| 摘要 |
PURPOSE:To enable the detection of a surface flaw over the entire circumference of a steel material, by detecting a leakage magnetic flux generated by a rotary magnetic field along the sectional circumference of the steel material. CONSTITUTION:A low-frequency three-phase AC power source 11 at a transmission circuit (B) generates an alternating current determining the revolutions of a magnetic field and a high frequency AC power source 12 does an alternating current as flaw detection frequency. then, the three-phase AC synthesized with a waveform synthesizer 13 is amplified 14 in the power to be applied to an exciting coil 10. At a detector section (A), a high frequency AC as flaw detection frequency rotates at the number of poles determined by the way of winding a coil 10 and at revolutions determined by the frequency of a low frequency three-phase AC. A leakage magnetic flux induced in the surface of a steel material 1 is detected by switching detection elements 8a-8c,... arranged in the circumferential direction of the steel material with a switching circuit 15 at a timing synchronizing the revolutions of a rotary magnetic field to be created by a synchronous control section 18 at a receiving circuit (C). After being differentially amplified 16, the presence of a surface flaw is judged to be outputted to a recorder 17.
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