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#127404 - 06/23/01 09:27 AM 1 phase schematic-single coils/tapped sec.
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA


Schematic of single phase 3 wire transformer with single winding primary and secondary.
Secondary has physical 50% tap [center tap], for multiwire use.
Note only one polarity marker per winding.

Any comments???

Scott SET... posted 06/23/01
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#127405 - 06/24/01 11:34 AM Re: 1 phase schematic-single coils/tapped sec.
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
Scott;
when would, or what situation would, depend on a field electrician as i to be aware of this polarity?
or am i simply unaware that i should be aware?

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#127406 - 06/25/01 12:01 PM Re: 1 phase schematic-single coils/tapped sec.
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Steve,

When using split-coil transformers, or even multi-secondary coil transformers, the polarity is needed to be observed. If you were setting up a "Buck/Boost" transformer, this is especially serious, otherwise the results will not be correct.

On the kind of transformer shown here, the polarity would not be a serious issue to be known in the field - mainy since there's only one position and one coil.

Basically, there's no real crucial need to know [even understand] the polarity under normal circumstances in the field. It's nice to know this stuff, but not absolutely necessary.
When it comes to altering outputs via connection arrangements, then it becomes crucial for the installing person to understand how this is done.

To show how polarity can be found, my next series of schematics to be uploaded here will show the various techniques [that I know of] to find polarity. Testing equipment is simply a scalable volt meter [not as simple as a "wiggy", but a simple analog meter will work, along with a low cost DVM].

Hope this message covered your Q to your satisfaction

Simply, this polarity stuff isn't important until the transformer is connected in a fashion that differs from it's "out-of-the-box" normal setup [like a Buck/Boost transformer]. That's when it's useful to know polarity.

Scott SET
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#127407 - 07/03/01 01:07 PM Re: 1 phase schematic-single coils/tapped sec.
Anonymous
Unregistered


Following on with the polarity finder techniques, would you give us some reference voltages for a typical residential transformer?

Assuming that X0 is bonded to H2 (= 0 V), H1 to H2&X0 = 7200 V, X1 to X0 = 120 V, X2 to X0 = 120 V, X1 to X2 = 240 V, what are the following voltages:

1. H1 to X1?
2. H1 to X2?

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#127408 - 07/07/01 03:14 PM Re: 1 phase schematic-single coils/tapped sec.
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Dspark,

I see where you're going here, a "Coil Blowover" situation!!! [is this correct?]

The situation would result with an Autotransformer connection - with the bonding connections being made to the "H2" lead of the primary coil, and the "X0" center tap of the secondary coil.
This is not something that could be thought of as "Rarely Happening", or "Not Very Likely To Happen", but more like "In Case It Happens", or "When It Happens".
This connection would function normally, as viewed on the primary side, but have catastrophic results on the secondary side.

As to your Q's regarding the output voltages from this type of coil blowover, here are some numbers for a 7.2 KV pri x 240/120 center tap:

  • H1 to X1 = 7320 VAC,
  • H1 to X2 = 7080 VAC,
  • X1 to X2 = 240 VAC,
  • X0 to X1 = 7320 VAC,
  • X0 to X2 = 7080 VAC


If the system is grounded on the secondary side, then there's less of a hazard for persons to be shocked from one line to ground.
This is the primary reason for grounding the secondary at both the transformer and the service - it reduces the potential difference that can be created between one line and ground by physically driving a conductor to the local ground level.
This helps reduce the levels of voltage and current that could be imposed on a person from just unplugging something, or flicking on a switch, while the system is in an overvoltage state as described here.
It kind of creates "Series-Parallel" connections on a person that would grab a cord, flick a switch, or similar - so their is a much simpler path to the ground reference through the physical ground connection at the service, than thru some unsuspecting person.

Wish I could explain things better, but this is the primary motivation behind grounding the low voltage AC supply.
Lightning is the next reason, but not necessarily the primary one [kind of a residual effect of the first one!].
Maybe another member can explain more clearly than I can!
My messages, in a Nutshell, are too much and , resulting in by members.

Just a little

Scott SET
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#127409 - 07/10/01 10:41 AM Re: 1 phase schematic-single coils/tapped sec.
Anonymous
Unregistered


>I see where you're going here, a "Coil Blowover" situation!!!
You are right about it being the blow over of something...

Actually, my messages to you were a series of carefully constructed questions to get you to back me up on the secondary having two waveforms of opposite polarity.

So when you say:
  • H1 to X1 = 7320 VAC,
  • H1 to X2 = 7080 VAC,
  • X1 to X2 = 240 VAC,
you basically just showed that X1 has opposite polarity of H1 (the voltages add) and X2 has the same polarity as H1 (the voltages subtract), which I think means that you swapped X1 and X2 from how they are in the diagram. Shall we edit?

The only possible inference from this is that the waves of X1 and X2 are 180┬░ apart.

(I am not ever trying to say that X1 lags or leads H1; X1's waveform is the simultaneously occurring property of an electromagnet, namely, the opposite pole).


[This message has been edited by Dspark (edited 07-10-2001).]

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#127410 - 07/10/01 04:17 PM Re: 1 phase schematic-single coils/tapped sec.
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
& but not


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#127411 - 07/10/01 04:34 PM Re: 1 phase schematic-single coils/tapped sec.
sparky66wv Offline
Member

Registered: 11/17/00
Posts: 2339
Loc: West Virginia
OK, Dspark, you've finally gotten my attention...

Definitely, & but not

-Virgil
_________________________
-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI

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#127412 - 07/10/01 06:52 PM Re: 1 phase schematic-single coils/tapped sec.
Anonymous
Unregistered


>you've finally gotten my attention...
You mean the 7200 minus 120 versus the 7200 plus 120 thingie?

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#127413 - 07/10/01 06:57 PM Re: 1 phase schematic-single coils/tapped sec.
sparky66wv Offline
Member

Registered: 11/17/00
Posts: 2339
Loc: West Virginia
Yep... I'm beginning to understand your perspective now...

[This message has been edited by sparky66wv (edited 07-10-2001).]
_________________________
-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI

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