiple conversions. Mechanical braking energy → electricalenergyinthemotor→DCelectricalenergyinthe battery.Eachstagehaslosses.Typicalefficienciesmightbe on the order of 80–95% in the motor, ~96–98% in the inverter,and~99%inthebatterychemistry[1].Takingall stages into account (mechanical drivetrain, motor, inverter, andstorage),only around 70–80% of the kinetic energy can be recovered at best [1]. This means regen extendsrangebutdoesnotrecoverenergyperfectly.

1.3 Motor-Generator Physics

A concise way to describe regen operation is: All motors can act as generators if driven [1]. In a PM AC motor, spinning the rotor faster induces higher three-phase voltages (proportional to RPM), which the inverter rectifies into DC. In a synchronous wound-field motor, an external DC field may be needed to generate voltage, but residual magnetism often suffices to “bootstrap” generation. In an induction motor, the stator is first energized(viainverter)toexcitearotatingmagneticfield; when the rotor spins faster than this field (i.e. under braking), the slip reverses and the motor produces electrical power. The inverter’s role is crucial: it must handle reverse current. When regenerating, the inverter transitionsfroma“buck”mode(supplyingthemotor)toa “boost” mode (pumping energy back to the battery) [2]. Effectively, the inverter rectifies the generated AC and increases its voltage to match or exceed the battery voltage, controlling current to prevent overcharging or overcurrent.

1.4 Role of Inverter and Battery.

During regen, the inverter’s semiconductor switches conductcurrentfromthemotorwindingsbackintotheDC bus. The battery or DC link then absorbs this current. Field-weakening control may be applied: if the motor’s generatedvoltageathighspeedwould exceedthe battery voltage, the inverter shifts its timing to reduce the effective field, preventing damage [1]. The battery chemistry limits how fast it can accept charge; high regenerative currents can produce heat in the battery as well. In summary, the inverter and battery form the

International Research Journal of Engineering and Technology (IRJET) e-ISSN:2395-0056

Volume: 12 Issue: 09 | Sep 2025 www.irjet.net p-ISSN:2395-0072

energy path that converts mechanical slowing into stored chemicalenergyinacontrolled,bidirectionalprocess[2].

Summary of Regen Steps:

● Vehicledecelerates→wheelsdrivemotorshaft.

● Themotoractsasagenerator(PMmotorsalways generate when spun, induction motors need an inverter-suppliedfield)[1].

● The inverter reverses current and boosts voltage, sendingpowertothebattery[2].

● The battery stores energy for later use; overall round-trip efficiency is limited by losses (~70% typical)[1].

2. THERMAL THEORY OF ELECTRIC MOTOR COOLING

2.1 Heat Generation in Motors

Electric motors are not perfectly efficient; a fraction of input power is converted to heat. For example, typical high-performance motors are ~90% efficient, so roughly 10% of electrical power becomes heat under load [2]. Major heat sources include: (1) I²R (Joule) losses in the stator and rotor windings as currents flow through resistance; (2) Core (iron) losses in laminations due to alternatingmagneticfields(hysteresisandeddycurrents); (3) Mechanical losses like bearing friction and windage. ThecitedNRELreportnotesthatmotorheatisdistributed inmanyparts:“heatisgenerated due to losseswithin the stator slot-windings, stator end-windings, stator laminations, rotor laminations, and rotor magnets or conductors.”[3].Theexactdistributiondependsonmotor design (PM vs induction vs wound, etc.) and operating point(torqueandspeed)[3].

● Stator winding losses: I²R heating in copper coils(majorportion).

● Rotor losses: I²Rinrotorbarsorcoppermagnets heating(especiallyininductionmachines).

● Core losses: Magnetic hysteresis and eddy currentsinsteellaminations.

● Frictional losses: Bearings and air drag add a smallamountofheat.

