Abstract: Taking parameter variation into account, a new model of the induced electromotive force relation of a permanent magnet synchronous motor (PMSM) was presented. Two kinds of induced electromotive force observers with high precision, differential and non-differential, were established. The stability and astringency of the induced electromotive force observers was proved. The effect of parameter variation on their position estimation was analyzed. For a sensorless vector control system, the interior permanent magnet synchronous motor for instance was systematically simulated. The results show that the induced electromotive force observers have excellent performance.