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Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
but an unloaded transformer, with good coupling, and operating at not too high frequency will transform voltage and current as determined by the turns ratio.
If you look ANSI/IEEE C57.13-1978 there is are zone boundrys for limits of accuracy where the ratio correction factor is applied (e.g. class 0.6) 0.994 to 1.006.
This is how manufacturers of PTs calibrate their units.
The statement is "the transformer characteristics shall lie with the limits of the parallelogram for all voltages between 90 and 100 percent of rated voltage"
To bring is down to earth the question could be, at any level of flux in a transformer core, can the absolute turns ratio translate into a voltage ratio which will hold constant as excitation levels change.
Because it is AC the phase angle (in the standard minutes of a degree are shown) changes because the inductance in a big core is voltage dependent).
It boils down to the international standard for trade in electrical energy.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
If you look ANSI/IEEE C57.13-1978 there is are zone boundrys for limits of accuracy where the ratio correction factor is applied (e.g. class 0.6) 0.994 to 1.006.
...
It boils down to the internation standard for trade in electrical energy.
... which isn't very relevant to the sort of hacking most amateurs want to do, especially when they ask questions on here.
If your transformer has enough turns, and you accept the losses as small, irrelevant, or something to learn about later, then the voltage ratio between the windings === the current ratio between the windings === the turns ratio between the windings. Period.
If you want to know how many turns is "enough", or compute and allow for the copper and or core losses, or allow for coupling and core inductance modifications, then there's a whole lot more study to do. But most of this stuff is second order, coupling =1, losses =0, inductance = infinity gets you most of the way there, the only thing that really bites is the "enough" turns.
How much extra (voltage, not energy) a flyback gives you has a whole new bunch of parameters.
Registered Member #3933
Joined: Mon Jun 06 2011, 04:45PM
Location:
Posts: 4
These are gorgeous answers, but can you just walk a dummy through the turns ratio please for 7.5v pri, 2000v secondary. What a fascinating thing this is
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The turns ratio for 7.5v primary, 2000 v secondary is
First approximation 2000 / 7.5 = 266.67
Second approximation 266.67 + a few more for losses, which will vary with load, core size and plenty of other factors
For a one turn primary, use 267 + a few more secondary turns. For a 10 turn primary, use 2667 + a few more secondary turns.
"A few" turns out to be in the 1% to 10% range, depending on the core size. Small wall wart cores go nearer 10% (or more) at rated load. Big utility transformers, as well as precision current or potential transformers which are inteneded to be used at a very low load, are sub 1%.
So do you use a 1 turn or 10 turn primary? Turns ratio is not enough to wind a transformer. In order to know how many turns is "enough" on the primary, you need to know the core area, operating frequency, and max flux you want to operate at. There are fairly simple ways to measure how many turns you need, if you have a few of a scope, variac, lamp, sig-gen, DVM.
Hint, with a typical MOT core at mains frequency, it's about 1 volt per turn maximum, at least that's what it will do with fan cooling, for most uses it's worth throwing on a few more turns and running at less than 1v per turn.
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Transformer ratios
