transformer primary winding turns

bob91343

Member
2010-03-11 10:43 pm
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.
 

Elvee

Member
2006-09-08 2:04 pm
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.
 
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.
 
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.
 
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.
 
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.
 
I have purchased transformers that were very much in saturation at normal line and also maybe with 20% to go. It's Totally up in the air. If you didn't design it you can't guess, although it's easy to tell once you plug them in. No load temperature rise and noise is a dead giveaway. I have some Talema 200VA toroids that must draw a watt or less at idle, and I think I paid 10 bucks a piece for them. Then I recently bought some Dayton plate amps and had to throw out the supplied transformers because they got hot just sitting there. I think they laminated them out of tin cans and put on about 3/4 of the required primary.
 
these are the traffos that i have made recently:
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