Design of an inductive load for D-C control circuit devices

The term “inductive load” is often seen in connection with the ratings of contacts. At times an attempt has been made to eliminate some of the ambiguity associated with this term by using a value of circuit “time constant” or “L/R ratio,” to specify the type of load which the contacts might be required to interrupt. When there are no iron or eddy currents present in the load inductance, this type of description is sufficient; but when the load consists of iron-core inductances, this type of description is not sufficient to describe the effects of the circuit inductance on circuit interruption. An interesting discussion on this subject can be found in reference 1. This paper will attempt to show why further factors must be considered when iron is present in an inductance, and will propose a method of specifying the characteristics of an inductive load. With the aid of such specifications, an equivalent test load for any type of inductive device might be built with some degree of confidence that the test load-interruption characteristics will be similar to the actual load-interruption characteristics.