Transformer 'Polarity' Confusion

DC - Direct Current - Electrons flow through a circuit in one direction.

AC - Alternating Current - Electrons change directions (Edit 120 = 2 x 60 times per second in US power systems).

To be continued...

In an AC system looking at the two posts at a given instant in time they will either be exactly zero volts; or one will be more positive than the other.  ~8 milliseconds later these polarities will reverse - the opposite pole will be more positive than the other.

In an AC system, if you have 2 power sources that are in phase and have the same polarity, their Commons will switch from positive voltage to negative voltages in sync with each other, as will the two Hots.

When the 2 power sources have opposite polarities, the two Hots are out of sync with each other, as are the 2 Commons.

This can be visualized by looking at their sine waves produced on a dual trace oscilloscope.  When they are in phase and have the same voltage the two sine waves will look identical.  When they have opposite polarities, they will be a mirror image of each other.

@SteveH,

Your explanation turned on the light!  I should have understood it from what I read about it in the last couple of days.

Thanks a lot.

To follow up on SteveH's opening response, let's start with the technical answer that, yes, strictly speaking "polarity" only applies to DC (direct current) voltages, where one terminal is typically labeled "plus" (+) and the other is labeled "minus" (-). Matching polarity is as simple as connecting "+" to "+", and "-" to "-" -- in effect, putting the 'batteries' in the 'holder' the right way!

Alternating current (AC), by contrast, sends the electrons back and forth, switching directions (actually 120 times per second in the USA, yielding 60 full cycles per second). So, there's no actual "polarity" to an AC output, but (just to make things confusing!) there *are* several concepts sometimes confused with (or mislabeled as) polarity:

- First, many AC distribution systems arbitrarily connect one pole of the system to ground -- literally driving copper rods into the earth to connect it to the ground, and sometimes relying on that return circuit to use only one transmission line. So, if you measure the voltage potential between each of the two power prongs of any receptacle and the ground, one will show line potential (120 VAC) and the other will show no voltage at all (the third prong, if any, is also grounded, or should be). Modern two-conductor plugs and receptacles are "polarized" in the sense that they can *only* be inserted in one direction, providing a reliable ground connection to the device even without the third wire. In modern devices, the chassis is often connected to ground, to prevent stray voltages from inadvertently contacting the user.

- Just to add to the potential confusion, sometimes the device makers create a common connection for distinct voltage outputs -- in multi-throttle train transformers, the "U" terminals are typically common for all outputs, with the other terminal for the various throttles labeled "A", "B", etc. This common terminal may or may not be "grounded" in the above sense, but all circuits will use the single and interconnected terminal as one side of the various power circuits.

- Which brings us to the final (sometimes confusing) term: "phasing". Phasing simply means matching the waveform of multiple AC sources, so the individual AC sources work together rather than against each other when connected in parallel or in series. In practice, this means that you can connect two small transformers in parallel to carry a heavier load than either could individually, or conversely connect them in series to produce the sum of their individual voltages. If the transformers are *not* properly phased before connection, they may tend to short each other out if connected in parallel, or cancel each other out if connected in series.

Oh, and one final confusion: electricity is literally the flow of loose electrons from atom to atom (and/or the reverse flow of "holes" which electrons can potentially occupy around atoms), but rather than "+" designating a surplus of electrons as you might imagine, it actually designated a *deficit* of electrons (i.e., "holes"). I can only ascribe the confusing reversal to the polarity signs having been assigned long before the understanding of subatomic physics clarified what "polarity" really meant.

There -- everything clear now?

