[In-depth Analysis of the Cooling System of the Drive Motor in New Energy Vehicles] Electric motors, as the driving force of electric vehicles, can achieve extremely low or zero emissions. During the operation of an electric vehicle for driving and energy recovery, the stator core and stator windings of the motor generate losses during movement. These losses are dissipated outwards as heat, requiring effective cooling media and methods to remove this heat and ensure the motor operates safely and reliably within a stable thermal cycle and ventilation system. The quality of the motor cooling system design directly affects the safe operation and lifespan of the motor. The diagram below shows the schematic of the drive motor cooling system for the BYD E6 model.
The cooling system for the electric motor and controller of an electric vehicle primarily relies on a cooling water pump to circulate coolant through cooling pipes. Through physical processes such as heat exchange in the radiator, the coolant carries away the heat generated by the motor and controller. To further enhance heat dissipation from the radiator, a fan is typically installed behind it.
I. Classification of Electric Motor Cooling Systems
When an electric motor is working, some of its energy is converted into heat. This heat must be continuously dissipated through the motor casing and the surrounding medium; this process of heat dissipation is called cooling. The main cooling methods for electric motors are natural cooling, air cooling, and water cooling. The composition, characteristics, and applications of each type of cooling system are shown in the following figure.
II. Common Vehicle Electric Motor Cooling Systems
1. Roewe E50
(1) Overview
The Roewe E50's electric cooling system consists of two independent systems: the inverter (PEB)/drive motor cooling system and the high-voltage battery pack cooling system (ESS). The high-voltage battery pack cooling system (ESS) has already been described in the relevant chapters of Module 3 and will not be repeated here.
The cooling system of the Roewe E50 inverter (PEB)/drive motor mainly consists of a radiator, cooling fan, expansion tank, coolant pump, coolant hoses, and coolant temperature sensor, as shown in the figure below.
The components and functions of the cooling system are shown in the figure below.
(2) Control of coolant flow
The circulation path of the coolant in the pipeline of the Roewe E50 cooling system is shown in the figure below.
As shown in the diagram above, the cooling principle of the drive motor is as follows:
Utilizing the principle of conduction, heat is transferred from the PEB/drive motor assembly to the coolant. The heated coolant flows through the evaporator pipes within the radiator, and is then dissipated into the atmosphere by airflow driven by a cooling fan. When the system is at a low temperature, the coolant pump does not operate. As the temperature rises, the coolant pump activates, and coolant flows through hoses into the radiator, which dissipates the heat into the air, maintaining the PEB/drive motor assembly at its optimal operating temperature.
The coolant flows from the upper right water chamber to the lower left water chamber through the radiator, where it is cooled by air passing through the core. The temperature of the cooling system is measured by an ECT sensor. This sensor sends a signal to the PEB, which controls the operation of the cooling fan as needed. The coolant temperature signal travels from the PEB through the CAN bus to the instrument cluster, where it is displayed in real time. If the coolant temperature becomes too high, warning lights and messages on the instrument cluster will alert the driver.
(3) Control of cooling fan
The control principle of the cooling fan in the Roewe E50 is shown in the figure below.
The PWM cooling fan is controlled by the vehicle control unit (VCU). When the cooling fan is working, the VCU controls the PWM module to make the cooling fan work at eight speeds within a duty cycle range of 20% to 90% to meet different cooling load requirements.
① Cooling fan activation conditions:
The operation of the cooling fan depends on two important factors: the A/C (Activated Coolant/Converter) and the PEB (Potentially Oxide/Boiler) coolant temperature. The coolant fan starts working when the A/C is activated or the PEB coolant temperature is above 52°C.
② Conditions under which the cooling fan stops working:
If the PEB coolant temperature is below 65°C and EAC is off, the cooling fan will stop working.
When the ignition switch is off and the EAC is off, and the PEB coolant temperature is above 65°C, the cooling fan will continue to work. If the ambient temperature is below 10°C, the cooling fan will work for 30 seconds; if the ambient temperature is above 10°C, the cooling fan will work for 60 seconds.
(4) PEB/Drive motor cooling system control
The operating temperature of the PEB should not exceed 75℃, and the optimal operating temperature should be below 65℃. Keeping the temperature below 75℃ can better extend the service life of both the PEB and the drive motor.
When the PEB starts working, the electric coolant pump will turn on immediately, and the coolant temperature sensor will provide a temperature signal to the ECT.
The PEB calculates the coolant temperature and compares it with the signal from the PEB coolant temperature sensor to determine whether the PEB coolant temperature sensor needs to be used.
2. GAC Trumpchi AG New Energy
The drive motor of the GAC Trumpchi AG new energy vehicle adopts a water-cooled cooling system. Its control principle and composition are basically the same as those of the Roewe E50; detailed principles will not be elaborated here. The following diagram shows the system composition.
The cooling system fluid flow path diagram is shown in the figure below.
3. BMW F18 (530LE)
To ensure effective cooling of the drive motor under any operating conditions, the BMW 530LE uses a cooling motor in its cooling system, which is connected to the engine's coolant lines. The coolant circulation paths for the BMW 530LE's engine and electric motor are shown in the diagram below.
To cool the stator windings, a cooling channel is provided between the stator support and the automatic transmission housing, through which coolant flows out from the engine cooling circuit. The cooling channel is sealed forward and backward by two sealing rings.
The transmission fluid cools the rotor; the oil mist of the transmission fluid absorbs heat and dissipates it into the atmosphere via the transmission fluid cooler.
The drive motor has an integrated thermostat that regulates the coolant flow temperature to an optimal range of approximately 80°C. This regulation is necessary because the motor's operating temperature is lower than the engine's. The thermostat is regulated by a paraffin thermostatic element that expands according to the coolant temperature. There is no electric control in this case. The thermostat's operating principle is shown in the diagram below.
The thermostat's operating status and coolant circulation path are shown in the following figure.
4. Toyota hybrid models
Toyota hybrid models, such as the Prius, Camry, and Corolla Hybrid, are equipped with a separate cooling system that operates independently of the engine cooling system, cooling the inverter, MG1, and MG2. This cooling system consists of a dedicated coolant reservoir, a dedicated coolant pump, a dedicated radiator, and dedicated coolant piping, as shown in the diagram below.
The cooling system is activated when the vehicle's power status is switched to READYON.
The dedicated radiators for the inverter, MG1, and MG2 are mounted on top of the condenser (air conditioner). The layout is made more compact by integrating independent inverter radiators, air conditioner condensers, and engine radiators.
