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Understanding Permanent Magnet Motors

Controlling the speed of ac motors is accomplished using a variable frequency drive (VFD) in most cases. While many scenarios involve using VFDs with induction motors with stator windings to generate a rotating magnetic field, they also can achieve precise speed control using speed or position feedback sensors as a reference to the VFD.

In some situations, it is possible to obtain comparably precise speed control without the need for feedback sensors. This is made possible using a permanent magnet (PM) motor and a process called the “high-frequency signal injection method.”

1. PM motors

A PM motor is an ac motor that uses magnets embedded 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.

2. Back-emf waveform

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Back emf is short for back electromotive force but is also known as the counter-electromotive force. The back electromotive force is the voltage that occurs in electric motors when there is a relative motion between the stator windings and the rotor’s magnetic field. The geometric properties of the rotor will determine the shape of the back-emf waveform. These waveforms can be sinusoidal, trapezoidal, triangular, or something in between.

Both induction and PM machines generate back-emf waveforms. In an induction machine, the back-emf waveform will decay as the residual rotor field slowly decays because of the lack of a stator field. However, with a PM machine, the rotor generates its own magnetic field. Therefore, a voltage can be induced in the stator windings whenever the rotor is in motion. Back-emf voltage will rise linearly with speed and is a crucial factor in determining maximum operating speed.

3. Understanding PM machine torque

An electric machine’s torque can be broken down into two components: magnetic torque and reluctance torque. Reluctance torque is the “force acting on the magnetic material that tends to align with the main flux to minimize reluctance.” In other words, reluctance torque is the torque generated by the alignment of the rotor shaft to the stator flux field. Magnetic torque is the “torque generated by the interaction between the magnet’s flux field and the current in the stator winding.”

Reluctance torque: Reluctance torque pertains to the torque generated through the alignment of the rotor that occurs when the magnetic field forces a desired direct flow from the north stator pole to the south stator pole.

Magnetic torque: Permanent magnets generate a flux field in the rotor. The stator generates a field that interacts with the rotor’s magnetic field. Changing the position of the stator field with respect to the rotor field causes the rotor to shift. The shift due to this interaction is the magnetic torque.

4. SPM versus IPM

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A PM motor can be separated into two main categories: surface permanent magnet motors (SPM) and interior permanent magnet motors (IPM). Neither motor design type contains rotor bars. Both types generate magnetic flux by the permanent magnets affixed to or inside of the rotor.

SPM motors have magnets affixed to the exterior of the rotor surface. Because of this mechanical mounting, their mechanical strength is weaker than that of IPM motors. The weakened mechanical strength limits the motor’s maximum safe mechanical speed. In addition, these motors exhibit very limited magnetic saliency (Ld ≈ Lq). Inductance values measured at the rotor terminals are consistent regardless of the rotor position. Because of the near unity saliency ratio, SPM motor designs rely significantly, if not completely, on the magnetic torque component to produce torque.

IPM motors have a permanent magnet embedded into the rotor itself. Unlike their SPM counterparts, the location of the permanent magnets makes IPM motors very mechanically sound, and suitable for operating at very high speeds. These motors also are defined by their relatively high magnetic saliency ratio (Lq > Ld). Due to their magnetic saliency, an IPM motor has the ability to generate torque by taking advantage of both the magnetic and reluctance torque components of the motor.

5. PM motor structures

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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 embedded in a spoke pattern.

6. PM motor inductance variation with load

Only so much flux can be linked to a piece of iron to generate torque. Eventually, the iron will saturate and no longer allow flux to link. The result is a reduction in the inductance of the path taken by a flux field. In a PM machine, the d-axis and q-axis inductance values will reduce with increases in the load current.

The d and q-axis inductances of an SPM motor are nearly identical. Because the magnet is outside of the rotor, the inductance of the q-axis will drop at the same rate as the d-axis inductance. However, the inductance of an IPM motor will reduce differently. Again, the d-axis inductance is naturally lower because the magnet is in the flux path and does not generate an inductive property. Therefore, there is less iron to saturate in the d-axis, which results in a significantly lower reduction in flux with respect to the q-axis.

7. Flux weakening/intensifying of PM motors

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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 the 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.

8. Self-sensing versus closed-loop operation

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 errors with the z-channel. Knowing the exact position of the magnet allows for optimum torque production resulting in optimum efficiency.

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