3 Phase to Single Phase Power

I'm a little confused about something power related. I'm sure this is all very fundamental so please take it easy on me. I believe I understand how transformers work (induction from one coil to another where a current exists through the primary and a load is put on the secondary) and the basic 3 phase (120 degrees out of phase with each other) system, and the sqrt 3*VAC source gives the per phase voltage (~277 per phase if Vs is 480). The thing I'm confused about is how a hot line can be considered a neutral after exiting the secondary of a transformer.

I'll elaborate: imagine a 3 phase 440VAC system comprised of L1, L2, L3 and GND... L1, L2, and L3 go off in one direction and in another direction L2 and L3 enter the primary of a step down transformer with a 115VAC (or 117VAC) secondary. So on each side of the primary winding you have a line (L2 and L3). The thing I'm confused about is how one of the sides of the secondary winding can be treated as a neutral. How do you go from a hot to a neutral through a transformer? I'm obviously missing something (probably something simple). The transformer in question is a 2TX 500 VA transformer. If someone can show me the math there going from 440 to 115 it'd be a welcomed review. Is it simply 440/2 = primary side = 220, 220/sqrt 3 = 127 secondary? 127 is fairly close to 115 or 117... Perhaps the ~10V difference can be attributed to I^2*R losses... What am I missing? Also what is to stop the ac secondary L2 and L3 from oscillating in the same (or opposite) fashion as the primary? The components connected to the single phase side are connected such that one of the secondary lines of the transformer is connected to the neutral pins.


Any explanations you can provide would be appreciated. Thanks!

The "neutral" is created by grounding one of the secondary leads. Technically this example you show is not a neutral but a grounded conductor. The secondary voltage is determined by the transformer turns ratio which is Vs/Vp = Ns/Np, where Vs = secondary voltage, Vp = primary voltage, Ns = number of secondary turns, Np = number of primary turns.
So in your example you have a 480VAC primary and 120VAC secondary. 120/480 = 4 to 1 step down. The secondary windings are not phisically tied to the primary windings, except for sometimes sharing the core of the transformer. The primary voltage is induced on the secondary windings through the magnetic field created in the inductor/transformer. The amount, or strength, of that field is related to the turns ratio.

BSomer said:

The "neutral" is created by grounding one of the secondary leads. Technically this example you show is not a neutral but a grounded conductor. The secondary voltage is determined by the transformer turns ratio which is Vs/Vp = Ns/Np, where Vs = secondary voltage, Vp = primary voltage, Ns = number of secondary turns, Np = number of primary turns.
So in your example you have a 480VAC primary and 120VAC secondary. 120/480 = 4 to 1 step down. The secondary windings are not phisically tied to the primary windings, except for sometimes sharing the core of the transformer. The primary voltage is induced on the secondary windings through the magnetic field created in the inductor/transformer. The amount, or strength, of that field is related to the turns ratio.

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Thanks for the reply. I wasn't aware of the "neutral" being created by grounding one of the secondary leads part. I'm looking at the schematic but I don't see any ground connections either on either of the secondary leads or anywhere on the 115VAC side of the electrical schematic. I'll go check with production but I think if there's a ground somewhere it would be indicated in the schematic. Also, where my source is 440VAC not 480VAC, and my secondary is 115VAC, wouldn't this be a 3.83 to 1 transformer? Or do we generally round up and call it a 4:1 transformer despite this discrepency?

I appreciate your replies!

transformers are not typically rated in ratio, but simply input/output. 'Nuetrals' only exsist in 3 wire circuits, and in the CEC, which closely resembles the NEC, if that wire is brought out, it must be grounded.

I wouldn't get too hung up on the turns ratio, I just put that in there to help you understand transformers a little better. As GetDeviceInfo stated, transformers are usually rated on input/output voltages and current rating/ VA, kVA. There may not be a connection to ground in your application, generally there is for safety reasons, it would depend on what the transformer is powering and your local codes.

If one of the leads of the secondary isn't grounded, wouldn't it cause problems? I think it would essentially mean you'd have a higher potential on each side for half the time (60 times per second for each of the two 115VAC lines). This would essentially mean reverse polarity for every component for approximately half of its continuous operation.

