[AMPS] Transformer Current ratings

[Ian White, G3SEK]

Jerry Flanders wrote:
>
>For "ordinary" old power transformers:
>
>Is there a rule of thumb to help estimate transformer KVA capacity from 
>the iron (or total) weight? If not, why?
>
>Prior to hypersil, weren't most transformers made from similar iron in 
>similar configuration? If so, wouldn't capacity be directly related to 
>core weight?

Only indirectly. Weight is a rough indication of core cross-section, 
which is important but still only one factor in the transformer's 
overall performance.

The rule of thumb was "Size of a six-pack = 500 watts, size of a 12-pack 
= 1500 watts, too heavy to lift = 2500+ watts!" This of course is very 
variable, and older transformers tend to be bigger and heavier for their 
power rating than modern ones.

However, I have been bitten several times in the past by building a 
whole PSU - or even a whole amp - around a transformer that passed the 
rule-of-thumb weight test but was quite unsuitable.

A *much* better way is to measure the winding voltages and resistances 
and look at the electrical performance. Rich gives some useful 
electrical rules of thumb, but before you commit to building a whole 
power supply around a particular transformer, you'd do well to run it 
through the 'PSU Designer' software.

'PSU Designer' is Windows freeware from an audio firm called Duncan 
Amplification and it's an easy way to put some real 21st-century 
accuracy into PSU design. Duncan's amps use relatively small PSUs by our 
standards, but the software  works just fine for big PSUs too.

I wrote a short article on it in my monthly magazine column, and there 
are links from: http://www.ifwtech.com/g3sek/in-prac/   Slide down to 
August 2001, where there are links to the download site and some extra 
files to download.

The extra files relevant to AMPS are the circuit files '3kV Bridge.psu' 
and '3kV Doubler.psu', and you'll also need the expanded RECTIFIERS.TXT 
file which contains a high-voltage rectifier.

The two .psu files are a basis for modeling normal high voltage 
supplies. Double-click on any item in the schematic to alter its 
properties, and then click Simulate to get a simulation over a chosen 
time period. You can change the load current by altering the current 
sink connected to the output.

Then click various items on the form below to see graphs over time. The 
most interesting item is probably V(C1) - the output voltage.

As you get further into using PSU Designer, check the Help files for 
more information.

A note about transformer modeling: PSU Designer only lets you model the 
secondary resistance of the transformer. You can include the effects of 
(primary + mains supply) resistance by multiplying those values by the 
square of the step-up turns ratio and adding the answer to the secondary 
resistance. There's more information if you double-click the transformer 
symbol, and then choose Help.

This really is a great program! It takes some practice, but I wouldn't 
ever build another PSU without running the components through PSU 
Designer.

Coming back to Jerry's original question about current ratings, if the 
winding resistances are low enough to give good voltage regulation at 
the currents you want, you pretty much have your answer about current 
ratings too.

-- 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)
                            http://www.ifwtech.com/g3sek

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