transformer primary winding turns

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AndrewT

Hi,
is there a method to test a transformer primary to ensure it has sufficient turns to match the supply voltage and supply frequency?

richie00boy

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.

DF96

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.

bob91343

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

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.

AndrewT

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.

sawreyrw

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.

AndrewT

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.

sawreyrw

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.

pieter t

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.

Andrew Eckhardt

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.

DF96

Saturation in a transformer is independent of the load current.
There was a long discussion about this somewhere. The conclusion was that load current reduces core flux by creating ohmic drop in the primary. Not a huge effect, but if a mains transformer is not saturating off-load then it will not saturate at any load.

TonyTecson

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?

yes.....no load current consisting of magnetizing(exciting current) and hysterissis/eddy current losses

TonyTecson

I wind power supply transformers with a maximum of 1,2 T at 230 Vac. Mostly not over 1 T.

i wind my traffos at 0.7T, cores available in my country are not so good compared to M6 irons from the states...

davidsrsb

Designing for 130/260V rms WHEN HOT is probably enough

AndrewT

That's the second time "hot" has been mentioned.
What is it about "hot" that makes the test condition more severe?

TonyTecson

there is such a thing as a 10 second rule.......if you hold the traffo in your hand and you are able to stand the heat for 10 seconds, then the traffo is not that hot.....

AndrewT

Hi,
all my own design gear that is mains driven run with the transformer feeling almost cold.

I have touched some plug in the wall type transformers that get too hot to touch !

TonyTecson

these are the traffos that i have made recently:

davidsrsb

That's the second time "hot" has been mentioned.
What is it about "hot" that makes the test condition more severe?
Winding resistance goes up with rising temperature, causing output drop and a potential runaway problem.

I don't think iron saturation varies too much with temperature, unlike ferrite which performs badly when it gets warmer eg Powdered-Iron and Ferrite Materials for Inductor Cores Page of

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