I have worked in many facilities in the past 20 years and have always switched control voltages from the orginal 115 VAC control to 24 VDC control, relating to refurbishing older equipment and systems. This was a natural selection especially when moving to PLC controls.

I currently have been involved with a plant that is resistant to switch to 24 VDC for operational concerns. I have never worried about this in the past and know of no reasons not to switch. I have always assumed that it was safer; however, I can find no details supporting my assumptions.

Any help would be appreciated....

 

That is a known fact 24vdc is better than 110vac I agree with you.

 

We have both here(a multi-plant facility) with voltage preferences dictated sometimes by equipment, sometimes by the old "That Is The Way We Always Done IT", and sometimes out of resistance(no pun intended) to change. The normal way to choose involves cable distances, control equipment, and power sources available. Example: we normally use 24VDC for most solenoids...but an exception is made for a nitrogen purge to a rotary feeder where the solenoid is switched via the switchgear(motor on = purge on). Here is one useful reference article:

http://www.manufacturing.net/ctl/article/CA188692?pubdate=5/1/2001

 

Modern trend is to use 24V DC control voltage. In some older plants still 115V AC is in use. If your existing plat have 115 V AC instruments like solenoid valves which shall be retained then yiu may keep this and use 115V AC. But for new instruments people prefers 24V DC.
I have experience in a refractory plant in Egypt (1992) where both 24VDC and 115V AC control voltages were used.But I feel comfortable with 24V DC system.

In case of 115V AC, don't forget to use arc supressor at PLC output.

 

24VDC is definitely and obviously safer. But 115 is much more tolerant of oxides and dirt on contacts. And 1 A at 115V can do a lot more than 1 A at 24V. So as long as we have relays and electro mechanical switches, etc. 115 will be preferred in some environments.

Regards

cww

 

Hi,

24v dc is more resistant to control circuit "drop out" caused by high inrush as associated with contactors starting motors ect causing a local volt drop, it is also safer for electric shock but not safer for fire.

Regards
Bob James

 

I am a 120VAC fan only for the reason that when something goes wrong with 24VDC supplies, they can do very strange and dangerous things.

For example, I witnessed a system with multiple 24VDC supplies where one of the supplies started folding back due to failure. Somehow, it started sending 24VDC down the common line. Things would just start up randomly and the hardwired emergency stop had no effect!

It is hard (possibly impossible) for a block-of-iron transformer to fail like that.

Regards,
Paul Karnopp

 

Responding to Paul Karnopp's comments:

Does your comment mean that you haven't heard about similar strange behavior of AC circuits?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

I had a similar problem once with a system where two 24VDC power supplies were joined by the DC common. The problem was a short circuit in one of the power supplies between part of the circuitry and the heat sink. The short was
caused by a strand of wire which had fallen into the power supply and welded itself into position. The result was one power supply feeding back into the other's circuit, and turning on valves in a pattern unrelated to what the PLC was trying to do.

The two power supplies in the above example were joined because the designer didn't want to spend a bit of money on isolators for handshaking lines between two machines.

I would say however, that I still prefer to work with 24VDC. 120VAC can be dangerous, especially the sensors (as they are in more exposed locations).

 

24 VDC from transformer isolated linear power supplies is less likely to do weird things than with the switchers that are driving the market now. Some have very little or no real isolation and don't degrade gracefully. I've seen a couple volts of noise on the DC from one unit after 2 or 3 years. And they can get erratic and slow to start as well. Their high efficiency means they are here to stay but they bear more watching than the linear types. Many
depend on very low ESR in the electrolytic caps to keep the noise down so their effective life is that of the capacitors which is very dependent on temperature, etc. Lotsa power problems down the road as these handly little blocks age. If you don't have a scope available, you should.

Regards

cww