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Godoh
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Joined: 26/09/2020
Location: Australia
Posts: 458

Posted: 10:32pm 24 Feb 2023

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I am having problems with figuring out how many turns to have on a transformer I have.
I bought one of those supposedly 5000 va peak torroid 230volt to 110 step down transformers from ebay.
The core measures

165mm OD
65mm hole
75mm high

It is supposedly rated at 3000 va , from what I have seen online transformers rated at 1000va are about the same size, so I am guessing that the ratings are very optimistic.

I want to wind a primary to use the transformer for a small inverter on 24 volt DC.
The problem is that the calculators I have found online give me nonsensical answers.

Can one of you knowledgeable people please point me to a calculator that gives me the area of the torroid and how many turns I need.

I wound one turn, around the core and read about 0.56 volts. when I powered it up.

Seems like I will need about 26 to 28 turns but from what I am reading here people are suggesting a saturation of 1 Tesla. How to get that is beyond me.
Hope you can help
Pete

phil99

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Joined: 11/02/2018
Location: Australia
Posts: 2135

Posted: 12:01am 25 Feb 2023

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Referring to this thread:-
https://www.thebackshed.com/forum/ViewTopic.php?FID=4&TID=15539#197484

I don't think you need the calculator unless you plan to strip or add turns to the 230V winding.
If we assume the manufacturer has already optimized the the number of turns for 230V there isn't much to do.

24V DC will give you roughly 15V RMS / 0.56V/turn = 27 turns, same as you got.

It should be easy to adjust the number by a turn or two by experiment if needed.

The power rating is determined by the temperature rise as you load it up.

Godoh
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Joined: 26/09/2020
Location: Australia
Posts: 458

Posted: 01:09am 25 Feb 2023

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Thanks Phil, that makes sense, now I can understand why the core is so small in the autotransformer.

I guess if I call it a 1.5 kva transformer in an inverter it will be fine.

It does seem that as I only got a fraction over half a volt for one turn on the transformer that they have a lot of turns on the 230 volt winding.

We have some hot days coming up so will hide in the shed and play with it to see what it does.

Thanks
Pete

rustyrotors
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Joined: 07/01/2023
Location: United States
Posts: 36

Posted: 12:30am 28 Feb 2023

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https://www.electricaltechnology.org/2014/02/maximum-flux-density-bmax-calculator.html

using 0.56 Vrms
0.000005 MHz
1 turn
100/2 * 75 = 37.5 cm2

it shows 0.67 Tesla, which is very low flux, idle current will be very low at 240V

phil99

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Joined: 11/02/2018
Location: Australia
Posts: 2135

Posted: 03:49am 28 Feb 2023

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"it shows 0.67 Tesla, which is very low flux"

For a GOSS material that is very low. It would be unusual for a manufacturer to be that conservative so I wonder if the measurements given are for the whole transformer.

On most toroidal transformers it isn't possible to directly measure the core. If the thickness of the windings is subtracted the reduced area might bring the result close to 1 Tesla.

Godoh
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Joined: 26/09/2020
Location: Australia
Posts: 458

Posted: 06:03am 28 Feb 2023

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Hi Phil and Rusty , thanks for the input. I suspect that you are correct in the core size, looking at it I am really only going to rate it at a 1kw inverter.
I have some new transformers coming that are much bigger, I will have to unwind them, hopefully the lower voltage windings are on the outside so I can leave the 230 volt windings in place. If not I will put the new cores together to make one larger transformer.
The new transformers are 220mm round and 80mm high( over the windings) still much bigger than the one I have now.
Basically they will be used to make spare inverters, I have had a few PowerJacks over the years but they tend to blow up a fair bit. Usually from loose or bad connections inside. And they don't like dips in battery voltage starting big loads.
So far the 8010 inverters I have have been very reliable, but as I live in a fairly remote place and fully off grid I like having spares.
Pete

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