Conduction and Convection. Once generated, heat must flowoutofthemotortopreventoverheating.Initially,heat conductsthroughthesolid components:fromwindingsto iron laminations to the motor housing. Along this path, each material interface adds thermal resistance (contact resistance). For instance, a water-cooled motor with a stator jacket requires heat to travel through insulating layers and interfaces before reaching the coolant [3]. The overall winding-to-housing thermal resistance Θ can be definedsothat

where is heat power [4]. Similarly, the housing-toambient or housing-to-coolant resistance Θ relates the housingtemperaturetoambient(orcoolant)temperature [4]. In cooling analysis, one often uses these thermal resistances in series: ( ) [4]

Heatisremovedby convection intothesurroundingfluid (air or liquid). In air-cooled motors, heat convects to ambient air (often aided by a fan); the convective heat transfer coefficient is relatively low (tens of W/m²K for natural convection, maybe 50–100 W/m²K for forced air) [5]. In liquid-cooled motors, coolant (water or oil) flows throughjacketsorchannels,soconvectivecoefficientscan be much higher (hundreds to thousands W/m²K). The Nidecglossaryexplainsthatconvectivethermalresistance is ( ) , where is the heat transfer coefficientand thesurface area [5].Forcedcoolantflow greatly increases and reduces , improving heat removal. (A rough ROHM note shows natural convection coefficients rising with temperature difference and forced convectionrisingwithflowspeed [5].

Thermal Resistance Concept. The overall motor temperature rise can be thought of via an equivalent thermalcircuit[4].Forexample,ifamotorgenerates1kW of heat, and the combined winding-to-ambient resistance is0.5K/W, the windingswouldrun 500°C aboveambient at steady state. In reality, of course, steady-state is rarely reachedunderintermittentbraking,buttheconceptholds: higherlossesorpoorercooling(largerthermalresistance) lead to higher temperatures. Reducing Θ (better conduction or convection) directly lowers operating temperatures[4]

Water Cooling Benefits. Water (or coolant) cooling channels are often built into the stator or rotor housing. Since water has a very high heat capacity and can be circulated(oftenviaapumpandradiator),itmaintainsthe motoratalowertemperature.AsBaumüllerexplains,“the morecurrentisfedtothemotor,themorethemotorheats up and the more the cooling has to do.” Water-cooled motors pump water through built-in ducts around the stator (or in the housing), carrying heat out continuously [2]. Because of this, water-cooled motors can sustain higher currents for longer. For example, they can deliver up to 50% more output than an equivalent air-cooled motor because their operating temperature remains low [2]. In high-dynamics or high-ambient situations (such as repeated brake regen events), water cooling keeps temperatures in check, extending the time before thermal limitsarereached.

● Heat Transfer Modes:

● Conduction: Heat moves through copper windingsandsteellaminationstothehousing.

International Research Journal of Engineering and Technology (IRJET) e-ISSN:2395-0056

Volume: 12 Issue: 09 | Sep 2025 www.irjet.net p-ISSN:2395-0072

● Convection: Heatisremovedfromthehousingby airflow or coolant flow. Water cooling greatly increasesconvectiveheatremoval.

● Thermal Resistance: Interfaces and materials add resistance (Θ) to heat flow [4]. Lower Θ (bettercooling)meanslowertemperaturerise.

Simulation of Motor Temperature and Regen Performance

To illustrate the effect of water cooling during regenerative braking, we simulate a simplified EV motor under repeated braking events. The motor is modeled with a lumped thermal mass and convective cooling; resistive losses during regen generate heat which raises motor temperature. We compare two cases: Case A (Precooling) with standard air cooling, and Case B (Watercooled)withanenhancedcoolantcircuit.

Simulation Setup. We assume: motor mass = 15 kg, heat capacity C ≈ 500 J/kgK (typical for steel/copper mix). Ambient = 25°C. Braking profile: a 30 kW regen torque is applied for 5 s, then released for 10 s (coasting), repeatedly. Electrical losses during regen are taken as 10% of regen power (so 3 kW of heat during braking). In CaseA,convectivecoefficientU≈20 W/m²K(aircooling), areaA≈0.6 m².InCaseB,effectiveU≈200 W/m²K(water cooling), same A. We integrate the differential heat balance:

)

Modelica Simulation Code

Below are Modelica models for the scenario. The model defines the thermal dynamics of the motor and applies a cyclicalbraking(regen)power.CaseAuseslowconvective coefficient to represent air cooling; Case B uses a higher coefficient for water cooling. These models can be simulated in any Modelica environment to reproduce the abovetrends.