@texgeekboy posted:

I have PW transformers, ZW and 1033.  They both have posts called 'U' or 'Common', and multiple posts that will output variable voltage.  I had an 80s era MW transformer that I bought new back then, and I've been using it for testing.  It died so I needed a new one.  To keep my CEO happy I researched all I could and decided on an affordable NIB 32923.  I know it's not big, but it should do what I need.  During my research I noted that others have asked about the 'polarity' of the posts on that transformer, and several knowledgeable OGRers replied there is no polarity on AC transformers.  In my very simple world I believe that electricity flows in one direction.  I've believed that the electricity flows from the 'A,B'C' posts on my transformers to the 'U' or 'common' posts.  Is that not true?  Does my 32923 transformer have an equivalent post to the 'U' posts?  If so, can it easily be determined?  I saw a tutorial of sorts from Marty about phasing transformers.  I'm not sure I understood it all.  BTW, the posts on the MW were not defined either.  It had a main output and an accessory output.  There were 2 unmarked posts for each output.

You mentioned having a 1033...to add to the potential confusion, i believe the A post is common and U is variable...for this model only.

@Farmall-Joe posted:

You mentioned having a 1033...to add to the potential confusion, i believe the A post is common and U is variable...for this model only.

ALL postwar Multi-Control single throttle transformers use "A"(or alternate posts for different voltages) as common and "U" to the center rail - so most transformers.

As @Steve Tyler points out above, only multi throttle transformers use "U" as common.

"ALL postwar Multi-Control single throttle transformers use "A"(or alternate posts for different voltages) as common and "U" to the center rail - so most transformers."

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I did NOT know that! I always thought that the U was ground, and went to the outside rails, and the variable voltage terminal (A,B, C, D) was to center rail. MTH uses red for center and black for gnd leads on outside rails.

So, a 1033 is as you say, A is gnd, U is center, but others such as the KW with 4 terminals like above is like what I thought above? Right? So....better not connect the U from one to the U on the other to get a "common ground?" Now, I am confused.

Meaning, for the OP, he must be careful when phasing as to which U terminal he picks for gnd?

And, what is a "Multi-Control single throttle transformers?" I thought single throttle meant one controller, and multi control meant "more than one controller>" ie: 1033 has one handle, the KW has 4 controllers and each has a U....or multiple U's tied together.

We are going down a deep hole here. Greg

This might help some people to better visualize phasing. The blue line represents the 120 volts at the wall socket. It is the same in both frames.  If your transformer doesn't have a polarized plug, you can change the output phasing, represented by the red line, by flipping the plug over at the wall socket.

@L & N posted:

This might help some people to better visualize phasing. The blue line represents the 120 volts at the wall socket. It is the same in both frames.  If your transformer doesn't have a polarized plug, you can change the output phasing, represented by the red line, by flipping the plug over at the wall socket.

Nice....but it fails to address the U is ground, the U is not ground issue....that is still up in the air......or down that deep rabbit hole.

You should also point out that "flipping the plug" does not always work - some transformers were sold with polarized plugs....and they were not the right polarity from the factory. That involves another fix.

Greg

The various output voltages from the post war transformers can be referenced with U as common, and also using post A through E if yours goes to E, as common.  This info is on page 536 of Greenbergs Repair and Operating Manual.  The manual has a good explanation of why there are multiple windings used in the transformer, most having to do with pull in and hold in of the whistle relay.  If you have Lionel, this book is probably one to have on hand, if not for repairing, to understand how things work.

@L & N posted:

This might help some people to better visualize phasing. The blue line represents the 120 volts at the wall socket. It is the same in both frames.  If your transformer doesn't have a polarized plug, you can change the output phasing, represented by the red line, by flipping the plug over at the wall socket.

I'm not sure what part of that was left out.

@cngw posted:

Nice....but it fails to address the U is ground, the U is not ground issue....that is still up in the air......or down that deep rabbit hole.

You should also point out that "flipping the plug" does not always work - some transformers were sold with polarized plugs....and they were not the right polarity from the factory. That involves another fix.

Greg

Agreed,  many transformers have been made inconsistently and some are more involved to correct than others.  I 'm loath to use terms like wired correctly or properly. Wired consistently, MTH Z4000's are wired with additive polarity and Lionel PH180's (all versions) are wired with subtractive polarity. Neither is right or wrong, in order for it to work on a layout they have to have the same type of polarity.

