In servo drive control systems, current sensing is a crucial component: acquiring the DC bus current enables short-circuit protection to ensure inverter safety; measuring the motor phase current enables current feedback control and motor overload protection to improve motor drive system performance. The classic current sensing method uses current sensors, but the high cost of these sensors increases system expenses. Another method utilizes multiple inexpensive linear resistors to obtain current information, but this is sometimes difficult to implement due to hardware limitations. To reduce system cost and size, using single-current sensing technology to obtain motor and drive system current information has become an effective approach. This paper, based on the analysis of inverter commutation under complementary PWM mode, proposes an AC motor phase current reconstruction technology under space vector PWM (SVPWM) control. This technology uses a linear resistor to sample the inverter's DC bus current, calculates the current in each phase based on the inverter's switching state and the three-phase current relationship, and realizes the phase current reconstruction of the AC motor. PWM Mode Inverter Commutation Analysis In modern AC servo drive control systems, complementary output PWM is generally used to control the inverter's power switching devices to adjust the inverter's output voltage amplitude and frequency. Complementary output means that when the upper bridge arm device is on, the lower bridge arm device is off, and vice versa. Figure 1 shows the main inverter circuit structure. In the inverter system shown in Figure 1, the switching variables are defined as either 0 or 1, where 1 indicates the upper bridge arm power switch is on, and 0 indicates the lower bridge arm power switch is on. Table 1 shows the switching states of the three-phase inverter. When (Sa, Sb, Sc) = (0, 0, 0) and (Sa, Sb, Sc) = (1, 1, 1), the inverter output voltage is zero. Therefore, these two switching states are defined as the zero state, and the remaining six states are defined as the active state. When the inverter switch is in the active state, for example, when (Sa, Sb, Sc) = (1, 0, 0), the current flow path is shown in Figure 2. As can be seen from Figure 2, in this state, the DC bus current Idc is the A-phase current of the AC motor. Figure 2 shows the current flow path when the switch state is 100. Figure 3 shows the current flow path when the switch state is 000. Figure 4 shows the current flow path during commutation. When the inverter switch changes from state (Sa, Sb, Sc) = (1, 0, 0) to (Sa, Sb, Sc) = (0, 0, 0), since the current cannot change abruptly, diode D4 conducts, and the A-phase current freewheels through diode D4. The current flow path is shown in Figure 3. At this time, the DC bus current Idc is zero, and the three-phase winding current flows inside the motor. When the inverter switch state changes from (Sa, Sb, Sc) = (1, 0, 0) to (Sa, Sb, Sc) = (1, 1, 0), due to the dead time set to prevent short circuits between the upper and lower bridge arms of the inverter, in the initial stage of commutation, both switching transistors T6 and T3 are simultaneously in the off state. The current flow path at this time is shown in Figure 4. When commutation ends, switch T3 is turned on, and the current flow path is shown in Figure 5. Figure 5 shows the current flow path when the switch state is 110. Figure 4 shows that at the beginning of commutation, the DC bus current Idc is the C-phase current of the AC motor. As can be seen from Figure 5, after commutation ends, the inverter DC bus current Idc is still the C-phase current of the AC motor. Phase current reconstruction technology According to the commutation analysis of the inverter under the complementary PWM method, except that the inverter DC bus current is zero in the zero state, there is always current flowing through the inverter DC bus in the effective state, which is equal to the phase current of the motor. Therefore, the phase current of the motor can be obtained by using the inverter main circuit structure shown in Figure 6, where the linear resistor R is the DC bus current sampling resistor. From the perspective of improving voltage utilization and reducing inverter switching losses, SVPWM is an optimized complementary PWM method. According to the SVPWM principle [1] and inverter commutation analysis, the correspondence between the DC bus current and the motor phase current shown in Table 2 can be obtained. Therefore, the phase current of the AC motor can be reconstructed based on the inverter switching state, the DC bus current Idc, and the relationship between the three-phase currents of the motor. Figure 6 shows the structure diagram of the main circuit of the single-current detection inverter. The controller of the experimental system is a TMS320F2407 DSP, the switching frequency of the inverter is 10kHz, and the experimental motor is a 500W AC asynchronous motor. In the experiment, in each effective state of the inverter switch, the DSP samples the DC bus current Idc, and in two adjacent bus current sampling cycles, according to the relationship ia + ib + ic = 0 of the three-phase currents of the motor, the instantaneous value of the phase current is calculated to realize the reconstruction of the motor phase current. (a) Phase current waveform (b) Phase current reconstruction waveform Figure 7 shows the experimental waveform of the motor phase current. Figure 7a) is the A-phase current waveform of the three-phase AC asynchronous motor, and 7b) is the playback waveform of the A-phase current after reconstruction and digital-to-analog conversion on the oscilloscope. Since the inverter's switching frequency is 10kHz, the sampling period for the inverter's DC bus current Idc is 0.1ms. The change in motor phase current within this 0.1ms time interval is very small, therefore the motor phase current reconstructed by the DSP has sufficiently high accuracy. Conclusion In many applications, there are strict controls on the cost and size of AC motor servo drive products. Using single-current sensing technology to obtain motor and drive system current information is an effective way to meet this requirement. The method proposed in this paper needs to be completed in two adjacent switching cycles, which is sufficient to meet the required current control accuracy for inverter drive systems with high switching frequencies.