What is inner voltage
This sounds like the EMF on the stator.  The rotating magnetic field in the machine induces an EMF on the stator windings which is more or less distributed along the length of the wiring.  Since the windings have a finite resistance, the stator windings can be modeled as an ideal voltage source (representing the induced EMF) in series with a resistor (representing the winding resistance).  The &quot;internal voltage&quot; is the magnitude of the ideal voltage source, which can be closely approximated by the open circuit voltage.