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Old 8th March 2009, 01:18 PM   #1
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Default piggyback transformers

hi guys

i would like a power transformer of 300v secondary

can i make this from 2 off the shelf power transformer?
PT1: pri 220v sec 0v 20v 1A
PT2: pri 220v sec 0v 15v 1A

is it possible that i do it this way:
connect the secondary of PT1 20v to secondary of PT2 15v and take my required 293v (if my calculation is correct and there is no iron loss) from the primary of 2nd PT?

and how to calculate the VA rating from this kind of setup if above setup is possible

TIA

Erwin
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Old 8th March 2009, 02:02 PM   #2
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Substantially, yes. You will get 220 * 20 / 15 = 293.3V

The consideration is that the 220V winding is rated 220V. It may withstand 293V, it may not. It's your call.

The VA rating will be that of the lower rated part.

w
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Old 8th March 2009, 02:20 PM   #3
AndrewT is offline AndrewT  Scotland
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Better still, buy a 1:1 isolating transformer.

Adapt any mains toroid transformer to make your own autotransformer to give any voltage you require.

Power the autotransformer from the isolated secondary and you are safe and completely flexible.

You can even use a Variac as your continuously variable autotransformer in complete safety.
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Old 8th March 2009, 02:27 PM   #4
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thanks wakibaki for the warning. its noted

how bout this:
piggyback the 15v 0v 15v with the 20v 0v 20v?

does the winding can withstand the 293v? does it become balance? as each end have half the voltage?

sorry if this question is ridiculous. i am learning. TIA
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Old 8th March 2009, 02:38 PM   #5
AndrewT is offline AndrewT  Scotland
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hi,
the turns ratio of the transformers is not what you might expect.
Let's take your first example using 15Vac and 20Vac transformers.
Both are small, 15VA and 20VA and likely to have regulation in the range 10 to 20%.

The open circuit voltage of these transformers will be ~15 * 1.15~=17.25Vac and ~20 * 1.12~=22.4Vac.
These can be used to determine the turns ratios.
220:17.25 and 220:22.4
Now use those ratios to determine the open circuit output voltage.
220*22.4/17.25~=286Vac.
The regulation of these cascaded transformers will be worse than the lower rated, i.e. worse than 15%. Let's assume 20%.
The rated output voltage is ~ 286 / 1.2 ~=238Vac not 293Vac.

Go back to post3 and build your own isolated autotransformer.

You can check the turns ratio by measuring the actual input and output voltages when the secondary has no load.
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Old 8th March 2009, 04:04 PM   #6
jcx is online now jcx  United States
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running the 2nd transfromer at higher V, same frequency also increases B in the core - power transformers are usually at the edge of acceptable loss with nominal line V for economic reasons

line power xmfr cores are operated at a very nonlinear part of their curve and Imag increases nonlinearly with V*t
the increased B in the core will draw lots more magnetizing I and increase losses more quickly than you might expect
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Old 9th March 2009, 05:18 AM   #7
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hi guys

thanks for all the help and pointers. will look into them

jcx, yep i tried piggyback 2 PTs to give out the same voltage as the main, trying to create isolation transformer from 2 off the shelf PTs, the output voltage is lower than mains.

at one times i try to draw only 1/2 of the rated current. the input volt is 220v but the output is only 190v. but they bring benefit to the circuit as long as lower voltage wont create problem.

Erwin
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