**3. Characteristics of MGs with respect to their use in EV application**

Two of the previously presented MG variants will be investigated here: the first one is an integrated motor-gear variant having an in-wheel configuration for the electric motor, and the second MG will be a high-speed variant with buried permanent magnets. Evaluation of the performances of these two topologies in terms of iron loss and torque (or output power) will be employed numerically, by means of finite element method (FEM) and the Flux2D software.

#### **3.1. Integrated in-wheel motor magnetic gear topology**

The considered in-wheel motor gear topology is the one presented in **Figure 8**. The motor is a fractional slot permanent magnet synchronous machine (FS-PMSM)—17 pairs of poles and 39 slots. Above the outer rotor of the FS-PMSM and attached to it, we have an inner rotor of the MG, having the same number of pairs of poles (*pin*=17), and a reduced number of pairs of poles for the outer rotor of the MG (*pout*=5). It means that the magnetic transmission will produce a higher speed, at a lower torque: it is a multiplier, or reversed gear [40]. Numerically computed results are shown in **Figures 10**–**12**.

**Figure 10.** Integrated motor-gear analyzed variant, based on in-wheel motor: field lines (left) and flux density distribu‐ tion (right) within the active parts.

State of the Art of Magnetic Gears, their Design, and Characteristics with Respect to EV Application 1385 http://dx.doi.org/10.5772/64174

**Figure 11.** Air-gap flux density in the integrated motor-gear studied variant [40].

*2.3.3. Analytical design of MGs through vector potential algorithm*

8412 Modeling and Simulation for Electric Vehicle Applications

previous references; thus, the reader can investigate this topic if needed.)

the influence of certain materials used in the construction of MGs.

**3.1. Integrated in-wheel motor magnetic gear topology**

results are shown in **Figures 10**–**12**.

tion (right) within the active parts.

The vector potential expressions, in both air gaps of the MG, have been detailed for radial and axial flux topologies in [28, 41]. Since the mathematical modeling for such an approach is quite laborious and goes beyond the scope of this state-of-the-art presentation of MGs, it will not be presented here. (It's worth mentioning that even the integral coefficients are detailed in the

With this, we have concluded a short presentation on the available analytical models used in the design of process of MGs. Next, we will consider two of the available configurations to evaluate their performances (in terms of iron loss and torque) and we will rapidly investigate

**3. Characteristics of MGs with respect to their use in EV application**

Two of the previously presented MG variants will be investigated here: the first one is an integrated motor-gear variant having an in-wheel configuration for the electric motor, and the second MG will be a high-speed variant with buried permanent magnets. Evaluation of the performances of these two topologies in terms of iron loss and torque (or output power) will be employed numerically, by means of finite element method (FEM) and the Flux2D software.

The considered in-wheel motor gear topology is the one presented in **Figure 8**. The motor is a fractional slot permanent magnet synchronous machine (FS-PMSM)—17 pairs of poles and 39 slots. Above the outer rotor of the FS-PMSM and attached to it, we have an inner rotor of the MG, having the same number of pairs of poles (*pin*=17), and a reduced number of pairs of poles for the outer rotor of the MG (*pout*=5). It means that the magnetic transmission will produce a higher speed, at a lower torque: it is a multiplier, or reversed gear [40]. Numerically computed

**Figure 10.** Integrated motor-gear analyzed variant, based on in-wheel motor: field lines (left) and flux density distribu‐

**Figure 12.** Performances of the integrated motor-gear studied variant: the speed-torque-power on the low speed rotor of the MG (a), for the high-speed outer rotor (b), and the iron loss in the active parts.

A first result of this analyzed integrated motor-gear variant is shown in **Figure 10**, where the field lines and flux density distribution within the active parts of the topology are presented. The reader can observe the influence of modulated teeth in the transfer of the flux (torque) from the inner to outer rotor. In the air gap layers of this machine-gear topology, the FEM software computed the following air gap flux densities, plotted in **Figure 11**: here, Gap1 refers to the in-wheel motor's air gap, Gap2 stands for the inner rotor air gap of the MG and Gap3 refers to its outer rotor air gap. More results are plotted in **Figure 12**.

