SOVs can only be of two types, either A/C or D/C. Now the question arises where to use A/C and where to use D/C valves. There are no guidelines for this. It solely depends upon the reliability of power supply. If you have good UPS power supply you can use A/C valves also.
Do you have a specific application in mind? Normally SOVs both AC & DC are used on control valves to operate as a Trip function or similar, the norm would be for 24VDC, however 24VAC and higher can be used.
It depends on the application but the choice often comes down to how much current the things need. If you have a bunch of 50W solenoids, that can be a real pain to power continuously at 24VDC. OTOH, if you only have to power them long enough to move a valve from one position to another, it may not be as big an issue.
DC solenoids tend to be low wattage low force. If you need more force AC solenoids are more common.
AC coils have the advantage that when the magnetic circuit is open (not pulled in) the impedance is lower so they pull more current. Once the solenoid pulls in and the magnetic circuit is complete the impedance rises and the current drops (I'm looking at the specs for an ASCO 8320 AC coil 70VA inrush, 40VA holding 17.1 Watts). This doesn't happen with DC so the coil always pulls the current required to close the magnetic circuit (ASCO 8320 DC coil 22.6 Watts).
If you look at any large DC solenoids you will likely find a contact that opens once the magnetic circuit closes introducing a resistor (economy resistor) in series with the coil.
In the US Automotive, Automation and Aerospace industries, we are trying to go all 24vdc systems.
There are many reasons for and against it but one thing most people don’t think about is personal injury. 110v in a ground fault condition can kill someone where 24v cant.
The low Voltage Directive in Europe and the USA addresses these issues.
Companies like SMC have very sophisticated 24VDC solenoid drive schemes (internally). There are models that have a large current spike to get the solenoid moving and then it decays electronically. There is some solid state pulser circuit in their valves to accommodate this. There are also air piloted valves that take very little current to drive that in turn move a big spool that does the real air flow requirements.
