Priniciples of Electromechanical Relay Operation


3.5 Matching Mechanical And Electrical Characteristics

The area under the load curve is a measure of the work that the electromagnet is required to perform. Curve B of Fig. 3.2 represents typical pull curve for an electromagnet. It shows the pull relative to the distance between the armature and core for a particular value or coil ampere-turns. This figure demonstrates that in order for the relay to operate, the magnetic force developed at all points of the armature travel will have to exceed the mechanical forces tending to restrain motion of the armature.
The force developed by the electromagnet may be expressed by:


in which N is ampere turns; A is pole piece area: x is distance between armature and core in de-energized position and Ro is reluctance of iron portion of magnetic circuit.
Thus the pull of an electromagnet of fixed dimensional constraints varies with the energizing ampere-turns squared. The ampere-turn value at which the relay just operates is known as the ampere-turn sensitivity. For many circuit applications, however, it is more convenient to express sensitivity in terms of power, P, required to operate (P=I^2R, in which R is the coil resistance). Whereas ampere-turn sensitivity is independent of coil dimensions, power sensitivity varies with volume and the proportion of conductor space occupied by the winding. For a given coil volume and proportion, the ratio N2/R is constant. It is known as coil conductance and is symbolized by Gc. By inversion, R = N2/Gc and P=I^2R=N^2I^2/Gc watts or power required is inversely proportional to Gc. Coil conductance may be expressed in terms of the dimensions of the coil by the equation,


in which e = winding space factor(decrease only slightly for fine wire windings); p = wire resistivity; l = length of winding cross section; h = depth of winding cross section and d = diameter of core.
Since power is N^2/I^2/Gc, it is clear that the power required to operate varies inversely with coil length and directly with winding depth. Do not assume that power sensitivity will be the same for all values of coil resistance. Using standard magnet wire sizes it is not possible to obtain equal fullness of winding for all desired resistance values.
The required ampere-turns of magnetizing force to be generated in a coil of given available volume may be obtained by many turns of fine wire or much fewer turns of coarse wire that carries correspondingly larger currents. Using wire of a given size and of a known resistance per unit length, the resistance of the coil can be computed from the number of turns that can be wound in the particular dimensions available.
The coil conductance Gc also relates to the amount of heat generated in the coil when providing a given magnetizing force, as well as to the inductance of the coil (the principle factor in determining the response speed of a relay of given design to a given electrical drive).
Scaling down the size of a relay proportionally in all directions affects the performance in a number of ways:
1. More magnetizing power (ampere-turns) is required for a given armature force and stroke.
2. Magnetic saturation of the iron circuit is reached at lower armature force values and little further gain in pull is possible.
3. The coil gives fewer ampere-turns of magnetizing force for a given power input.
4. The ability of the relay to dissipate internally generated heat fall off sharply.
5. The relay is more resistance to vibration, particularly at the higher frequencies, because of the lower moments of inertia of its various parts.
Changing the coil wire one gauge size finer is a coil of given dimensions increases the coil resistance by approximately 60% if the bobbin is wound to the same fullness. Such a change does not affect the power required for a given number of ampere-turns nor, therefore, the corresponding switching performance. It does reduce the current required for equal ampere-turns by 20% and increases the voltage required by 25%. Incremental changes of this order do not ordinarily introduce circuit mismatch inefficiencies of sufficient amount to be of concern, but the use of standard wire sizes (and resistance tolerances) automatically sets the resistance interval for fully wound coils at approximately 60% and the resistance tolerance at plus or minus 10% fir all but the very fine wire sizes (above No. 45 AWG, a tolerance of 15% is sometimes encountered).
Consideration of the force curves shown in Figs. 3.02 and 3.04 indicates that the ease of obtaining contact action with large forces and adequate travel is dependent on how low the dropout power may be as well as the value of pickup power available. Also, the choice of the return spring force and contact spring rates permits the designer to make a trade-off between armature travel and the contact forces for given pickup and drop out values.
In such a trade-off, there are least five variables related to the desired relay performance: 1) pickup values, 2) dropout values, 3) contact gap, 4) pole gap, and 5) contact pressure force. Three may be selected arbitrarily. The remaining two then become dependent variables and must be accepted, or else the first three choices must be revised until all five variables are satisfactory.
A simple relay with a compliant normally open contact has certain predictable characteristics:
1. Pickup values are adjustable within the limitations of spring force and contact position stability.
2. Dropout values cannot be precisely adjusted without special provisions.
3. Contact erosion results in variations in both pickup and dropout values and usually determines the effective end of contact life.
4. Contact impact usually results in some bounce and provides some degree of contact wipe, roll or scrubbing motion.
5. The normally closed contacts become less stable as the relay coil current approaches the pickup value. This condition tends to make many relays prone to contact chatter when subjected to environmental vibration or to coil current ripple when partially energized close to the pickup value.
6. The normally open contact set, commonly having greater overtravel, is less subject to chatter and bounce.
7. Erratic pickup and dropout values can result from switching loads with heavy inrush current transients sufficient to cause contact sticking. Use of modern magnetic iron, annealing techniques, and relay design has resulted in relays seldom troubled with erratic performance due to magnetic hysterics.
Dropout. Curve C Fig. 3.2 represents the pull of a relay electromagnet in the release or dropout range. As indicated, the armature will restore to its de-energized position when the energizing ampere-turns produce a pull throughout the release stroke that is less than the mechanical load.