# Thread: 3 phase 208 to 110 single transformer question.

1. ## 3 phase 208 to 110 single transformer question.

Hey guys I have another project Im working on in which I what to use the machines current 110V transformer to supply voltage to a DC PSU. The current transformer takes 2 legs off of 208V 3 phase and is wired to the terminals labeled "0" and "200" on primary side. On the secondary side is the wire are connected to "0" and "100". Now this transformer has 4 terminals on each side.

Primary equals 0,20,200,240 and the secondary side has 0,10,100,120. Now the machine is using the 0 and 100 which I am assuming is 110V due to the +/- 10% tolerance. But my questions are:

1. Is the terminal labeled "0" the Neutral/L2 for my signle phase circuits?

2. Is there a way to find the nuetral verse hot using a multimeter with AC?>

3. On this transformer with the machine using the "100" could I tap into the 120 for my 120VAc -24VDC power supply at the same time or are these type of transformer an either or deal?

2. 1) sure
2) there is not one
3) confusing question but sounds like u r asking if u can power something requiring 120vac from the 100v side. sure

there is no nuetral when u have 2 wires coming out of a transformer; just 2 wires. u can call one nuetral if u want, call it L1 or L2. whatever.

reread and it sounds like u r not going to remove this xfmr from ur machine; just want to tap into it for 120v to feed a 24vdc power supply. so yes, just wire across the 100v side with 2 wires that go to the 120v input of your power supply. dont ground either, just hook them up and dont worry about n vs L noemclature.

sometimes msch builders tie one side of 120v to machien ground, so if u care, yes, put ur meter from machine case or ground to each side of the 100v and see if one is 0 volts; if so it is the 'neutral' side. other would show 100 v to ground.

3. Originally Posted by mike_Kilroy
1)
reread and it sounds like u r not going to remove this xfmr from ur machine; just want to tap into it for 120v to feed a 24vdc power supply. so yes, just wire across the 100v side with 2 wires that go to the 120v input of your power supply. dont ground either, just hook them up and dont worry about n vs L noemclature.

sometimes msch builders tie one side of 120v to machien ground, so if u care, yes, put ur meter from machine case or ground to each side of the 100v and see if one is 0 volts; if so it is the 'neutral' side. other would show 100 v to ground.
Thank you. But the question above was refering to the fact that the machine's transformer is using the 100V for its 110V devices,but this transformer obviously has the ability to output 10V,100V or 120V. I was asking if I left the transformer in the machine as is with the 100V wired, could I also tap into the 120V terminal as well or is this transformer such that you can only choose one voltage to use on the primary at a time. In other words could I tap 10,100 and 120V all at the same time without making the transformer off balance?And is it no problem with wiring a 120V devive to the transformer terminals rated at 100V?This is why I am asking this question to know if I need to use the 120V side and if I even can use it with the 100V wired up already at the same time?

Also I have recently seen an instance where the nuetral was tied to equipment ground and it puzzled me coming from a DC world. DC is straight foward but with AC changing polarity I would think tying anything to it like a piece of equipment would fry you? Now I know the equipment is bonded to earth ground but lets say the earth ground connection in the plug came undone. Would you be getting zapped everytime you touched the machine? I mean I wouldnt stick a screw driver in a outlet but this is making me think I can as long as its in the nuetral side.Just Kidding.

4. now i get it, sorry. YES absolutely u can wire ur 120v load from 0 to 120 terminals while the 0 & 100v terminals are being used by the machine. there would only be 2 possible limitations:
1) u cannot exceed the rated kva of the xfmr: so if the mach uses 80% of the rated kva for the 100v, then u can only pull ant=other 20% of its kva rating for your load; i assume ur 24vdc psu is small enough to not matter much?? maybe u should say what the xfmr kva rating is, what the 24v psu rating is, and maybe see if u can figure out the load on the 100v already - u can do this by a clamp on ac ammeter around one of its 2 output wires, or if less than 10 amps unhook on and put it thru ur dgital meter set to 10 amp ac current scale.
2)if one side is grounded make sure ur 24v psu dont care.

