Transformer theory

[Jnewell]

Can someone please explain how a transformer can be multi tapped for voltage from 480 or 240 or 208 or 120 to 24v, because according to theory the secondary voltage is a direct proportion to the number of coils on the primary? But the secondary side has only one set of taps? Thank you

 

[LarryFine]

Can someone please explain how a transformer can be multi tapped for voltage from 480 or 240 or 208 or 120 to 24v, because according to theory the secondary voltage is a direct proportion to the number of coils on the primary? But the secondary side has only one set of taps? Thank you

Let's make up a transformer with 100 turns on the primary and 10 on the secondary. If you supply 120v to the primary, you will get 12v from the secondary. That's 1.2v per turn, which applies to every winding on the transformer.

Now, let's suppose you attach a wire (tap) at every other turn on the secondary. You'd be able to take 2.4v from each segment, so you could get 4.8 between one end and the second tap, 7.2 from the third, 9.6 from the fourth, etc.

If it helps, picture a string of batteries in series, with a wire sticking out from where each battery touches the next one. Those are your taps.

 

[Jnewell]

Right so if the secondary side is 10 turns to get 24 from a primary of 200 turns at 480 v, but off that same transformer there is 3 other taps at 240 at 100 turns,208 at almost 86.6 turns and 120 at 50 turns but the secondary windings doesn’t have taps according to the primary voltage so it stays at 20 turns.

 

[ActionDave]

Your question has many layers to it. You are correct that the secondary voltage is a direct proportion to the number of coils on the primary, but you can't just buy a transformer and hook it up to get whatever voltage you want. You choose a transformer based on the application and they are manufactured for the voltage you want and the power you have available whether it's to power a whole building or a doorbell.

 

[LarryFine]

Right so if the secondary side is 10 turns to get 24 from a primary of 200 turns at 480 v, but off that same transformer there is 3 other taps at 240 at 100 turns,208 at almost 86.6 turns and 120 at 50 turns but the secondary windings doesn’t have taps according to the primary voltage so it stays at 20 turns.

Exactly, and now I understand the voltages in your first post. I didn't realize you were describing a multi-voltage primary, and I described a multi-voltage secondary.

Yes, the primary has a full winding for the highest voltage, and taps at proportionate points on the winding for lesser voltages. The output remains constant because the turns ratio of the energized portion to the secondary is constant.

By the way, when you apply, for example, 120 volts between the common (white) and the 120v tap, there will be 208 volts at the capped 208v tap, 240 volts at the capped 240v tap, etc. That's why all of the unused taps must be capped.

 

[synchro]

The primary is tapped so you can maintain the same "volts per turn" with different supply voltages. By doing this you keep the maximum level of the magnetic field in the iron core at the same value that it was designed for (enough to fully utilize it but not go overboard and "saturate" it).

 

[Jnewell]

I finally understand, the multi tap thing threw me off I wasn’t comprehending that the colts per turn stayed the same as the voltage went up. Thank you for the help.