## Operating Principle
### System Modeling  
First, we will introduce the concept of mid-point voltage. By equally dividing primary power source and secondary power source into two parts according to their voltages, we can acquire virtual mid-points m and n. Voltage difference between each inverter leg's output and corresponding mid-point is mid-point phase voltage, and voltage difference between mid-points of the two power sources is mid-point voltage difference. As shown in Figure 2, \(V_{dc1}\) and \(V_{dc2}\) are the DC-bus voltages of primary power source and secondary power source respectively. \(u_{Am}\), \(u_{Bm}\) and \(u_{Cm}\) are the mid-point phase voltages of inverter1. \(u_{Xn}\), \(u_{Yn}\) and \(u_{Zn}\) are the mid-point phase voltages of inverter2. \(u_{mm}\) is the mid-point voltage difference between the two power sources.  
The phase voltages of OW-PMSM are affected by both inverters' switching states. Based on [PERSON]'s law, the phase voltages of OW-PMSM are determined by Equation (1):  
\[\left\{\begin{array}{l}u_{AX}=u_{Am}-u_{Xn}+u_{mm}\\ u_{BY}=u_{Bm}-u_{Yn}+u_{mm}\\ u_{CZ}=u_{Cm}-u_{Zn}+u_{mm}\end{array}\right. \tag{1}\]  
Figure 2: Schematic diagram of mid-point voltage.  
In motor stator plane, the stator voltage vector of OW-PMSM \(\overset{\rightharpoonup}{u_{s}}\) is synthesized by the three phase voltages as follow:  
\[\overset{\rightharpoonup}{u_{s}}=\sqrt{\frac{2}{3}}(u_{AX}e^{j0}+u_{BY}e^{j \frac{2\pi}{3}}+u_{CZ}e^{j\frac{4\pi}{3}}) \tag{2}\]  
where \(e^{j0}\), \(e^{j\frac{2\pi}{3}}\) and \(e^{j\frac{4\pi}{3}}\) are spatial operators, pointing the directions of the three phases' axes respectively. \(\sqrt{\frac{2}{3}}\) is the coefficient of equivalent power conversions. Similarly, the output voltage vector of the two inverters \(\overset{\rightharpoonup}{u_{s1}}\) and \(\overset{\rightharpoonup}{u_{s2}}\) expressed by mid-point phase voltages are given as:  
\[\left\{\begin{array}{l}\overset{\rightharpoonup}{u_{s1}}=\sqrt{\frac{2}{3} }(u_{Am}e^{j0}+u_{Bm}e^{j\frac{2\pi}{3}}+u_{Cm}e^{j\frac{4\pi}{3}})\\ \overset{\rightharpoonup}{u_{s2}}=\sqrt{\frac{2}{3}}(u_{Xn}e^{j0}+u_{Yn}e^{j \frac{2\pi}{3}}+u_{Zn}e^{j\frac{4\pi}{3}})\end{array}\right. \tag{3}\]  
This indicates \(\overset{\rightharpoonup}{u_{s1}}\) and \(\overset{\rightharpoonup}{u_{s2}}\) are only determined by switching states of the corresponding inverter, and not affected by the other side. Thus, \(\overset{\rightharpoonup}{u_{s1}}\) and \(\overset{\rightharpoonup}{u_{s2}}\) can be generated by two independent space vector pulse width modulation (SVPWM) controllers.  
Substituting Equations (1) and (3) into (2) results in an expression for \(\overset{\rightharpoonup}{u_{s}}\) in terms of \(\overset{\rightharpoonup}{u_{s1}}\) and \(\overset{\rightharpoonup}{u_{s2}}\), shown by Equation (4):  
\[\overset{\rightharpoonup}{u_{s}}=\overset{\rightharpoonup}{u_{s1}}- \overset{\rightharpoonup}{u_{s2}}+\sqrt{\frac{2}{3}}u_{mn}(e^{j0}+e^{j\frac{2 \pi}{3}}+e^{j\frac{4\pi}{3}})=\overset{\rightharpoonup}{u_{s1}}-\overset{ \rightharpoonup}{u_{s2}} \tag{4}\]  
Because the two power sources are electrically isolated, the mid-point voltage difference \(u_{mn}\) is floating and varying along with the switching states of dual inverter. However, when synthesizing the motor stator voltage vector \(\overset{\rightharpoonup}{u_{s}}\), the floating \(u_{mn}\) is eliminated as shown in Equation (4). That is because the three phase voltages contain identical component of \(u_{mn}\), which just counteract each other in motor stator plane. In essence, when the loads of three phase windings are completely symmetrical, the synthesized motor stator voltage vector should be free of zero-sequence component, which means the following equation holds:  
\[u_{AX}+u_{BY}+u_{CZ}=0 \tag{5}\]  
By substituting (1) into (5), the value of \(u_{mn}\) can be obtained.