Development of the Technique for Measuring the Magnetic Flux in the Motor Core under Operating Conditions

Abstract

The increasing need for EV, PHV and HV has led to an intense research and development in the field of high efficiency motors. In order to develop the high efficiency motors, we have to evaluate magnetic properties of the core materials that consider the influences of manufacturing processes and actual motor operating conditions accurately. It is useful to develop designing techniques that can consider those evaluated properties accurately. The evaluations are carried out with a single sheet specimen by simulating stress [1], rotating magnetic field [2], direct flux superimposition [3] and so on, because emphasis is placed on their simplicity and versatility of the measurement. However, evaluations with a single sheet specimen have problems because they can⁈t evaluate actual magnetic phenomena occurred in the motor core under operating conditions completely. In order to solve these problems, we have examined some techniques to visualize magnetic phenomena occurred in the motor core. In this paper, we introduce the newly developed technique to measure the magnetic flux flow in the motor core. In order to measure the magnetic flux in the motor core, the magnetic flux sensor that does not disturb the magnetic flux flow and has electric noise tolerance in operating motor is required. Newly developed three directional (radial, circumferential, and axial direction of the motor core) thin magnetic flux sensor to locate in the motor core is shown in Fig.1. This sensor consists of two-layered flexible printed board. Thickness of the sensor is about $170 \mu \mathrm {m}$, thinner than an electrical steel sheet. Therefore, the sensor is able to set easily between the steel sheets without additional modification of the motor core. The magnetic flux in the radial direction and circumferential direction are measured by the needle probe method [4], which does not require any hole-drillings for measuring. The flux in the radial direction can be calculated by measuring the induced voltage between $\mathrm {r}_{1}$ and $\mathrm {r}_{2}$, the one in the circumferential direction can be calculated by the induced voltage between $\theta_{1}$ and $\theta_{2}$. The magnetic flux in the axial direction is calculated by measuring the induced voltage of the square search coil located on the flexible printed board. In order to make the higher tolerance for electric noise depending on the leakage flux, we constructed twisted pair line by using through holes in the two-layered circuit board, as a lead line connected to the sensor and the terminal point. Then, measurement results with developed sensors and magnetic field analyses results under the motor operating condition are compared. The magnetic flux density loci comparisons at the local area on the tooth in the stator core are shown in Fig.2. The solid line means measured results. The dotted line means analyses results. According to Fig.2, the measured magnetic flux density at the tip part of the teeth has elliptical locus. And the calculated result at the same position has alternating locus. Occurrence factors of the difference between measurement and analyses are considered as follows: (1) influence depending on an anisotropic of the magnetic material, (2) influence depending on manufacturing process such as form shaping, sheet lamination, coil winding. Analyses results ignore the influence (1) and (2), on the other hand, measurement results reflect them all. In other words, the proposed measurement technique can grasp the behavior of the magnetic flux flow, which is not able to grasp by analyses results. In conclusion, the newly developed measurement technique can measure the magnetic flux flow in motor core under operating condition, and visualize the behavior of the magnetic flux flow which cannot be considered by calculation. In the future, we will evaluate the magnetic properties of the motor core in detail by using developed technique.