# Design and Verification of a Modular Reconfigurable Test Platform for Electric Tractors
## 4 Case Study
### Results and Discussion  
Before 160 s, the test platform simulated a pure electric tractor with a dual-motor independent drive architecture under rotary tillage operations. At 75 s, the PTO motor was provided the reference torque to simulate the resistance torque of the rotary blade shaft. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Values** & **Units** \\ \hline Tractor mass & 3000 & kg \\ Driving wheel radius & 0.5 & m \\ Forward speed & 2.4 & km/h \\ PTO motor speed & 540 & rpm \\ Engine working speed & 1600 & rpm \\ Charging current & 15 & A \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Primary setting parameter values of the test scenario. Figure 8: The curve of the power takeoff (PTO) reference torque cycles. After 160 s, the test platform simulated the series hybrid electric tractor under rotary tillage operation, and the genset began to charge the battery at 225 s.  
The experimental results under the rotary tillage operation are illustrated in Figures 9 and 10. The curves of the torque and speed for each electric machine (e.g., the two LMs, the PTO motor, and the DMs) are displayed in Figure 9. What needs to be explained here is that the result of DM1 is the same as the PTO motor, which is not shown in Figure 9. This is because DM1 and the PTO motor are fixed directly to each other without a reducer (this is a limitation of this study and will be fixed in future work). As can be clearly seen in Figure 9a, whether in the purely electric mode or the series hybrid mode, the torque of DM2 and the two LMs responded almost simultaneously, and the control precision of the PTO motor and DM1 remained excellent compared to the reference torque cycle, which all indicates that the control system for this test platform has a good control effect. In addition, in the series hybrid mode, the genset received a constant charging current control command. The genset torque quickly reached a particular value, started to decrease slowly, and finally tended to fluctuate in a stable range, as shown by the green curve (Figure 9a). This may have occurred because the generator cooling system was not run before the genset worked for about 25 s, but the temperature of the genset system kept rising, which led to an increase in the thermal energy of the system. To generate a constant charging current value, the engine was forced to provide a larger torque. When the cooling system started to work, the temperature of the genset system gradually cooled down, and the thermal energy decreased to a stable range. This is why the output torque of the engine slowly decreased and tended toward a certain range. The operating state of the genset also became gradually stable, further indicating that the platform can operate normally. As shown in Figure 9b, the speed response time of the DM2 was faster than that of the two LMs. The same phenomenon occurred in the DM1 and the PTO motor due to the motor characteristics of PMSMs and AMs. However, the speed response time between the two LMs was not the same. Specifically, the response speed of the right load motor was less than that of the left load motor. This was caused by the different frictional resistance of the mechanical connection between the left and right load motors via the drive axle differential. Moreover, the engine speed increased from idle speed to the set value of 1600 r/min, and the speed fluctuation range of the genset was 1590 to 1606 r/min. Further, the maximum relative error compared to the set value was approximately 0.625%. The speed fluctuation of motor 2 was 354-355 r/min, and the maximum relative error with a theoretical value of 353.6 r/min was approximately 0.396%. The left load motor speed fluctuation was 223-227 r/min, and for the right motor, the fluctuation was 222-225 r/min. The maximum relative errors with a theoretical value of 353.6 r/min were approximately 0.889% and 1.333%. The PTO motor was run using variable torque, which made the speed fluctuation larger.