Current Transductors

In some countries the term transductor is used to denote any magnetic amplifier. Here it denotes a saturable reactor circuit for measuring direct current. A current transductor is hardly an amplifier; it is a metering device. A transductor circuit is shown in Fig. 213.

Fig. 213. Current transductor circuit.

It is similar to that of Fig. 210 but with feed-back windings and a-c load removed. Operation is entirely different. Cores are circular or square, and are wound in-and-out toroid-ally in a manner resembling through-type current transformers. The heavy d-c bus then may be inserted through the toroid to form a single turn on each core. In Fig. 213 the d-c load windings are shown aiding, and the a-c windings bucking; this accomplishes the same core flux polarities as for Fig. 205. Load direct current is determined by the load resistance, which is large compared to the reactance of the transductor. Control circuit impedance multiplied by the turns ratio is large; magnetization is constrained. It will be recalled from Section 110 that, under this condition, even current harmonics cannot flow. Therefore a-c winding current is flat-topped. After this flat-topped current is rectified, it flows through the ammeter as smooth direct current.⁽¹⁾

At any instant one reactor of the pair is saturated, and the other unsaturated. On each a-c half-cycle the unsaturated reactor maintains the output current constant. Total output d-c ampere-turns of course must equal twice the load direct current at all times. Transductors are like simple magnetic amplifiers as far as the relations of load and output currents are concerned. They have been built to measure currents of 10,000 amp or more, with good linearity.

(1)	For a description of the current and flux conditions, see "Magnetic Amplifiers," by S. E. Tweedy, Electronic Eng., February, 1948, p. 38.