Understanding Permanent Magnet Motors

An ac induction machine (IM) also is commonly referred to as an ac motor. A rotating field is generated by the stator winding. The rotating field induces a current in the rotor bars. The current generation requires a speed difference between the rotor and the magnetic field. The interaction between the field and the current produces the driving force. Therefore, ac induction machines are the predominant motor operated by adjustable speed drives.

A PM motor is an ac motor that uses magnets imbedded into or attached to the surface of the motor’s rotor. The magnets are used to generate a constant motor flux instead of requiring the stator field to generate one by linking to the rotor, as is the case with an induction motor. A fourth motor known as a line-start PM (LSPM) motor incorporates characteristics of both motors. An LSPM motor incorporates a PM motor’s magnets within the rotor and a squirrel cage motor’s rotor bars to maximize torque and efficiency.

In geometric terms, the “d” and “q” axes are the single-phase representations of the flux contributed by the three separate sinusoidal phase quantities at the same angular velocity. The d axis, also known as the direct axis, is the axis by which flux is produced by the field winding. The q axis, or the quadrature axis is the axis on which torque is produced. By convention, the quadrature axis always will lead the direct axis electrically by 90 deg. In simplistic terms, the d axis is the main flux direction, while the q axis is the main torque producing direction. Salience or saliency is the state or quality by which something stands out relative to its neighbors. Magnetic saliency describes the relationship between the rotor’s main flux (d axis) inductance and the main torque-producing (q axis) inductance. The magnetic saliency varies depending on the position of the rotor to the stator field, where maximum saliency occurs at 90 electrical deg from the main flux axis.

According to the injection angle, rotor impedance varies. Interior permanent magnet (IPM) motor terminal impedance decreases when the high-frequency signal injecting axis and the magnetic pole axis (d-axis) are aligned, i.e. at 0 deg. The impedance is maximum at ±90 deg. Using this characteristic, the drive can detect the rotor position without pulse encoders by injecting high frequency ac voltage/current to the IPM motor. Moreover, the high-frequency signal injection method can be used for speed detection in the low-speed region where typically full-load torque control is very difficult because the motor’s back-emf voltage level is too low.

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