Overview

The continuous operation characteristics of motors for EV/HEV drives are extremely important as they are directly linked to the vehicle’s high-speed cruising performance and continuous climbing performance. When determining an efficiency map that considers continuous operation, it is necessary to consider the mutual influence of the magnetic field and heat, such as how much the temperature rises due to losses and how much torque can be exerted at high temperatures.
By using an efficiency map that takes temperature dependency into account, simulations can consider the mutual influence of the magnetic field and heat, thereby creating an efficiency map that considers continuous operation. In JMAG, thermal circuits can be configured using cooling models that consider changes in the coolant temperature, such as cooling jackets and shaft cooling combined with sprays.
In this example, we will create an efficiency map for an IPM motor that considers continuous operation and imposes temperature constraints on the components.

Analysis Flow

When creating an efficiency map that considers continuous operation, magnetic field analysis (efficiency map analysis) and thermal analysis are performed sequentially.
In magnetic field analysis (efficiency map analysis), multiple efficiency maps that include loss information are generated according to the combination of coil and magnet temperatures.
In thermal analysis, operating points are assigned based on the efficiency maps generated in the magnetic field analysis (efficiency map analysis), and steady-state thermal analysis is performed for each operating point. At each operating point, the steady-state thermal analysis is iteratively calculated to align the temperature dependency of the heat generation of the components in the magnetic field analysis (efficiency map analysis) with the steady-state temperature. From this, maps of current, losses, etc., considering continuous operation are obtained. By setting temperature constraints on these maps, operating points exceeding the temperature constraints are excluded, and NT characteristics and efficiency maps during continuous operation with temperature constraints are obtained.

Continuous Operation Characteristics

Fig. 2 shows the N-T curve for short-term operation at room temperature (20 deg C) and continuous operation considering temperature constraints (coil temperature limit 140 deg C, magnet temperature limit 180 deg C). Fig. 3 shows the efficiency map under the same conditions. For continuous operation, the coolant flow rate is set to 2 L/min.
From Fig. 2, it can be seen that the torque during continuous operation is reduced due to the increase in magnet temperature and the temperature constraints.
From Fig. 3, it can be seen that the maximum efficiency during continuous operation is reduced by approximately 1 point compared to short-term operation.

Impact of Coolant Flow Rate on Continuous Operation Characteristics

Efficiency maps considering continuous operation were created at two different levels of coolant flow rate (0.5 and 10 L/min). Fig. 4 shows the efficiency map with temperature constraints for the coil and magnet, Fig. 5 shows the coil temperature map, and Fig. 6 shows the coil copper loss map.
From Fig. 4, it can be seen that as the coolant flow rate, i.e., the cooling capacity of the motor, increases, the continuous operation torque also increases. Additionally, when the coolant flow rate is low (0.5 L/min), torque decreases in the ultra-low-speed region where the cooling capacity of shaft cooling decreases due to reduced rotational speed.
From Fig. 5, it can be seen that the coil temperature is kept lower at the same operating points due to increased cooling capacity. Furthermore, regardless of the coolant flow rate, the coil temperature near the outer N-T curve points is close to the constraint. This indicates that the expanded region due to increased coolant flow rate is freed from constraints because the improved cooling capacity lowers the temperature.
From Fig. 6, it can be seen that when the coolant flow rate is high (10 L/min), operating points with higher coil copper loss are displayed on the map compared to when the coolant flow rate is low (0.5 L/min). This means that increased cooling capacity allows for higher heat generation at operating points with greater coil copper loss.