AC or DC Solenoid Valves

[parky]

There is a marked difference in speed of DC operated valves, also solid state outputs are faster than relay.
I have recently been doing some experiments on AC vers DC.
A control system we have had for about 5 years uses ac 24v solenoids about 26 of them, over a year we found we replaced at least 9 of these (sometimes the same ones 2-3 timesbut no pattern) so I guess we have replaced all of them a couple of times
18 months ago I went to replace one & guess what none in stock & a lead time of 7-10 days, as a quick fix I fitted a 24v dc power supply & fitted a dc coil (same manufacturer so fitted perfectly).
the following week before we got the replacements we had two more go down so replaced these with dc types.
since then all have been replaced with dc types & the fault rate has fallen to none so far.
We have since retrofitted a number of other systems with dc types & so far the reliabillity seems to have improved no end.

 

[OkiePC]

Mac Valve Specs for 82 and 92 series

 

[milldrone]

Response times for Parker hydraulic Valves were mentioned earlier

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[kamenges]

The differences between OkiePC's and milldrone's posts tend to show the differences between air pilot activated valves and direct solenoid activated valves. Since pilot valve activation forces on the pilot operated valves are so low the differences in response are much less and the total valve response in governed more by the pilot airflow than the solenoid. With direct solenoid activated valves the higher forces from the AC solenoids really start to show through.

Take a look at this PDF. It is a more apples to apples comparison to OkiePC's valve as they are both pneumatic valves.

Keith

 

[cornbread]

I vote for 24vdc solenoids, you will avoid any issues with NFPA 70E, making troubleshooting and repair much easier.

 

[gas]

This thread was about shifting a pneumatic solenoid valve.

It really is a moot point. When the machine calls for a valve to shift nothing happens until the next PLC scan. When the valve eventually does shift air has to gather speed and flow thru the tubing, fill the cylinder, overcome the cylinder breakaway force and only then does the cylinder start to move. In human terms this happens very fast but if you were to time every bit of the above you will find that the actual time that it takes to shift the valve spool is a very small percentage of the time from signal to stroke.
Tell the customer that he is of course right and he has created a brilliant piece of engineering and give him what he wants.

 

[kamenges]

gas, it depends on what you consider a 'small percentage'. We have timed cylinder actuations on high speed video in the 50 msec range from signal output from a dedicated CPU running a 1 msec loop to full stroke. This was using a DC version of the MAC 82 valve the OkiePc posted and a 1.5" bore x 0.5" stroke cylinder. So the listed 9 msec valve actuation time is just a bit less than 20% of the total actuation time. And 50 msec isn't really all that fast. There are much faster applications than that.

If you are closing a nip or pivoting a large conveyor then valve actuation time doesn't matter. But there are applications where it does.

Keith

 

[gas]

kamengas

All I can say is wow. Companies that I have worked for have always gone with mechanical mechanisims when reliable high speed was required.
Obviously that picture is changing. Thanks for the insght.

 

[Keystone]

In the facility I work at we have a mix of both 120VAC and 24VDC, both work equally well. We are however changing over to 24VDC as we upgrade machinery. The primary reason is NFPA 70E as "cornbread" mentioned.

It allows for much easier troubleshooting and repair (esp. when you follow NFPA 70E to the strict letter of the law) with the PPE and guarding issues associated with 120V.

 

[milmat1]

Pnuematic delays and unpredictability will far outway any response difference between AC/DC coils..
DC is pretty much the standard anymore though,And most plc outputs are protected from inductive kick...

 

[scottezell]

I spoke with a fella from ross valves a few years back and what he told me is that many moons ago there was a speed difference in AC vs. DC and DC was the winner. BUT....the new coil manufacturing processes with the shading coils and what not the time difference is neglegible. If you really need to know the on and or off times for a coil the only way to find that is to call the manufacture and speak directly with an engineer....good luck