Update – March 1, 2023: AxSTREAM NET is our legacy software replaced by AxSTREAM System Simulation. System Simulation was born out of the union of the legacy AxCYCLE and AxSTREAM NET software packages.
Electric motors are all around us. They feature prominently in every major industry, and in many of the devices we use daily. For instance, this author’s personal morning routine relies on electric motors when using a coffee grinder, when turning on a desktop computer to read the news, and even when setting up an automatic cat feeder. Electric motors convert electrical energy into mechanical energy through interaction between the magnetic fields generated in the motor’s stator and rotor windings. To meet the power requirements of different industries and applications, electric motors are available in a variety of strengths and sizes.
Electric motors can have remarkably high efficiency ratings of over 90 percent. In other words, a large portion of the electrical energy that is supplied to the motor is successfully converted into mechanical output. The approximately 10 percent remaining is lost in the form of heat. Regardless of the application, one of the main challenges that motor designers face is that of thermal management.Electric motors can have remarkably high efficiency ratings of over 90 percent. In other words, a large portion of the electrical energy that is supplied to the motor is successfully converted into mechanical output. The approximately 10 percent remaining is lost in the form of heat. Regardless of the application, one of the main challenges that motor designers face is that of thermal management.
Selection of the right electric motor is often based on a particular work or load requirement. When an electric motor is in operation and high performance is needed, the motor’s load can be increased (letting the motor draw more current), and greater heat is generated due to increases in rotor and stator losses. Since the heat flux in a system influences its thermal behavior, the motor’s temperature evolution depends on these losses.
Loads are limited by a motor’s thermal limit conditions—especially the maximum temperature allowed inside the motor, where the windings and permanent magnets reside. If the temperature is not controlled, materials can exceed their normal operating temperatures and experience phase change, softening, melting, or other forms of degradation. Thermal stresses that can cause fatigue, cracking, and material deformation don’t only shorten a motor’s lifetime, but can also lead to serious safety issues. For example, some electric motors use rare earth magnets that can overheat to the point that they become demagnetized. Thus, maintaining optimal temperature levels is necessary for the sake of avoiding efficiency reduction and ensuring a more reliable and robust motor. To that end, the generated heat must be managed by an appropriate cooling system.
Several types of cooling systems are available for electric motors, including air cooling, liquid cooling, heat pipes cooling, and hybrid cooling with heat pipes and liquid. These four types of systems are presented in Figure 2 as (a), (b), (c), and (d) respectively. The optimal cooling system choice depends on the intended application, motor mounting location, operating environment, and other factors (see SoftInWay’s past blog entries on thermal management in electric propulsion to learn more about different cooling systems for electric motors).
One example of an electric motor liquid cooling system with cooling channels is presented in Figure 3. There, the system provides frame liquid cooling via liquid jackets (stator cooling channels) around the motor corner, and rotor liquid cooling via the motor shaft (shaft cooling channel). The coolant flows through the stator and shaft cooling channels to absorb the thermal flux, while the external environment acts as a heat evacuation medium to dissipate the absorbed heat. In the stator channel, the liquid flows axially from the front to the rear of the motor. The liquid in the channel enters through the frame lateral surfaces at the front of the motor and exits through the corresponding surfaces at the rear. This improves the heat transfer from motor cavities to the coolant directly through those lateral surfaces. In the rotor shaft, the cooling channel has a circular cross-section. As in the stator channel, the liquid flows axially along the motor’s rotation axis from the front to the rear of the motor shaft. The coolant pipe from the heat exchanger is connected to the motor cooling channels at the inlet and outlet for thermal behavior evaluation.One example of an electric motor liquid cooling system with cooling channels is presented in Figure 3. There, the system provides frame liquid cooling via liquid jackets (stator cooling channels) around the motor corner, and rotor liquid cooling via the motor shaft (shaft cooling channel). The coolant flows through the stator and shaft cooling channels to absorb the thermal flux, while the external environment acts as a heat evacuation medium to dissipate the absorbed heat.In the stator channel, the liquid flows axially from the front to the rear of the motor. The liquid in the channel enters through the frame lateral surfaces at the front of the motor and exits through the corresponding surfaces at the rear. This improves the heat transfer from motor cavities to the coolant directly through those lateral surfaces. In the rotor shaft, the cooling channel has a circular cross-section. As in the stator channel, the liquid flows axially along the motor’s rotation axis from the front to the rear of the motor shaft. The coolant pipe from the heat exchanger is connected to the motor cooling channels at the inlet and outlet for thermal behavior evaluation.
