24.18.227.0
In Reply to: RE: Heater Current and Transformer Ratings posted by dbishopbliss on May 04, 2009 at 17:01:40
I'm taking this as an opportunity to bloviate on transformer design. If you want to skip the long part, yes you can meet the rating exactly with perfect safety, but should not exceed them.
Transformers are rated for the maximum current they can sustain continuously without compromising their lifespan and safety. There is a safety margin built-in for things like fluctuating power-line voltage or a reasonable amount of nearby hot components (tubes, resistors, etc.) and there is an assumption that reasonable cooling airflow is available, i.e. if in a box there are vent holes.
To put it crudely, if you don't exceed the ratings and something goes wrong, you can sue the manufacturer.
Since transformers die by excess heat, there is a balancing act done by power transformer designers, among core losses and copper losses. Core losses increase as the magnetic flux level increases, and copper losses are just the resistive heating of the copper wire. More expensive laminations such as M6 can take more flux, and more expensive bobbins, wire, etc. can take more heat, so there's an economic balancing act going on as well.
High-temperature materials are affordable these days - unlike the old days of 50 years ago. So modern, economically efficient designs will be smaller and run hotter than classic designs.
OK, that sets the stage. Now many DIYers prefer to keep the transformer temperature down. There are legitimate reasons like avoiding burns and keeping other components cool, or from a mistaken belief based on the lower safe temperatures of vintage parts. The easy way to do that is to reduce the current draw. This will not affect the core losses, only the copper losses, so it is no longer an optimal compromise - but it's the one that is easy to implement with commercially available components.
For the Bottlehead custom transformers, I have specified much lower flux levels than usual, which minimizes the external magnetic field and any transformer vibration. I also have kept the copper losses as much as possible in balance with the reduced flux, so as a net effect they do run cooler than modern designs run at their specified limits. We have retained the industry-standard high temperature parts so there is a temperature safety margin from both reduced core and copper losses.
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