The rotor impedance varies with injection angle. When the high-frequency signal injecting axis and the magnetic pole axis (d-axis) are aligned, i.e. at 0 deg., the Interior Permanent Magnet (IPM) Motor terminal impedance decreases. The impedance is greatest at 90 degrees. By injecting high-frequency ac voltage/current to the IPM motor, the drive can detect rotor position without using pulse encoders. Furthermore, the high-frequency signal injection method can be used for speed detection in the low-speed region, where full-load torque control is typically difficult due to the motor's low back-emf voltage level.
Permanent Magnet Motor structures are classified into two types: interior and exterior. Each category has a set of subcategories. To increase the robustness of the design, surface PM motors can have their magnets on or inset into the surface of the rotor.
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