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Overvoltage on transformer secondary

Started by Unknown November 28, 2017
On Tue, 28 Nov 2017 23:50:25 GMT, Steve Wilson <no@spam.com> wrote:

>John Larkin <jjlarkin@highland_snip_technology.com> wrote: > >> On Tue, 28 Nov 2017 14:51:06 +0000 (UTC), antispam@math.uni.wroc.pl >> wrote: > >>>I his article about troubleshooting () Bob Pease writes: >>> >>>: If the power line switch was turned off at exactly the wrong time of >>>: the cycle, the flux in the transformer steel core could be >>>: strored at high level. Then, if the line power switch was >>>: reconnected at exactly the wrong time in the cycle, the flux >>>: in the transformer would ontinue to build up until the >>>: transformer saturated and produced a voltage spike of 70 to 90V on >>>: its secondary. > >>>Possibility of saturation is well-known. However, getting voltage >>>spike on secondary due to saturation looks strange: > >>>1) saturation means that high current in primary gives only tiny >>> increase of flux. SEM is proportional to derivative of flux, so SEM >>> is limited. In fact, high current in primary is because SEM is to >>> small to oppose line voltage. >>>2) Ignoring stray inductance SEM on the secondary is transformer >>> constant times SEM on the primary. Ohmic losses mean that >>> SEM on primary is lower than voltage on the primary, SEM on >>> secondary is higher than voltage on secondary. I would expect >>> similar effect from stray inductance. > >>>So I do not see how saturation can lead to overvoltage on secondary. > >> Not on the half-cycle that saturates the core. But maybe on the next >> opposite-sign half cycle, the one that yanks the core out of >> saturation. > >> That will happen when there is a lot of magnetizing (actually >> demagnetizing) current. > >Instead of speculating, the correct procedure would have been to measure >the pulse with a scope, something that Bob P. should have done in the first >place.
Why do you think he never scoped the voltages? He mentions "extensive investigations", not speculation.
> >Guessing at the problem was very poor troubleshooting technique, as well as >increasing the size of the capacitor by some arbitrary amount. Yhat was >very poor engineering, as shown by the residual failure rate.
He didn't say the cap value was increased by "some arbitrary amount." The residual failure rate was "near zero", whatever that means. No failure rate will ever be zero.
> >Another problem was the cascading failure. If the cap failed, it would >destroy the regulator. This could damage the following circuits which >depended on the regulator output voltage. One of the tenets of good >engineering is to prevent cascading failure, something that Bob P. should >have emphasized.
Different people have their own definitions of "good engineering." To me, a failed box is failed, and it doesn't matter much how many parts are failed. The best design minimizes the field falure rate, not the details of a failure. I do try to prevent part failures from starting fires or blowing traces off boards, but if a failed mosfet might take out a gate resistor, I'm not going to make a big effort to prevent that. A lower system failure rate but more parts lost per failure sounds good to me.
> >So in this case, Bob showed the worst possible examples of bad >troublehooting technique and poor engineering. Not a very good result for a >book on troubleshooting technique.
Bob was a pretty good engineer. He just had bad taste in cars. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On 2017-11-28 16:12, Steve Wilson wrote:
> Ubuntu 10.04
Are you kidding? This hasn't received updates for 2.5 years, 4.5 if you're on the desktop version. Do you still use Windows 98 as well?
On Wednesday, 29 November 2017 00:23:57 UTC, John Larkin  wrote:

> Different people have their own definitions of "good engineering." To
highly dependant on what you're designing for. NT
Ant <usenet@antiphase.eu> wrote:

