Nowadays, many different technical solutions of hybrid excited machines, including the claw pole machines with permanent magnets are mentioned in literature [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. Some of this type electrical machines are used in vehicle drive [9, 10, 11, 12]. This article concerns to claw pole machines with permanent magnets and excitation control coil on the rotor. Many of known claw pole type machines have substantial limitations – the excitation flux is non-controllable [13, 14, 15, 16] or, for hybrid excited ones, it is necessary to introduce special areas inside the machine, to limit the excitation flux leakage . For this reason, a new claw pole machine with hybrid excitation has been proposed, whose first construction and research have been discussed in . Present paper is the continuation of the former Hybrid Excited Claw Pole Generator (HECPG) research. In order to increase the back-emf value in comparison to the model discussed in  permanent magnets are also placed on the second part of the claw pole rotor . These magnets were placed at the appropriate angle in order to minimize the cogging torque.
2 Machine design and simulation results
To carry out the research a simulation model of the HECPG, which is 1/6 of the whole machine has been prepared. The model, with the distribution of the magnetic flux, is shown in Fig. 1. This machine has non-skewed permanent magnets (PMs) on claws of one part of the rotor, and on the second part permanent magnets are skewed.
The HECPG consists of six pair of poles and 36 stator slots, so the number of slots per pole and phase is 3. The outer diameter of the rotor is 101 mm, the stator inner diameter is 102 mm, therefore the air gap is equal 0.5 mm. An active length of the HECPG is 30 mm. On the rotor poles of the machine rare earth magnets NdFeB were used, whose remanence was 1.2 T. The basic data of the tested prototype are summarized in Table 1.
A simulation model has been developed according to parameters presented in Table 1. Simulation research has been conducted in ANSYS Maxwell program. The magnetic flux distribution (both axial and tangential) within the machine is very complex, so it was necessary to perform three-dimensional simulations. The calculations were done using self-developed unique numerical procedures for the mesh optimization and for the torque and back-emf evaluation.
During tests, the angle of permanent magnet skewing on one part of the rotor was changed from γ = 0 (Figure 2a) to γ = 15° (Figure 2c). The aim of the research was the choice of such the angle γ that the cogging torque, after the addition of permanent magnets, would be as small as possible. As a result of the research which will be presented in following chapters, it was decided to build an experimental model with the angle of the PM rotation γ = 9° (Figure 2b).
An exploded view of the developed hybrid excited claw pole generator parts are shown in Figure 3. As had been mentioned above, the machine has two parts of claw pole rotor. Non-skewed permanent magnets are mounted on one of these parts, and the another claw pole part have skewed permanent magnets. The HECPG also includes: housings, bearings, voltage regulator, rectifier, brush holder, slip rings, carbon brushes, stator core, rotor excitation control coil, claw poles and permanent magnets. The polarity of permanent magnets is the same as polarity of claw poles on which they are placed during flux straightening.
3 Cogging torque in the HECPG
During the simulation research the cogging torque tests were performed depending on the skew angle of the magnets on one part of the rotor. Figure 4 shows the cogging torque waveforms depending on the angle position of the magnet γ and the angle position of the rotor relative to the stator α. The study was conducted for one tooth pitch.
The root mean square and maximum values of the cogging torque are summarized in the Table 3 and visualized in Figure 5. The results show that the optimum angle, from the point of view of the minimum cogging torque, is γ = 9°, because in this case both the maximum value and the rms of the cogging torque are the smallest. That’s why it was decided to build an experimental model with such a skew of magnets as told in previous chapter.
4 Back-emf in the HECPG
Next, the influence of angle γon back-emf has been examined. Figure 6 presents waveforms of the 1-phase back-emf in the form of a three-dimensional graph when the rotational speed is n = 1000 rpm. Table III lists the maximum and rms values of back-emf in the HECPM. The next Figure 7 shows the back-emf dependence of the angle γ.
It should be noted that unfortunately the back-emf value always decreases with increasing of the skew angle γ. Nevertheless, due to the need to reduce the cogging torque, it was decided to perform an experimental model with skew angle γ = 9°, despite a decrease in back-emf rms value of about 17%.
5 Machine construction and experimental validation
According to above considerations after simulation tests it was decided to build the experimental model in which permanent magnets on claws of one part are non-skewed, but on the second part PMs are skewed with angle γ = 9°. Figure 8 shows the rotor of the experimental model of HECPG.
A number of tests have been carried out at the test stand (Figure 9). These results have confirmed the theoretical and simulation considerations. For example a comparison of back-emf waveforms obtained during numerical and experimental studies for n = 1000 rpm are given in Figure 10. As shown in this figure, relatively high convergence results were obtained.
Presented results show that when skew angle increases the cogging torque first decreases, achieving a minimal value in case γ = 9°, and then increases. It should be noted that back-emf value always decreases during skew angle increasing.
The obtained values of the cogging torque are relatively high. Various techniques for reducing cogging torque pulsations are known, including the use of magnetic wedges or changing shapes of rotor poles [20, 21]. The future work will be devoted to the selection and development of optimal construction of the claw pole machine with permanent magnets in which the cogging torque will be minimized without the deterioration of the voltage parameters.
This work has been supported with the grant of the National Science Centre, Poland 2015/17/B/ST8/03251.
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Published Online: 2017-12-29