AC or DC Solenoid Valves

I recently had a discussion with a Machine Builder for whom we are building a PLC control. He said that he would be using 24 VDC air valves on the machine as he needed them to be fast acting. I questioned whether 24 VDC was any faster than 120 VAC. He replied that he always used 24 VDC for high speed applications.

I have never used 24 VDC valves. What do others think about AC vs. DC for fast response? I cannot find anything from the valve manufacturer (Mac) indicating one type of solenoid is faster than another.

What type of PLC output would you recommend for the 24 VDC valves, solid state or relay contact? I realize that solid state is faster and more reliable but how do they hold up with the inductive spike when the output is shut off? I would like to put free wheeling diodes across the valve coils but this would really slow down the response. How do others control DC valves?


Thanks for any comments or experiences.

I had an application once that needed a fast acting pneumatic valve. We used a nominal 24 VDC valve and cranked the voltage up as high as the power supply would go, about 28 volts. We used a solid state ouput on the PLC's high speed counter card but put an SSR between the PLC output and the solenoid.

For really fast solenoids I use an isolating relay on the PLC to switch 240V AC to a 24V AV coil. This is used on carton erector and sealers where the on time is extremly short. I do replacce the coils but only every couple of years. As for which is faster AC or DC my money is on DC but I don't know for sure............

Peter

Vic,

We normally use 24Vdc in our hydraulic apps. and have just switched to solid state relays driven by the PLC outputs for longer switching life. The only thing to remember is to use a suppression circuit.

Most cable/plug manufacturers will do you a premade lead for standard DIN plugs. I buy ours from Murr, but Hirschman et al will have offerings as well.

Jon.

VicI have had those same discussions with customers and vendors.
How fast is relative. How far away is the use from the valve? Is there enough air? Are other valves cycling at the same time? And the big one. Does he understand the concept of scan time?
We have found plain ol off the shelf valves work reliably if all the other details are worked out.
24vDC will probably cycle much faster than the fluid dynamics can respond.

24DC is common, fast, and safe. It's a standard for me.

keithkyll

24DC is common, fast, and safe. It's a standard for me.

Click to expand...

What kind of PLC outputs do you use to control the valves? Do you use any suppressors? Do you have many output failures?

i think the only ac solenoid i've seen in over a decade is in my dish washer. all machinery we build (and not just us) seam to use 24VDC. AC can be better choice if it is bigger load or really long wiring (same drop is less significant on circuit with higher voltage). as gas mentiones there are many factors to consider but if they are taken care of, DC will always be little bit faster.
for AC to respond fast, output would have to turn near AC peak. PLC doesn't guarantee that. also on AC circuit output will be either relay or triac. relay is mechanical device and comparably slow. triacs are solid state devices but can't turn off on their own. normally they have to wait for zero cross to turn off and this may be as long as half AC cycle.

on a dc version one can use relay or transistor outputs. relays are slow, transistors are fast. transistor outputs have built in reverse biased diode for protection. you don't have to put external one. the reverse biased diode actually slightly slows turning off of the dc solenoids but since it's built in, there is no practical way to remove it (you would have to hack output cards - or buffer outputs so those built in diodes are not connected to load). even though those dodes are integrated, it is quite common to see suppression on dc coils, specially if you are interested in (normally installed right across inductive dc load).
normally diodes only work as suppressors in DC circuits (that is if we are talking about standard rectifying diodes such as 1N400x for example).
some people prefer more universal solution which will work for both AC and DC and they don't need to worry about polarity - in that case check MOV.

of course it the load exceeds rating of the output, or if that this happens to be customer's spec for example, you have to use interposing relays. as usual solid state beats electromechanical in terms of speed.

Transistor for small valves. SMC have supressors built-in. Built-in is typical for any valve that has an LED indicator. PLC outputs are designed to drive inductive loads. They don't have failure problems. Most have MOSFET output. The MOSFET's intrinsic diode helps protect it. For larger valves, use 1N4937 (fast version of 1N4007). The diode will slow switch off time.
The characteristics of a fast switching valve are mostly mechanical. Keyword is 'pulsed'.

Hi Vic

Do a search and you will find other discussions on this subject. What it came down to was that some DC valves are faster than some AC valves, and some AC valves are faster than some DC valves. You have to read the manufacturers blurb to know which is which. Adding a simple diode on a DC valve will slow the switch off operation, but, making the suppressor circuit more complex using a mix of zener and diode and resistor helps with that issue. In terms of switching the supply to the valve, DC will usually be more consistent because it is at full voltage all the time whereas AC is only at full voltage twice in each Hz frequency, though you are only talking 10 milliseconds here (UK 50Hz).

A big advantage as Keith mentioned is safety, 24Vd.c. is a lot less likely to kill someone than 120/240V a.c.

Bryan

Primarily 24Vdc valves unless we have to use Parker Hydraulic Valves and then we'll use 110VAC. We've had problems in the past with very short life spans with low VDC Parkers for some reason and usually they are the only brand allowed in customer specs.

Talking only about the solenoid, an AC coil is likely to operate slower than an equivalent DC coil because the AC version will have a shading ring on the magnetic pole to keep the solenoid from chattering at 60hz.

This shading ring slows down the decay of magnetic flux enough to "bridge" the gap between AC cycles and, at the same time, will delay the release of the coil when power is removed.

I'm not sure if the pickup time of the solenoid is affected by the shading ring.

Based on experience I do not see any difference in operation between AC and DC valves or pneumatic actuators. I think it is a mute point with exception of needing a power supply for DC.

What will make a HUGE difference in operating time and reliability with pneumatics is dirt in the air system and lack of oiling. Funny how people expect equipment with high tolerances to operate well with dirt and no lubrication.

Dan Bentler

MAC Valves offers 24vdc solenoids with built in surge suppression. They also offer very low power DC solenoids. I use the standard power dc solenoids with built in surge suppression in their 82 and 92 series valves, driven with normal DC output cards and have had no trouble.

In general, all other things being 'equal' an AC actuated coil will reach higher armature speeds than an equivalent DC operated coil. This is because AC coils are generally designed to use the armature inductance to provide increased inductive reactance after the armature is pulled in. This limits the current in the coil after the armature is pulled in to a safe, constant level. However, this also allows much higher current flow when the armature is NOT pulled in, creating a larger magnetic field. This is also why you can burn out a coil on an AC valve if the spool doesn't move. This doesn't happen on DC coils.


However, it is the response consistency of AC versus DC that will ultimately determine your success. The actuation to actuation consistency with DC is much better than with AC. This consistent delay allows the user to account for the delay and know it is close to the same every time. So while it in not technically faster it is much more repeatable.

Keith