Permanent Magnet Demagnetization

PM motor structures can be separated into two categories: interior and surface. Each category has its subset of categories. A surface PM motor can have its magnets on or inset into the surface of the rotor, to increase the robustness of the design. An interior permanent magnet motor positioning and design can vary widely. The IPM motor’s magnets can be inset as a large block or staggered as they come closer to the core. Another method is to have them imbedded in a spoke pattern.

Flux in a permanent magnet motor is generated by the magnets. The flux field follows a certain path, which can be boosted or opposed. Boosting or intensifying the flux field will allow the motor to temporarily increase torque production. Opposing the flux field will negate the existing magnet field of the motor. The reduced magnet field will limit torque production, but reduce the back-emf voltage. The reduced back-emf voltage frees up voltage to push the motor to operate at higher output speeds. Both types of operation require additional motor current. The direction of the motor current across the d axis, provided by the motor controller, determines the desired effect.

The angle of excitation is the angle at which the vector sum of the d-axis and q-axis waveforms are excited to the motor with respect to the d axis. The d axis is always viewed to be where the magnet exists. Maximum magnetic flux is achieved at the q axis, which is 90 electrical deg from the d axis. Therefore, most references of the angle of excitation already take into account the 90-deg difference from the d axis to the q axis.

Magnetic torque is maximized when the stator field excites the motor rotor 90 electrical deg from the d axis (motor magnet position). Reluctance torque follows a different path and is maximized 45 electrical deg past the q axis. The maximum magnetic torque takes advantage of both the motor’s reluctance and magnetic torques. Shifting further away from the q axis reduces magnetic toque, but is far outweighed by the gain in reluctance torque. The maximum combined magnetic and reluctance torque occurs near 45 electrical deg from the q axis, but the exact angle will vary based on the characteristics of the PM motor.

A PM motor’s power generation depends on the configuration of the motor magnets and the resulting motor saliency. Motors with a high saliency ratio (Lq > Ld) can increase motor efficiency and torque production by incorporating the motor’s reluctance torque. An inverter can be used to change the angle of excitation with respect to the d axis to maximize both the reluctance torque and magnetic torque of the motor.

Recent advances in drive technology allow standard ac drives to “self-detect” and track the motor magnet position. A closed-loop system typically uses the z-pulse channel to optimize performance. Through certain routines, the drive knows the exact position of the motor magnet by tracking the A/B channels and correcting for error with the z-channel. Knowing the exact position of the magnet allows for optimum torque production resulting in optimum efficiency.

Servomotors are permanent magnet motors used for motion control applications. Typically, in an interior/internal permanent-magnet motor design, these motors are paired with a specific amplifier as part of a matched set to maximize performance. The amplifier has been fine tuned to the PM motor to reach optimum performance by its manufacturer. The motion amplifier/servo configuration typically uses motor feedback, which also provides a magnetic pole position and speed feedback.

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