3 phase to 24 VDC and instrumentation question

rozen

Member
Join Date
Sep 2004
Location
So. Cal.
Posts
2
Please excuse if this is a naive/silly question, but...

We have a panel with 208 VAC/3 phase coming in. I would like to use 24 VDC for all control, using a DC power supply, which will accept 85-264 VAC. I also have an instrument (flow meter), which requires VAC, 85-250VAC.

Is there anything wrong with powering these 2 devices (flow meter and DC power supply) with 2 legs of the 3 phase input? The naive point of view is that the voltage accross the 2 legs is 208VAC, so should work?

Thank you very much for all input.
 
Go for it man!

We have a panel with 208 VAC/3 phase coming in
And since you have the 3 phase 208 you also have 120V line to neutral on that Y transformer. So you are good to go on all the uses you expressed intrest in using. I say, "go for it man"!
 
The use of 2 legs to drive an appropriatly sized stepdown transformer is normal in control cabinets I've seen. For that matter, I've NEVER seen a 3 phase 208-480 stepdown to 120 single.

If the device to be driven accepts 208-480 two leg then just use appropriate fusing. Many step downs have fuse block mounted on the input/output block.
 
Re: Go for it man!

randylud said:

And since you have the 3 phase 208 you also have 120V line to neutral on that Y transformer

Interestingly, that is what I thougth, i.e. 4th leg in a typical 208/3 phase is neutral, which is great, cause you get 120VAC for "free". However, the plant electrician which will be the lucky recipient of this device claims that there is no neutral and that all they provide is 208VAC on 3 legs.

BTW, micron transformer makes a 208 to 120 step down...

thanks
 
Ungrounded three phase is not uncommon. OK, at 208, it is more common to be grounded, but it is not necessary.

Most electicians call three phase with a neutral "four wire" and three phase without a neutral "three wire."

Without a wire from the center tap to reference (typically ground), then any leg reading to neutral would depend on leakages, parallel paths and other "intangibles" such as phase loading.

Bottom line:
Without a grounded center tap, do not take any given phase to ground as a given. Use such "three wire" circuits by connecting phase to phase, or through a transformer (which is also connected phase to phase, but the secondary can be connected with a center-tap to ground or single phased with one side to ground or other common).
 
rozen said:


However, the plant electrician which will be the lucky recipient of this device claims that there is no neutral and that all they provide is 208VAC on 3 legs.

You probably know by now how "Sparkies" are. I bet what he meant was there is 208 potential at all three pairs: L1/L2, L2/L3, and L1/L3. You indeed get 120 from any single lead to neutral. Like Rod said, I've not seen a three-phase xfmr in a control panel and it might make nervous if I did.
 
Rube said:
Like Rod said, I've not seen a three-phase xfmr in a control panel and it might make nervous if I did.
Rod said:
For that matter, I've NEVER seen a 3 phase 208-480 stepdown to 120 single.

All 208 Y transformers are 120V line to neutral. Most 208 Y transformers have primary windings at 480V or higher. All industrial and most commercial 120V distribution comes from a 3 phase transformer of the 208-120V variety. The distribution breaker panel that is used is a 3 phase panel with individual 120V branch circuits. In this case, the neutral of the Y is grounded for obvious reasons. Not the least of which is that the NEC in this country requires it. Many control circuits come from distribution systems just like this whether it is a known fact or not.
 
You probably know by now how "Sparkies" are. I bet what he meant was there is 208 potential at all three pairs:

There is a difference between a Delta and Wye secondary on a three phase transformer.

The good news though is:

I've not seen a three-phase xfmr in a control panel and it might make nervous if I did.

"Sparkie"
 
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Randy- the original post did not mention a transformer on the input. I am not sure about SoCal, but in some areas, there is "grounded B" power, with 1 leg grounded. In this situation there is no neutral.
 
WILD LEG
I have dealt with systems that had 208 across any 2 of 3 legs but only 2 of the 3 legs would give 120 to neutral/ground. The third leg was what we refered to as the WILD LEG. A Master Electrician fried our bosses living quarters/office complex two years in a row at this site.

Whatever the incoming supply is verify the transformer connection and type. If your devices, ALL DEVICES...ie any device powered from this cabinet, can accept the incoming 208 then its acceptable. You must maintain balance of the currents on the incoming supply.
 
Watch the Neutral Load!...

rozen said:
Interestingly, that is what I thougth, i.e. 4th leg in a typical 208/3 phase is neutral, which is great, cause you get 120VAC for "free".

Be careful with the 'for free' statement. You have to always keep in mind how much current the neutral will be carrying. Let's say you have a 120/208Y supply on a 20A breaker. 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!... :eek:

All too often, I've seen guys run 120V devices from a 120/208V 3-wire (no neutral) feed by grounding the neutral wire of the device. Yes, you get 120V, but now you have current flowing on the ground wire, or more often, the conduit!!!... :eek:

beerchug

-Eric
 
Nothing like this would surprise me anymore.

I have seen 480 delta with one leg as ground, and heard of 240 delta with one leg as ground. I would be suspicious of 208 delta with one leg grounded.

I have seen 208 delta, but very rarely.

I have seen small 3 phase transformers in control panels many times, but never 3 phase to single, or a 3 phase control transformer.

In one modern plant, there were differing voltages in various parts of the production floor. Some were 480/277, some 240 3 phase, some 240/120 single, some 208 three phase, some 208/120 2 wire and nuetral, some 277 single, and some 120 single.

It seemed like each new engineer picked something, or the tied into whatever was close. It did keep machinery from being moved around very much.

Their I/O cards were about the same way. At least everything was 9030's. I did get a standard set for 16 point input and 6 point relay output cards. The engineering manager hot a standard set for the top of the line CPU card, with full network capabilities. Nothing like a $2,400 cpu on a machine with less then 16 total I/O.

Another vote for Micron!!! I got to special order a lot of thier control xfmr's at GE. GE was switching over to Hammond at the time, and there was a lot of quality problems with the Hammond units. Possibly becuase they were being made to GE specs. Lots of failures, screws TOO TIGHT, broken wires and terminal strips, etc. "Important" customers got the micron's.

Wouldn't it be nice if we all woke up in the morning, and all plc inputs were 24vdc, all outputs were 10 amp relays, all plant power was ___/___v 3 phase, you know, standardized. Like Louis Armstrong sang: "What a wonderful world it would be!"

Lets not argue 24v or relay.

regards to all, keep 'em running,.....casey
 
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control

Go ahead, control transformers for 208/120 are easy to find.
Just remember to fuse BOTH primary lines. Many come with a secondary
fuse built on the unit. The secondary should be grounded on one leg, the unfused leg, and make sure that what you grounded to is also bonded to a panel ground somewhere.

Bruce :)
 

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