24v AC or 24v DC - Which is Preferable?

wwsmith

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May 2010
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I have an application with sixteen 3 watt proportional Belimo LMB24SR actuators that can be powered by either 24v ac or 24v dc. If I want to fuse protect each one from a common power supply, is there any advantage to powering them with one or the other type of voltage? Maybe lower cost and greater range of selection of power supplies with one or the other? Maybe less interference issues with one type of voltage vs. the other?

BTW, I tried searching for past threads addressing this issue as I am sure it must have come up before, but I couldn't come up with the right combination of key words to find anything relevant. So I am sorry if this has been addressed before.
 
The only place I've ever seen 24VAC used was in some HVAC stuff. I believe 24VDC is more common in an industrial machine application. If you are building something new, I would go 24VDC. The only time I'd use 24VAC is if I were adding onto something that already used 24VAC.
 
are you using 0 to 10 or 4 to 20? are you using a plc to control these? what is your output divice/plc voltage/current rated at? I think these are rated dc at least thats what the pdf I found says.
Best regards, Steve
 
Transformers are more reliable than power supplies. AC is generally better suited for long distances. I like AC for relay coils better than DC too, and many HVAC systems favor 24AC.

Those are the only reasons I can think of for using AC. If those factors don't weigh heavily in your application, I would stick with DC.
 
are you using 0 to 10 or 4 to 20? are you using a plc to control these? what is your output divice/plc voltage/current rated at? I think these are rated dc at least thats what the pdf I found says.
Best regards, Steve

My output device is this 4-20ma USB powered module http://www.mccdaq.com/usb-data-acquisition/USB-3106.aspx Of course, that module just sends the signal to each HVAC damper actuator about what position they should be in. A separate power supply is needed to power the damper actuator motors. This can be either 24v dc or 24 ac.

Transformers are more reliable than power supplies. AC is generally better suited for long distances. I like AC for relay coils better than DC too, and many HVAC systems favor 24AC.

Those are the only reasons I can think of for using AC. If those factors don't weigh heavily in your application, I would stick with DC.

This is for the HVAC portion of a complete building automation system (BAS). The distances involved are not long. None are more than 120' away. There are other DC power needs in the complete BAS so I guess I should stick to DC as much as possible.

I appreciate everyone's comments. Okie's points about transformers being generally more reliable and AC being generally better for distance and relay coils is also very helpful to know. I may very well include a few AC powered items in other parts of the BAS for the reasons Okie mentioned.
 
your output is way overdone, but it will work.
16 bit is heavy.


I would give you one power supply of 24 volts as the units must be grounded proper for this output module.
as the units are only very small one fuse is good enough make it 1 amp as it is enough for even very small wires.
fusing lower is not practical as the belimo will take current only when moving. Is it really needed to have all this analog stuff in?
why not use simple DC motors they do not have any electronics onboard, and in HVAC the pipes will get moist, and will eat this.
Why not just open close, or even with a timer a little open and close.
 
your output is way overdone, but it will work. 16 bit is heavy.

I would give you one power supply of 24 volts as the units must be grounded proper for this output module.
As the units are only very small one fuse is good enough, make it 1 amp as it is enough for even very small wires.
Fusing lower is not practical as the belimo will take current only when moving. Is it really needed to have all this analog stuff in?
why not use simple DC motors they do not have any electronics onboard, and in HVAC the pipes will get moist, and will eat this. Why not just open close, or even with a timer a little open and close?

Shooter, to be honest, this office building automation system is being way overdone on many levels. The HVAC portion of this BAS system will have analog everything including a Copeland Digital scroll compressor with 10 to 100% modulation capabaility and a vfd driven air handler blower fan with full modulation. Since the money and desire is there for all this, why not go all the way?

The building is using insulation applied underneath the roof decking so all components are within conditioned space so there are no moisture or corrosion worries.

You make a good point about the low amp draw of the damper actuators such that maybe only one fuse is needed to cover all of them at once. From a gold standard perspective it is always best to protect every power circuit individually, but that may be too much overkill even for this project.
 
This is for the HVAC portion of a complete building automation system (BAS).
For a HVAC system, use 24 VAC transformers to power your Belimo actuators. It will eliminate one electronic device, and 24 VAC is what most HVAC repairmen will expect to see.
 
For a HVAC system, use 24 VAC transformers to power your Belimo actuators. It will eliminate one electronic device, and 24 VAC is what most HVAC repairmen will expect to see.

This system is going to be so unique that I am not expecting any regular HVAC repairman to be able to work on it. Only me and my staff will know enough about it to repair anything on it as we are not only doing the control system, but also assembling all the HVAC components ourselves. Most of us are mechanical engineers and my company is www.elitesoft.com which specializes in HVAC design software.

But as you say, 24v ac is a simple way to go. Instead of using 16 little wall wart transformers for each of these actuators, what do you think of using a cabinet power supply like this that has 32 power limited 24v ac outputs for only $141 ? http://www.spytown.com/altronix-altv2432300ulcb.html This module would allow for easy power of many other 24v ac devices such as smoke detectors and security cameras.
 
I designed and programmed a PLC based HVAC control system for a new paper mill last year. The Air and make up units were all built by Trane Custom. There standard for controlling dampers and chilled water valves were belimo actuators. I faced the same decision you are making now as to whether to us 24AC or DC for actuator supply voltage. The only difference I saw between AC and DC was operating voltage range of the actuators. With 24VAC you have a +/- 20%voltage tolerance, with DC its +/- 10% I went with a single 24VAC transformer to power each area of actuators. i.e. Wet end, Dry end, Basement. Position was controlled using a 4-20mA plc output signal.
 
I designed and programmed a PLC based HVAC control system for a new paper mill last year. The Air and make up units were all built by Trane Custom. Their standard for controlling dampers and chilled water valves were belimo actuators. I faced the same decision you are making now as to whether to us 24AC or DC for actuator supply voltage. The only difference I saw between AC and DC was operating voltage range of the actuators. With 24VAC you have a +/- 20%voltage tolerance, with DC its +/- 10% I went with a single 24VAC transformer to power each area of actuators. i.e. Wet end, Dry end, Basement. Position was controlled using a 4-20mA plc output signal.

Hey Gabsdad, thanks so much for your input on this. Its always great to hear from someone who has dealt with a similar situation already. The more generous tolerance with 24v ac is a factor I wasn't aware of. Definitely adds a plus for going with 24v ac. Did you have any concern that any one actuator could encounter a short and damage anything?
 
If your application is noise sensitive or are working in a place with noisy power, i'd prefer to go with DC power supply. Kindof depends on your application though.
 
If your application is noise sensitive or are working in a place with noisy power, i'd prefer to go with DC power supply. Kind of depends on your application though.

There will be a 10 hp and 3 hp VFD running frequently, but good line reactors will be used on them. We will do whatever is necessary to keep clean power and minimize interference.
 

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