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Solid State Relays - Selection Process


Load considerations

Most difficulties arising from the application of AC solid state relays are due to an incomplete analysis of the operating conditions that specific loads impose upon the relays.

Resistive Loads

Loads of constant value resistance are probably the simplest application of AC solid state relays. Observing the steady - state current and blocking voltage specifications will normally result in a trouble free application. The rate-of-rise of current (di / dt) in a purely resistive load is limited only by the line impedances and the turn-on characteristics of the output thyristor. It is possible, particularly in high current applications, to exceed the di / dt to within the relays rating. The addition of some series inductance especially in high duty cycle applications, may some times be necessary to limit the di / dt to within the relay rating. The use of relay with zero-voltage switching is an effective way of keeping the di / dt within the rating of the output thyristor. With zero-voltage switching is an effective way of keeping the di / dt within the rating of the output thyristor with zero-voltage switching relay turn-on occurs at a point near the zero crossing of the voltage and, therefore, it is very difficult to have a high di / dt through the relay.

Lamp Loads

Incandescent lamp loads, though basically resistive, present some special problems. Because the cold resistance of a tungsten fi lament is only 10% or less of the hot resistance, a large inrush current can occur. The duration of the inrush current can range from one half cycle to several cycles, depending on the thermal time constant of the fi lament. It is essential to verify that this inrush current is within the surge rating of relay. Because of the unusually low fi lament resistance at the time of turn-on potential problems with di / dt may be more severe with lamp loads. A zero-voltage switching relay is particularly desirable with tungsten fi laments because of the ability to reduce the di / dt stress impesed on the relay and to increase lamp life. Certain types of lamps can momentarily apply near short circuit conditions on the relay at the moment of burnout. This occurs if a mechanically failed fi lament falls back across itself or the input lines, in such a manner as to result in a greatly reduced impedance, or if a low impedance gaseous discharge path exists, as it does in some lamps at burnout. The lamp characteristics at the moment of burnout should be carefully investigated and adequate precautionary measures taken to assure reliable operation, fast acting semiconductor fuses, of some series impedance line can be used to limit fault current to within capability of the relay.

Capacitive Loads

Capacitive loads are not extremely common but they are encountered in applications such as switching capacitor discharge banks or capacitor input power supplies. Caution must be used with low impedance capacitive loads to verify that the di / dt capabilities of the relay are not exceeded. The di / dt of a discharged capacitive load without external limited impedance can approach infi nity. The valuable means of limiting di / dt with capacitive loads. Particular attention should be given to the safety margin on the relay blocking voltage rating, and voltage transients must be limited when switching capacitive loads. False operation at near peak line voltage into a discharged capacitive load can result in very large and potentially damaging di / dt values. The addition of series line impedance or absolute voltage clamping may be necessary to limit di / dt and protect the relay against the inevitable, occasional large voltage transient on the line.

Inductive Loads, General

Inductive loads are commonly encountered and they present some special operating conditions for an AC Solid state relay. As a result, most application problems with Solid state relays probably occur when inductive loads are being switched. The most basic inductive load problem is associated with AC Solid state relays that have a triac as the output thyristor, and it is the failure of the relay to commutate (turn-off)properly. This occurs because at the instant of turn-off (zero current through the thyristor) of a lagging power factor load (inductive), the instantaneous value of line voltage can be very high (peak voltage if load is purely inductive). This instantaneous value of line voltage is immediately applied across the triac in the relay when the current goes to zero and can appear as a very high rate of rise of voltage (dv / dt). The high dv/dt can causes the triac to immediately return to the On-state and a "lock-on" conditions occurs. The input circuit no longer has control of the relay and power must be removed from the load circuit in order to turn-off the load. This potential problem is worsened by the fact that the high dv / dt is applied immediately after current conduction and it is the lower value commutating dv / dt capability (typically 4V/u sec) of the triac, not the higher value off-state dv / dt capability (100 V/u sec) which determines if successful commutation will take place. This problem is overcome by the use of R-C snubber network (discussed in another section of this manual) which limits the dv / dt applied to the relay at turn-off to a level within the commutating dv / dt capability of the triac. Alternatively, two SCR's connected in inverse parallel may be used to form the output switch in the relay. This technique allows the much higher off-state dv / dt value to the limiting factor in assuring turn-off. Snubber networks are also used with dual SCR outputs in extremely high dv / dt applications. An inductive load inherently tends to limit the rate-of-rise of current (di / dt) and, therefore, di / dt problems are relatively uncommon with inductive loads.