A schematic diagram of a complete liquid cooling network can be seen in Figure 4. The diagram includes end-windings potting and windings channels. Potting involves filling electronic assemblies with a compound (typically an epoxy resin) that protects components. For end-windings potting, a solid connection between the end-windings and frame is made using a highly conductive resin, which allows efficient conduction of heat through the potting material—thus lowering the temperature in the end-windings critical zone.
To allow the evacuation of heat to the outside, the cooling system in this diagram employs frame liquid cooling with liquid jackets around the motor core, as well as end-windings potting and rotor liquid cooling. In addition to these methods, direct cooling of windings through the windings channels allows dissipation of the high heat generated in windings and surroundings (due to Joule and iron losses).To allow the evacuation of heat to the outside, the cooling system in this diagram employs frame liquid cooling with liquid jackets around the motor core, as well as end-windings potting and rotor liquid cooling. In addition to these methods, direct cooling of windings through the windings channels allows dissipation of the high heat generated in windings and surroundings (due to Joule and iron losses).
Different types of electric motor cooling flow systems can be accurately modeled and analyzed with AxSTREAM NET™, by creating a cooling flow passage using a 1D thermal-fluid network approach. Figure 5 shows an AxSTREAM NET project modeling a 1D thermal-fluid network of an electric motor liquid cooling system with cooling channels and solid walls.
Here, fluid flow in the frame, windings, and shaft is simulated using pipes and annular channels. Surface and thermal elements are added and connected to the fluid network to simulate convective heat transfer between the fluid flow and the pipes’ solid walls. Wall elements are used to represent the motor’s solid parts and are connected to each other to model conductive heat transfer between them. In this way, motor cooling systems can be modeled and analyzed with AxSTREAM NET, thus providing accurate predictions of coolant temperatures, motor wall temperatures, and flow rate distribution in cooling channels.
Selecting the right cooling system for an electric motor is far from easy. Many factors go into the decision, as the optimal cooling system depends on the application, operating environment, lifetime requirements, machine configuration, classification, power level, and more. SoftInWay offers consulting and software solutions to help engineers who face such decisions make the right choice. The technical team has extensive experience and a thorough understanding of the most advanced cooling methods—along with their pros and cons.
Are you interested in learning how AxSTREAM NET can help you design or analyze an electric motor cooling system? Reach out to us at Info@Softinway.com to schedule a demo!
References
After more than a century and a half of development and improvements, the internal combustion engine’s thermal efficiency is still a joke compared to electric motors. That’s because of the more complex design of the ICE, and the high temperatures, pressure, and friction it must endure. Nevertheless, electric motors’ thermal management is not as simple as you might think. And oil is a big part of the equation.