> On 2017-11-28 16:12, Steve Wilson wrote: >> Ubuntu 10.04
> Are you kidding? This hasn't received updates for 2.5 years, 4.5 if > you're on the desktop version.
I don't care. I never use it to scan the web, but it is needed to run with Virtualbox 4.3.36 for legacy software. I have tried later versions but always ran into some incompatibilty problems, so I'll stay with 10.04.
> Do you still use Windows 98 as well?
I did for a long time, then switched to XP. I like it a lot better than Win7. Just to terrorize you a bit further, my main browser is Firefox 12. It has a phenomenal range of add-ons that are invaluable for browsing the web. I can control javascript, CSS, page color, and kill tabbed browsing which is not meeded on XP. I also control the keybindings, use custom toolbar buttons, have cert bypass, and a password editor. None of these are available in later versions. I also depend on the Nir software utilities for many other invaluable functions. Also, I never use critical passwords in FF12. They are all in another VDI that is protected by Sticky password manager. There are a few websites that refuse to use SHA-1, and won't work with FF12. For these, I use Palemoon 26.5.0, but I reaally don't like it. None of the Nir Software works with it, so it is almost useless. So, you say, you have no protection. How do you prevent malware? My response is, you can't, even with the most advanced operating system and anti-virus software available. You need to recognize how vulnerable you are, and create a system where it doesn't matter if you get infected. For me, the system is excellent backups on a platform that is not accessible to malware. That is extremely easy with Virtualbox. The backups are through Ubuntu, not Windows. Since I use RAID0, it is not possible for Windows malware to get to the backups. In Windows, if you get infected, the usual remedy is to pave over and rebuild. This means you have to reinstall all your programs, and configure everything back to the way it was. This also happens if you lose a hard disk drive. It can take a very long time to reinstall and reconfigure, and you will never get everything back as it was before. With Ubuntu, if you lose the operating system, all you have to do is reinstall it, and run a simple bash file to reinstall all your programs. All your config files are in the Home directory, and they will survive the reinstall. In Windows, all I have to do is copy the backup files. This restores the operating system and all the software and config files. It only takes a few minutes for 100GB of files, then I am back to exactly where I was before the failure.
Steve Wilson wrote:

-----------------------

> > Instead of speculating, the correct procedure would have been to measure > the pulse with a scope, something that Bob P. should have done in the first > place. > > Guessing at the problem was very poor troubleshooting technique, as well as > increasing the size of the capacitor by some arbitrary amount. Yhat was > very poor engineering, as shown by the residual failure rate. >
** As I read the account, fitting a 1000uF electro in lieu of 10uF solved the problem completely.
> Another problem was the cascading failure. If the cap failed, it would > destroy the regulator.
** Bob never said the electro failed. When over-voltaged, electros draw large currents immediately. Seems this zener like action did not help, likely because the instant failure voltage of the reg IC was less than the max voltage of the electro.
> This could damage the following circuits which > depended on the regulator output voltage. One of the tenets of good > engineering is to prevent cascading failure, something that Bob P. should > have emphasized. >
** My reading is that the reg IC failed and the 10uF was unharmed.
> So in this case, Bob showed the worst possible examples of bad > troublehooting technique and poor engineering. Not a very good result for a > book on troubleshooting technique. > >
** Bob was looking for a rare event causing spike over-voltages on the secondary at switch on, so one related to inrush surge core saturation sprung to mind. OTOH, it is very unlikely for an iron core supply tranny to have higher voltage at the secondary than its turns ratio dictates - so another thing to look out for is a rare or intermittent cause of PRIMARY overvoltage. Aside from a lightning hit, this is most only likely to happen at the moment the AC switch on the particular unit is opened OR and much worse if a switch or breaker opens that controls the AC supply to MANY units. All the units then have their AC supply terminals connected in parallel with no actual supply in place. Imagine some of there units are inductor ballast fluoro tubes. If, as is normal, the supply switch contacts arc upon opening - a burst of noise with peak voltage up to double the supply peak occurs at the secondary of an *unloaded* transformer. I have tried this on the bench with a 100VA e-core and found that bridging a 10uf film cap across the secondary had little effect. IME, the most likely scenario is due to arcing at switch off, possibly aided and abetted by other items powered from by the same supply switch or breaker. The reg IC fails instantly due to overvoltage and of course this is not discovered until the next time the unit is powered up. Using a 1000 uF electro on the secondary fixes the problem. Other fixes like primary side varistors and X1 caps may help too. .... Phil
On Tue, 28 Nov 2017 17:45:52 -0500, "P E Schoen" <paul@pstech-inc.com>
wrote:

