Non-destructive Measurement of Magnetic Properties of Claw Pole

The claw pole is a necessary component of a synchronous generator with direct-current (DC) excitation, which is commonly used in the automobile industry because of its low cost and ease of regulation [1]. Claw poles are mainly manufactured by hot forging: common forging schemes include closed-die forging [2], casting–forging [3], single forging with a spring-controlled die [4], and radial forging with forward extrusion [5]. After forging, claw poles are heat treated and machined for subsequent assembly.

Two claw poles cooperate with each other to form the magnetic poles of the rotor, as shown in Figure 1. They are magnetized under the excitation of the energized magnetizing winding and then generate a strong magnetic field [6]. The fundamental magnetization curve of the material is the most important curve characterizing the magnetic properties of a claw pole, reflecting its ability to be excited and magnetized under the action of an external magnetic field. The greater the magnetic induction of the material under the same excitation current, the greater the output current of the claw pole generator, and the better the power generation performance.

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Magnetic properties can be measured in a closed or open circuit. The simplest example of a closed magnetic circuit is a ring, which need not be limited to a circular cross-section [7]. It is also possible to obtain the magnetization curve and hysteresis loop using a hollow square or other closed geometrical shape [8]. The effect of precipitate element addition on the magnetic properties of the magnetostrictive Fe₈₃Ga₁₇ alloy was studied with ring samples: the results showed that the coercivity significantly increased with the addition of such elements [9]. Addition of boron caused undesirable deterioration of the magnetic properties of a permalloy, while the effects of magnesium and calcium on its magnetic properties were small [10]. A ring sample can easily form a closed magnetic circuit, but the disadvantages of this method include: (1) considering the thermal effect, it is not possible to apply very large circumferential fields [11]; (2) for a real part like a claw pole, the measured results are closely related to the sampling position; (3) it can be time-consuming because primary and secondary windings must be applied to each sample.

Magnetic measurements in open circuit are usually performed with a vibrating-sample magnetometer (VSM) [12], alternating-gradient magnetometer [13], fluxmeter, or superconducting quantum-interference device magnetometer [14, 15]. Of these the VSM is most popular. The magnetic flux density and coercivity of new Fe84xNb2B14Cux nanocrystalline alloys were measured with a VSM under an applied field of 8×10⁵ A/m, and it was found that the magnetic properties were enhanced with appropriate Cu addition and annealing conditions [16]. The magnetic hysteresis loops of Co/CoFe₂O₄ nanoparticles were measured using a VSM with a maximum field of ± 1.6×10⁶ A/m, and a demagnetization jump at low fields was found [17]. A low-cost VSM was successfully constructed by using an electromagnet and an audio loud speaker, which could record the magnetic hysteresis loop up to 6.6×10⁵ A/m at room temperature [18]. The magnetic properties of Tb32Co68 after adding a [SiNx/Co]_n underlayer were analyzed by VSM with a maximum applied field of 1.6 × 10⁶ A/m, and the saturation magnetization value of the multilayer films was enhanced [19]. The ferromagnetic behavior of Ni–Zn ferrite-epoxy composite thin films was revealed by VSM, and the degree of saturation magnetization of the composites increased with the addition of NiZn ferrite nanoparticles [20]. An optimized VSM design was determined using numerical calculations, based on the principle of reciprocity and the Biot–Savart law, and was able to detect the magnetic moment of a 100 nm-thick Fe₁₉Ni₈₁ film and its change of ~ 2 × 10⁻⁷ A m² [21]. However, VSMs of normal design are only applicable to samples with small dimensions, thin films, or other small electronic components.

Magnetic parameters can also be measured using a permeameter [22], in which a closed magnetic circuit is formed by attaching a yoke, or yokes, of soft magnetic material to the sample. Various types of permeameters have been proposed, which differ in the size and shape of the sample, yoke, magnetizing winding, and other characteristics. A semi-rigid cable loop was proposed to work as a single-turn shielded-loop pick-up coil for a high-frequency permeameter, and it could measure permeability up to 3 GHz with a simple structure [23]. A fully automatic system for measuring magnetic properties of soft and hard ferromagnetic materials was designed and it can adopt a variety of test configurations, including a bar permeameter, ring samples, or Epstein frame [24]. A technique was developed to calibrate the performance of a special permeameter for measuring soft magnetic materials by using cylindrical standard samples, and the true M(H) curve for standard samples of large length-to-diameter ratios was obtained by the demagnetizing-corrected solenoid method [25]. Because a non-measured medium exists in the magnetic circuit using a permeameter, the abscissa of the measured B–H curve is not the actual external magnetic field intensity applied to the measured medium. This instrument is not suitable for evaluating the magnetic properties of a complex part.

