Scenario: a 300 watt toroidal transformer delivering 50-0-50 volts. If there was a break in the output paths to load. So say a current of 2 amps was interrupted, would a high DC voltage , much higher than mains level, be generated momentarily across the primary ?
N Cook schrieb:
Yes. Due to the leakage inductance of a transformer a suddenly interrupted current could cause a short voltage _spike_ but normally the spike would be damped by the low impedance of the main voltage net or be absorbed through the filter network normally found on the primary side.
- Udo
much
With no filter caps etc in place, straight off the mains, would you care to put a figure on the peak voltage of the spike generated in the primary, appearing at the primary , rather than down the supply feed ? The secondary interrruption is abrupt due to a faulty broken connection that sometinmes makes sometimes breaks.
The spike will be about big enough to stop all but the magnetising current from flowing in the primary.
but you won't see much voltage spike on the mains.
Bye. Jasen
much
No. Interruption of the secondary current would not affect the primary voltage. However, such an interruption could momentarily upset the current balance depending on where in the cycle it occurred.
What you are driving at is the stored energy in the transformer sufficient to cause a high voltage spike when the secondary current is interrupted? As Phil points out above, that energy only comes from the magnetizing current in the primary not from the "transformed" current, primary to secondary. The magnetizing current is small and the stored energy low.
The magnetizing current is not interrupted in your example, therefore no collapse of field occurs and no voltage spike results. The flux in the transformer remains the same.
The transformed current, primary to secondary does not affect the flux in a transformer. The secondary amp-turns exactly cancels the primary amp-turns. The flux in the core comes ONLY from the magnetizing current and nowhere else.
"N Cook" schrieb im Newsbeitrag news:g83b44$mmu$ snipped-for-privacy@inews.gazeta.pl...
Hello,
You should be concerned about a high voltage spike and ringing at the secondary side when the current will be interupted, because of the stray inductance. This high voltage will stress/damage devices directly connected on the secondary side. You will have no problem on the primary side as others already explained.
Helmut
Yes.
I don't want to get into it how ever, if the primary side was being supplied via a thyristor type system for example, the primary would see high voltage at times when the thyristor was in the off state if proper use of caps to protect them aren't in place.
In many designs where thyristors are in play, a RC network is coupled over the thyristor device. I guess the same could hold true with non thyristor devices like MOS/IGBT/etc...
much
I find it a bit too much of a coincidence that there are weak contacts to 2 fuses on the secondary of such a transformer. Blackened sputter marks on fuses and fuseholers and then the primary side of the transformer ends up with shorted turns and blows the mains fuse. The amp , which this transformer was in, had not been abused , no overheating , being dropped etc.
I'm still in the process of unwinding the transformer to explore, 3 of the 4 windings unwound via shuttle/bobbin so far.
In a 240V UK situation if that is relevant
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
Not only is the primary still connected to a low impedance source, the other half of the secondary is still connected to a load. This gives paths for the field energy to go to. There will be a blip, just not very large.
was a
, ?to
that
I unwound the transformer yesterday and it is bad winding practise. To save on shuttle through passes the primary is wound as 2 wires together. Then 2 ends brought together and enclosed and the other 2 become live and neutral. About half way along the wires , not at a bend , was the neat little "weld" point between the 2 runs, and a very small smoke trail. Presumably where there was most potential difference caried by these 2 wires. Asking too much of 2 thicknesses of lacquer to resist. Maybe problem seen by the secondary was the final straw , but primarily bad primary winding practise. Could have occured due to mains spike at any time.
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.