When we say control the first thing comes in theory or practical is that it should have a low value of noise which is apparently achieved by DC.
- 24V is far safer limit than compared to higher values 115V AC.
- 24V DC is more resistant to control circuit drop out caused by high inrush using contactors used for starting motors or any capacitive load as well.
- Modern day electronics can now be operated at lesser voltage, Optos, relays, improving efficiency of the system and lot cheaper as compared 110 V solenoids, saves cash.
Historically, 24VDC has been used for relays in Industry, and this has carried over: when PLCs and Computer-control moved in, they adhered to the existing standard. But the reason for 24V DC may be muddled in time. Certainly, DC as direct current (rather than continuous voltage) has higher immunity to a large number of industrial safety, reliability and convenience issues. Here are a few that haven't been mentioned:
- If AC, what frequency is appropriate? Military systems prefer 400Hz because generators are lighter and smaller, but getting a shock from a 400Hz circuit has caused some victims' hearts to try to sync with 400Hz. (This is not good.) 60Hz requires larger filter components where the final target works on DC than 400Hz as well, but both can be a source of noise far greater than impulse noise, just from lines running parallel.
- The safety issue is also a big one, but at lower current levels, DC might burn, but AC, as mentioned, can have physiological effects leading to death.
- DC can be filtered anywhere with a simple capacitor or RC circuit. Filtering AC and restoring the amplitude can be problematic.
- If AC arrives from two directions, and each acquires different lag, the AC waveform can be degraded significantly, or if phase approaches 180 degrees, the actual control signal can be nullified.
Some control systems are threatening to move to 48V DC (mostly automotive). There are some good reasons for this: greater noise immunity (for the same absolute level of noise) is a big one. Life is hard enough with industry happily adhering to 5V, 12V, 24V and maybe soon, 48V sensors. Why complicate life with AC?