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.
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.
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).
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.