Hybrid-Excited Permanent Magnet-Assisted Synchronous Reluctance Machine

Abstract

This paper presents the results of simulation tests of a unique hybrid-excited permanent magnet machine operating in in different working regimes. The common feature of analyzed machine is a presence of magnetic barriers in the rotor structure. Structurally, this machine combines the advantages of the PMa-SynRM machine (Permanent Magnet-assisted Synchronous Machine) and a wound synchronous machine. The paper presents, among other results, the voltage and torque characteristics as a function of the current in the stator and the additional DC control coil. Selected results of experimental studies are also shown.

1. Introduction

The subject of this research is a special electric machine with a hybrid excitation and an appropriately shaped magnetic circuit. This machine combines all advantages of a permanent magnets (PM) assisted synchronous reluctance machine and a classic synchronous machine. Recent years have brought enormous progress in the production of high-energy permanent magnets that are perfect for use in electrical machines. This increase is caused due to the growing costs of remaining raw materials for magnet production—rare earth elements. The advantage of the PMa-SynRM machine is the decrease of permanent magnets’ volume. The torque generated in this machine consists of two parts: the synchronous torque from the permanent magnets and the reluctance torque, which is the result of different magnetic circuit reluctances along the d- and q-axes. Machines of this type have also good control properties, especially in operation at constant power (in the field, which is being weakened by the current in d-axis). This paper concerns a machine with an additional field winding in the rotor area. This solution allows for further limitation of the expensive permanent magnet usage and additionally extends a range of machine control—in the range of low rotational speeds, it is possible to strengthen the machine, and in the high range, to weaken it. This type of machine can be successfully used in drives of electric vehicles, which require a wide range of rotational speed with an appropriately large torque. For the purposes of the analyses, the prototype of the Hybrid-Excited Permanent Magnet-assisted Synchronous Reluctance Machine (HEPMa-SynRM) was tested in the generator regime.

2. Hybrid-Excited Machines

In the literature, many varieties of hybrid-excited machines can be found. Due to their common features of the structure and principle of operation, the hybrid-excited machines can be divided into some categories:

• Synchronous machines with permanent magnets [1,2]. The hybrid-excited synchronous machine with contactless energy transfer can be a representative of this group of machines (Figure 1a). It has the ability to adjust the excitation flux by using an additional coil on the machine rotor. Supply coils for wireless power transfer relate to the housing of the machine.
• Flux-switching machines [3], represented by the hybrid-excited switched-flux permanent magnet machine (Figure 1b), where the iron flux bridge adjacent to the DC filed winding slots is utilized as well to enhance flux-controllability. It has high torque density, sinusoidal back-EMF, and hence, lower torque ripple. In [4], the influence of stator and rotor pole combinations on machine parameters have been theoretically and experimentally tested. Similar designs have been presented in [5,6]. In these papers, the simulation and experimental research of hybrid-excited flux-switching PM machines with iron flux bridges have been presented.
• Doubly salient machines [7] that look like the hybrid-excited asymmetric stator pole doubly salient machine from Figure 1c, have a unique ratio of the number of stator and rotor poles. The angle between the two stator poles on adjacent segments of π-shaped iron cores α is different from that between the stator poles on the same segment, resulting in a non-uniform distribution of stator poles along the circumference of the stator yoke. A similar design has also been presented in [8].
• Axial flux hybrid-excited machines with core-wound coils [9] (Figure 1d). This machine is integrated with a planetary reducer. The transmission shaft connects the output of the motor to the input of the reducer, resulting in the function to slow down and increase torque. Similar solutions have also been presented in [10,11].
• Axial-radial flux machines [12,13], such as the double excitation synchronous motor (Figure 1e), where the air-gap flux in the machine can be regulated by controlling the field currents. The gap is affected by magnetic saturation and thermal limits of machines.
• Dual rotor/stator machines [14]—the representative of which is the dual-stator-switched flux consequent pole permanent magnet machine (Figure 1f), with an unequal number of teeth. This machine has two stators, the outer and inner stator, and a sandwiched rotor. The outer stator has three-phase armature windings and semi-closed slots, and the inner stator has the iron core and field winding for providing excitation and flux regulation. The sandwiched rotor has the consequent pole PMs to improve the torque density. It should be noted that the inner stator has an unequal length of teeth, and the long-length teeth align with the outer stator slots.
• Machines with DC winding on the stator [15]. An example of this group of machines is the Hybrid-Excited Permanent Magnet Machine with additional DC field windings (Figure 1g). This machine has PMs located on the rotor side and DC field windings in the stator side to realize the flux adjustability while ensuring high torque density.
• Axial flux switched reluctance machines [16], such as the double stator inner rotor axial airgap flux switching permanent magnet machine (Figure 1h). Each of the two stators contains some modular U-shaped blocks between which magnetized permanent magnets are placed.
• Claw pole machines [17], such as the hybrid-excited claw pole generator with skewed and non-skewed permanent magnets on the rotor (Figure 1i). The novelty of this machine is the existence of non-skewed permanent magnets on claws of one part of the rotor and skewed permanent magnets on the second one.
• Hybrid-excited Vernier machines are another group of machines. The authors of [18] presented typical structures of these machines with PMs mounted on the surface and using magnetic concentrators (Figure 1j). The authors of [19] presented a Vernier permanent magnet machine with homopolar topology, while the authors of [20] presented research of the machine where rotor consists of two parts: with permanent magnets and the wound electromagnets.
• Consequent-pole permanent magnet machines [21]—a machine having a structure similar to a consequent pole permanent magnet motor (Figure 1k), where the magnetic circuit of the rotor iron core is necessarily imbalanced, but the impedance of the stator windings can be balanced if the two windings (on the real PM pole sides) and the other two windings (on the image pole sides) are connected in series.
• Very interesting and slightly different approaches have been presented in [22,23], where the authors showed different line-start synchronous machines (Figure 1l). A novelty in this case is the addition of a starter cage to the rotor of the reluctance machine.

