Impact of modern development

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Ralph Müller (1940) stated "That the history of physical science is largely the history of instruments and their intelligent use is well known. The broad generalizations and theories which have arisen from time to time have stood or fallen on the basis of accurate measurement, and in several instances new instruments have had to be devised for the purpose. There is little evidence to show that the mind of modern man is superior to that of the ancients. His tools are incomparably better."
Davis Baird has argued that the major change associated with Floris Cohen's identification of a "fourth big scientific revolution" after World War II is the development of scientific instrumentation, not only in chemistry but across the sciences. In chemistry, the introduction of new instrumentation in the 1940s was "nothing less than a scientific and technological revolution" in which classical wet-and-dry methods of structural organic chemistry were discarded, and new areas of research opened up.
As early as 1954, W A Wildhack discussed both the productive and destructive potential inherent in process control. The ability to make precise, verifiable and reproducible measurements of the natural world, at levels that were not previously observable, using scientific instrumentation, has "provided a different texture of the world". This instrumentation revolution fundamentally changes human abilities to monitor and respond, as is illustrated in the examples of DDT monitoring and the use of UV spectrophotometry and gas chromatography to monitor water pollutants.
Construction
Generally, a soft starter is constructed with three pairs of SCRs reverse parallel connected to allow the current to flow to or from the motor. Soft starters can be made by controlling just one or two phases, but this paper will focus on the most prevalent implementation, three-phase control. Each phase of a soft starter can be controlled with an SCR pair reverse parallel connected (Fig. 2), an SCR/diode pair reverse parallel connected  depending on cost and/or quality. The most prevalent switch in industry is probably the SCR pair and will also be the focus of this paper. Soft starters are used almost exclusively for starting and stopping and not during the run time because of the heat loss through the SCRs from the voltage drop across them.
A standard assembly of a soft starter uses one SCR pair per phase and once the voltage gets to within approximately 1.1V of full voltage (depending on the voltage drop across the SCR) a bypass contactor (internal or external to the soft starter), running parallel to the SCR pairs, pulls in (Fig. 5). Once pulled in the SCRs stop firing. Typically, the bypass contactor is much smaller than compared to what is needed for a full voltage start as the contacts only need to be able to handle the full load current of the motor. Since the mechanical contacts cannot handle the inrush current, the SCRs must be sized correctly to handle the motor’s locked-rotor current.

Control
A soft starter reduces the voltage by “notching” the applied sinusoidal waveform, simulations of which can be seen in Figures 8-13. A notch is a non-technical term for the zero voltage area in the middle waveform seen in Fig. 9. Fig. 9 also shows the forward firing SCR pulses in green and the reverse firing pulses in blue to notch the output voltage. Fig. 10-13 show a progression from a 90º firing angle to 30º. As the notch decreases in size, the Vrms increases along with Irms. An initial voltage, determined by the user, is ramped up to full voltage by varying the firing angle depending on the preset profile of the soft starter. Soft starters can be controlled via open-loop or closed-loop control.

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