It won't cause any problems as it is still 115 VAC. If you measure the voltage with your meter from lead to lead it is 115 VAC. If you were to test the voltages from one lead to a ground (earth) connection you may get different reading on your meter. Remeber it is AC as in Alternating Current, so the reversing of the polarity is what it does to work. The reason that one side is normally grounded is to provide a safe path to ground (earth) for faults. If that isn't there, any fault current can go through the user of the device that is connected to the power source.

Thanks for the clarifications. That certainly helps. So basically the second line is not a neutral, but a second hot line, as the schematic indicates. I was told by a designer that it was a neutral, which is what prompted all my confusion. He is not an electrician or an EE so perhaps he was mistaken.

AutoNub said:

Thanks for the clarifications. That certainly helps. So basically the second line is not a neutral, but a second hot line, as the schematic indicates. I was told by a designer that it was a neutral, which is what prompted all my confusion. He is not an electrician or an EE so perhaps he was mistaken.

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By definition, a neutral carries the imbalanced current, which of course is impossible in a two wire ciruit, where both conductors carry all of the current. It is common practice however to tie one of the secondary lines, in a two wire circuit, to ground, and then fuse the other. Another somewhat common pratice is then to color code the grounded conductor white, to identify it. In the CEC we refer to this as the 'identified conductor', while the NEC refers to it as the grounded conductor. This then leads to the misconception that it is equal to or equivalent to a neutral (white by code). I've heard many Electricians refer to it as such, but there is important differences when spec'ing and troubleshooting a circuit.

GetDeviceInfo said:

By definition, a neutral carries the imbalanced current, which of course is impossible in a two wire ciruit, where both conductors carry all of the current. It is common practice however to tie one of the secondary lines, in a two wire circuit, to ground, and then fuse the other. Another somewhat common pratice is then to color code the grounded conductor white, to identify it. In the CEC we refer to this as the 'identified conductor', while the NEC refers to it as the grounded conductor. This then leads to the misconception that it is equal to or equivalent to a neutral (white by code). I've heard many Electricians refer to it as such, but there is important differences when spec'ing and troubleshooting a circuit.

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Both lines of the secondary are immediately fused. Also, one line of the primary is red and the other is white. The white wire might suggest neutral but it (as denoted by the letter T in the schematic in accordance with the R, S, T, G source convention) goes to the L3 terminal of a power supply in addition to going to one of the two leads on the primary of the transformer.

So I believe the fact both lines of the secondary of the transformer are immediately fused is further indication that this is indeed a second hot line. Perhaps I should suggest grounding this wire to improve safety and removing the fuse to reduce the cost. Do you agree with this idea?

Just out of curiosity, is it ever a good idea or standard practice in any scenario to fuse a neutral wire or line? Thanks!

It is never a good idea to fuse a true neutral. If that fuse blows and the line side does not, the current is looking for a path to return and that could end up being earth/ground through someone using the equipment.

Thanks for all the great replies! Just out of curiosity, what do you do for work (BSomer and GetDeviceInfo)?

Would you ever tie a neutral wire to one of only two terminals/leads on the primary of a transformer?

I used to be an electrician doing new construction and remodel work. Did that for a little over 16 years. Now I work for a company that repairs industrial electronics, like PLCs, VFDs, and associated control systems. Quite a daunting task when a lot of the items that I get in for repair are obsolete and hard to find any information on.

As to your last question there. That depends on the transformer in question. It all goes back to what the rated voltages are for the primary of the transformer. If it is a 120 VAC primary, then yes. If it is a 277 VAC primary, again yes. Everything I have mentioned in this thread is based on my knowledge of things, however limited it may be, and being in the USA where the power systems are based on a 60Hz system.

Hope this helps you a little.

-Brian

and I worked in manufacturing on the technical side as a Millwright/Electrician, as well as the managerial side for about the same time. For the last ten years I've freelanced, helping manufacturers bring thier operations into modern times.

My approach would be to tie a secondary line to ground and fuse the other line. This may or may not be the approach your engineers decide upon. As far as using neutral on the primary, not a problem if the voltage meets your needs. If you don't need a neutral at the location, pulling it there raises your costs, as the transformer would cost roughly the same for the phase to phase voltage, and maybe more with a neutral considering commonly stocked items (347/277/120 is not common).