Magnetic flowmeter

Abstract

A magnetic flowmeter whose electromagnet is excited by a low-frequency excitation current that is derived from an on-off alternating current whose frequency is higher than that of a commercial power line source, the on-off current being modulated by a low-frequency wave. A reference voltage-detecting transformer is interposed between the excitation coil of the electromagnet and an excitation current source. The primary of the transformer is constituted by a pair of windings wherein the excitation current is caused to flow in one direction in the first primary winding and a current produced by the counter-electromotive force generated in the excitation coil in every on-off transient of the excitation current is caused to flow in the opposite direction in the second primary winding. A reference-voltage a-c signal changing in both positive and negative directions is derived from the secondary winding of the reference transformer, whereby a reference voltage having a waveform similar to that of the excitation current actually flowing in the excitation coil is obtained by rectifying the reference-voltge a-c signal, thereby avoiding saturation of the transformer core resulting from a d-c component included in the excitation current.