Photo: GM
ICE
cooling circuit, but it is kind of tricky. For instance, water-glycol-based cooling systems are very limited because of their electrical conductivity. This means the risk of anPhoto: Bosch
Photo: IDTechEx
EV
market in all transport areas is expected to generate $2.6 trillion (€2.4 trillion) by 2042.Photo: GM
When people talk about electric cars, most of the debate is about batteries and their shorter range compared to conventional cars. Generally, battery improvements are related to their thermal management.Active cooling with water-glycol-type coolants increases batteries’ performance, durability, and fast-charging capabilities. By comparison, air-cooled batteries – which means they don’t really have a thermal management system – are much more prone to critics.On the other hand, a thermal management system makes the cost of a battery go higher and requires regular maintenance. This is an uncomfortable truth for proponents of EVs. And there’s more. The electric motor and power electronics also require some thermal management.While it’s true an electric motor has no moving parts that get in contact with one another – thus no losses because of the heat and no intensive wear or degradation – it’s also true an electric motor gets hot and it should be cooled down.Usually, electric motors and inverters’ optimal operating temperature is around 60°C (140°F). The cooper windings in the stator generate the electric fields used to drive the rotor. Consequently, a high amount of heat is generated, and it needs a cooling circuit.You might think it’s not as complicated as ancooling circuit, but it is kind of tricky. For instance, water-glycol-based cooling systems are very limited because of their electrical conductivity. This means the risk of an electrical short circuit is serious if it gets in direct contact with electrical components.That’s why the most common way to cool an electric motor is to use a 'water jacket' around the outside of the stator. But this simple solution is almost of no use for high-power electric motors, which require better cooling on the inside for improving their efficiency as much as possible.It turns out that water-glycol coolants can be replaced by oils. As a matter of fact, engineers used it for decades in industrial electric motors, while in a conventional car, the oil in the transmission is used both for lubricating and cooling the moving parts.Inside an electric motor, direct contact with oil poses no short circuit risk and it removes the heat from internal parts more effectively, especially from a high-speed moving rotor. The oil also lubricates the rotor’s bearings, while stator windings’ dilatation and wear caused by the heat is greatly minimized.Oil cooling can successfully replace the 'water jacket' and this can lead to smaller and more power-dense electric motors . Oil-cooling system can remove double or triple the heat compared to a water-glycol cooling system and even ten times more than an air-cooling system.In simple words, using oil for cooling can allow a serious increase in the electric motor’s power, without the need to radically change the design. However, other issues arise. For instance, the heated oil must come out of the motor and be cooled before re-entering the motor.As such, oil-cooling thermal management adds a lot to the complexity of modern and powerful electric cars. This also means an increase in the price, which is still a burden for EVs’ competitiveness, despite the performance benefits outweighing the complexity of oil cooling.In recent years, many electric motors design were changed to benefit from oil cooling. Research company IDTechEx reveals that in the first half of 2022 oil cooling for electric motors was adopted by 50% of the new electric cars sold worldwide.In the meantime, only one-third of EVs were using the water-glycol coolant solution, while electric motors cooled by air had a small 13% share market. In the next ten years, forecasts favor the oil-cooling solution market share increase, while air-cooled electric motors are expected to almost be phased out from cars.The market evolved quickly, and engineers created integrated thermal management systems for both battery and electric motor, which are more efficient overall by combining oil cooling and water-glycol cooling. But things will get even better.The industry is working hard to find the best way to integrate inverters into electric motors. This way, the drive system would be more compact and more efficient, and also suitable for a larger number of applications.Nowadays, inverters are mostly cooled by water-glycol coolants. In the near future, integrating inverters into electric motors would provide the benefits of direct oil cooling, further enhancing EVs’ performances and range.These forecasts are particularly useful for the oil industry . The mass electrification of road transport and heavy investments in hydrogen and near-zero-emissions fuels for ships, airplanes, trains, and heavy trucks and machinery will lead to a massive drop in petroleum fuel demand.Oil companies are already changing their priorities to favor the petrochemical sector, but the oil business is nothing to ignore. According to predictions, themarket in all transport areas is expected to generate $2.6 trillion (€2.4 trillion) by 2042.As oil-cooled electric motors will most probably account for more than three-quarters of electric vehicles, the demand for oil coolants is expected to be a very profitable business. Oil coolants are either mineral oils or synthetic oils, which are sourced from crude oil.Of course, the industry will still have to apply best practices for recycling these oils or for near-zero-emissions sourcing of raw materials, and also for carbon-free technologies for manufacturing it. For now, I’m sure this might be another reason for EV antagonists to attack electromobility – just like the battery manufacturing-related pollution debate.There are still challenges along the way, but at least using oils in electric motors’ thermal management system is much more common sense than burning them in combustion engines. And that really is part of the evolution to a more sustainable transportation system.