Priniciples of Electromechanical Relay Operation

3.11 Timing Characteristics

Operate Time. Determination of operate time of electromagnetic armature type relays is treated comprehensively in various tests. This discussion is limited, therefore, to circuit applications using the circuit parameters and basic characteristics or a relay to predict the probable range of operate time. Although the method neglects some secondary factors, it is sufficiently accurate for most purposes. Operate time is a function of relay adjustments and coil conductance, Gc = N2/R, which varies with fullness of winding but is substantially constant for a given fullness or winding. The resistance term includes any resistance, which varies with temperature, i.e., operate time will vary with temperature. The following discussion of operate time considers only time to first closure and disregards contact bounce time, which is influenced by many factors relating to the dynamics of a relay design. Operate time of a relay is comprised of waiting time and transit time. Waiting time is the interval, after closure of the winding circuit, in which the flux builds up sufficiently to start the movement of the armature toward the core. Transit time is the interval between the start of movement of the armature and the completion of the switching function. As the energizing power is increased, waiting time decreases but finally levels off. Transit time also decreases somewhat with an increase in energizing power, but it, too, finally tends to become a constant at a certain power level. Thus, total operate time decreases with increased power and approaches a value at which there will be little or no benefit derived from increasing the driving power.

Fig 3.8 illustrates typical operate time curves obtained at three ambient temperatures on several samples of a particular type of four transfer (4 form C) relay. The data were obtained using a bounce free, mercury wetted contact to control the winding circuit. In this figure, Q is the ratio I_j/I_a (or E_j/E_a, in which I_j (or E_j) is the measured pick-up point in current (or voltage) and I_a (or E_a) is the current (or voltage) at which the winding is energized. The upper and lower limits of operate time for a family of relays at given values of Q are illustrated by sets of curves, A and B. Curves such as these provide the basic data for predicting operate time. A family of relays refers to a group of relays having essentially the same armature travel and load and the same fullness of winding or coil conductance (G_c=N^2/R). For another fullness of winding, the timing characteristic will be different. For example, for shallower windings, operate time will be shorter because of N^2 term of G_c decreases more rapidly than the resistance, and varies directly with G_c. As Q decreases (it is the equivalent of increasing power), operate time decreases until it approaches a minimum time. At this point, very little increase in speed is derived from large increases in power. At the other extreme, operate time approaches maximum at Q=1, the point at which energizing current (or voltage) is equal to the value at which the relay just picks up. It is inadvisable to operate a relay in the steep region of these curves if operate time is critical.


If a relay is to be used over a range of ambient temperatures, curves should be obtained at the applicable temperature extremes, as illustrated by Fig. 3.8. Likewise, if a winding reaches an elevated temperature as the result of internal heating, and then is de-energized and re-energized shortly thereafter, timing data should be obtained for this condition. Heating effects are particularly important in rapidly pulsing circuits. It may be seen from Fig. 3.8 that for the same value of Q, operate time is shorter at higher temperatures and longer at lower temperatures. The effect is explained by G_c or N^2/R, being inversely proportional to resistance and absolute temperature. As G_c decreases, operate time becomes faster. With the nonpick-up and pick-up points controlled by initial requirements and stability limits, it is possible to use operate time versus Q data to predict, with reasonable accuracy, the range of operate time for relays adjusted within the specified nonpick-up and pick-up limits. Using curve A fo Fig. 3.8, maximum operate time for this family of relays will result when a relay adjusted to pick-up at the specified pick-up value (I_j is maximum) is energized the minimum circuit current.

In a similar manner, minimum operate time is obtained from curve B when Q is minimum, that is, when a relay adjusted to pick-up at the minimum value, just above the nonpick-up requirement, is energized at maximum circuit current. The operate time range can be determined in this manner for any combination of ambient temperatures by obtaining Q curves at the applicable temperature extremes. The curves of Fig. 3.8 were obtained with no resistance in series with the winding. For the series resistance case, G_c=N^2/(R_c+R_s); R_c is the winding resistance and R_s is the series resistance. For a given Q and a given winding, the relay will be faster when resistance is in series with the winding. This method is used to reduce operate time and avoid the overheating that would result from applying a higher voltage directly across the winding. Operating time characteristics for the series resistance case can be obtained by using the method illustrated in Fig.3.8
Operate time curves expressed in terms of input watts frequently offer greater flexibility in circuit design. The Q data of Fig.3.8 may be converted into watts versus time curves (see Fig. 3.9) as explained in the following Paragraphs.

