Priniciples of Electromechanical Relay Operation

3.9 Winding Resistance and Sensitivity

To define relays either as voltage sensitive or current sensitive devices is to use common misnomers. When a relay is to be operated from a power source with no series components, it is convenient to specify performance requirements in terms of voltage. When the winding circuit includes other fixed or variable components, however, it is preferabble to specify relay and circuit requirements in terms of current. In either case, it is the ampere-turns-the product of turns and current (NI)-that determines the sensitivity and operating characteristics or a relay. Only for convenience, or to reflect circuit usage, are relays designated voltage or current relays.

Efficiency or sensitivity of an electromagnetic armature type relay may be expressed in ampere-turns or watts required to get full armature closure. For a particular design and contact arrangement, ampere-turn sensitivity is dependent of wire size or fullness of winding. On the other hand, power sensitivity (I^2R or EI) varies with the fullness of winding since it is a function of coil conductance, G_c=N^2/R^2. In the absence of information on a desired winding, it is important that the manufacturer be consulted with respect to sensitivity or operating characteristics.
Almost all electromagnetic relay coils are wound with copper magnet wire. Since winding resistance is proportional to the absolute temperature for copper windings:


In these expressions, T_O and T₁ are winding temperatures, and 234.5 is the implied point of zero resistance on the Celsius scale. Since the current required to perform a particular function remains constant, assuming no instability, the power required (I^2R) is proportional to absolute temperature. Ampere-turn sensitivity, again assuming no instability, is unaffected by temperature.