>Waldek wrote in message news:ovjt4q$oth$1@z-news.wcss.wroc.pl... > >> I his article about troubleshooting () Bob Pease writes: > >: If the power line switch was turned off at exactly the wrong time of >: the cycle, the flux in the transformer steel core could be >: strored at high level. Then, if the line power switch was >: reconnected at exactly the wrong time in the cycle, the flux >: in the transformer would ontinue to build up until the >: transformer saturated and produced a voltage spike of 70 to 90V >: on its secondary. > >> Possibility of saturation is well-known. However, getting voltage spike >> on secondary due to saturation looks strange: > >> 1) saturation means that high current in primary gives only tiny > increase of flux. SEM is proportional to derivative of flux, > so SEM is limited. In fact, high current in primary is because > SEM is to small to oppose line voltage. >> 2) Ignoring stray inductance SEM on the secondary is transformer > constant times SEM on the primary. Ohmic losses mean that > SEM on primary is lower than voltage on the primary, SEM on > secondary is higher than voltage on secondary. I would expect > similar effect from stray inductance. > >> So I do not see how saturation can lead to overvoltage on secondary. I >> can imagine getting overvoltage on secondary for different reasons. Pease >> wrote that overwoltage was releated to having very small filtering >> capacitor and that bigger capacitorsolved the problem. AFAIC bigger >> capacitor would solve problem regardless of reason... Has anybody >> experienced such overvoltage? Can you explain why saturation could lead to >> overvoltage on secondary? > >I have experienced the effects of remanent magnetism and DC offset in >circuit breaker test sets, which use large step-down transformers to drive >high currents (up to 100,000 amps) into circuit breakers. The load as seen >by the mains power (usually 480 VAC) is mostly inductive, so the output is >controlled by an SCR switch that fires close to the waveform peak, resulting >in minimal DC offset. > >If the output is initiated at a zero crossing, the first half-cycle may have >a peak as much as twice normal, and the waveform then decays until it is >symmetrical about the zero voltage level. > >When an odd number of half-cycles is applied to the transformer, there is a >net DC component that creates remanent magnetism in the core. If the next >application of voltage is in the same phase, the core will saturate and >cause a high current spike in the primary. Since under such circumstances, >the core is already saturated, the output voltage on the secondary would be >expected to be lower than usual, except perhaps for a very brief spike. >However, if the phase is opposite to the magnetization, It might be possible >for the output to see a higher-than-normal voltage, especially if the >voltage is applied at or near a zero crossing to produce DC offset. > >At least, that's how I understand it and what I have observed. > >Paul
Yep, in high power transformer based supplies you can get core saturation. One way around it is to remember which cycle (pos, neg) was last applied. It has been done. Cheers
On Tue, 28 Nov 2017 23:31:40 -0500, Martin Riddle
<martin_ridd@verizon.net> wrote:

