DocumentCode :
1379286
Title :
Measurements Section: Chairman´s address. Magnetic amplifiers
Author :
Coales, J.F.
Volume :
101
Issue :
80
fYear :
1954
fDate :
4/1/1954 12:00:00 AM
Firstpage :
83
Lastpage :
99
Abstract :
The use of the non-linearity of the magnetization curves of transformer cores appears to have first been used in this country by the author´s father for testing large power transformers in 1907?08.* This and bridge methods employed more recently work on the change with magnetizing field of the incremental permeability of the magnetic core. The advent of high-permeability materials with a sharp knee in the magnetization curve has resulted in the development of the transductor, which essentially consists of two cores which are alternatively driven far into saturation in each half-cycle of the supply frequency. Transductors are essentially d.c. amplifiers, the magnetization of the cores being biased by input windings connected to the excitation windings in series opposition. The operation of a transductor is similar to that of a pair of grid-controlled rectifiers in that as soon as saturation of one core is reached the inductances of the windings on it decrease to a low value and the currents rapidly increase, driving the core far into saturation. The impedance of the windings must then remain small until the current through them has decreased to a very small value; since the impedance is small this can result only from the excitation voltage decreasing almost to zero. Another important principle in the operation of transductors is that the current through the load is only significant when one core is saturated and the other is not, in which case there is tight coupling between the input and excitation circuits, and ampere-turn balance must be maintained between them. Thus the current gain of a simple transductor is always equal to the ratio of the numbers of turns on the excitation and input windings, and the transductor is equivalent to a current amplifier of high gain with 100% negative feedback. This results in a highly linear relationship between input and output but a low power gain. As might be expected, the power gain can be increased by feeding the rectified - load current back to the input in the positive sense. This is known as self-excitation and results in some reduction of the linear range and increased drift. There is, however, considerable improvement in transient response, since, independent of the amount of feedback employed, the time-constant of the transient response is almost directly proportional to the current gain, whereas the power gain is proportional to the square of the current gain. It has been shown that, provided 100% self-excitation is required, this can be more simply achieved in parallel-connected transductors by the use of rectifiers to suppress in each half-cycle the excitation current in the winding which will not be saturated. Self- or auto-excitation results in increased standing current, and, as might be expected, this results in reduction of stability. It is easily shown that the chief cause of drift is change of the rectifier characteristics, and with the best rectifiers available at present the smallest input signal that can be discriminated with this type of transductor is 10-7 watt. The simple two-core transductor does not distinguish between positive and negative input currents; to do this, a push-pull arrangement results in some improvement of stability, and the smallest input power which can be discriminated is of the order of 10-9 watt. By connecting the load to additional windings, magnetically similar to the input windings, and suppressing by means of a high impedance the even harmonics which would otherwise be induced in the input circuit, these harmonic currents can be forced into the load circuit. This circuit then has no standing current, but, since ampere-turn balance must be maintained, it has the same signal current as before. This results in a very great improvement in stability, and by the use of elaborate valve circuits a short-term stability of 10-18 watt can be achieved. If, however, the magnetic modulator is to be followed by a magnetic amplifier, since the greatest
fLanguage :
English
Journal_Title :
Proceedings of the IEE - Part II: Power Engineering
Publisher :
iet
Type :
jour
DOI :
10.1049/pi-2.1954.0017
Filename :
5240708
Link To Document :
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