Priniciples of Electromechanical Relay Operation

3.28 Operating Characteristics

Adjustments of thermal relays are subject to variation upon exposure to temperature extremes and dynamic environments such as vibration and shock. Operate time depends on the magnitude of variation in energizing voltage.
The operating characteristics of thermal relays can be predetermined for various environmental conditions and adjustments from the "basic time" of a particular structure. Basic time is that setting of a particular relay structure at which the operate time and the saturate release time are approximately equal (Fig. 3.13).

The time interval of any thermal relay is a function of voltage applied to the heater element, as shown in Fig. 3.13. As indicated in Fig. 3.14 a thermal relay will operate at any voltage above the critical voltage. Below this voltage the relay will not operate no matter what the duration of applied voltage may be.



A thermal relay is said to have reached thermal saturation when it has been energized long enough for all elements to have reached thermal equilibrium. This saturation time may be 12 to 15 times the basic time.
Recovery and reoperation of a thermal relay depend upon the basic time of the relay and upon its adjustment within its operating range. A particular time delay can usually be provided by any of several different relays. Choosing the most suitable relay for a particular application, however, involves choosing basic time that offers a balance between recovery and reoperation characteristics on the one hand and the effect of voltage change on the other.

The instantaneous release and recovery times are important factors for some applications. Normally, the instantaneous release time is less than 1 or 2 seconds, but it may be a few seconds longer when a thermal relay is set to a small portion of its basic time. Any thermal relay will recover a fixed percentage of the time for which it has been adjusted after a definite cooling interval. The time after de-energization to regain a given percentage of initial operate time is a function of the adjustment and the basic time of the relay (see Fig. 3.16).

In some circuits it is necessary to have current applied to the heater of the thermal relay as long as the equipment is in operation. These applications sometimes involve reoperation after de-energization. In these cases it is important to know the time interval required, after removal of power from a saturated relay, for the relay to cool enough to permit reoperation. This time depends upon the basic time of the relay and its adjustment in the timing range. Fig. 3.17 illustrates this factor. Obviously the longer the relay is permitted to cool, the longer will be its operated delay time. In effect, release time after saturation plus recovery time is the time required for reoperation.