标签：三相,produces,current,电路,谐振,change From： https://www.cnblogs.com/luobaigu/p/17158703.html
- The change in current produces a change in magnetic field（磁场）, which in turn creates an induced(感应的) voltage.
- If we change the surround dirction of the current,as let it flows from the back that is actually equivalent to flowing from below.
- And we define when the port current flow in is