The delivered torque and power, as well as the iron loss within the active parts of the integrated motor-gear studied variant are plotted in **Figure 11**. Some torque ripples can be identified, both on the inner and on the outer rotors of the integrated motor-gear variant. Since the major loss component within an MG is the iron loss, it is obvoius that the efficiency of the transmission is very low. One could sum the iron loss components (for the inner and outer rotor, as well as for the static part poles or teeth—SPP) and of the in-wheel motor to get 130 W. These results are not optimized. Based on the image of the flux density distribution (**Figure 10** right), we could consider to reshape the teeth's geometry and the outer rotor yoke, in the perspective of increasing the efficiency.

In order to prove the important advantage of MGs against classic mechanical gear, that is the possibility to obtain a very high transmission ratio for high-speed applications, and to get a very compact magnetic traction system with very good power density, the reader's attention will be oriented toward a high-speed MG.

## **3.2. MG with buried permanent magnets for high-speed applications**

The second analyzed MG is with high-speed inner rotor, running at 26,000 r/min (with *pin*=1), and outer rotor for low-speed rotor (with *pout*=15) running at approximately 1500 r/min. The number of static part iron teeth is *Ns*=16. The magnets are buried in order to avoid the surfacemounted pieces which need a consolidating ring. A preliminary check on the mechanical resistance of the inner rotor steel needs to be employed numerically, to check if the iron bridges have not been damaged during MG's operation. A cross section of the high-speed MG is shown in **Figure 13**.

**Figure 13.** The high-speed MG studied variant.

This MG will be evaluated for different steel material characteristics (M335, M400, and Vacoflux48), and permanent magnet types (Nd-Fe-B, Ferrite, Alnico). As a reference variant, the MG with Nd-Fe-B and M400 steel is considered. The first result of this configuration is shown in **Figure 14**, where the field lines and flux density distribution are shown. An obvious saturation is found on the inner rotor core, on the iron's bridge, which is normal behavior since the flux needs to pass the air gap.

State of the Art of Magnetic Gears, their Design, and Characteristics with Respect to EV Application 1587 http://dx.doi.org/10.5772/64174

**Figure 14.** The field lines and flux density distribution within the active parts of the high-speed MG.

The delivered torque and power, as well as the iron loss within the active parts of the integrated motor-gear studied variant are plotted in **Figure 11**. Some torque ripples can be identified, both on the inner and on the outer rotors of the integrated motor-gear variant. Since the major loss component within an MG is the iron loss, it is obvoius that the efficiency of the transmission is very low. One could sum the iron loss components (for the inner and outer rotor, as well as for the static part poles or teeth—SPP) and of the in-wheel motor to get 130 W. These results are not optimized. Based on the image of the flux density distribution (**Figure 10** right), we could consider to reshape the teeth's geometry and the outer rotor yoke, in the perspective of

In order to prove the important advantage of MGs against classic mechanical gear, that is the possibility to obtain a very high transmission ratio for high-speed applications, and to get a very compact magnetic traction system with very good power density, the reader's attention

The second analyzed MG is with high-speed inner rotor, running at 26,000 r/min (with *pin*=1), and outer rotor for low-speed rotor (with *pout*=15) running at approximately 1500 r/min. The number of static part iron teeth is *Ns*=16. The magnets are buried in order to avoid the surfacemounted pieces which need a consolidating ring. A preliminary check on the mechanical resistance of the inner rotor steel needs to be employed numerically, to check if the iron bridges have not been damaged during MG's operation. A cross section of the high-speed MG is shown

This MG will be evaluated for different steel material characteristics (M335, M400, and Vacoflux48), and permanent magnet types (Nd-Fe-B, Ferrite, Alnico). As a reference variant, the MG with Nd-Fe-B and M400 steel is considered. The first result of this configuration is shown in **Figure 14**, where the field lines and flux density distribution are shown. An obvious saturation is found on the inner rotor core, on the iron's bridge, which is normal behavior since

**3.2. MG with buried permanent magnets for high-speed applications**

increasing the efficiency.

in **Figure 13**.

will be oriented toward a high-speed MG.

8614 Modeling and Simulation for Electric Vehicle Applications

**Figure 13.** The high-speed MG studied variant.

the flux needs to pass the air gap.

