Estimation and control of the operate time of relays

The dynamic equations applying to the operation and release of electromagnets are derived in a form in which the armature position and the flux linkages of the coil are the variables to be determined as functions of time. The effective magnetomotive force and pull are taken as given by the static magnetization relations. This formulation is valid if the dynamic field pattern is substantially that of the static field, and it is shown that this condition is satisfied if the effective conductance of the eddy current paths is small compared with that of the coil or other linking circuits. This is usually the case in operation, but is not the case in normal release. Approximate solutions are obtained for both fast and slow operation, giving the time as related to the mechanical work done, the mass and travel of the moving parts, and the steady state power input to the coil. Expressions are derived for optimum coil design and pole face area, and design requirements for fast operation are discussed. An approximate expression is derived for the release waiting time, the initial period of field decay prior to armature motion. The effect on this decay of contact protection in the coil circuit is discussed. The slow release case is treated in another article in this issue. An analysis is given of the armature motion in release, and the results of an analog computer solution to this problem reported. These show the effect of design parameters, particularly the armature mass, on the velocity attained in release motion, and hence on the amplitude of armature rebound.