Motors

Motors frequently present some problems in addition to those of passive inductive loads. Specifically, motors often have severe inrush currents during starting and produce unusual voltage during turn-off. The Inrush currents of motors connected to mechanical loads having high starting torque or high inertia should be carefully determined to verify that they are within the surge capabilities of the relay. Both the envelope and the duration of the Inrush current should be examined using an oscilloscope. Frequently, applications require motor starting at short intervals (pulsing) and the effect of the repetitive inrush current, on the thermal operation point of the relay must then be carefully weighed. The possibility of stalled rotor conditions where current may be six times higher than normal, should be taken into account. An extended stalled rotor condition may require an over sized relay or fuse protection. The EMF generated by certain motor circuits can require a relay to have a blocking voltage rating greater than would normally be expected based upon the steady-state line voltage applied. This matter can become quite complex, and the voltage applied. This matter can become quite complex, and the voltage applied to a relay by a motor circuit during turn-off an oscilloscope to verify that it is safely below the rated blocking voltage of the relay. Otherwise, “lock-on” or erratic turn-off of the motor may occur. Some motor circuits may require relays with higher than normal blocking voltages, transient limiting devices, or other techniques to withstand the voltage which is produced by a motor during deceleration or reversal.

Transformers

In switching the primary of a transformer, the characteristics of the secondary load should be examined because they are refl ected as the effective load on the relay. Voltage transients from secondary load circuits, likewise, are frequently transformed and can be imposed on the relay. Transformers, present a special problem in that, depending on the state of the transformer fl ux at the time of turn-off, the transformer may saturate during the fi rst half-cycle of operation at the next turn-on. This saturation can result in a very large current (commonly 10 to 100 times rated primary current) through the relay that could exceed the half cycle surge capability. Relays having random switching may have a better chance of survival than those with zero voltage switching because they commonly conduct for only a portion of the fi rst half-cycle of the voltage. On the other hand, a random switching relay will frequently turn-on at essentially the zero voltage crossing and then the relay must sustain the worst- case saturation current. Zero-voltage switching relay has the advantage that it turns on in a known, predictable mode and will normally immediately demonstrate (depending on turn-off flux polarity) the worst-case condition. The use of an oscilloscope to study the first half-cycle worst-case condition is advised to verify that the half-cycle surge capability of Application Notes 85 the relay is not being exceeded. The severity of the transformer saturation problem varies greatly, dependent on such things as the magnetic material in the transformer core, the saturated primary impedance and the line impedance. A safe rule of thumb in applying an AC solid state relay to transformer primary is to select a relay having a half-cycle surge current rating greater than the maximum applied line voltage divided by the transformer primary resistance, or I Inrush (Peak) = V Line (Peak) R Primary Where I Inrush (Peak) = Worst-case transformer peak half-cycle Surge current. V Line (Peak) = Peak value of applied line voltage. R Primary = Primary resistance of transformer. The transformer primary resistance is usually easily measured and can be relied on as a minimum impedance limiting the current during the first half-cycle of conduction. The presence of some residual fl ux plus the saturated reactance of the primary will further limit, in the worstcase, the half-cycle surge to safely within the capability of the relay.

Solenoids

In switching AC solenoids, an inrush current occurs until the plunger is seated. The longer the stroke (travel distance of the plunger from rest to seated position) the higher the inrush current. The relay selected, in addition to handling the steady state current (plunger seated), must also have a surge rating capable of handling the inrush current. The operating characteristics of the solenoid should be studied to determine the amplitude and duration of the inrush current. A safe approach to selecting a relay to switch a solenoid load is to select one with a load current rating equal to, or greater than, the inrush current of the solenoid. If the inrush current is not known, a worstcase current can be calculated from the following. V Line (RMS) I Coil - R D C Where I Coil - Worst-case solenoid coil current V Line (RMS) - RMS value of applied line voltage R D C - DC resistance of solenoid coil a realy is selected based upon the worst-case condition, it will withstand the high current that will result if a mechanical malfunction occurs preventing the solenoid plunger from pulling in.