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Author Topic: Question for engineers versed in transformer design  (Read 4386 times)
W6EM
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Posts: 1641




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« Reply #15 on: October 22, 2017, 09:51:38 AM »

Good to know that. I had a house with 130volts at the wall receptacles. ...

........ They eventually found that my town was loading the 3 phase distribution network unevenly.  Took them several weeks to get it straightened out. 
Music to my ancient ears....  Cheesy  Many years ago, as a wet behind the ears engineer, I worked in a rural office of my employer.  One with many rural and forested overhead lines.  Unbalanced loading was a frequent problem that caused unequal voltage drops on 3-phase feeders and gave me fits trying to set regulators along the lines.

The reason was that linemen who hung a lot of the transformers didn't want to bother connecting the same relative number and size to all three legs.  They found it easier just to connect "road and field" phases and forget about the middle phase.  So, I got even.  Sent them out "phase balancing."  And, the superintendent and I checked the work to make sure it was done.  It took care of a lot of high and low voltage complaints.
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W6EM
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Posts: 1641




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« Reply #16 on: October 22, 2017, 10:33:37 AM »

The transformer presents inductive-reactance to the source. A capacitor presents capacitive-reactance. When you provide correction via the addition of the capacitor you are pulling the power factor back to "1". An ideal load would be purely resistive with no inductive or capacitive component.

Excessive inductive-reactance can cause heating of conductors.
......So, a given reactance connected on the primary of a 120v to 12v transformer would be 1/100th of same value if moved to the secondary side and looked at from the primary.......
  I got it backwards.  The reactance of the same capacitor connected to the secondary would be seen as 100 times the value, when at the primary looking inwards.  Too many inversions....in my head.  But, no excuse.
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N2EY
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« Reply #17 on: October 23, 2017, 08:20:11 AM »

Hi,
    What is the advantage of using an AutoTransformer configuration as shown in the schematics?   I always thought a bucking transformer wired the 120 Volt Primary of the Transformer across the power line.  One side of the load would go to one side of the primary.  The other side of the load would be in series with the other side of the primary phased "opposite" to the primary to reduce the output voltage.   See my cheesy diagram below.  (The \ and / represent coil windings.

<-------------|-----|
                   \  ||  |
                   /  ||  |
120 AC         \  ||  \
Primary         / ||   /  6 Volt Secondary
to house       \  ||  \
power           /  ||  |--------------->
                    \  ||              To Load
<---------------|---------------------> 

I'm always ready to learn something new.

73
Ken
KB3MDT
OK.  I'll give it a try.  "Auto-ing" means connecting the secondary and primary in series.  Not as your sketch shows, which simply connects the secondary to one of the primary leads as a series bucking winding.

Auto-ing, as shown in the schematics above yours, series-connects the primary and secondary transformer windings.  Doing it so the windings are "additive," as he has done (even though the result is a "substractive" output voltage) will do two things to the transformer's advantage.  First, it reduces primary winding current by adding the secondary turns so that the input voltage seen by the actual primary is reduced by the secondary winding voltage.  And, a lower primary voltage means less risk of insulation failure (important to electric utility substation transformers).

All true. The stress on the autotransformer is less, and that's good.

A disadvantage, although small, is regulation is not as good as the separate-winding- only involvement.  The reason being that whatever the buck winding voltage drop is will be reflected in lower primary winding voltage.  So, if significant, it will aggravate voltage drop across the transformer and make actual voltage sags larger.

But is that really true? How big is the difference, really? Let's do some math....

Suppose we have a transformer whose primary is 120 volts and secondary is 6 volts. Then it's a 20:1 transformer.

First we hook it up in the "bucking" connection - input across the 120 volt primary, secondary in series with the output, so we get direct subtraction.

If the line voltage is 120 volts, the secondary is 6 volts, and the result is 120 - 6 = 114 volts.
If the line voltage rises to 130 volts, the secondary rises to 6.5 volts, and the result is 130 - 6.5 = 123.5 volts
If the line voltage drops to 110 volts, the secondary drops to 5.5 volts, and the result is 110 - 5.5 = 104.5 volts

So, for a 20 volt variation (10 volts above and below) the nominal 120 volts, we get a total output voltage variation of 19 volts, and 120 volt output voltage of 114 volts.

Now try the auto-transformer connection. Since it's a 20 to 1 transformer, the output voltage is 20/21 of the input voltage.

If the line voltage is 120 volts, the output voltage is 120 times 20 divided by 21 = 114.2857....volts.
If the line voltage rises to 130 volts, the output voltage is 130 times 20 divided by 21 = 123.8095....volts.
If the line voltage drops to 110 volts, the output voltage is 110 times 20 divided by 21 = 104.7619....volts.

So, for a 20 volt variation (10 volts above and below) the nominal 120 volts, we get a total output voltage variation of 19.0475.... volts, and 120 volt output voltage of 114.2857... volts.

I'd go with the autotransformer connection because it stresses the transformer less. If high line voltage causes the transformer to saturate, all sorts of problems happen.



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KM1H
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Posts: 2435




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« Reply #18 on: October 23, 2017, 06:06:35 PM »

Here is a rather lengthy but good discussion on the subject that I suggest be read thru.

Carl
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