3 phase to 24 VDC and instrumentation question

[Lancie1]

Switching Power Supplies: 480 VAC to 24 VDC

While we are discussing control power supplies of varying voltages, I have a question. Has anyone had any bad experiences with using a 3-phase 480 volt-to-24 volts DC power supply? I am looking at a Sola Hevi-Duty unit (switching type direct 3-phase-to-DC conversion) for a project. The reason is that this project is mounted on a couple of skids (portable), there is very limited space, and I must give up something. I am trying for only two voltages, 480/277 to run the motors and lighting, and 24 vdc for everything else (controls, valve operators, instruments), and no transformers at all.

 

[Ken Moore]

I have one (same brand) that has been in service for almost three years without any issues. It was a first for me also, and so far it has been trouble free.

 

[Pego]

Realize that the neutral carries ALL the 120V load. You have the 'possibility' of three 20A 120V circuits (one for each phase to neutral). BUT, if you pull 20A on each of these, the neutral will be carrying SIXTY AMPS!, yet the breaker won't trip!...
-Eric

In a three phase system, if all phases are laoded equally, the neutral carries ZERO amps.
Worste case, two phases loaded 20 amps and one phase without load, the neutral carries 20/1.73 = 11.6 amps.

 

[leitmotif]

No transformer ??

While we are discussing control power supplies of varying voltages, I have a question. Has anyone had any bad experiences with using a 3-phase 480 volt-to-24 volts DC power supply? I am looking at a Sola Hevi-Duty unit (switching type direct 3-phase-to-DC conversion) for a project. The reason is that this project is mounted on a couple of skids (portable), there is very limited space, and I must give up something. I am trying for only two voltages, 480/277 to run the motors and lighting, and 24 vdc for everything else (controls, valve operators, instruments), and no transformers at all.

Lancie
What is wrong with transformers?? If I had to pick the one piece of electrical equipment for best efficiency and reliability it would be the transformer.

If I read you correctly you want to go from 480 to 24 VDC with just solid state switching and not transform down to around 24 AC
prior to rectifying ??

If it were mine I would be much more comfortable (and whole bunch safer) with 480 to 24 volt transformer and 3 phase full wave bridge.

Dan Bentler

 

[rdrast]

While we are discussing control power supplies of varying voltages, I have a question. Has anyone had any bad experiences with using a 3-phase 480 volt-to-24 volts DC power supply? I am looking at a Sola Hevi-Duty unit (switching type direct 3-phase-to-DC conversion) for a project. The reason is that this project is mounted on a couple of skids (portable), there is very limited space, and I must give up something. I am trying for only two voltages, 480/277 to run the motors and lighting, and 24 vdc for everything else (controls, valve operators, instruments), and no transformers at all.

Yep. I use both Siemens and Phoenix 3 Phase, 480 VAC in, 24VDC out supplies all the time. I love em.

A motor-starter protector on the input protects the input side, and I generally use fast acting DC breakers on the output side. I've never had a problem with one.

 

[rsdoran]

The unit Lancie is discussing has a transformer built in to step down the voltage, probably something along the lines of an SDN 5-24-480
http://www.solaheviduty.com/products/powersupplies/sdn/index.htm

They are small space saving and very reliable.

 

[504bloke]

We use Siemens only 380-415Vac 3 Phase In 24Vdc 20,30 & 40Amps OUT

There are superb, never changed one yet

 

[MarkTTU]

Has anyone had any bad experiences with using a 3-phase 480 volt-to-24 volts DC power supply?

I use a Weidmuller on one of our products and haven't had a problem yet. The electricians even like it better; before they had to supply 480 & 120 as seperate power sources...

 

[darrenj]

In a three phase system, if all phases are laoded equally, the neutral carries ZERO amps.

In a perfect world..However with invention of drives and switching power supplies come Harmonics..So even on perfectly balanced system you still have neautral current..

Worste case, two phases loaded 20 amps and one phase without load, the neutral carries 20/1.73 = 11.6 amps

Somewhat true..To find out the true neatral current you need to the Vector diagram and a hell of a lot of time on your hands...Hell half the engineers out there dont remember how to work out 3 phase neatral loads! and aggain harmonics plays a role..

also concerning the first question asked...It is possible that the 208 volt system is grounded however they just dont have a neatral at the location where the equiptment is going..??

 

[Eric Nelson]

You caught me...

In a three phase system, if all phases are laoded equally, the neutral carries ZERO amps.
Worste case, two phases loaded 20 amps and one phase without load, the neutral carries 20/1.73 = 11.6 amps.

Yes, I was WAY off with that...

Amazing no one caught that before now. I am confused by your result though. I thought the formula for 3-Phase neutral current was:

Square root of ((IA² + IB² + IC²) - ((IA * IB) + (IA * IC) + (IB * IC)))

Should there be a /1.73 tagged onto the end?...



-Eric

P.S. to Ron (Doran). I don't see a neutral current formula on your site. It might be a good addition (especially for me!)..

 

[leitmotif]

"P.S. to Ron (Doran). I don't see a neutral current formula on your site. It might be a good addition (especially for me!)"

Me too

Dan Bentler

 

[Lancie1]

Thanks for all the replies about the 3-phase DC power supply. Dan, I like plain old transformers also, but in this case I need to save all the physical space for other equipment. I have had to adjust my thinking to be "space-saving" for all devices. For portable skids made to be hauled on a flat-bed trailer, there is a known amount of space and not an inch more.

 

[leitmotif]

Packin it in

Thanks for all the replies about the 3-phase DC power supply. Dan, I like plain old transformers also, but in this case I need to save all the physical space for other equipment. I have had to adjust my thinking to be "space-saving" for all devices. For portable skids made to be hauled on a flat-bed trailer, there is a known amount of space and not an inch more.

Was on submarines - know ALL ABOUT 5 lbs in the 3 lb bag - was the smallest electrician - amazing where they put junction boxes and terminal covers.

Dan

 

[rsdoran]

I think I do but not seeing it, ok tackle this while I see what is happening. For single phase the neutral will always carry the unbalanced load..ie if one leg uses 50 amps and the other leg 30 amps then neutral will carry 20 amps.
The following is for 3phase with a neutral. There are conditions like harmonics that may require a more indepth formula.

the neutral current is the sum of the phase currents

since R=1

using angle subtraction formula

 

[leitmotif]

Ron Doran

I plotted this out in Excel showing balanced 3 phase wye and also with one phase opened. I think it would really be neat to show this also. I tend to understand the graph better than all the trig.

I think so much of it I would be willing to do the graphs for you.

Dan Bentler