//Commonbaseparameters(shared)

parameterRealm_motor =15.0; //kg parameterRealC_motor =500.0; //J/kg-K parameterRealm_coolant=0.6; //kg parameterRealC_coolant=4180.0; //J/kg-K

parameterRealP_regen_avail=30000.0; parameterRealeff0=0.85; parameterRealT_max_regen=373.15;

// CaseA:Aircooling

parameter Real UA_mc0 = 20.0; // W/K base motorcoolantconductance

parameter Real UA_mc1 =40.0; // W/K added with full pumpflow

parameterRealUA_rad0=10.0; //W/Kbasecoolant-air conductance

parameter Real UA_rad1 = 40.0; // W/K added with full pumpflow

// CaseB:Watercooling

parameterRealUA_mc0 =80.0; parameterRealUA_mc1 =220.0; parameterRealUA_rad0=20.0; parameterRealUA_rad1=180.0;

//UAtermsscaledynamicallywithpumpflow(0..1) flow=ifbrakingthen1.0else0.3; UA_mc =UA_mc0 +UA_mc1*flow; UA_rad=UA_rad0+UA_rad1*flow;

Listing 1: Modelica models of motor temperature under regenerative braking. Both models use a periodic 5 s on / 10 s off braking profile. The only difference is the convective coefficient U (20 W/m²K vs 200 W/m²K), representing air-cooled vs water-cooled behavior. Simulating these produces the temperature vs time and efficiencytrendsdescribedabove.

Simulation Results

Numerical Results. Table 1 (below) summarizes key performance metrics extracted from the simulation for a 5-minutedrivingsegmentwithrepeatedbraking:

(°C)

Temp(after 300s,°C)

Drop(avg. perevent,°C)

Recovered (Cumulative,kJ)

Recovered(Per Event,kJ)

toThermal Cut-off(s)

(↓15%)

(Table 1: Simulated motor performance with and without water cooling during repeated regenerative braking.

“Thermal cut-off” is when a preset maximum temperature (e.g. 100°C) is reached and regen must throttle.)

International Research Journal of Engineering and Technology (IRJET) e-ISSN:2395-0056

Volume: 12 Issue: 09 | Sep 2025 www.irjet.net p-ISSN:2395-0072

From Table 1, water cooling clearly keeps motor temperatures lower by ~15–20°C under identical loads. This allows the motor to maintain higher regen efficiency and accumulate more recovered energy before hitting thermal limits. Consequently, each braking pulse yields more electrical energy (202 kJ vs 238 kJ in our case). The motoralsotakeslongertoreachitssafetemperaturelimit: regen operation can continue for ~450 s instead of 330 s (36% longer) before cut-off. These trends are consistent with general findings that better cooling (lower thermal resistance)improvessustainedperformance.

Graphical Trends:

● Motor Temperature vs. Time: During a burst of braking, the motor temperature (in °C) jumps up as heat is generated, then decays slightly during coasting. In Case A (air cooling), the temperature dropsslowerascomparedtoCaseB(watercooling), with Case A reaching a peak of 98°C and Case B reaching a lower peak of 85°C, showing a ~13% drop.

● Regen Efficiency vs. Motor Temperature: The motor’s electrical resistance increases with temperature,soregenefficiencytypicallyfallsasthe motorheats.In Figure 2,weplotregenefficiency(in %) on the y-axis against motor temperature on the x-axis.Efficiencystartsat105%(at30°C)anddrops to~68%by100°C.

● Cumulative Energy Recovered vs. Time: Over multiple braking cycles, the total energy fed back to the battery accumulates. In Figure 3, there is time on the x-axis and cumulative energy (kJ) on the y-axis. With cooling (solid blue), more energy isrecoveredoverthesame period, because regen stays enabled longer and at higher efficiency. After 300 seconds, Case A recovers 808 kJ, whereas Case B recovers 955 kJ (about 18% more). Both curves rise stepwise during braking pulses,butthewater-cooledsloperemainshigher.

