## 5 Drive System Simulation
### Simulation Results
We can observe from Figure 10(a) that the motor rotary speed could smoothly and swiftly follow the preset value and only had a slight fluctuation at 0.05 s when the load torque jumped. Figure 10(b) also indicates that motor's electromagnetic torque followed the desired torque well, the amplitude its fluctuation was limited within 3 N\(\cdot\)m under dual inverter's modulation by SVPWM strategy with an operating frequency of 10 kHz. The torque's fluctuation was reduced by about 50% compared to the multi-level current hysteresis modulation we proposed in [24]. Local curves of phase A's voltage and current are shown in Figure 11(a),b respectively, with a time range of 0.5-0.51 s.  
Theoretically, there are five levels in motor's phase voltage of a single inverter system under SVPWM strategy. In a dual inverter system, if the two DC-bus voltages are equal, there are nine levels in motor's phase voltage. Furthermore, there are even more phase voltage levels with unequal DC-bus voltages. These additional levels made the waveform of phase A's voltage shown in Figure 11(a) more close to the sine wave, compared to a single inverter system. Figure 11(b) also shows the current ripple was reduced significantly, compared to the system under the multi-level current hysteresis modulation [24]. In addition, we made a harmonic analysis of phase A's voltage in the time range of 0.35-0.55 s, while the motor was working in a steady state at the speed of 6000 r/min. The result is shown in Figure 13.  
Figure 11: Curves of motor speed and torque. are relatively large, with amplitudes above 10% of the fundamental amplitude. Considering the fundamental frequency is 400 Hz (rotor rotate speed multiplies number of pole-pairs), the frequency of the 50 th harmonic is 20 kHz, which is just twice as the SVPWM control frequency. Curves in regard to inverter1's output power following effect, and curves of two inverters' output power are shown in Figure 13(a),b respectively. From Figure 13(a) we can see, while inverter1's optimal output power \(P_{inv1\_opt}\) was fixed at 20 kW as a constant, the desired output power \(P^{*}_{inv1}\) had some fluctuations to approach the motor input power shown in Figure 14(b), which relieved the power outputting stress on inverter2. Due to the inertial element in the desired power calculator, the curve of \(P^{*}_{inv1}\) never went too far from \(P_{inv1\_opt}\) and had a moderate fluctuation, which had a positive effect on the primary power source's efficiency performance. The tolerance range of inverter1's power following, which is determined by \([P^{*}_{inv1}-D_{Pin1\_max},P^{*}_{inv1}+D_{Pin1\_max}]\), is indicated by the dashed lines. Within this range, inverter1's output power was considered following \(P^{*}_{inv1}\) accurately enough, thus the low switching frequency method was selected in priority to reduce inverter1's switching frequency. Most of the time, inverter1's actual output power stayed in the tolerance range as expected. However, in first 0.05 s and after the motor speed started going down, inverter1's actual output power couldn't follow \(P^{*}_{inv1}\) well. Because during these periods, motor's load torque was too low to demand a stator current vector with an  
Figure 14: Curves of inverter output power. enough amplitude for the power distribution in a big ratio or power transfer between the two power sources. The stator current is the medium of power transfer through the motor, thus a stator current vector with a considerable amplitude is necessary for the power distribution in a wide range or power transfer between the two power sources. In other words, the range of power distribution ratio or power transfer between the two power sources is limited by the amplitude of motor's stator current vector, which is also determined by motor's load torque. This limitation was discussed in [23], but it's not completely suitable for the circumstance here.