I dont understand this transformers taps labels, please help

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roscoe0

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Hello, first time poster here.

Can somebody please tell me what the numbers next to 'Taps' mean. 2.5% means there is a 2.5% increment between taps, but what does the 4 and 2a, 2b mean?

https://gyazo.com/d6c949ad55d91e47c92297a06db98e63

p.s i dont know how to insert an image, when i click the insert image button it asks me for a url...

Thank you!!
 
Thanks for answering!

If 4 means 4 total taps, then I dont understand why there are 7 taps on the diagram...
 
roscoe0 said:
Hello, first time poster here.

Can somebody please tell me what the numbers next to 'Taps' mean. 2.5% means there is a 2.5% increment between taps, but what does the 4 and 2a, 2b mean?

https://gyazo.com/d6c949ad55d91e47c92297a06db98e63

p.s i dont know how to insert an image, when i click the insert image button it asks me for a url...

Thank you!!
Click to expand...

What it means for there to be taps on a transformer, is the option to decide how many windings to engage on the primary coils. Suppose there are 40 windings (arbitrary number, that makes the math easy for us) that are engaged on the primary coil, at the nominal 600V primary voltage. Your transformer ratio between each pair of windings is from 600 to 120, which is 5:1. This means there are 8 windings on each secondary coil in this example. Each winding corresponds to 15 Volts, on both the primary and secondary in this example. You don't have taps on the secondary, so it will always be 8 windings. The primary by contrast, has the ability to adjust.

At the +5% tap setting, you engage two additional windings, to have 42 primary windings engaged. These windings are dead-end circuit sections which carry no current if you select a lower tap setting. The ratio becomes 42:8 instead of 40:8, so that you can compensate for the primary voltage being 630V instead of 600V like it is supposed to be.

Likewise, the +2.5% tap setting, engages just one additional winding beyond the nominal amount, for 41 windings engaged. The ratio becomes 41:8, to work with 615V primary voltage.

The -2.5% tap setting, disengages a winding, to get 39 windings engaged. The ratio becomes 39:8, to work with a 585V primary voltage.
Finally, the -5% tap setting, disengages a winding, to get 38 windings engaged. The ratio becomes 38:8, to work with a 570V primary voltage.

In all of the above cases, the secondary voltage is still 120V phase-to-neutral, with the taps allowing you to fine-tune the transformer to adapt to primary voltages that deviate from nominal.
 
If I may add, changing primary taps actually slightly alters the volts/turn ratio, which alters the output voltage.

So, if you feed a given voltage to a different number of primary turns, the secondary output changes inversely.
 
Carultch said:
In all of the above cases, the secondary voltage is still 120V phase-to-neutral, with the taps allowing you to fine-tune the transformer to adapt to primary voltages that deviate from nominal.
Click to expand...

Hello, thanks for replying.

In your explanation, the primary voltages deviate from nominal 600V.

But can it also happen that secondary voltages deviate from nominal while the primary voltages are nominal? Say you get 115V instead of 120V while your primary is at a fixed 600V, can you change the taps to increase secondary to get as close to 120V? I have made a drawing with the same number of windings as your example (40 and 8) to explain what im trying to say.

transformer taps.jpg
 
roscoe0 said:
But can it also happen that secondary voltages deviate from nominal while the primary voltages are nominal? Say you get 115V instead of 120V while your primary is at a fixed 600V, can you change the taps to increase secondary to get as close to 120V?
Click to expand...
While we've been discussing primary voltage variations, you're 100% correct. Good observation.

Obtaining the desired secondary voltage despite primary deviations is the reason for primary taps.

Who cares what the primary supply and transformer taps are? We care about the output voltage.
 
roscoe0 said:
But can it also happen that secondary voltages deviate from nominal while the primary voltages are nominal?
Click to expand...
As Larry said, in a perfect world the purpose of the taps is to compensate for primary voltage being off nominal. Some transformers have the voltage shown for the tap positions and they show what the primary voltage would have to be to get nominal secondary for that tap position. You can't have nominal primary voltage and non nominal secondary voltage, except for the voltage drop thru the transformer, so one may use taps to compensate for that but probably only in the case of a consistently heavily loaded transformer. Another use of taps may be in PV systems where the voltage at the inverters is running at the top of their voltage window. This is a fairly common issue.
 
My explanation assumed an ideal transformer, that has zero internal losses and perfect coupling. It was also assumed that our goal was to maintain the nominal secondary voltage as the primary voltage is off from nominal. The taps could be used if desired for adjusting the secondary voltage as well, and compensating for voltage loss on the secondary side of the circuit. In any case, what the taps do is adjust the voltage ratio between the primary and secondary of that particular pair of coupled windings. It is most common that they are on the primary side.
 
Simply put, the voltage applied to a given number of turns in the energized winding sets the volts-per-turn ratio for every winding on the same core (ignoring losses - we're talking theory).

When you change either (or both, of course) the applied voltage or the number of turns to which that voltage is applied, the volts-per-turn ratio changes, as does the output voltage(s).
 
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