What does the dot mean on a single phase transformer wiring schematic?

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KyleFowler

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Electrician
Got a 0.5kva 480/240 to 240/120 single phase Hammond transformer to wire up and the diagram has this unexplained dot next to the windings of H1 and X1. On the Hammond website they actually do elaborate on the dot but for practical purposes their explanation isn't very helpful to me. Below is their explanation:

"The placement of these dots next to the ends of the primary and secondary windings informs us that the instantaneous voltage polarity seen across the primary winding will be the same across the secondary winding. In other words, the phase shift from primary to secondary will be zero degrees, which is important for some types of circuits. If the wiring to the dots is reversed on one side, the primary and secondary will be 180 degrees out of phase."

I want to know when I wire this transformer to step 480 volt down to 120 volt should the ungrounded conductor be X1 since it has the dot? That's my interpretation of the dot. I'm sure I really don't understand what Hammond is trying to tell me. Just trying to do my due diligence and not guess/test.

For a scope of work we are wiring a 0.5kva transformer to step down 480v to 120v to refeed a road sign that had it's original underground line cut underneath the building the sign is fed from. This apparently happened 20 years ago and no one cared until now. The 480 volt supply we are sourcing is from a pole light 50 feet away and the only power source available without cutting the parking lot or an underground bore etc. The road sign was converted from t12 HO to LED by a sign company recently with 120 volt drivers.
 

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Not sure what the dots mean. I'm remembering something about "direction" of something, but I could be way off

wouldn't it be cheaper and easier to change the drivers?
 
Joethemechanic said:
Not sure what the dots mean. I'm remembering something about "direction" of something, but I could be way off

wOULDN'T IT BE CHEAPER AND EASIER TO CHANGE THE DRIVERS?
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was told by our lighting rep he didn't have any 480 volt LED drivers compatible with the existing setup. In general 480 volt LED stuff is always special order and expensive with long lead times
 
You can connect either the X1 or the X2 to ground.

The dots are particularly important when you connecting several transformers together. They serve about the purpose as the designations H1 and X1. They are important when connecting multiple windings in parallel or series (connecting two dots together would be a subtractive arrangement). They can also be found on current transformers, where they are used often.
 
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You can ignore the dots for your application. Your road sign will not care what the phase shift is across the transformer. But if you're curious about the dots, there is further info below.

The dots are just providing information about the the polarity between the primary and secondary windings. You can have either 0 or 180 degrees phase shift in the voltage from the input to output. When a positive half-cycle of the AC voltage waveform is applied to the "dotted" side of the primary relative to the non-dotted side of the primary, the "dotted" side of the secondary will also have a positive half-cycle. And vice-versa during the negative half-cycle of the AC voltage. Therefore there will be zero degrees phase shift between the primary and secondary dots. You can then reverse either the primary or secondary to get 180 degrees phase shift.

Also, when the instantaneous load current flows out of the dot on the secondary winding during half of the AC cycle, the current will be flowing into the dot on the primary winding. And it's reversed on the other AC half cycle.
 
Transformer polarity

You get something like an open delta wired up with the polarity swapped you won’t get 240,240,240,120,120.
You will get 120,120,240,316,415

Think of it as a way to draw out the coils on paper.
As Jim D said, it doesn’t matter about the dots unless you start banking the transformers
Here is a slide I made on a presentation I did a couple of years back.
IMG_4700.jpeg
 
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" If the wiring to the dots is reversed on one side, the primary and secondary will be 180 degrees out of phase."

Not to be starting a Range War here, BUT.....

They will be of opposite polarity, which is indistinguishable from a 180 degree phase shift on a sine wave.

A transformer is incapable of inducing a 180-degree phase shift from primary to secondary ( or secondaries).

Polarity reversal yes, phase shift no.

Sorry, one of my Big Red Buttons
 
OldBroadcastTech said:
" If the wiring to the dots is reversed on one side, the primary and secondary will be 180 degrees out of phase."

Not to be starting a Range War here, BUT.....

They will be of opposite polarity, which is indistinguishable from a 180 degree phase shift on a sine wave.

A transformer is incapable of inducing a 180-degree phase shift from primary to secondary ( or secondaries).

Polarity reversal yes, phase shift no.