Case A: Maximum operate time at a given temperature (D curves of Fig. 3.9). An input wattage, W_a, is selected and a value for Q is calculated using either of the following formulas, depending on whether the operating voltage characteristics are expressed in current or in voltage:


Imax = specified pick-up current, plus applicable stability limit
Rmax = nominal winding resistance plus specified tolerance (corrected to applicable temperature)
Emax = specified pick-up voltage plus applicable stability limit (corrected to applicable temperature)
Rmin = nominal winding resistance minus specified tolerance (corrected to applicable temperature)
To obtain the maximum operate time value for the calculated Q, use curve A of Fig. 3.8. This value is plotted against W_a as one point of curve D of Fig. 3.9. In the same manner other points may be calculated using several values of W_a.
Case B: Minimum operate time at a given temperature (C curves of Fig. 3.9). As in case A, an input wattage is selected and the corresponding Q is calculated from the following formulas:


In this case, E_min is the specified nonpick-up voltage minus the applicable stability limit (corrected to the applicable temperature). Likewise I_min is the specified nonpick-up current minus the applicable stability limit.
Minimum operate time is obtained for the calculated Q from curve A of Fig 3.8 and then plotted against W_a as one point of curve C of Fig 3.9.
1. Input power operate time curves established, it is possible to determine minimum and maximum operate time for the applicable temperature by calculating maximum circuit watts and reading the minimum time from curve C and calculating minimum circuit watts, and reading maximum time from curve D.
When timing is critical, a relay should be used only within the fairly flat range of the upper and lower curves.
The methods described also may be used to determine operate time of relays employing copper slugs or short circuited turns to a limited range of the delays.
Conditions requiring special study are:
1) Timing during shock or vibration.
2) Operation in capacitor charging or discharging circuits.
3) Operation when a relay is in parallel with a capacitor and in series with a resistor.


Release Time. If you disregard the dynamic considerations affecting contact bounce, release time of an electromagnet relay is comprised of two stages, waiting time and transit time. The former is the time, after the opening of the winding circuit, for the flux to decay to a level at which the magnetic pull can no longer sustain the mechanical force acting on the armature and the magnetic pull can no longer sustain the mechanical force acting on the armature and the armature starts to move from its operated position. Unless the relay is equipped with a sleeve, copper slug, or short circuiting winding to delay decay of flux, waiting time on dropout will be appreciable shorter than on pick-up since the rate of decay of flux is faster than flux buildup. The second stage, transit time, is the interval between the start of movement of the armature and contact actuation. Transit time usually is shorter during release than during operate because of the more rapid flux decay and because the forces acting on the armature aid release and oppose operation. Release time for many designs, therefore, is shorter than operate time. Contact protection in the form of semiconductor diodes, short circuiting windings, or capacitor resistor networks may substantially increase release time by retarding decay of flux but occasionally a capacitor resistor network may decrease release time.

In a particular design, the parameters affecting release time are variations in the armature residual gap, armature load, contact separation, and magnitude of the interrupted energizing current. Waiting time will be maximum when the interrupted energizing current is maximum and the residual gap and armature load are minimum. It will be minimum when the interrupted energizing current is minimum and the other parameters are maximum. In the case of unprotected windings, this effect usually is small. It may be appreciable, however, for protected winding.

Usually when protection is employed, the range of release current adjustments has an appreciable effect on release time. Transit time will be minimum when the armature load is maximum and the contact separation of normally closed contacts is minimum.

Thus in evaluating release time characteristics, it is essential that studies be conducted on samples representing the two extremes of adjustments for production relays. From this type of study it should be possible to obtain correlation between dropout current or dropout ampere turns and release time.

By specifying dropout and hold requirements and controlling their stability, it is possible to predict the range of release time within limits suitable for practical application. Any variation in dropout current caused by environment, operation, or acceleration will affect release time.
In determining release time capability of relays equipped with time delay features such as sleeves, copper slugs, short circuited windings, and the like, the maximum conductance sleeve, copper slug or short circuited winding should be used on relays adjusted to provide minimum release time.