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# Step up & Step Down of Auto Transformer

Discussion in 'Power Electronics' started by Abdul Salam, Mar 6, 2018.

1. ### Abdul Salam Thread Starter New Member

Jul 15, 2017
10
0
Hi,
I have an auto transformer with three winding:
1- 0 ~ 15 for circuit power supply
2- 0 ~ 20 for step up or step down to main winding (0-40-140-220)
3- 0 ~ 40 ~ 140 ~ 220 main winding

this can take input from 90 volts to 265 and produce 220 volt output.

Problem: I do not understand how it can step up and down in mentioned range?
I do not have electronics background, just programming!

Last edited: Mar 6, 2018
2. ### MrChips Moderator

Oct 2, 2009
19,132
6,148
And the question is?

Jul 18, 2013
18,847
5,890
Secondary windings can be connected additive or subtractive, depending on polarity connected.
Max.

4. ### LesJones Well-Known Member

Jan 8, 2017
2,304
693
An auto transformer only has one winding but it can have a number of taps on the winding.

Les.

5. ### crutschow Expert

Mar 14, 2008
23,124
6,858
An auto transformer can step up or down depending upon the secondary tap as compared to the primary.
As shown below, the top tap steps up the voltage and the bottom four taps step the voltage down.

Feb 20, 2016
2,927
815
If it is actually an Auto Transformer, that is, one single multi tapped winding, be very careful how you use it as there is no mains isolation.
You need to have an isolation transformer for safety reasons.
Do you understand how a transformer works?
This may help. It is just one of the first search results.
https://www.quora.com/How-do-transformers-work

7. ### Abdul Salam Thread Starter New Member

Jul 15, 2017
10
0
Thanks all of you for reply .

There are two winding on toroidal transformer 1) is 0 - 20 Volts and 2) is 0 - 40 - 140 - 220 volts.
these both work together to give stable output of 220 volts if input volts are in between 90 and 265 volt.

Question is how it step down from 265 to 220 and step up for 90 input to 220 output?

I think it is simple but not for me

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8. ### Ramussons Well-Known Member

May 3, 2013
794
144
Such transformers were / are in use to Reduce the voltage Fluctuation on TV's, Refrigerators to reduce the possibility of damage. The output Voltage was maintained around 220 +/- 10 for an input variation of 90 to 265.

Depending on the Input Voltage, the Transformer Tappings are selected by Relays to Boost or Buck to reduce the output fluctuations.

9. ### Abdul Salam Thread Starter New Member

Jul 15, 2017
10
0
That's what I do not know " how to select tappings?"

10. ### LesJones Well-Known Member

Jan 8, 2017
2,304
693
With the description of the two windings on the transformer in post #7 your statement "these both work together to give stable output of 220 volts if input volts are in between 90 and 265 volt." CANNOT be correct.
You can't supply the transformer with or get 265 volts out as the most you can achieve is 240 volts. (220 + 20 = 240 volts.)
You could not step up 90 volts 220 volts but you could get close. If you connected the 90 volts between the 40 volt tap and the 140 volt tap for the input. Then connect one end of the 20 volt winding to the 220 volt tap (Phased so the voltages add.) and take the output from the 0 volt tap and the end of the 20 volt winding that is not connected to the 220 volt tap you would get close to 220 volts output.
(240 x 90)/100 = 216 volts. NOTE THAT neither the neutral or the live on the output would be at the same potential the neutral or live on the input.

Les.

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