Mains Transients from vacuum cleaners, showers, tumble dryers etc

[coppercone2]

lol I would just call it a clothing dryer. I dunno a washing machine tumbles too but you don't call it a tumble washing machine?

[ebastler]

lol I would just call it a clothing dryer. I dunno a washing machine tumbles too but you don't call it a tumble washing machine?

Just curious -- which part of the US are you from?
I have come across so many pieces of clothing which say "do not tumble dry" on their care labels that I have to conclude it's a very common term.

[IDEngineer]

"Tumble dryer" is the term of art in the appliance industry. And yes, that's why the labels on clothing refer to "Do (or "Do not") tumble dry" with some sort of heat setting recommendation (usually low or medium).

The alternatives are line drying and flat drying, neither of which requires a machine, so it does seem a bit redundant to say "tumble" dryer when referring to the appliance. But that's what they call them. 

[Siwastaja]

In my son's racing drone community (I'm talking international here, so folks from everywhere), there's a persistent rumor that hanging a bit fat electrolytic cap across the LiPo battery "kills noise" and suppresses transients.   Meanwhile, my son installs an actual Littelfuse tranzorb, plus small ceramic caps at the power inputs to each PCB on his aircraft. Guess which approach has proven the most reliable?

Folks also believe that those big fat electrolytics provide makeup power under short-term heavy load conditions (such as extreme acceleration). These aircraft can burst 150+ amps from a LiPo, and both he and I have tried to explain to people that the ESR of the battery is WAY lower than the ESR of the electrolytic cap... but do they listen? AND run the numbers, just how much actual energy is available from that electrolytic as compared to the LiPo even if the ESR wasn't dissimilar?

It could further help if you explained them why the electrolytic capacitors are used, then, in addition to just pointing out what they are not used for.

You know, because the "persistent rumor" of electrolytic caps being used to "kill noise" and "suppress transients" is quite sound and well founded, even if not fully relevant in your specific case. Although, the "noise" referred to is relatively low-frequency, but can be very high-energy.

The electrolytic caps are there to dampen any parasitic LC ringing which would happen with the extremely low-ESR ceramic capacitors oscillating with the input leads; without the elcaps, the DC bus can peak up to 2x the input voltage, necessitating oversizing the Vds spec of the switching MOSFETs, and building a nice EMI transmitter.

For such high power inverters, you need quite a bit of very low-L, low-R ceramic bypassing, and then to dampen it, you need higher ESR capacitance in parallel, providing multiple times (preferably at least 3-5x) the capacitance of the low-ESR ceramics. This may be impractically expensive to provide using additional ceramic capacitors and separate series resistors, so the cheap, ubiquitous electrolytic capacitors to the rescue! Of course, the modules should already take care of all this. Probably adding the external elcap isn't helping, because there probably isn't a problem to begin with.

Although, in an RC drone setting, the battery lead lengths and thus inductance can be minimized. It would still need to be analyzed properly.

In the end, adding a large elcap seldom is a bad idea if you just need a generic "rule of thumb". If anything, it tends to stabilize things, by having the ESR. It very rarely causes any issues; unlike adding a "better" big ceramic capacitor on the inputs, which can be, and often is, a total disaster for at least two completely different reasons (left as homework for the reader to figure them out ).

[ocset]

Hi

Thats good to hear about the electrolytic situation etc, thanks.

Do you agree that for mains transient protection only, the only bit of the damping/transient circuitry on the 28th page of the below datasheet that is  needed is the part as shown in the attached?
Ie, most of that protection circuit on the  28th page is for damping when a triac dimmer is used….the damping resistor is not needed for mains transient protection. Do you agree?
LYTswitch 4 datasheet
https://led-driver.power.com/sites/default/files/product-docs/lytswitch-4_family_datasheet.pdf?download=1

LTspice sim of transient protector also attached

[IDEngineer]

In the end, adding a large elcap seldom is a bad idea if you just need a generic "rule of thumb". If anything, it tends to stabilize things, by having the ESR.

Except that it's paralleled by the LiPo's own far lower ESR, negating any benefit.

