涡流引起的钢棒磁化滞后时间理论计算

Theoretical Calculation of Eddy Current Induced Time Lag in Magnetization of Steel Rod

在漏磁检测中,当检测速度增大时,漏磁场随之减小。针对这一现象,对钢棒在瞬变磁场下的磁化模型进行分析,在线圈中通入方波电流激励模拟外磁场瞬间变化的情况。通过求解谐波形式的Maxwell方程组得到单频电流激励下钢棒内磁矢量及磁场强度分布;对方波激励情形,通过Fourier变换将方波分解成谐波的叠加,计算得到各频率下的响应函数,再通过逆变换得到磁场的时域响应;计算电导率、磁导率对磁场达到稳定所需时间的影响。结果表明,在外磁场瞬间变化时,钢棒表面处的磁场几乎随外磁场同步变化,而中心处的磁场明显滞后于外磁场的变化,这一滞后效应导致了高速检测时钢棒内磁化不足,从而进一步影响了漏磁场的大小。

In magnetic fiuxleakage testing, the testing signal decreases as the testing velocity increases. To explain this phenomenon, the change of magnetic field inside a steel rod under suddenly changed field is studied. When conductive metal experiences a sudden change of external magnetic field, the magnetic field inside the conductor will lag behind the change of external magnetic field. In order to calculate the lagging time, a model of conductive rod magnetized by a coaxial coil is built. The distribution of magnetic field inside the conductor is obtained by solving the harmonic Maxwell＇s equations. The sudden change of magnetic field is simulated by rectangular waveform excitation. The rectangular wave is decomposed to harmonic waves by Fourier transform, then the time domain response of magnetic field is obtained by inverse Fourier transform. The magnetic field responses for different parameters are calculated. The results show that the magnetic field on the conductor surface almost changes with external field simultaneously, while the time lag is strongest in the rod center. The time lag of magnetic field in the rob center approximately increases with conductivity and permeability linearly.