As far as "U" (common) goes, Lionel has been consistently inconsistent, see page 6 of the PS attachment that identifies  which post is the common terminal for many postwar transformers.

Steve

In my opinion, confusion often comes from referring to Common as Ground.  Ground can mean a lot things to a lot of people.  Ground is often meant to be a zero voltage reference point.  It can also be used to describe a common voltage reference point in one or more circuits.  In a building's electrical system, Ground is literally that - a metal stake in the earth ground electrically connected to the building's ground circuit.  At only one point, near the building's electrical service entrance, Neutral is also supposed to be tied to Earth Ground.

However, an AC transformer decouples (isolates) the input circuit from it's output, thereby removing a direct connection to the original building Ground.  Specifically, current flowing through the transformer's primary coil creates a magnetic field around it (alternating positive to negative and back 60 times per second).  This magnetic field induces current flow in the transformer secondary winding(s) flowing in the same direction as in the primary.  This isolation/magnetic coupling blocks direct current flow to/from the Earth ground path.

Reversing polarity of a transformer's connections at either the input or the output will reverse it's measured phase (polarity) relationship to a second transformer.

When we have 2 transformers, and for simplicity of explanation, we'll say each transformer has one output, each with two posts.  On each transformer, at any given instant, one terminal post's voltage will be trending towards positive relative to the other.  Phasing variable voltage transformers is a simple a two part test.  Part one is to see which posts on opposing transformers are trending positive and which are trending negative in sync.  Part two is to find the post on each transformer that does not have a significant voltage difference between it and it's counterpart on the other transformer as the adjustment handle is moved.  The two terminal posts (one on each transformer) with consistently close to zero voltage between them are considered Common.  These two post are then connected to each other and to the outside rails of a 3 rail track system, making them Track Common.  Each transformer's remaining post is Hot.

On multi-output transformers, Common is the post or posts to which some or all other posts' voltage is referenced.  Note that the Lionel Multi-Control 1032, 1032M, 1033, and 1232 are an exception as can be seen in this schematic:

When a TMCC/Legacy Base is introduced into the mix, the distinction between Common and Earth Ground is important.  The base transmits it's 455kHz signal between the building's Earth Ground and Track Common.

On a DCS system, Track Common should be connected to the TIU/WTIU Black post and Transformer Hot to the Red Input Post.

@ADCX Rob posted:

ALL postwar Multi-Control single throttle transformers use "A"(or alternate posts for different voltages) as common and "U" to the center rail - so most transformers.

As @Steve Tyler points out above, only multi throttle transformers use "U" as common.

@cngw posted:

I did NOT know that! I always thought that the U was ground, and went to the outside rails, and the variable voltage terminal (A,B, C, D) was to center rail. MTH uses red for center and black for gnd leads on outside rails.

So, a 1033 is as you say, A is gnd, U is center, but others such as the KW with 4 terminals like above is like what I thought above? Right? So....better not connect the U from one to the U on the other to get a "common ground?" Now, I am confused.

Meaning, for the OP, he must be careful when phasing as to which U terminal he picks for gnd?

And, what is a "Multi-Control single throttle transformers?" I thought single throttle meant one controller, and multi control meant "more than one controller>" ie: 1033 has one handle, the KW has 4 controllers and each has a U....or multiple U's tied together.

We are going down a deep hole here. Greg

I would be more comfortable using "common" where you are using "ground" for building toy train layouts.

@cngw posted:

So, a 1033 is as you say, A is gnd, U is center, but others such as the KW with 4 terminals like above is like what I thought above? Right? So....better not connect the U from one to the U on the other to get a "common ground?"

"A" is common on the 1033 for the purpose of phasing with other transformers on larger layouts. While "polarity" isn't the issue, and you could phase the 1033 "U" with the 4 ZW "U" terminals, the problem is that the throttle, whistle, and directional controls are all connected to and operate through the "U" post.

@cngw posted:

... what is a "Multi-Control single throttle transformers?"

Throttle/Whistle-Horn/Direction... 1033 for example.