In industry, more attention is paid to the overall magnetic properties of claw pole. If the above methods were used, the sampling process would destroy the part and the specific sampling position would directly affect the measurement. In this work, a Non-destructive Test for Evaluating the Magnetic properties of claw pole (NDTEM) was studied. In the following sections, the design concept of this new measurement method is introduced in detail. In Section 3, a finite-element (FE) model is described, which was constructed according to the concept, and an attempt to verify the correctness and effectiveness of the simulation results is made. Section 4 describes a group of experimental tests that was performed to validate the NDTEM, and specific schemes for both the NDTEM and ring-sample methods are presented. In Section 5, the consistency evaluation, accuracy analysis, and stability analysis of the two methods are discussed, and the magnetic properties of different claw poles are briefly evaluated based on the results measured by the NDTEM. Finally, the conclusions drawn from this research are summarized in Section 6.

In the test method using a ring sample, the ring itself forms a closed magnetic circuit. In the method using a permeameter, a rod sample with a yoke in the permeameter constitutes a closed circuit. In the proposed NDTEM, a claw pole to be measured, a support base, and a guide ring are used to construct a complete magnetic circuit, as shown in Figure 2. Here, the support base and guide ring were made of electric iron DT4 that has good magnetic properties to ensure that the support base, guide ring and claw pole can form a closed magnetic circuit. The positioning axis and winding ferrule were made of aluminum alloy 2A12 that has a magnetic permeability of around 1 H/m, so that they have almost no influence on the magnetic circuit. The magnetic line of force, indicated by small arrows, was vertical to the end face of the boss on the claw pole and went through the claw part to the outer edge surface and the air, then penetrated to the guide ring. After passing over the outer ring and inner boss of the support base, the magnetic line of force entered the boss of the claw pole again, forming a closed magnetic circuit.

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The designed magnetic circuit had a high degree of similarity with the magnetic circuit formed by a claw pole working in a motor. The magnetic field of a claw pole in operation goes through the air gap after passing through the claw part to the outer edge surface, enters the stator yoke, passes through the air gap again, and then penetrates the outer edge face of the adjacent claw of another paired claw pole. Therefore, the magnetic force lines are twisted between the paired claw poles. However, for each claw pole, the magnetic line of force always passes from the outer edge of the claw part and the air gap to the stator yoke.

The winding ferrule had two spaces for coils. The magnetizing winding was wound on the lower space where the inner boss of the support base was covered, and the search coil was wound on the upper space where the inner boss of the claw pole was covered. The excitation effect of the magnetizing winding was the same as that of the magnetizing winding inside the rotor of a claw pole generator. When the magnetizing winding was connected to the inrush current, a voltage and current were induced in the search coil. The relationship between the inductive voltage and time was obtained using the galvanometer. By integration, the magnetic flux passing through the cross-section of the inner boss of the claw pole was collected, and the corresponding magnetic induction intensity was calculated by division by the area of the cross-section. The curve of magnetic induction intensity in the magnetic circuit that varied with the excitation current could then be obtained, which was used to characterize the magnetic properties of the claw pole. In the process of measurement, if the same support base and guide ring were used, claw poles with the same outer diameter could be interchanged, so the magnetic properties of the same type of claw poles produced by different manufacturing processes could be evaluated. Of course, the dimensions of the support base and guide ring can be designed based on different sizes of claw poles for testing of their magnetic properties.

To verify the correctness and effectiveness of the NDTEM, numerical analysis of the magnetic measuring process was carried out by FE simulation. To simplify the modeling process and reduce the calculation time, an equivalent static magnetic analysis model was used to solve the transient magnetic field problem for the entire measurement system.

