Why do AC SSRs have lower AC Voltage limits and what does this mean?
Why is there a lower output voltage limit (24 VAC)?


Because some voltage is required to operate the triac.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. An instant turn-on SSR features an opto-triac and a regular triac.

TR1 requires some headroom to turn on. The SSR may work at lower voltages but the turn-on point will be late in the cycle because the voltage has to rise high enough for TR1 and TR2 to turn on.



Figure 2. Resultant waveform if supply voltage is too low. Image mine.

The same effect is seen at higher voltages but because the voltage is rising at a much faster rate the turn-on happens much earlier in the cycle and is barely noticible in the overall waveform.



Figure 3. The zero-cross SSR features circuitry to suppress turn-on if the voltage has risen above a certain level. Source: How opto-triacs and zero cross works.

You can read more in my article linked above.