The comparison of the most important features of the discussed machines is summarized in

Table 1. Comparison of presented designs.

No.	Design	Advantages	Disadvantages
1.	Hybrid-excited synchronous machine	very good back-EMF value regulation	complicated design, large mass, the need to perform 3D simulations
2.	Hybrid-excited switched-flux permanent magnet machine	simply design	average use of weight (volume) in relation to power
3.	Doubly salient machine	simply design	average use of weight (volume) in relation to power
4.	Axial flux hybrid excitation motor	high torque	complicated design, the need to perform 3D simulations
5.	Axial-radial flux machine	use of axial and radial fluxes	complicated design, the need to perform 3D simulations
6.	Dual rotor/stator machine	simply design	average use of weight (volume) in relation to power
7.	Hybrid-Excited Machines with DC winding on stator	simply design	average use of weight (volume) in relation to power
8.	Stator of double stator inner rotor axial air gap flux switching permanent magnet machine	high torque	complicated design, the need to perform 3D simulations
9.	Hybrid-excited claw pole generator	simply design	the need to perform 3D simulations
10.	Hybrid-excited Vernier machine	simply design	average use of weight (volume) in relation to power
11.	Consequent-pole permanent magnet machines	simply design	average use of weight (volume) in relation to power excitation flux cannot be adjusted
12.	Line-start interior permanent magnet synchronous motor	simply design line-start machine	excitation flux cannot be adjusted

.

Some other papers also provide a broader overview of different types of hybrid-excited machines [24,25,26]. The aim of the research was to develop a compact machine structure with the possibility of easy manufacturing and having regulation of the magnetic flux. The next steps are presented in the following chapters.

3. Machine Design and Mathematical Model

The concept of the HEPMa-SynRM, which was the subject of research, has a structure that combines the features of a PMSM and a reluctance machine, and additionally has the option of regulating the flux with an additional excitation coil [27,28,29]. All the important machine data are listed in

Table 2. Machine data.

No.	Parameter	Description	Value
1.	Pn	Nominal power	6.5 kW
2.	In	Nominal current rms	8.4 A
3.	Un	Nominal voltage rms	400 V
4.	Y	Phase connection	star
5.	Rso	Stator’s outer radius	110 mm
6.	Rsi	Stator’s inner radius	68.0 mm
7.	Rro	Rotor’s outer radius	67.5 mm
8.	ag	Air gap length	0.5 mm
9.	l	Machine’s length	199 mm
10.	p	Number of pole pairs	3
11.	ks	Number of stator windings	35
12.	kIexc	Number of rotor DC windings’ wires	29
13.	s	Number of slots	36
14.	lPM25 lPM30	Length of PMs 1st type Length of PMs 2nd type	25 mm 30 mm
15.	hPM	Height of PMs	2.4 mm
16.	wPM	Width of PMs	7 mm
17.	kPM25 kPM30	Number of PMs lPM25 = 25 mm Number of PMs lPM30 = 30 mm	245 185
18.		Steel sheet type—silicon steel	M400-50A
19.	Hs2	Slot body height	14.8 mm
20.	Bs0	Slot opening width	3 mm
21.	Bs1	Slot wedge maximum width	5.4 mm
22.	Bs2	Slot body bottom width	8 mm

(see also Figure 2). The outer radius of the stator is 110 mm, the inner stator radius is 68 mm, and rotor’s outer radius is 67.5 mm; thus, the air gap is 0.5 mm. The length of machine is 199 mm. Stator and rotor laminations have been made of silicon steel M400-50A. Two types of high-energy rare earth NdFeB PMs were used in the machine, i.e., with dimensions of 25 × 7 × 2.4 mm (245 pcs.) and 30 × 7 × 2.4 mm (185 pcs.).

