PURPOSE:To reduce the number of detecting means, by controlling the feed of power to primary side coils positioned at the same position as that of the detecting means of a detected carrier, at the following step, and at the previous front step, based on the detected result of the carrier. CONSTITUTION:Sensors S1-S4 detecting a carrier 2 are respectively arranged to be placed at a central section in the longitudinal direction of respective primary side coils M1-M4. When the carrier 2 runs on the primary side coil M1 and comes to the upper section of the primary side coil M2, then detection signal is fed from the sensor S2 to a controller C. Then, by the controller C, a solid state relay R1 is turned OFF, and a solid state relay R2 is turned ON, and the under these states, a solid state relay R3 is also turned ON. As a result, three-phase alternating current is fed to the primary side coils M2 and M3.
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