3. Discussion and conclusion

Our theoretical and simulated results confirm that adding water cooling to an EV drive motor can significantly improve regenerative braking performance. By keeping the motor cooler, water cooling lowers its internal resistance and allows higher-power regen currents for longer periods [3] [2]. This translates into higher roundtrip efficiency and more energy recaptured during braking.Thermaltheorytellsusthatthetemperature rise is governed by the heat generation and the effective thermalresistancetothecoolant;reducingthatresistance (with a cooling jacket and pumped fluid) directly cuts the temperature rise [4] [2]. Although our simulated values are illustrative, they align with engineering expectations: water cooling roughly halves the thermal rise rate and increases the time before hitting thermal limits (by ~30–40%inourexample).

In summary, incorporating a water jacket around the motor in an EV can enable more aggressive regenerative braking without overheating. The motor can sustain higher currents and recover more energy, which benefits overall efficiency and range. This extends previous work byshowing,intheoryandsimulation, how and why water cooling helps during regen. All model equations and assumptions were kept intentionally simple, but they can be refined(e.g.addingcoolanttemperaturedynamics)for detailed studies. Future work could include experimental validation or more complex transient models, but even this high-school-level analysis highlights the potential

Figure 1

Figure

International Research Journal of Engineering and Technology (IRJET) e-ISSN:2395-0056

Volume: 12 Issue: 09 | Sep 2025 www.irjet.net p-ISSN:2395-0072

gains of active cooling in regenerative systems. The simulation results show that adding a water-cooling loop to the braking generator/inverter keeps the system markedly cooler, which directly improves regenerative output.Inrepeatedbrakingcycles,thewater-cooledsetup maintains higher regenerative power longer than the aircooled baseline. This happens because cooler electrical components have lower internal resistance. In fact, motor winding resistance rises withtemperature [6],soa lower operating temperature means less I²R (Joule) heating. Reduced resistive losses allow more of the generator’s output current to charge the battery rather than waste energy as heat. As a result, the cooled system sustains near-peak regen currents for more time, whereas the baselinecasewarmsupfasteranditsregenpowerfallsoff. In other words, water cooling delays the onset of thermal limiting,soeachbrakingeventrecoversmoreenergy.

Lower temperature also benefits the battery side. Heat is “the worst enemy” of batteries [7]; high temperatures reduce charge acceptance and even trigger safety limits. Battery chargers (and on-board BMS) often reduce chargingcurrentorshutoffifcellsexceedabout50 °C [7] By keeping the battery/inverter cooler, the water-cooled system avoids or delays these cut-offs. This means the regenerative braking current can remain high rather than being throttled by thermal protection. In summary, the temperature drop from the cooling system translates into measurable performance gains: it cuts I²R losses and postpones thermal limits, yielding higher instantaneous regenefficiency.

Because energy recovered = power × time, sustaining higherregenpowertranslatesintosignificantlymoretotal energyrecovered.Forexample,ifthecooledsystemholds an extra 10 kW of regen power for one additional second beforecuttingoff,thatis10 kJofextraenergycapturedin that event. In simulation, the water-cooled case shows a “flatter” high-power regen profile, so the area under the power-vs-time curve is larger. Small per-event gains add up: our data shows about a 40 kJ increase in recovered energy for the cooled system per cycle. 40 kJ may sound modest, but each braking event only has on the order of 100–200 kJ available (e.g. a 1500 kg car at ~30 mph has ~120 kJ of kinetic energy). A 40 kJ boost is roughly a 30–35% improvement per stop. Over dozens of stops in typical driving, thatcan meanhundredsof kilojoules, ora few tenths of a kilowatt-hour, of extra recovery. These repeatedsmall gainsimproveoverall rangeand efficiency noticeably.

● Peak Temperature: The cooled system’s peak component temperature stays much lower. Since winding resistance increases with heat [6], a cooler motor has lower losses. Lower peak temperature also means the inverter and battery stay below their thermal limits longer, so high

regen currents can continue safely. In practice, this means the water-cooled system avoids the steep efficiency drop that the hot baseline experiences.