In order to carry out experimental tests, the proprietary rotor of the machine, containing permanent magnets, air barriers, magnetic bridges, and an additional excitation system in the form of DC coils was designed and constructed. Each of the 6 in series-connected DC coils is built of 29 winding wires with a diameter of 0.9 mm, powered by slip rings. The rotor model of the machine is shown in Figure 3. The HEPMa-SynRM machine has a classic stator design derived from the ABB M3AA 132 MC8 squirrel cage induction motor with a power of 6.3 kW. It has 3-phase windings placed in 36 slots (12 per phase). The stator simulation model is shown in Figure 4.

It is still necessary to formulate the appropriate mathematical model of the machine to develop a control strategy in two cases: steady and transient states. This model is also necessary to realize a new power supply control system: in a fault mode and in field weakening and strengthening operations. For this purpose, typical assumptions and simplifications were adopted: armature windings symmetry, constancy of windings inductances and resistances, constancy of the PM flux, and omitting of magnetic flux density’s higher harmonics. This allows to write the equations for d- and q-axis voltages and the electromagnetic torque of hybrid-excited machines as follows [1]:

$$U_d=R{I}_d+\frac{\mathrm{d}{\psi }_d}{\mathrm{d}t}-p{\Omega }_m{\psi }_q,$$

(1)

$$U_q=R{I}_q+\frac{\mathrm{d}{\psi }_q}{\mathrm{d}t}+p{\Omega }_m{\psi }_d,$$

(2)

$$T_e=\frac{3}{2}p\left(I_q{\psi }_d-I_d{\psi }_q\right)=\frac{3}{2}p\left[\stackrel{T_{pm}}{\overbrace{{\psi }_{pm}{I}_q}}+\stackrel{T_{DC}}{\overbrace{M_{DC}{I}_{DC}{I}_q}}+\stackrel{T_{rel}}{\overbrace{\left(L_d-L_q\right)I_d{I}_q}}\right]$$

(3)

where:

• p—number of pole pairs;
• Ψ_d, Ψ_q—d- and q-axis magnetic fluxes;
• Ψ_pm—flux generated by PMs;
• I_d, I_q—d- and q-axis stator currents;
• L_d, L_q—d- and q-axis inductances;
• M_DC—mutual inductance of the additional coil;
• I_DC—current in the additional coil.

Equation (3) gives the way of forming a required electromagnetic torque with the values of the stator current components in d- and q-axes and additional DC current. The value of the electromagnetic torque achieved by the HEPMa-SynRM machine is influenced by 3 factors:

• Torque produced by means of a flux from permanent magnets (T_PM);
• Torque generated by the flux coming from the additional excitation coil (T_exc);
• Reluctance torque resulting from the difference in inductances in d- and q-axes of the machine (T_rel).

If it is necessary to obtain a high torque, the excitation flux of the machine should be increased, which at the same time allows to reduce the stator current. Due to the increased induced voltage, high rotational speeds of the rotor make it necessary to weaken the excitation flux. This can also affect the reduction of losses in the whole magnetic circuit. All this increases the efficiency of hybrid-excited machines.

Considering the statements above, the value of the electromagnetic torque generated by the machine can be described by the formula:

$$T_e=T_{PM}+T_{exc}+T_{rel}.$$

(4)

4. Simulation Results

Many simulation tests were performed for the purpose of the research. They were performed using the ANSYS Maxwell software. Finally, the relationships between the back-EMF as a function of the current in the additional coil have been determined and the influence of the excitation current on the cogging torque and the electromagnetic torque has been also investigated. Figure 5 presents a FEM (Finite Element Method) model of the machine (the mesh with 49,519 elements) and the magnetic field distribution within the machine, which was the basis for further calculations. The presented picture shows the motor in a no-load state (zero current in the stator and rotor coils). Due to the small amount of magnetic material, the induction is not high and—as it can be seen in Figure 5—the machine is not saturated. The simulation tests of back-EMF presented in Figure 5 have been performed with the assumption that the rotor was rotating at a speed of 1000 rpm. The results have been obtained for the time of 20 ms with the step 0.1 ms (200 steps). On the other hand, the values of the electromagnetic and cogging torque were determined for load angles in the range from 0 to 180 electrical degrees with the step of 1 degree.

Figure 6 shows the shape of the back-EMF induced in phase A of the machine for different values of the excitation current in the DC coil. The back-EMF has the shape of a strongly distorted sinusoid, which results from the influence of the machine teeth and slots. The noticeable increase in the induced voltage from 95.9 V to 134.2 V, i.e., by approx. 40% can be observed, while de-excitation reduces the voltage to 89.8 V, i.e., by approx. 7%.

