Ferrite motors have the advantage of low cost but also have disadvantages such as low power/torque density, easy demagnetization, complex mechanical structure, large torque fluctuation, and large temperature coefficient. Taking advantage of the advantages while reducing/eliminating the disadvantages is the diligent pursuit of electrical machine researchers and engineers. Shortcomings can be made up by using advanced design technology, so to a certain extent, the design difficulty of the optimal solution for ferrite permanent magnet motors is greater than that of rare earth permanent magnet motors.

Demagnetization

The demagnetization rate can be defined as the amount of irreversible demagnetization divided by the total amount. Ferrite permanent magnet motors generally have higher demagnetization rates compared to rare earth permanent magnet motors. Compared with NdFeB, the Curie temperatures of different grades of ferrite permanent magnets are relatively high, and the usual temperature rise of ferrite motors will not cause irreversible demagnetization. However, due to their low coercivity, ferrite permanent magnets can be easily demagnetized by a demagnetizing field.

Temperature affects the coercivity of NdFeB and ferrite permanent magnets in two opposite ways. Typical NdFeB permanent magnets have higher coercivity at low temperatures, while ferrite permanent magnets have lower coercivity when the temperature drops. The coercive force temperature coefficient of ferrite permanent magnets is positive, while the coercive force temperature coefficient of NdFeB permanent magnets is negative. In addition, as the temperature decreases, the critical knee point on the ferrite permanent magnet magnetization curve moves upward (from the third quadrant of the BH plane to the second quadrant). Therefore, low temperatures in ferrite motors are more dangerous than high temperatures in terms of irreversible demagnetization. Figure 22 shows the demagnetization curves of ferrite permanent magnet materials at different temperatures.

Fig.22 Variations of demagnetization curve of a Fe-PM (Grade:Y33BH) with temperature.

Temperature, motor geometry, number of winding turns, current amplitude, current phase angle and rotor position can all have a serious impact on the irreversible demagnetization of ferrite motors. Therefore, they should be carefully taken into account when studying demagnetization effects. To achieve these ends, it is necessary to consider worst-case scenarios.

Working conditions that may cause demagnetization:

lowest possible operating temperature

Maximum possible current

Worst current phase angle and worst rotor position (resulting in lowest flux density).

Special events, such as overloading or temporary short circuits at the motor terminals, especially at low temperatures, can demagnetize the ferrite permanent magnets. Therefore, this unusual operating condition should also be taken into account. The final design should have good demagnetization tolerance.

Another parameter that has a major influence on demagnetization is the electrical conductivity of the permanent magnet material. For a typical ferrite permanent magnet, the electrical conductivity is very low. In other words, the eddy current inside the ferrite permanent magnet is negligible.

For applications with considerable ambient temperature changes or necessary field weakening control, the evaluation of demagnetization intensity requires more attention.

The demagnetization effect of ferrites is usually evaluated based on the magnetic flux density distribution inside the permanent magnet.

One evaluation criterion: Assume that any part of the permanent magnet with a magnetic density lower than the knee point of the BH curve is demagnetized (see Figure 23). Therefore, the minimum magnetic density constraint needs to be considered during design.

Fig.23 Demagnetization curve of a Fe-PM material at -40℃

The induced voltage in the motor winding can also reflect the demagnetization effect of PM, and this quantity can be used as an indirect demagnetization evaluation criterion.

A good design should provide nearly identical no-load back EMF waveforms under several different operating conditions. For example, Figure 24 shows the calculated and measured no-load electromotive force waveforms of a well-designed ferrite-assisted synchronous reluctance motor. Not only are the calculated and measured values very close, but the electromotive force waveform does not change significantly after the demagnetization test (test under harsh operating conditions).

Fig.24 The EMF waveforms of a Fe-based PMa-SynRM at no-load condition.

The demagnetized area is removed according to the magnetic field distribution. For example, Figure 26a shows the demagnetization part of three types of ferrite permanent magnets used in PMa-SynRM (see Figure 19 for structure).

Increasing the chamfer of the permanent magnet reduces the demagnetization area, as shown in Figure 26b, thereby reducing the demagnetization rate. While this technique is useful in terms of quality and cost of the ferrite permanent magnet material, it does not improve the motor’s torque capabilities. In fact, the useless demagnetization part in the PM is removed, and theoretically, the generated torque will not change. In practice, this may even lead to a small reduction in torque density.

Fig.19  A PM-assisted synchronous reluctance motor with a three-dimensional trench air gap(a) overall view(b) salient pole of the rotor(c) cross-sectional view.

