There are many mathematical models for the position sensorless control of permanent magnet synchronous motors (PMSMs), and various position sensorless control methods have already proposed. However, it is necessary to select a control model according to the objective from characteristics of the EMF phase and the robustness against to the motor parameters. An Imaginary Electromotive Force (IEMF) expression can clarify the differences of these models. This paper discusses the characteristics of each model including the Extended Electromotive Force model for position sensorless control that can be applied to the standstill / low speed range from the middle / high speed range. Finally, we show that this IEMF expression is extremely useful in discussing and designing characteristics of the control models of PMSMs.