Hi, I'll try and explain the current rating the way I understand it using mostly non-electrical engineering terms.
The power rating of a transformer is based on the core size and material. Core size is measured using the cross sectional area among other things but we'll ignore that for the moment. Take a look in any transformer catalog and you'll see that the VA rating of the transformer regardless of output or input voltages remains the same across transformer core sizes. The reason for this is quite simple. If you try to increase the current for a given transformer past a certain point the core material becomes saturated with the magnetic field and the 'impedance' to the input line voltage drops to a very low value, allowing an inrush of current which usually burns out the windings. Back to the input impedance for a moment. The input/output wire size, the number of turns etc, are all set up to provide an impedance to the power line that prevents the transformer from overheating. Let's say for example, the transformer at 60Hz and 240VAC draws 1A. That means the impedance (not resistance) is 240 Ohms and the transformer is a 240VA unit. Now put that transformer on a European grid at 50Hz. At this point the impedance of the transformer will be lower because the frequency is lower hence the transformer will draw more current and as a result run warmer exceeding the input VA rating. In fact, reduce the frequency to 0 (DC) and 120V in will toast the primary in very short order. The impedance is dependant on the wire size, number of windings on both the primary and the secondary and the load on the secondary. Short the secondary and the impute impedance changes again (lower) and once more you get overheating on the primary and secondary or even core saturation. I believe that's why wall warts fail by toasting their small input fuse inside the primary windings when the output is shorted. I have a few 12V Halogen lamp transformers where the lamp suddenly decreased in resistance as the filament failed. Blew the input fuse making the transformer useless. A temperature resetable fuse would have opened until the bulb was replaced and then stayed operational. When we look at center-tapped verses full wave systems we find that in fact once again the VA rating determines the output power of the unit and determines how hot the transformer gets. The primary and core are sized for the VA rating. The secondary wire size for the current rating. The ratio between the primary and secondary determine the output voltage. A center tapped transformer is really a transformer with two secondaries each rated at a particular amperage and voltage. If each winding can deliver 1A then in parallel you get two amps at the rated voltage. If the windings are put in series then you get 1A but at twice the voltage. The difference is the center tap connecting to the two windings is buried inside the core somewhere rather than bringing out two wires that are then connected externally. You see the same thing on transformers destined for 120/240VAC. The input side windings are put in parallel for 120VAC or in series for 240VAC. In series, the input impedance of each winding is added just like resistors so if each winding draws 1A at 120VAC then they'll still draw 1A at 240VAC for 240VA. For the 120VAC circuit, they'll be put in parallel and each draw 1A at 120VAC. That's a total of 2A at 120VAC for both windings and again 240VA. Hope that clears things up. John Dammeyer Wireless CAN with the CANRF module now available. http://www.autoartisans.com/products Automation Artisans Inc. Ph. 1 250 544 4950 Addresses: FAQ: http://www.ktmarketing.com/faq.html FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/ Post Messages: [EMAIL PROTECTED] Subscribe: [EMAIL PROTECTED] Unsubscribe: [EMAIL PROTECTED] List owner: [EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED] Moderator: [EMAIL PROTECTED] [EMAIL PROTECTED] [EMAIL PROTECTED] [Moderators] URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO OFF Topic POSTS: General Machining If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble. http://www.metalworking.com/news_servers.html http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list. NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........ bill List Mom List Owner Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
