Dynamic Measurements on Electromagnetic Devices

The application of common control methods to telephone switchingsystems has led to the widespread use of high-speed relays. The actuation time of these relays is affected by many parameters such as the power supplied, how far the armature has to move, the mechanical work the armature has to perform during its motion, the winding design, the magnetic structure, and eddy currents introduced in the magnetic members caused by the application of current to the winding. The eddy currents act to oppose a magnetic flux change and hence retard a building or decay of flux. This causes the actuation time to be increased compared to a relay without such effects. An analytical determination of the development and effects of eddy currents can be made for simple sym1413 1414 T H E B E L L SYSTEM TECHNICAL JOUIINAL, N O V E M B E R 1 9 5 3 metrical magnetic structures having an infinitely long or torus shaped round or rectangular cross-section, assuming linear magnetization characteristics. However, for relay-like structures having air gaps, leakage flux which only partly completes its circuit through the magnetic material, varying cross-section so that boundary conditions become complicated, and non-linear magnetic properties, an analytical approach becomes unmanageable. For a fundamental study and direct measurement of eddy current effects, a test set has been developed to measure the dynamic flux rise and decay characteristics of relays, and similar structures. This test set is electronically operated on a synchronous switching principle.