One big 24V power supply or several smalls

Plc_User · Mar 19, 2011

In a rather big installation where you should use a 40A 24V power supply if you should a single power supply, and where there are multiple remote IO, do you prefer one big power supply or multiple smaller power supplies in the field?
For the last option, in case of the last option, can I
be sure to have no potential differences, even with good grounding?

kb9ttx · Mar 19, 2011

There are pros and cons to everything.
I would select multiple power supplies myself. One reason is because if your one supply fails, then the whole system will likely fail. Another reason is that I would probably select 460 VAC to 24VDC three phase supplies and this gives me the opportunity to "balance" my feeds. Another reason, is that multiple supplies enables me to use the supplies for specific functions. If you have a lot of noisy devices like servos with electric brakes, then you can isolate your feedback devices on other supplies. You can use a specific supply or a group of supplies and isolate it to be your safety power. This way other less essential loads will not affect the integrity of your safety circuit power supply.

In regards to potential differences, I'd say that is a con to the multiple supply approach.

Tharon · Mar 19, 2011

If you go the route of multiple supplies, I would suggest keeping the positive of the power supply output seperate for each supply, don't parallel them (even if you buy parallel rated supplies). You can tie the commons together if you wish, to avoid later issues of using the wrong 24V source on a section of the machine. But from a purely troubleshooting point of view, I've always had more headaches when the power supplies are paralleled. This would also make the differences in potential a non-issue, since they wouldn't ever contact each other.

OkiePC · Mar 19, 2011

Distance and independence are the two main considerations. Looking at the whole machine, do you have to transmit 24VDC a great distance? Can the machine benefit by having several stations that can still function while one (or more) is down?

I also would not parallel them, but prefer to have all commons tied to instrument ground if possible.

Steve Etter · Mar 20, 2011

One other plus in favor of multiple supplies, especially if the supplies are mounted remotely from each other, is troubleshooting. In the event that something becomes grounded or pulls the current down, it can be quite time consuming to find on a single supply system.

Typically on systems with remote I/O racks, I will install a 480VAC to 24VDC supply for each rack. This eliminates most, if not all, of the parallel wire issues and makes troubleshooting much easier.

Steve

OkiePC · Mar 20, 2011

I had a machine with a 40a main supply and multiple 20a supplies remotely located. One circuit off of the main supply was for safety and went from end to end of this machine (about 200' long). There was probably no more than 250' of 12 gauge wire carrying the DC current to each remote j-box, which was fine when new. But when branches "decayed" loose terminals or "bruised" 22awg device cables, or when devices began to fail, the far ends of the system could drop as low as 18volts, and it was tricky to figure out where the problem existed. Having them fused in groups of 16 was okay, we could open fuses to find out which group was pulling it down, but sometimes there would be more than one...fusing in groups of 8 or fewer is so much cheaper in the long run (for troubleshooting).

JeffKiper · Mar 20, 2011

OT but I like having each device of a safety circuit loop back to the main panel.it does cost more but in troubleshooting you can't beat it.

gbradley · Mar 21, 2011

I see that [480V >>> 24V] was mentioned a couple of times in this post.

Is [120V >>> 24V] or [220V >>> 24V] more common for a power supply?

Steve Etter · Mar 21, 2011

I would say that 120VAC to 24VDC is probably the most common. I like 480 -> 24 in panels where 480 is already required - eliminates the need for a transformer if there is no other use for one required.

Steve

Jeff23spl · Mar 21, 2011

a good pratice where critical, is to mount at each far location a separate pair of powersupply tied toghether so if 1 fail, the other one take the lead and you can keep the load ratio lower on both...(sized for just 1) This is the main goal to have some connected parrallele...
Because you can easily find a 40a 24vdc powersupply form a 3phase sources voltage.
As stated:
At 24v, wire lenght is important so spread the powersupply at each end will diminish the drop. And even in between if you need to.

I would also separate 24v load on different line if i got noisy device and transmitters

caddymac · Mar 21, 2011

Jeff, how are you parallel the power supplies? With diodes? Or using parallel rated power supplies?

gbradley · Mar 21, 2011

kb9ttx said:
...this gives me the opportunity to "balance" my feeds. ...

So if you are running three or more Power supplies, you get the direct to line type (480V)
Like the right side of the picture correct?
Is this what you mean by balancing the load?

keithkyll · Mar 22, 2011

Wire size for 40A is quite large. Wire, lugs, and terminal blocks get expensive. Takes up more real estate too.
Cost of wiring alone should justify multiple supplies.

jstolaruk · Mar 22, 2011

keithkyll said:
Wire size for 40A is quite large. Wire, lugs, and terminal blocks get expensive. Takes up more real estate too.
Cost of wiring alone should justify multiple supplies.

In my experience, a short run of large wire to a pair of power distribution blocks has been cheaper than multiple power supplies, fuses and holders.

If the foot print of the machine is not large, (1) 480vac to 24VDC supply has been the most economical (and preferred by my customer base). If it wasn't for the 120vac convenience panels for laptops, 480V & 24V is all we have in a panel.

One big 24V power supply or several smalls

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