>On Tue, 28 Nov 2017 17:45:52 -0500, "P E Schoen" <paul@pstech-inc.com> >wrote: > >>Waldek wrote in message news:ovjt4q$oth$1@z-news.wcss.wroc.pl... >> >>> I his article about troubleshooting () Bob Pease writes: >> >>: If the power line switch was turned off at exactly the wrong time of >>: the cycle, the flux in the transformer steel core could be >>: strored at high level. Then, if the line power switch was >>: reconnected at exactly the wrong time in the cycle, the flux >>: in the transformer would ontinue to build up until the >>: transformer saturated and produced a voltage spike of 70 to 90V >>: on its secondary. >> >>> Possibility of saturation is well-known. However, getting voltage spike >>> on secondary due to saturation looks strange: >> >>> 1) saturation means that high current in primary gives only tiny >> increase of flux. SEM is proportional to derivative of flux, >> so SEM is limited. In fact, high current in primary is because >> SEM is to small to oppose line voltage. >>> 2) Ignoring stray inductance SEM on the secondary is transformer >> constant times SEM on the primary. Ohmic losses mean that >> SEM on primary is lower than voltage on the primary, SEM on >> secondary is higher than voltage on secondary. I would expect >> similar effect from stray inductance. >> >>> So I do not see how saturation can lead to overvoltage on secondary. I >>> can imagine getting overvoltage on secondary for different reasons. Pease >>> wrote that overwoltage was releated to having very small filtering >>> capacitor and that bigger capacitorsolved the problem. AFAIC bigger >>> capacitor would solve problem regardless of reason... Has anybody >>> experienced such overvoltage? Can you explain why saturation could lead to >>> overvoltage on secondary? >> >>I have experienced the effects of remanent magnetism and DC offset in >>circuit breaker test sets, which use large step-down transformers to drive >>high currents (up to 100,000 amps) into circuit breakers. The load as seen >>by the mains power (usually 480 VAC) is mostly inductive, so the output is >>controlled by an SCR switch that fires close to the waveform peak, resulting >>in minimal DC offset. >> >>If the output is initiated at a zero crossing, the first half-cycle may have >>a peak as much as twice normal, and the waveform then decays until it is >>symmetrical about the zero voltage level. >> >>When an odd number of half-cycles is applied to the transformer, there is a >>net DC component that creates remanent magnetism in the core. If the next >>application of voltage is in the same phase, the core will saturate and >>cause a high current spike in the primary. Since under such circumstances, >>the core is already saturated, the output voltage on the secondary would be >>expected to be lower than usual, except perhaps for a very brief spike. >>However, if the phase is opposite to the magnetization, It might be possible >>for the output to see a higher-than-normal voltage, especially if the >>voltage is applied at or near a zero crossing to produce DC offset. >> >>At least, that's how I understand it and what I have observed. >> >>Paul > >Yep, in high power transformer based supplies you can get core >saturation. One way around it is to remember which cycle (pos, neg) >was last applied. It has been done. > >Cheers
Forgot to mention 'Core walking' is a somewhat milder case. Capacitive coupling can alieviate that. Cheers
"Martin Riddle"  wrote in message 
news:s2es1dt2bqsu2ngljgfn04j0inbahdaekv@4ax.com...

>> Yep, in high power transformer based supplies you can get core >> saturation. One way around it is to remember which cycle (pos, neg) was >> last applied. It has been done.
> Forgot to mention 'Core walking' is a somewhat milder case. Capacitive > coupling can alieviate that.
I think one company's circuit breaker test set incorporated a gradually changing phase firing on the controller to demagnetize the core. But IIRC it had to be done after the breaker tripped. The test set is designed to apply an equal number of positive and negative half-waves, at least when applying pulses of six cycles or less to determine instantaneous trip point. But when it tripped, there could be DC offset because it might trip in 1.5 cycles, or even less than 1/2 cycle. Paul
On 28/11/2017 23:50, Steve Wilson wrote:
> Instead of speculating, the correct procedure would have been to measure > the pulse with a scope, something that Bob P. should have done in the first > place. >
Why do you say it was speculation? Seems to me just as likely that Pease did scope and that was how they found the magnitude and timing of the transient. Nothing in the writing says they speculated. piglet
On 28/11/2017 14:51, antispam@math.uni.wroc.pl wrote:
> So I do not see how saturation can lead to overvoltage on > secondary. I can imagine getting overvoltage on secondary > for different reasons. Pease wrote that overwoltage was releated > to having very small filtering capacitor and that bigger capacitor > solved the problem. AFAIC bigger capacitor would solve problem > regardless of reason... Has anybody experienced such overvoltage? > Can you explain why saturation could lead to overvoltage on > secondary? >
Saturation could have occurred rapidly and caused large dI/dT ? piglet