3.1 Finite-Element Modeling

According to the above concept, a corresponding FE model was created in ANSYS-Maxwell, as shown in Figure 3. The main dimensions of the claw pole to be measured are illustrated in Figure 4. The maximum outer diameter and height were 98.30 mm and 42.20 mm, respectively, and the magnetization curve of the raw material of the claw pole was input. The materials data for the support base, guide ring, and positioning axis were selected from the database of the software. In the entire solution domain, the parameter of air was also defined. In the simulation, all magnetic parameters of the materials were assumed to be isotropic.

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The magnetizing winding, which consisted of twisted copper conductors with a diameter of 0.8 mm, is equivalent to a ring: it was set as a “stranded” type, in which the eddy current effect was ignored. The corresponding magnetic parameters of the copper conductor were defined using the material database. The number of magnetizing winding turns in the claw pole motor was 360; an appropriate increase in the number of turns could reduce the output current of the equipment. The number of magnetizing winding turns used for the designed measuring system was 400, and the excitation current could be varied in the range of 0.00–3.75 A. Adaptive analysis technology was used to mesh the tetrahedral elements for all parts. This was an iterative process to form an initial mesh with fewer elements and subdivide the mesh of some regions according to the calculated magnetic distributions. The specific parameters of the solution setup are shown in Table 1.

Table 1

Parameter values for finite-element model

Parameters	Values
Maximum number of passes	12
Percent error	1
Refinement per pass	30%
Minimum number of passes	2
Minimum converged passes	1
Nonlinear residual	0.001
Relative residual	1×10−6

In the actual measurement process, the average magnetic induction intensity was obtained. In the simulation, the magnetic flux was the integration of the magnetic flux density through the cross-section of the inner boss in the claw pole, so the average magnetic induction intensity was calculated according to Eq. (1):

$$\overline{B} = \frac{{\int {B \cdot {\text{d}}s} }}{S},$$

(1)

where B is magnetic flux density, S is the cross-sectional area of the inner boss in the claw pole, and \(\overline{B}\) is the average magnetic induction intensity.

By changing the input excitation current from 0.00 A to 3.75 A, a magnetization-like curve of the claw pole could be measured. The average magnetic induction intensity was selected as the ordinate, while the total current (which is the product of the excitation current and the number of turns of the magnetizing winding) was defined as the abscissa.

The relationship between the magnetization curve and the magnetization-like curve is determined by Ampere circuital theorem, as shown in Eq. (2):

$$\oint {H \cdot {\text{d}}l = l_{e} } \cdot H = \sum {NI,}$$

(2)

where H is the intensity of external magnetic field, \(l_{e}\) is the length of equivalent magnetic circuit, N is the number of turns of the magnetizing winding, I is the excitation current.

So, there exists a ratio \(l_{e}\) between the abscissa of the two type of measured curves. In this work, the magnetization-like curve was used to reflect the overall magnetic performance of the claw poles.

3.2 Simulation Results

To analyze the measurement process in detail, specific magnetization curves, which were measured using the ring-sample method, of two claw poles made by different manufacturing processes (shown in Figure 5) were imported into the constructed FE model to simulate the static magnetic field.

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Figure 6 shows the simulated result of the magnetic field distribution of the measuring system when the magnetizing winding was energized. The magnetic flux density through the cross-section of the inner boss of the claw pole was quite uniform and the absolute value was larger than that of other zones.

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This also proved that the use of Eq. (1) for the calculation of magnetic induction intensity was reasonable. In this case, the magnetic induction intensity obtained from the cross-section was much more representative. Therefore, the magnetic properties of claw poles could be evaluated by comparing the measured magnetization-like curves.

Figure 7 shows the magnetization-like curves obtained from simulation results. The magnetic induction intensity of the claw poles increased with the total current, as expected. Compared with the input magnetization curves shown in Figure 5, there is good agreement in the trend of the curves, and the difference in magnetic properties of the two claw poles can be clearly identified. Therefore, the NDTEM can be used to measure the magnetic properties of claw poles.

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In actual practice, the manufacturing process and installation for measurement will introduce some errors in positional deviations of the claw pole. In the simulation, the claw pole position was assumed to be offset by 0.5 mm in both the X and Y directions. These results are also illustrated in Figure 7. The measured magnetization-like curves after offset were almost identical to the original curve. There were only subtle differences in individual points: this showed that the positioning error in the measuring process had almost no effect on the results. Therefore, the proposed NDTEM has good stability.