5. didnt have time to finish my comment... no, if u loose ur grnd connection on machine plug, it does mean u will get shocked

u need current to flow to get shocked. means u need to put urself in series with the voltage source; the ONE sire from the 100vac going to ground, then on to plug ground as u say, is NOT in series with the voltage. so if that ground wire at plug goes away, and u put urself in place of it by touching the machine whilst standing in puddle of water in barefeet, u are still not in series with the voltage; the voltage is from the 100vac terminal to the 0 v terminal, not from the 0 terminal to ground. to put voltage across u, u wud have to touch the 100v terminal and 0 terminal at the same time; altho u dont want to experiment and try to prove it, u could touch that 100v terminal whilst standing in the puddle and not get shocked IF that ground wire from machine to ground WAS off since there would (should) be no path for the voltage to flow current thru u (dont try this at home)

6. Just to expand on the grounded neutral, it is optional in the code that if you have a isolated control transformer, that it sounds like you have, it is optional whether you setup a grounded neutral, IOW re-reference the secondary to ground.
When choosing which conductor you ground is purely optional as at that point they are totally identical.
The conductor chosen to be earth grounded should be done right at the transformer and no where else.
This terminal will be secured to chassis and a ground wire and a neutral conductor will be taken from this point and be the ONLY place where the neutral and ground conductor connection is made.
As to using it without grounded neutral it is possible but there should be fusing in both secondary conductors as opposed to just one with grounded neutral.
But if you are using the 100 and 120v tap then the '0' conductor would be the one to ground in that case.
BTW, all ground and machine bonding conductors together with shields should go to a central ground 'star' point where the service ground should also be terminated.
Al.

7. Thankx guys. Like I said Im from the DC world. Well amatuer DC world, where polarity is constant. AC with changing polarity like in this situation confuses me. In DC negative is ground ,period. But the term nuetral I never under stood because AC constantly changes polarity. So too me earth ground is negative. This may be incorrect but when I think ground I think negative. So with an AC circuit changing polarity I was thinking any connection of the AC circuit(hot or nuetral) to a grounded machine would cause a short because I thought even the nuetral changes polarity from negative to positive. This is what I thought with AC. You see my logic of why I thought I would get shocked? But your syaing nuetral does carry voltage at all untill the hot side(potential voltage source)is connected to ground or am I see looking at this all wrong?

But are you guys saying there is no voltage on the nuetral wire? If I stand in a puddle and stick a fork in the nuetral side of an outlet I am okay,but its not untill I stick the fork in the hot side and become the nuetral(aka path to ground or least path of resistance) that I will get shocked,correct?

8. c ur logic but u r telling ur age....

my first vw beetle was 6 volt POSITIVE ground!

my ford 8N tractor was 6v POSITIVE ground up till 2009 when I changed it to 12v negative ground

9. Originally Posted by Skiroy
Thankx guys. Like I said Im from the DC world. Well amatuer DC world, where polarity is constant. AC with changing polarity like in this situation confuses me. In DC negative is ground ,period. But the term nuetral I never under stood because AC constantly changes polarity. So too me earth ground is negative. This may be incorrect but when I think ground I think negative. So with an AC circuit changing polarity I was thinking any connection of the AC circuit(hot or nuetral) to a grounded machine would cause a short because I thought even the nuetral changes polarity from negative to positive. This is what I thought with AC. You see my logic of why I thought I would get shocked? But your syaing nuetral does carry voltage at all untill the hot side(potential voltage source)is connected to ground or am I see looking at this all wrong?

