# Research on Constant Power Control Strategy of Pure Electric Excavator
## 4 EM Overload Constant Power Control  
When SOC of the battery pack is larger than 30%, the maximum allowable discharge power of battery is about 63 kW. The rated power of the EM is 49 kW. In order to improve the dynamic performance of the system, the EM can be operated above the rated power range in a short period of time. However, the overload operation of the EM will lead to a rapid increase of EM heat. Furthermore, the EM should also be operated within the maximum power range of EM. Therefore, in order to ensure the reliable operation of the power train system, the impact of the maximum allowable output power of the battery, the maximum power, and the temperature of the EM should be comprehensively considered in the constant power control. The temperature curve of the stator of an EM measured with different load rates is given in Figure 6, which is provided by the EM manufacturer. As can be seen, the stator temperature of the EM increases with the increase of the load power. A permanent magnet synchronous motor (PMSM) has a higher power density, which easily leads to an excessive temperature rise in high-power operation. Excessive temperatures will lead to permanent magnet demagnetization or serious life degradation. Therefore, in the overload power mode, it is necessary to restrict the power according to the temperature of the EM. Figure 6: Temperature variation of an EM stator with load change. As shown in Figure 7 and measured by an experimental platform, the temperature change of the EM driver before and after 2 min of operation at rated power is measured in the uncooled state. It can be seen that the temperature rise of the EM driver is large in case of the heavy duty load. The IGBT module is the key unit in the EM driver. Its current capacity decreases with the increase of temperature, which leads to a decrease of the driving ability of the EM driver. When the temperature rises to a certain extent, the failure rate will increase or even burst. Therefore, the working temperature of the EM driving system is another important factor affecting the dynamic performance and safety performance of the EM power train system. The IGBT module in the EM driver and permanent magnet in PMSM have their normal working temperature range. When their temperature reaches the critical temperature, constant power control should be employed to restrict the output power of the EM. By modifying the proportional coefficient \(k\) between the system pressure drop and the pilot pressure, the system power is restrained, so as to reduce the temperature of the IGBT module of the EM driver and the permanent magnet of EM, so that it can operate in the allowable temperature range. The constraints boundary of the temperature in the EM driver can be described as:  
\[T_{pmc}<T_{pmcmax}, \tag{12}\]  
where \(T_{pmc}\) is the working temperature of the EM driver. \(T_{pmc}\) is the upper limitation of the EM driver. The constraints boundary of the temperature in the EM can be described as:  
\[T_{pm}<T_{pmc}, \tag{13}\]  
where \(T_{pmc}\) is the working temperature of the EM. \(T_{pmc}\) is the upper limitation of the EM. In order to satisfy the dynamic performance of the power system, when the temperature of the EM and its driver is within the allowable range of normal operation, the output power of the hydraulic pump is limited, so that the current power of the EM does not exceed the maximum allowable power. The constraints of overload constant power control can be expressed as:  
\[\left\{ \begin{array}{l}T_{pmc}<T_{pmcmax}\\ T_{pm}<T_{pmcmax}\\ SOC\leq 30\%\end{array} \right.. \tag{14}\]  
When the constraints of overload power constant power control are satisfied, the output power of the control hydraulic pump can be expressed as:  
\[P_{p}\leq P_{mp}\cdot\eta_{m}\cdot\eta_{p}. \tag{15}\]  
Figure 7: EM driver temperature.