Clamp-On DC Current Transducer Employing Mixing Effects in Nonlinear Ferromagnetic Materials

A clamp-on transducer is described that is based on nonlinear mixing effects for measuring dc currents in the range of 0.1 mA to several amperes. The present device consists of a closed magnetic circuit which surrounds a wire carrying the measured current. The transducer consists of three thin strips arranged in a U form, while the fourth strip serves for closing and opening the magnetic circuit. Two excitation coils placed on each of the U arms are supplied by an amplitude modulated current. The excitation magnetic fields of these coils oppose one another. A low-frequency magnetic field (at the modulation frequency) would appear in the ferromagnetic core only if the dc magnetic field due to the measured current is present in the magnetic circuit. This is due to the odd nonlinearity of the magnetic characteristic of the core material. In contrast with the open magnetic circuits employed for the excitation fields, the magnetic circuit for the low-frequency field, as well as the one for the dc field associated with the measured current, are closed ones. The low-frequency flux is sensed by pickup coils located underneath the excitation coils. Experimental results obtained for various sensors of different magnetic core materials are described. It is shown that the transduction gain of the sensor can be interpreted and evaluated by assuming the effective cross section of the nonlinear core to be periodically varied by the amplitude modulated excitation field. This process is assisted by the nonlinear eddy currents effects present in the core.