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Old July 29th, 2004, 09:04 AM   #1
georgeofthejungle02
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120VAC or 24VDC power on new PLC?

Hello All:
I am doing a comparision between 110VAC vs 24VDC power systems.
DO any of you know what are some of the pros/cons involved in this?Are there any literature or anything you are aware of that have a
detailed analysis of the advantages/disadvantages
110VAC versus 24VDC power systems? For example, why would you pick a 24VDC PLC instead of 120VAC PLC. What about reliability and redundancy?

From my work experience with manufacturing plants here in
Canada, decisions made on new installations are not
based on a solid understanding of the two systems, but
simply what electricians/maintenance are familiar
with. I eagerly look forward to any information you
can provide.
Regards,
George
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Old July 29th, 2004, 11:52 AM   #2
Rube
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George, there are gonna be several different opinions of why to use each voltage but my feelings are 24 volts in any wet environment or situations where there is a high likelihood of damage or human contact with wires exposed by repeated motion (and lack of maintenance). I have used both and in my buildings now, we use mainly 120 volt components.
Also, if the maintenance staff has a real problem understanding AC versus DC, or Sinking/Sourcing, you might want to stay with 120, if it's a dry environment.
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Old July 29th, 2004, 01:04 PM   #3
Jim Dungar
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Cost

It seems most PLC and sensor manufacturers seem to be forcing inputs to be 24VDC. This is evident in their costing and availability of input options.

Most of my customers are acceptable to the idea of 24VDC inputs and pilot lights but they still want 120VAC outputs especially for motor starter coils.


In my area the predominate 24VDC input wiring is to switch the positive and use the negative as a common (PNP sensors).
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Old July 29th, 2004, 01:06 PM   #4
georgeofthejungle02
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Would anyone agree that 24VDC design philosophy comes from Europe?
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Old July 29th, 2004, 02:24 PM   #5
CaseyK
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Lightbulb

As far as the main PLC power supply, 120 volts by all means. One less piece of equipment to worry about.

As far as input voltage, I've used a lot of both, sometimes on the same PLC. Some applications, such as a diesel generator, 24vdc is a must. Many industrial applications require both. Varies from machine to machine.

Many years ago as an electrician in a food cannery, I wish they would have had 24vdc, as I got hosed down while troubleshooting more than once.

regards.....casey
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Old July 29th, 2004, 02:52 PM   #6
Hardwerk
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24dc
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Old July 29th, 2004, 04:56 PM   #7
bobwithdana
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The last 2 companies I worked for have corporate standards that require all I/O to be 24 vdc. In fact , The Company I work for now have all the plant floor I/O on Devicenet at 24VDC. The enclosures dont even require disconnects because of the voltage.
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Old July 29th, 2004, 06:50 PM   #8
waggs
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Quote:
Many years ago as an electrician in a food cannery, I wish they would have had 24vdc, as I got hosed down while troubleshooting more than once
Thanks for the visual, casey!~!~!

Bob
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Old July 29th, 2004, 08:54 PM   #9
guest_007
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I have worked with both ac and dc I/O's,and I MUCH prefer dc, simply because it's less dangerous to troubleshoot in a plc that's crammed full of wires.
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Old July 29th, 2004, 09:39 PM   #10
gbradley
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From the Ab Website
PanelView 300 and 300 Micro terminals are 24V dc only.
Of course just because the HMI is 24VDC doesn't mean that the PLC needs to be.
You gotta look at the entire system.
Good Luck
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Old July 29th, 2004, 11:59 PM   #11
allscott
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There are pros and cons to both

120V advantages;
It can be easier to troubleshoot. If you run +24V DC out to say a push button station without the common it can be tough to check for voltage (if the supply is floating). You can avoid the sinking/sourcing discussion with your electricians with a 120V system. You may not need to buy a DC power supply for certain systems.

24V DC advantages;
Some sensor manufacturers have been telling me that switching is faster on 24VDC. DC is less stimulating to hook up hot. DC sensors are solid state, cheaper, and offer more variety.
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Old July 30th, 2004, 03:57 AM   #12
ArikBY
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Long time I thought 110AC input is better to avoid noises until I fount it not true by hard way.
So I recomend DC supply to the PLC and DC inputs/outputs
if someting go wrong with your service it better the DC power supply would demage then the PLC. Sinking/Sourcing it just matter of training.
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Old July 30th, 2004, 04:18 AM   #13
Peter Nachtwey
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Quote:
Originally posted by allscott
Some sensor manufacturers have been telling me that switching is faster on 24VDC.
On a motion control project, 120 vac is unacceptable for photocells or any other device where fast feedback is required. Why?

Quote:
Would anyone agree that 24VDC design philosophy comes from Europe?
Why. 24 volt systems are much easier to get certified by UL and CSA.
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Old July 30th, 2004, 05:06 AM   #14
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Quote:
Originally posted by Peter Nachtwey


On a motion control project, 120 vac is unacceptable for photocells or any other device where fast feedback is required. Why?
Hrm... Because it probably takes at least 16ms to determine that a signal is present (or absent)(60Hz systems)? Plus, there is a further time delay in that the AC is usually rectified and filtered a bit to prevent dropouts from noise and low lines.

Switched DC signals only have the speed of the switching device, capacitance of the signal lead, and propogation delay of the lead length to deal with .

Quote:
Why. 24 volt systems are much easier to get certified by UL and CSA.
Very true here.

I much prefer 24VDC systems myself. Safer, overall lower energy, and as a side benefit, it's quick and simple to check if you are getting noise on a DC signal; just measure the signal with your multimeter set to AC.

Casey, what piece of equipment is eliminated by using 24vdc power? If you mean the power supply, well, whether you use 110VAC, or 24VDC, you still have another power supply before the CPU. Add to that, that a seperate 24VDC supply (even if sourced by a 110VAC transformer) is a huge level of protection from AC Line spikes.

24VDC systems can also be powered reliably directly from batteries. I can't count how many times UPS systems have failed, but float-charged battery systems almost never do.
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Old July 30th, 2004, 05:40 AM   #15
JohnW
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24V DC power

I use only 24V dc control voltage now. It is a lot safer and means that there is no need to worry about risks of electric shock on pushbutton stations etc. Panels are safer to work in live as long as you take care to seperate your 415V power from the control equipment. I use Siemens Sitop power supplies as these are SELV rated so there is no need for a seperate isolating transformer so I have more space and less heat in the panel. I have used 24V dc control on contactors up to 300A. The 24V dc power supplies I use are generally very fault tolerant. The only disadvantage is that dc powered contactors are more expensive, if this is an issue then I use 24V dc control circuits and interface relays to drive 110V ac contactors. I never put 110V onto any control items like pushbuttons or limit switches.

Use a seperate dc power supply for the PLC so a control line fault doesn't affect it.
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