Fig.26. Decreasing demagnetization ratio by removing specific regions of PMs (a) before (b) after.

Structural modifications to prevent irreversible demagnetization of ferrite permanent magnet motors.

Choosing thick permanent magnets in brushless motors reduces the risk of demagnetization. Likewise, optimizing the magnetic grid tip angle in a ferrite-assisted synchronous reluctance motor can reduce the effects of armature reactions and thus prevent severe irreversible demagnetization.

Fig.27 Effect of structural modifications on demagnetization ratio reduction

• conventional motor configuration of a spoke PMBM
• modified rotor configuration
• load lines
• speed-torque characteristics.

The traditional spoke structure of Figure 27a has been replaced by an improved structure in Figure 27b. This modification c moves the load line of the permanent magnet away from the knee region, as shown in Figure 27c, thereby reducing the risk of demagnetization. It also extends the range of the motor’s speed-torque characteristics, as shown in Figure 27d.

Therefore, every modification to improve torque should be done carefully. For example, in a mixed ferrite + NdFeB permanent magnet motor, special attention should be paid to the demagnetization effect of the ferrite permanent magnet. In fact, the leakage flux of NdFeB permanent magnets may cause severe demagnetization of the ferrite. Therefore, this effect should be minimized when choosing the relative positions of the two types of permanent magnets.

Structure

Ferrite and rare earth permanent magnet materials have some different structural (mechanical) properties. For example, ferrites are lower density, harder, and more brittle than rare earth permanent magnets. The tensile strength of ferrite is approximately one-seventh that of a similarly sized neodymium permanent magnet. In addition, due to the limitations of the processing technology, it is difficult to manufacture very thin ferrite sheets (thickness less than 1 mm). Currently, it is difficult to make anything less than 2 mm in China.

But ferrite is not easily corroded. Therefore, compared to other permanent magnets, they can be used in almost all applications without the use of coatings. Physical properties limit the use of ferrite motors in applications with high mechanical stress. A well-designed ferrite motor (especially a high-speed motor) must have acceptable mechanical strength and should be able to withstand centrifugal forces. This strength is usually evaluated by analyzing the distribution of von Mises stress at maximum rotational speed.

There are techniques that can improve the stress tolerance of ferrite motors. For example, Figure 28 illustrates changing the geometric details of a ferrite-assisted synchronous reluctance motor to reduce stress. In Figure 28a, the width of the central magnetic bridge of all three-layer rotors is basically the same, and the outer magnetic bridge (the distance between the magnetic barrier and the rotor surface) of all layers has the same thickness. It can be observed that the central magnetic bridge and its outer magnetic bridge of the third layer have high von Mises stress.

As shown in Figure 28b, changing the central magnetic bridge width and outer bridge thickness of each layer can greatly reduce the maximum Von Mises stress (58%), ultimately improving the mechanical strength of the rotor. However, this has a negative impact on the motor’s torque capability (approximately 3%). Because the increase in magnetic bridge thickness leads to an increase in magnetic flux leakage, and an increase in Ld leads to a decrease in the salient pole ratio.

Fig.28. Distribution of Von Mises stresses at 10000 rpm for a PMa-SynRM (a) initial rotor design (b) improved rotor design.

The magnetic properties of the ferrite may also affect the mechanical design of the motor. Due to the low residual magnetism of ferrite, the size of ferrite motors is relatively large. As the axial length of a cylindrical motor increases, its critical speed decreases. In high-speed applications where the outer diameter of the motor is not strictly constrained, it is a good idea to select a higher diameter to shaft length ratio. However, this may have a negative impact on the balance of copper and iron losses.

From another perspective, it would be better to increase the length of some ferrite motors. The amount of permanent magnets per unit length of each motor is limited, and the number of series turns should not be very high to avoid demagnetization. Therefore, the length of the motor must be increased. Therefore, the designer may face some kind of conflict when choosing the aspect ratio of the ferrite.

Other considerations

Due to the large variation in the magnetic properties of ferrite permanent magnets, the performance characteristics of ferrite motors change with temperature (even if the irreversible demagnetization of the ferrite is ignored). For example, at a given output power, when the temperature of the permanent magnets of a surface-mounted permanent magnet motor decreases, the stator phase current decreases, and the power factor increases. The temperature reduction of ferrite BLDC motors limits the maximum speed of the motor. As shown in the speed-torque characteristics of Figure 29, outside the constant torque region, the temperature effect is obvious.

Fig.29. Speed-torque curves of a Fe-based BLDC motor for different PM temperatures.