Priniciples of Electromechanical Relay Operation

3.6 Dynamic Considerations

In addition to satisfying the requirements for operations under various steady-state conditions of circuit and environment, the relay must be considered in its dynamic characteristics, e.g. operate time, tendency to chatter caused by current ripple, and bounce caused by contact impact. Relay response to a step change in applied voltage is subject to delay because of two factors:
1) The first is the time required to change the flux in the magnetic circuit from some initial value to whatever value will cause the desired operation (pickup or dropout). This delay is related to inductance (L) and resistance R in the coil circuit and to applied voltage (E) through the expression for current:


Since, however, the value of inductance (L) is highly variable as a function of the degree of magnetic saturation of the iron, an empirical solution is normally required.
2) The second is the mechanical transfer time of the moving elements. The time required for the accelerated motion of transfer in most small relays is short compared to the time required for the inductive current buildup except for applications in which the effect of the relay inductance is minimized by the other circuit parameters.
The pickup delay due to coil inductance, particularly for sensitive relays with a large value of coil constant, can be considerable, with 5 to 50 milliseconds being common, whereas transfer times will usually fall within the range of 1 to a few milliseconds. For relays of a given design, the speed of response can be improved by:
Although relay inductance is quite variable, dependable operate time curves can be made that are based on the coil conductance relationship, G_c = N^2/R, in which the actual time for operation is plotted against coil "overdrive". Coil overdrive consists of two factors- the ratio of the final steady-state current to the relay pickup current, and the ratio of the open-circuit voltage to the voltage actually required for pickup current, and the ratio of the open-circuit voltage actually required for pickup of the relay. Graphic representation of coil overdrive versus operate time requires a family of curves; one form is shown in Fig. 3.7. Using these curves for a given design of relay, one can start with the desired performance and determine the necessary circuit power as well as the relay specifications or start with given circuit conditions and determine the speed of response after the coil current is reduced below the dropout value. If, however, a relay is shunted with a diode such as might be used for transient voltage suppression in transistor or integrated circuits, the delay can be as great as 20 to 300 milliseconds or longer. This is expressed approximately by the conventional, logarithmic, current decay curve from the steady-state value to the relay dropout value.
In the design of a relay for high speed service, the bounce of the contacts resulting from the closure impact is another dynamic characteristic to consider. In general, some mechanism for the absorption of the moving parts kinetic energy at the instant of impact must be provided in order to keep bounce time short compared to the operating time. This is normally accomplished by providing a small amount of spring compliance and contact rubbing action in addition to making the movable contact member as light as possible and its resonant frequency high. In any power switching application, bounce control is one of the most important factors in obtaining good contact life. Visible arcing at contact closure is normally evidence of contact bounce.