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Old 14th December 2010, 01:06 PM   #1
AndrewT is offline AndrewT  Scotland
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Default transformer primary winding turns

Hi,
is there a method to test a transformer primary to ensure it has sufficient turns to match the supply voltage and supply frequency?
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Old 14th December 2010, 02:10 PM   #2
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Insufficient turns will cause saturation of the core at higher loads, so you could look at the current with a scope and increase load step by step. The wire diameter will also be a factor, i.e. you may not get core saturation but you may get copper loss.
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Old 14th December 2010, 05:48 PM   #3
DF96 is offline DF96  England
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Bear in mind that some cheaper transformers may be on the verge of core saturation when used off-load at their rated voltage and frequency anyway. Saturation reduces as the secondary load is increased.
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Old 14th December 2010, 06:15 PM   #4
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The way to tell is to bring up the voltage slowly with a Variac while measuring primary current. If the current curve starts to go up steeply, you are hitting core saturation.

To make sure, keep raising the voltage until you get the sudden rise; that will tell you the highest safe primary voltage. If you want to allow safety for a different power line frequency, scale the voltage accordingly; for 50 Hz operation only operate at 5/6 the voltage that is safe for 60 Hz.

The way power lines have been going up through the years, I'd make sure all was well at least up to 130 V input on a USA power line.
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Old 15th December 2010, 07:31 AM   #5
Elvee is offline Elvee  Belgium
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The no-load primary current is a good indicator: depending on the construction quality and VA rating, it can range between 1% of the nominal current for a large (~500VA) transformer to tens of % for a small one (some VAs).
You can make two measurement: one at the mains voltage, and the other at Vmains + 5 or 10%.
If the current increase is approximately proportional, this means the core is far from saturation. If the current shows a marked increase, this means the primary is under dimensioned.
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Old 15th December 2010, 01:38 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi all,
thanks for the suggestions.

To clarify one point.

Measuring the Primary current with the secondary open circuit. Is that the same as measuring the primary current with the secondary removed?

I can use my Variac for applying test primary voltages.

Since my normal mains voltage range is 226Vac to 254Vac, do I check to 254Vac+5%? i.e. 267Vac.
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Old 15th December 2010, 02:45 PM   #7
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Saturation in a transformer is independent of the load current. In other words, the flux density is a function of the applied voltage, but not load current. So it will be the same with the secondary open or with it removed entirely.

Due to core tolerances and other factors, I would recommend the transformer be designed to support at least 120% of the maximum voltage you expect to see. In this specific case that is 305 volts.
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Old 15th December 2010, 02:52 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by sawreyrw View Post
Due to core tolerances and other factors, I would recommend the transformer be designed to support at least 120% of the maximum voltage you expect to see. In this specific case that is 305 volts.
I like the idea of individually set Factors of Safety (FoS), but a random FoS=1.2 for a transformer is probably never achieved for a commercially available 220/240 or 230Vac transformer for domestic use.
I guess and it's only a guess that every 220Vac, or 230Vac, or 240Vac transformer I have, will show the saturation knee well before 305Vac is reached.
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Old 15th December 2010, 03:04 PM   #9
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I agreee that 20% is pretty conservative. In addition to core tolerances, you also have to consider what happens to the peak flux density the core can handle at increased temperature.
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Old 15th December 2010, 03:19 PM   #10
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[QUOTE=AndrewT;2399427]I like the idea of individually set Factors of Safety (FoS), but a random FoS=1.2 for a transformer is probably never achieved for a commercially available 220/240 or 230Vac transformer for domestic use.

Not quite so Andrew.
I wind power supply transformers with a maximum of 1,2 T at 230 Vac. Mostly not over 1 T.
So there is headroom, and also the transformer will not get hot, and have low strayfields.
As I use c-cores exclusively, I apply a very small airgap to prevent the core from appproaching saturation caused by DC like line pollution, which seems to be pretty common these days. I also apply an electrostatic shield (copper foil) between primary and secondary windings.
A good quality power supply transformer is the basis for a good amplifier.
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