But are you guys saying there is no voltage on the nuetral wire? If I stand in a puddle and stick a fork in the nuetral side of an outlet I am okay,but its not untill I stick the fork in the hot side and become the nuetral(aka path to ground or least path of resistance) that I will get shocked,correct?
Be very carefull with the neutral, althought it is normally connected to ground ,if the neutral circuit is open you will get zapped if touching the neutral and ground.
As far as your DC analogy, think of it as the neutral connected to ground , while L1 is going from 0 volts to +169volt peak to 0volt to -169 volts peak. (peak to peak values for 120vac )

The neutral is carrying the same current as the L1 line as they are connected to both side of the transformer secondary.

The number on your transformer primary and secondary are turns ratio more than voltage. If you use the 200 primary tap with 200volts you get 100v on the 100 secondary tap.In your case you are using 208volt on the 200 primary tap so you should get 104volt on the 100 secondary tap.

hope this help.

10. Originally Posted by Skiroy
AC with changing polarity like in this situation confuses me. In DC negative is ground ,period. But the term nuetral I never under stood because AC constantly changes polarity. So too me earth ground is negative. This may be incorrect but when I think ground I think negative. So with an AC circuit changing polarity I was thinking any connection of the AC circuit(hot or nuetral) to a grounded machine would cause a short because I thought even the nuetral changes polarity from negative to positive.?
But are you guys saying there is no voltage on the nuetral wire? If I stand in a puddle and stick a fork in the nuetral side of an outlet I am okay,but its not untill I stick the fork in the hot side and become the nuetral(aka path to ground or least path of resistance) that I will get shocked,correct?
The secondary of your 120v transformer Alternates in voltage polarity (AC) the current also reverses direction at a frequency of the supply, if you earth ground one of these conductors, nothing changes except the conductor that is grounded becomes, ideally, at the same potential as the planet earth and You, hence you do not sense anything when touching the neutral in spite of this 'alternating' voltage because there is (hopefully) zero potential between you and mother earth.
The theory is that the earth consists of an homogenous mass that has zero resistance anywhere in its total area.
The big problem I have with modern notation is that commons & grounds are used interchangeably without due indication as to what respective supply system it applies to and whether or not it is actual Earth Ground referred to??
Al.

11. I have read where there can sometimes be low voltage up to 40V on a nuetral but that depends on the substation. So is this why you can only tie your nuetral to ground at an "isolated transformer" to clean any stray voltage off the nuetral?

Also I see a flaw in my fork in the nuetral side of the outlet scenerio. Assuming that the nuetral side is a clean 0V I see you wont get shocked because no voltage can make it to the nuetral as it is physically disconnected from the voltage source. But in my scenerio with the machine transformer is a loop because the machine's devices are in series with the transformer. So it makes the connection from the hot side of the transformer to the device as the conduit and back to the neutral side so I would expect electron to be present at the nuetral side of the transformer, meaning there should be electrons flowing into the machines metal casing because its tied to the casing. I dont know why Im having such a hard time with this.

This to me would be like connecting a lightbulb to L1 and L2 and then touching the Neutral side(L2) on the lightbulb. Im still going to get shock because there is a load connecting L1 and L2 so the nuetral no longer is dead,voltage is now flowing through it and you will get shocked regardless of if you touch the nuetral or hot side at the bulb or at the voltage generator(this is of course assuming you are standing in a puddle and are grounded). PLEASE HELP THE FUZZY GO AWAY!

12. Originally Posted by Skiroy
I have read where there can sometimes be low voltage up to 40V on a nuetral but that depends on the substation. So is this why you can only tie your nuetral to ground at an "isolated transformer" to clean any stray voltage off the nuetral?
See post #10.
There should NEVER be any potential between neutral and earth ground. when the substation or supply transformer is grounded, in N.A. the neutral is re-referenced to ground at the service supply entrance, therefore if done correctly the neutral ground path should ideally have low ground path resistance back to the substation ground.
The method of re-referencing an isolated control transformer is to re-establish a grounded neutral which otherwise would not be present due to the isolation effect of the transformer.
Al.

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