Sorry, one of my Big Red Buttons
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I agree with you. You cannot get a broadband constant phase shift with just a transformer.

But if you really get down to the details, a practical transformer will introduce at least a very small phase shift under load because of its leakage inductance. And the phase shift in the voltage will have some dependence on the power factor of the load. But it will have a small lagging phase shift even with a resistive load. And of course the phase shift will be a function of frequency as well as the load applied.

Some transformers subsrantially increase the leakage inductance using magnetic shunts or other means in order to provide a current limiting function. For example, some arc welders and neon sign transformers. That extra leakage inductance will significantly increase the amount of phase shift under load.

A major contributor to the impedance of a transformer is its leakage inductance. And so a transformer with a higher impedance will experience more phase shift under load.

The small phase shift from leakage inductance will not be important in power applications unless matching is required, such as when paralleling transformers.
 
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OldBroadcastTech said:
" If the wiring to the dots is reversed on one side, the primary and secondary will be 180 degrees out of phase."

Not to be starting a Range War here, BUT.....

They will be of opposite polarity, which is indistinguishable from a 180 degree phase shift on a sine wave.

A transformer is incapable of inducing a 180-degree phase shift from primary to secondary ( or secondaries).

Polarity reversal yes, phase shift no.

Sorry, one of my Big Red Buttons
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Mine as well; it sometimes raises its ugly head in audio circles. Some folks insist on referring to an audio signal polarity flip as being "180 degrees out of phase", but the two are indistinguishable only when you are speaking of a single frequency.
 
A concrete example, let me know if I got it right or if I have it backwards:

Simplify the transformer in the OP to a 480V:120V transformer with just 4 terminals, H1 (dot), H2, X1 (dot), X2. Apply 480V to H1 and H2.

Then if you connect X2 to H1, the voltage H2 to X1 will now be 480V + 120V = 600V. Whereas if you connect X1 to H1--the voltage H2 to X2 will now be 480V - 120V = 360V.

Cheers, Wayne
 
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I have read about something similar when using a 120/24volt transformer for HVAC equipment. I don't claim to uderstand it and I have never run into this issue. See Attached. Don't know if we are talking about the same thing.Transformer phasing (939x1280).jpg
 
ggunn said:
Some folks insist on referring to an audio signal polarity flip as being "180 degrees out of phase", but the two are indistinguishable only when you are speaking of a single frequency.
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It's a little more complicated than that. Say you have two stereo channels. They both have to be in phase with each other, all the way to the speakers. There are test sets that will "ping" an audio channel and listen to the speaker. Then perform the test on the opposite channel and tell you if the signals and speakers are in phase. Makes a big difference with stereo imaging and surround sound.

My experience is with commercial audio for restaurants, stores, etc. Never do we ever use stereo, if we get it that way both channels are always summed or combined into a mono signal simply because stereo is useless in that kind of environment. I remember once a restaurant using an FM radio station. We took the stereo channels from the tuner and combined them into mono. Then one day, they complained that with the announcer and most commercials (which were mono) the volume dropped way down. What had happened was the station somehow flipped the polarity of one channel (both channels were the same signal on their end because the mic and commercials were mono) and because they were 180 deg out of phase when combined with the other on my end, they pretty much cancelled out. I even called the radio station and of course they didn't know what I was talking about. If you were listening in stereo you wouldn't notice a thing, except possibly weird sounding stereo if you had a decent system.

With sound systems it's important to maintain polarity from source to speakers.

-Hal
 
Joethemechanic said:
Way back then. Did the restaurant look like this, with a radio in a finished wooden case?
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Actually, that would be legal.

Would still be doing that today if I was still in business. Some owners are too cheap to pay for a music service and legally have music with multiple speakers in their venue. Until they get caught. Sirius XM, Apple Play, Spotify and many others offer royalty paid music services.

-Hal
 
Yeah, my friend up here on the other side of town Is a Podieetrist. He subscribes to a doctor's waiting room music service. I was teasing him about that being what he listens to while he drives around in his 1970's Rolls, Bently , or other British thing it is he has at the moment (Yeah he's a motorhead, but weird stuff). But even though he's pushing 70, he is still "RJ" and he's a kid to most of his patients because he's a local, born and raised here
 
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