DC Frequency? - Page 1
Forum section: electronics / general

author: orbiter
author_rank: Frequent Contributor
timestamp: April 01, 2013, 11:12:35 am
content: Hi fellas..Saw a transformer today that had specs stating that it was a 125KV - 15kHz - 'DC' transformer..I am trying to understand the DC frequency part. Am I correct In thinking that a DC frequency is the amount of pulses per second in the DC flow, as I can't see otherwise how you would see this frequency? Also how would you measure such a Frequency please?Thank you.

author: ErikTheNorwegian
author_rank: Frequent Contributor
timestamp: April 01, 2013, 06:38:33 pm
content: Found this..http://www.zena.net/htdocs/FAQ/whatisHFDC.shtmlI hope that is the info you need..ErikIn the ZENA mobile welding system, the term high frequency DC (HFDC) has a very special meaning:With the ZENA welding system, the term high frequency DC is used to describe a special condition where the direct current flow in the welding circuit (arc) is caused to vary, very slightly, in a special way, under electronic control, with the tiny variations happening at a very high speed, so as to produce greatly improved welding characteristics when compared to a conventional welding current generating system of the type used in standard engine driven welders, or in welders which use automotive alternators as welding current generators (which often refer to high frequency DC in their specifications).In the ZENA welding system the high frequency current transitions / variations occur at least 18,000 times per second, and will typically exceed 21,000 times per second.HFDC current provides a three very substantial benefits in the ZENA welders:HFDC creates electrical agitation of the welding puddle (you will hear a high pitched whistling/whining tone coming from within the puddle while you are welding). This puddle agitation process:works to facilitate the removal of impurities from the weld puddle, and toimprove weld penetration by moving hot metal from the center of the puddle towards the edge of the puddle and cooler metal toward the center, allowing the arc to melt additional metal in the areas surrounding the puddle -- for a given current setting, and tosignificantly improve over all weld quality by insuring that the metal which makes up the bead and the metal surrounding the bead are more homogeneously mixed with each other and that heat within the bead is more evenly distributed.HFDC also improves the ability of the welding cable to transmit DC welding power from the generator to your electrode -- reducing the effects of welding cable DC resistance and internal impedance -- allowing us, of a given welder, to use welding cable with a smaller cross sectional diameter than would other be necessary for proper welder performanceHFDC also works to increase the stability of the welding arc and allows an operator to achieve excellent welding results with both a shorter than normal welding arc (down to 0", in many cases) OR a longer than normal welding arc (up to 2X welding rod diameter). Inexperienced welders find the welder to be much easier to use and with much better results than a conventional welder can provide -- pros find it much easier to create x-ray quality welds and, at the same time, find that top quality results take less effort.

author: orbiter
author_rank: Frequent Contributor
timestamp: April 01, 2013, 06:56:15 pm
content: I did see that article thank you.I'm still only under the assumption that the DC frequency is just separate pulses in a DC signal. S I guess it still leaves me wonderinghow this is measured, and now I'm wondering where these pulses originate from in the first placeIIRC the transformer I saw these specs on was an X-Ray transformer.

author: IanB
author_rank: Super Contributor
timestamp: April 01, 2013, 07:05:27 pm
content: Quote from: orbiter on April 01, 2013, 11:12:35 amWhen I was a boy I had a 12 V DC "transformer" for my model railway set.It was of course a normal AC transformer followed by a rectifier. On the other hand, it did in fact transform 240 V AC into 12 V DC, so it could certainly be called a transformer in a general sense.As I understand it, X-ray tubes require a DC supply. So you could be looking at a box that uses AC internally but produces a DC output.

author: orbiter
author_rank: Frequent Contributor
timestamp: April 01, 2013, 07:35:44 pm
content: Here are the transformers specs... Primary input is indeed AC and secondary 150KV, however it's the 15KHz DC that I'm confused about.Is this DC voltage internal somewhere at a 15KHz frequency? It's the signal itself I'm trying to figure outin my head, because if it's DC.. How does it have a frequency, and assuming this is just a pulsing DC frequency how is it measuredso that you can see it?

author: IanB
author_rank: Super Contributor
timestamp: April 01, 2013, 07:41:51 pm
content: I'd guess the output is pulsed in some way? Maybe it uses a flyback transformer internally? It would be similar to the way the HT voltage in a car ignition system is "DC", and yet it is pulsed rather than continuous.

author: orbiter
author_rank: Frequent Contributor
timestamp: April 01, 2013, 07:52:32 pm
content: Ah yes... That's a very good point.So.. The pulses would actually make up the DC 'frequency' even though the voltage was a flat line?Thanks Ian.. I'll be able to sleep now

author: codeboy2k
author_rank: Super Contributor
timestamp: April 03, 2013, 10:52:12 am
content: Maybe it's simply an unfiltered PWM signal. This way, the average DC voltage can still be adjusted with the controls, but there's a 15-20kHz pwm clock.

author: G7PSK
author_rank: Super Contributor
timestamp: April 03, 2013, 11:15:44 am
content: The Zena welding system that was quoted is a square wave or PWM system. Most tig welders these days use a AC with independent variation of the positive and negative going pulses, as this aids the cleaning and heating action of the arc.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 03, 2013, 11:45:05 am
content: A square wave is used because the sharp rising/falling edges prevent the arc from breaking.