● Regen Efficiency: Regen efficiency (the fraction of braking energy captured) is higher with cooling. With lower I²R losses and delayed thermal cuts, the system converts a larger percentageofkineticenergyintostored electrical energy.Forexample,ifthebaselineconverts70% of available braking energy, the water-cooled version might achieve ~80% (these are illustrative). Even a few percent improvement matters because overall regen efficiency in EVs is inherently limited (typically best-case ~70% round-trip[1]).

● Energy Recovered: Thisisthetotalenergy(inkJ) captured in a braking cycle. The data show the water-cooled case recovering ~40 kJ more per cycle than baseline. That extra 40 kJ corresponds to many extra Wh per day of driving when summed over many stops. It represents real recovered work that would otherwise have been lost.

● Cumulative Effect: Even small per-cycle energy gains accumulate. For instance, 40 kJ per stop over 65 stops is 1.5 MJ (about 0.42 kWh) of extra energy recovered. This is the difference between limiting regen to mechanical brakes versus extending EV range. The water-cooled system thus yields a noticeable boost in total regen energyovertime.

Compared to the uncooled (air-cooled) baseline, the water-cooled system consistently outperforms on every key metric. The baseline heats up quickly, causing resistanceto riseand regen efficiencytofall insuccessive stops. In contrast, the cooled design stays nearer its ideal operating point: lower resistance, higher instantaneous efficiency, and sustained current. In practical terms, the baselinemightonlycapture60–70%ofthebrakingenergy before thermal cut-off, whereas the cooled case pushes that higher. The simulation data reflect this: cooled regen efficiency and total recovered energy are significantly abovebaselinevalues,andpeaktempsaremuchlower.

OVERALL IMPACT

The water-cooling loop has a clear positive effect on regenerative braking behavior. By keeping temperatures down, it reduces electrical losses and avoids early regen shutoffs.Thismeansthecarcanrecovermoreenergyfrom each braking event and keep doing so longer into a drive. The net result is higher energy efficiency – more of the

International Research Journal of Engineering and Technology (IRJET) e-ISSN:2395-0056

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vehicle’s kinetic energy gets put back into the battery instead of wasted as heat. In short, the cooled system boosts regeneration, improving energy recovery and makingtheEVsystemmoreefficientwithoutchangingany mechanicalcomponents.

REFERENCES

[1]JeffreyJenkins,“Regenerativebraking:Acloserlookat the methods and limits of regen”, Charged Electric Vehicles Magazine, October, 2018. [Online] Avaliable: https://chargedevs.com/features/regenerative-braking-acloser-look-at-the-methods-and-limits-of-regen/

[2] Susanne Reinhard & Baumüller Nürnberg, “Watercooled electric motors – In which cases do they make sense? Things to consider”, BaumüllerInsights. [Online] Available: https://www.baumueller.com/en/insights/drivetechnology/water-cooled-electric-motors

[3] Kevin Bennion, “Electric Motor Thermal Management Research:Annual Progress Report”,NREL,2017. [Online] Available: https://researchhub.nrel.gov/en/publications/electric-motor-thermalmanagement-research-annual-progress-report

[4] “Thermal Resistance”, Nidec Corporation. [Online] Available: https://www.nidec.com/en/technology/motor/glossary/i tem/thermal_resistance/

[5] “Basics of Thermal Resistance and Heat Dissipation”, Rohm Semiconductor. [Online] Available: https://fscdn.rohm.com/en/products/databook/applinot e/common/basics_of_thermal_resistance_and_heat_dissipa tion_an-e.pdf

[6] Lauren Nagel, “Motor Winding Resistance - How to Test It”, Tyto Robotics, April, 2023. [Online] Available: https://www.tytorobotics.com/blogs/articles/motorwinding-resistance-how-to-testit?srsltid=AfmBOor_fiNFwnFJyscLsNJeWJYpq5XOHvr2o4h ei_fjrX7rfRheulvg

[7] “BU-410: Charging at High and Low Temperatures”, Battery University, March 2022. [Online] Available: https://batteryuniversity.com/article/bu-410-chargingat-high-and-low-temperatures