LF choke schematic question. - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 05, 2014, 08:16:18 am
content: I am continuing to trouble shoot my single phase TIG welder and have a question regarding a schematic in a description file. The choke is shown withe a "Z" line joining the core. What does this mean please? The file should be attached as a .jpg image I also attach an image of the original choke. It has been rewound since then due to an internal short. Thanks.

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2014, 10:52:34 am
content: The Z line indicates that both windings are on a common core.Glad to see you back, now all we have to do is fix it!

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 05, 2014, 10:58:19 am
content: Work has intervened, not given up yet, (having paid 500 UK Sterling for the choke to be rewound I cannot afford to give up on it.....).I was loking to see if there was any way the new choke could have been installed incorrectly, but I believe it is all well.I will post more info as and when I can get back to it! Cheers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2014, 11:05:03 am
content: Not only that, but it indicates the phasing of the windings.If you imagine the one coil moved so it sits adjacent to the other, with the common core being a straightened line, the windings read plus-to-minus, plus-to-minus. So, it should be a common mode choke.At least, I should imagine that's what they mean. Dem crazy euro schems, who knows...Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 06, 2014, 10:58:50 am
content: Thanks for the further replies. I Googled choke and phase and got sod all info. Which makes me think a series connected choke cannot change phase, and only a parallel connected transformer can? Or have I got that totally wrong?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2014, 12:19:20 pm
content: Try transformer phasing, or winding directions. Usually indicated with a dot.A choke with more than two terminals is always a transformer of some sort, but the practical description may still be a choke, depending on which effect is more important in the end use, the inductance or the transformation.Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 06, 2014, 12:30:53 pm
content: OK, got you, thanks. An acquaintance has just e-mailed his take on this and I feel that I now have the more complete picture in a form I find easier to follow than the description from the mnaufacturer.:Yes, the two chokes are in series with the power flow from the mains transformer, however they are also "cross-coupled" to one another because they share a common magnetic core - so L1a and L1b share a mutual transformer action. Energy in one choke can transfer to the other choke!My understanding of how it works is this: on +ve half-cycles of the mains, current flows through the upper choke L1a and energy is stored in the core, particularly when the upper thyristor turns off to control the power delivery and the current flow is altered abruptly; on -ve half-cycles that energy stored in the magnetic field is then coupled via the common core to L1b where it contributes to the power deliver via the "lower" thyristor. It's a way of smoothing the energy demand by storing energy during that part of a half-cycle when it's not needed by the load, and delivering it when needed during the other half cycle.Obviously the phasing of the two chokes is important so that the energy stored during one half-cycle is released in the correct sense during the opposite half-cycle.The two chokes are acting mutually as a transformer even though individually they are in series with the main current flow - one winding on that "transformer" is 1/7(orand the other winding is 2/8(or 7). To check the relative phase of that cross-coupled energy delivery you need to apply ac across 1/7(orand see what you get out on 2/8(or 7); I believe they should be anti-phase.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2014, 11:36:54 pm
content: Another way to think of it is this:Since they are common mode, you can merge them into a single inductor on whichever branch shares that current. So, it's equivalent to a single choke on the transformer CT, or the DC output, or... In which case, it's simply a choke input filter with SCR rectifiers instead of regular diodes.The difference, I think, is the transformer's output voltage riding on top of each winding, which changes when the SCRs can be switched on and off. Hence the waveforms being as they are. I'd have to look at it a little while to be more precise but I think this is the jist of it....No, actually that doesn't make sense. Because suppose you transpose the SCRs and windings, but leave them separate otherwise. Now the two windings share a common node, the output. But they have the same voltage, and therefore can be merged into one. So it is no different than a single filter choke.Give or take leakage inductance, which it can be easily seen amounts to a choke in series with each SCR. Which is a common way to limit dI/dt to keep them happy (don't forget the damping resistors or other snubbers to absorb the inductive energy on turn-off!).Tim