These special-purpose analog computers were an outgrowth of the DC calculating boards used in the very earliest power system analysis.
Laboratory investigations of the stability of multiple-machine systems were constrained by the use of direct-operated indicating instruments (voltmeters, ammeters, and wattmeters).
Generators, transmission lines, and loads were represented by miniature electrical components with scale values in proportion to the modeled system.
The phase angle and terminal voltage of each simulated generator could be set using rotary scales on each phase-shifting transformer unit.
Model circuits were energized at relatively low voltages to allow for safe measurement with adequate precision.
A complete network analyzer was a system that filled a large room; one model was described as four bays of equipment, spanning a U-shaped arrangement 26 feet (8 metres) across.
Although expensive to build and operate, network analyzers often repaid their costs in reduced calculation time and expedited project schedules.
[9] The network analyzer installed at Massachusetts Institute of Technology (MIT) grew out of a 1924 thesis project by Hugh H. Spencer and Harold Locke Hazen, investigating a power system modelling concept proposed by Vannevar Bush.
Instead of miniature rotating machines, each generator was represented by a transformer with adjustable voltage and phase, all fed from a common source.
The 1925 publication of this thesis attracted the attention at General Electric, where Robert Doherty was interested in modelling problems of system stability.
[10] The analyzer occupied four large panels, arranged in a U-shape, with tables in front of each section to hold measuring instruments.
American Gas and Electric Company, the Tennessee Valley Authority, and many other organizations studied problems on the MIT analyzer in its first decade of operation.
It was targeted at utility companies to solve problems too large for hand computation but not worth the expense of renting time on a full size analyzer.
Similarly to an AC network analyzer, they represented apparatus and lines with scaled inductances and resistances.
A synchronously driven switch repeatedly applied a transient impulse to the model system, and the response at any point could be observed on an oscilloscope or recorded on an oscillograph.
Some transient analyzers are still in use for research and education, sometimes combined with digital protective relays or recording instruments.
[20] The Westinghouse Anacom was an AC-energized electrical analog computer system used extensively for problems in mechanical design, structural elements, lubrication oil flow, and various transient problems including those due to lightning surges in electric power transmission systems.
The system was periodically updated and expanded; by the 1980s the Anacom could be run through many simulation cases unattended, under the control of a digital computer that automatically set up initial conditions and recorded the results.
[9] Since the multiple elements of the AC network analyzer formed a powerful analog computer, occasionally problems in physics and chemistry were modeled (by such researchers as Gabriel Kron of General Electric), in the late 1940s prior to the ready availability of general-purpose digital computers.
The forces and displacements of a mechanical system could be readily modelled with the voltages and currents of a network analyzer, which allowed easy adjustment of properties such as the stiffness of a spring by, for example, changing the value of a capacitor.
Unlike AC analyzers used for power systems work, the exciting frequency was made continuously variable so that mechanical resonance effects could be investigated.
Even during the Depression and the Second World War, many network analyzers were constructed because of their great value in solving calculations related to electric power transmission.
The same general-purpose digital computer hardware that ran network studies could easily be dual-tasked with business functions such as payroll.
The analyzer purchased by American Electric Power was replaced by digital systems in 1961, and donated to Virginia Tech.