Mains Transformers Power Ratings

[barry wilkins]

I have a Newnes Engineering Data book 1951 .
There is a mains transformer table in it ,if I look up 230volts 50 cps (Hz) it says a constant of 1840 divided by the size of the transformer core in square inches .So a small transformer say .5 by.5 inches = .25 sq inches = 7360 turns.
Obviously there is more to a transformer than this ,how do you work out the core size in regards to the power required ?. I know the wire sizes are determined by the current capacity of the wire ( thicker the better) and the number of turns you can actually fit on the bobbins.
Does any body have any easy tranformer info. that can help me.
Thanks
Barry Wilkins

[ben7]

Re: Mains Transformers Power Ratings

I have a Newnes Engineering Data book 1951 .
There is a mains transformer table in it ,if I look up 230volts 50 cps (Hz) it says a constant of 1840 divided by the size of the transformer core in square inches .So a small transformer say .5 by.5 inches = .25 sq inches = 7360 turns.
Obviously there is more to a transformer than this ,how do you work out the core size in regards to the power required ?. I know the wire sizes are determined by the current capacity of the wire ( thicker the better) and the number of turns you can actually fit on the bobbins.
Does any body have any easy tranformer info. that can help me.
Thanks
Barry Wilkins

Take a look at this great page!

-Ben

[eccerr0r]

Re: Mains Transformers Power Ratings

Speaking winding coils, anyone seen how a machine toroid winder works, even for a small number of turns? Winding by hand would be a major PITA and have to go with a bobbin + split core (for high freq)/EI system(for low frequency)...

[redwire]

Re: Mains Transformers Power Ratings

Transformer design is really complicated, although they are constructed quite simply. Everything is a trade-off with something else and the math can be intense. In the 1950's they ran (AC mains) transformers quite conservative, they kept everything really simple and over-designed.

Operating magnetic field strength (flux density) and frequency sets power output. If you push the core harder, it can be smaller in size (saves $) but runs hotter due to eddy current losses. There are several different types of steel laminations, some better than others. Another trade off is a smaller core being able to fit the windings that you need.

There's some good info here, the excel spreadsheet calcs help: http://ludens.cl/Electron/trafos/trafos.html

[barry wilkins]

Re: Mains Transformers Power Ratings

Speaking winding coils, anyone seen how a machine toroid winder works, even for a small number of turns? Winding by hand would be a major PITA and have to go with a bobbin + split core (for high freq)/EI system(for low frequency)...

I have a small hand winder ,its actually a grindstone about 30 years old and has a 9 to 1 ratio OK for little coils and the like .I dont wind many coils these days ,but I do need to make an inverter for my caravan lights.
Barry

[barry wilkins]

Re: Mains Transformers Power Ratings

Thank for the info ,I did read quite a good article on the internet,but there is something wrong with the final equation .I need to work on it.
I have been trying to make a new lamp inverter for my caravan as the ones that came with it fail very quickly .I just cant understand how they get away with such a small transformer with so few turns on it . I dont think there any shorted turns in the transformers but the units draw an awful ammont of current !

I have made a unit with a design a saw on the internet that uses a ferrite rod ,this used a single transistor ,but my circuit is push pull not exactly like the Royer design but close.
This is performing very well driving two lamps in series about 25W and drawing around 1/2 Amp at 14volts.
The only draw back it is a bit big (about 3 times).Also the flyback across the transistors is very large it can easily kill the BUX 85 trnasistors I am usuing.
Anyway I have ordered an E133 core on ebay ,when I have some spare time I will try to design a much neater one.
Barry Wilkins

[redwire]

Re: Mains Transformers Power Ratings

Barry, it sounds like you have a saturated-core oscillator for your fluorescent inverter. I find these have a couple transistors and a tiny transformer. Since the transformer is driven to its max. (then) causing the transistors to switch, it really does get the most from the transformer. Frequency also changes based on the load. Lowest-cost circuit but it does mid-efficiency, since the transformer is run hard and the transistors don't switch cleanly.

A bit more here: http://ludens.cl/Electron/Fluolamp/fluolamp.html