Some transient simulations were carried out in Flux2D. In **Figures 15**–**17** are presented the simulated results for the case of an MG with M400 steel used for all cores and armature, and Nd-Fe-b as magnet material. Here, the output power and torque as well as the iron losses in the active parts of the structure are plotted. Very smooth mechanical performances have been obtained with such configuration of MG, with 1/16 ratio. The amount of iron loss in this case is about 800 W.

**Figure 15.** Simulated results for the MG with M400 steel and Nd-Fe-B magnet: input and output power [40].

**Figure 16.** Simulated results for the MG with M400 steel and Nd-Fe-B magnet: input and output torque [40].

**Figure 17.** Simulated results for the MG with M400 steel and Nd-Fe-B magnet: iron losses in the active parts [40].

Now, let us see what happens if better material is used, such as the Vacoflux48 steel, which has a very narrow hysteresis curve. The iron loss results are given in **Figure 18**. (The mechanical output performances are not given here; only the amount of iron loss in the fixed iron teeth, inner, and outer rotor of the MG.)

**Figure 18.** Simulated iron losses for the MG with Vacoflux48 steel and Nd-Fe-B magnet [40].

An amount of 365 W of iron loss was computed here. By comparing these iron losses (for the M400 steel, given in **Figure 17**, and the ones from **Figure 18**, for Vacoflux48), the huge advantage of the structure equiped with Vacoflux material is obvious, for which the iron loss is less than half of the M400—which is neither a bad material nor a cheap one. Of course, products based on Vacoflux48 are rather exclusive units and meant for special applications (like racing cars)—the cost of one prototype is almost prohibitively expensive. For ordinary applications, even the M400 steel is considered an expensive material—it is usually used for prototypes and low series manufacturing. Otherwise, a cheaper variant based on M335 is suitable in terms of output performances, giving similar results as the ones using M400.

A more complete comparison of the performances of the high-speed MG, when equipped with different types of materials and their magnetical characteristics is given in **Tables 4** and **5** the width of the considered steel sheet (0.2, 0.35, and 0.5mm) is given also in **Table 4**.


**Table 4.** The properties of the materials used in the construction of the high-speed MG.

**Figure 16.** Simulated results for the MG with M400 steel and Nd-Fe-B magnet: input and output torque [40].

**Figure 17.** Simulated results for the MG with M400 steel and Nd-Fe-B magnet: iron losses in the active parts [40].

**Figure 18.** Simulated iron losses for the MG with Vacoflux48 steel and Nd-Fe-B magnet [40].

inner, and outer rotor of the MG.)

8816 Modeling and Simulation for Electric Vehicle Applications

Now, let us see what happens if better material is used, such as the Vacoflux48 steel, which has a very narrow hysteresis curve. The iron loss results are given in **Figure 18**. (The mechanical output performances are not given here; only the amount of iron loss in the fixed iron teeth,


**Table 5.** Comparison of different configurations (material based) of MGs.

Some comments need to be made with respect to the Alnico and ferrite, the results of which are summarized in **Table 4**: for this specific configuration of the MG, because of the iron bridge which needs to support important centrifugal forces (we recall that the high-speed rotor is running at 26,000 r/min), and the low level of remanent flux density (for Ferrite material) or coercivity (for Alnico), the magnetic flux is not sufficiently strong to cross the air gap. Different rotor configurations (with concentrated flux or halback array) need to be considered—which means that a topology with more than one pair of poles is requested. To prove this, one can investigate the field lines and the flux densities depicted in **Figure 16**, given within the active parts of the MG while ferrite or Alnico materials are used (**Figure 19**).

**Figure 19.** Investigation of the MG field lines and flux density distribution, when ferrite (top) and Alnico (bottom) magnets were considered.

As 15 years have passed since the first efficient MG proposition, we could expect, in the next years, improvements in the field of MGs, especially for the ones with variable transmission ratio, which could be extremly useful in transportation and aeronautical domains. With the advancement in the field of materials, for rare earth materials and ferromagnetic steel, the future of fully electromagnetic propulsion systems could be considered as a viable industrial solution, and not only a new research subject.