Figure 7 shows the characteristics of the cogging torque value for different values of the DC coil current. It should be noticed that the positive value of the current in the DC coil first causes the reduction of the maximum cogging torque value and then causes the cogging torque to increase. The minimal value of cogging torque is obtained for a current of 4A in the DC coil. It is also significant that in the range of field weakening, the cogging torque always increases with the increase of the current in the additional rotor coil.

Figure 8 shows the FEM results of the rms value of the back-EMF as a function of the rotational speed of the machine. It can be noticed that the excitation system has better control possibilities in terms of increasing the RMS value of the back-EMF than of decreasing it.

The relationship between the maximum value of the electromagnetic torque T_e and the stator current I_s and the current in the additional coil I_exc has also been analyzed during simulation calculations. The results are shown in Figure 9.

The tests of the maximum electromagnetic torque show that the torque value can be adjusted with the current in the additional coil, which is also consistent with Equation (4). One should keep in mind that flux weakening with the current in the additional coil will also reduce the electromagnetic torque, which is obvious from the point of view of electrical machines control. In other words, the energy used on flux weakening will reduce the motor torque. However, such a state of operation is necessary only at high rotational speeds, where—in traditional control strategies—the field weakening with the d-axis current would be used, which also requires energy.

5. Results of Experimental Tests and Comparison with FEM Results

The experimental tests have been performed using a dedicated laboratory stand presented in Figure 10 and Figure 11.

The whole test stand (Figure 10 and Figure 11) consisted of:

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Network parameter analyzer NORMA 5000 (1);

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Three current coils (2);

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Two oscilloscopes Tektronix (3);

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Laboratory power supply (4);

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Computer station (5);

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Servo control system Bernecker&Reiner with a control panel (6);

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Drive unit Bernecker&Reiner (7);

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Torque meter Dataflex 32 (8);

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Slip rings (9);

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HEPMa-SynRM prototype (10).

As a result of the research, Figure 12 shows the change of the RMS value of the back-EMF as a function of the current in the additional coil current for selected rotational speeds of the rotor (n = 1000 rpm; 2000 rpm; 3000 rpm) in comparison with simulation results. The graph shows the results of both simulation and experimental tests. The differences in the test results between simulations and experiments do not exceed 5%.

In the next test, the relationships between the torque generated by the machine and two currents—the effective value of the stator current (three-phase supply) and the value of direct current in the additional coil—were determined. The results are shown in Figure 13.

The comparison of the experimental and simulation research shows that a quite good convergence of the test results was obtained. Slightly worse results of the experimental tests in comparison with the simulation tests are also visible. In our opinion, this is caused due to the fact that the rotor plates were made by laser cutting, which oxidizes the edges of machine parts, thus deteriorates the magnetic properties of the steel. Moreover, the assumed parameters of the magnets may differ from the actual ones. Despite this, the differences between the measurements do not exceed 8%, which is a satisfactory result.

6. Conclusions

This paper presents the principle of operation of the HEPMa-SynRM machine, which uses an additional excitation coil in the rotor. A characteristic feature of the proposed machine is the usage of two excitation sources: PMs and an additional DC-powered coil. Furthermore, it should be noted that the electromagnetic torque is basically the result of three components: the synchronous torque from the permanent magnets, the reluctance torque resulting from the shape of the rotor’s magnetic core, and the torque from the additional excitation coil. With the use of additional electromagnetic excitation, the machine has gained an additional degree of freedom in terms of voltage and torque regulation on the shaft.

The research shows that the field control range was equal to 1.5 for experimental tests and 1.55 for simulation research. An increase in the electromagnetic torque was also noticed, with the greatest increase for lower values of the stator current (by over 50%), and for higher values—by approx. 8%. In the authors’ opinion, it results from the increasing saturation of the magnetic circuit of the machine.

The aim of the work was to develop a structure that would effectively control the magnetic flux in the air gap. The presented machine enables a significant increase in the magnetic flux and a slightly lower reduction thereof. The area of more effective machine flux weakening will be the subject of future research. During the process of reaching the final machine topology, efforts were made to ensure that the machine would enable an effective strategy for controlling the excitation flux with the current in the d-axis, as well as “adding another degree of freedom”, which is the possibility of regulating the excitation flux with an additional electromagnetic circuit in the machine rotor. Moreover, the proposed structure is characterized by a compact rotor structure.

The proposed design can be successfully used in electric vehicle drives, which require the highest possible starting torque to enable a dynamic start of the vehicle.

At the later stage of the research, it is planned to perform extensive optimization tests in order to obtain a structure with the largest possible range of back-EMF and electromagnetic torque control. Research of thermal phenomena is also planned. Because these phenomena may be dangerous from the point of view of PMs, which may be damaged when their limit temperature would be exceeded. In addition, tests will be carried out of the mechanical strength of the most critical parts of the rotor—especially in the area of magnetic barriers in the vicinity of the shaft.