It very rarely causes any issues

The primary problem, in addition to the simple matter of radial lead devices hanging off the sides of things without proper support, is that the folks habitually underspec the voltages. Example: ~30V electrolytics on a LiPo that charges to 25.2V baseline (and they often HV them to 26.1V). Now, abruptly stop those motors by running into something such that their field collapse dumps right across the cap, and that cap is seeing voltages WELL in excess of its rating. And then folks come back from the flight line saying "I blew another cap". Sheesh, ya think?!? Sometimes it's mechanical failure due to relying solely on the radial leads, sometimes it's literally a blown cap from the field collapse. But still they install them.

It's approaching the level of snake oil in the audiophile industry. There's now a "product" being sold called a "Cap Cap", which is nothing more than a small PCB with two traces on it. You're supposed to solder your electrolytic to the PCB, then solder wires to the PCB. Note this makes absolutely zero electrical difference, but folks "feel" better not soldering directly to the cap leads. So now the total mass dangling off the side has INcreased - but I guess it's better because the strain is borne by wires instead of the component leads! 

[coppercone2]

I never read a clothing label other then the percentage of materials. You might be right.

[Siwastaja]

In the end, adding a large elcap seldom is a bad idea if you just need a generic "rule of thumb". If anything, it tends to stabilize things, by having the ESR.

Except that it's paralleled by the LiPo's own far lower ESR, negating any benefit.

Oh, so you really don't know this! Not all is lost, though: you can always learn. You aren't alone, either - recently we had this, which was a great learning experience for the OP:
https://www.eevblog.com/forum/projects/the-tlv760xx-regulators-are-a-nice-piece-of-crap/


Please reread my explanation; it's about the inductance of the li-ion battery and the related wiring, and about damping the oscillation at the inverter, not at the battery. Having low ESR at both sides actually makes it worse - so the hi-ESR capacitors (crappy electrolytics, for example) are exactly needed because of the low-ESR battery and the low-ESR ceramic capacitors! The location of the capacitor matters - it's not paralleled by the li-ion's ESR, there's an inductance inbetween!

This might not be an issue at all with a local battery with extremely short, thick wires routed properly, but anyway, you should understand the mechanism. This is a classical trap for the young players, and you'll find a lot of appnotes about this.

The primary problem, in addition to the simple matter of radial lead devices hanging off the sides of things without proper support, is that the folks habitually underspec the voltages. Example: ~30V electrolytics on a LiPo that charges to 25.2V baseline (and they often HV them to 26.1V). Now, abruptly stop those motors by running into something such that their field collapse dumps right across the cap, and that cap is seeing voltages WELL in excess of its rating.

In a properly designed inverter, there should be, IMO, enough DC bus capacitance to suck the energy stored in the motor inductance without exceeding ratings. A TVS diode might not be bad idea, as well, but they are fairly inaccurate, preventing optimized design, so an active crowbar circuit might be an attractive alternative. By the way, 30V electrolytics practically do not exist. The next size up is 35V, which is just fine for a 6s li-ion, given that there is enough capacitance to handle the motor inductance discharge. You still need active circuitry (such as the microcontroller reading the DC bus voltage) to freewheel the motor to prevent regeneration, but this can be fairly slow (even up to ~1ms) if you have enough capacitance on the bus. To me, it sounds like these inverter modules have design issues.

It's approaching the level of snake oil in the audiophile industry. There's now a "product" being sold called a "Cap Cap", which is nothing more than a small PCB with two traces on it. You're supposed to solder your electrolytic to the PCB, then solder wires to the PCB. Note this makes absolutely zero electrical difference, but folks "feel" better not soldering directly to the cap leads. So now the total mass dangling off the side has INcreased - but I guess it's better because the strain is borne by wires instead of the component leads! 

You are probably completely right about the bullshit level of this market. And, RC folks are notorious for not understanding what they are doing.

Actually understanding the design would, however, help you explain others why they are seeing the same electrolytic capacitors in properly designed motor drives, and why retrofitting them has probably, in some cases, actually helped.

This appnote may be helpful to describe the phenomenon in question:
https://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdf
(Table 3 especially shows how well a simple, cheap elcap performs! It outperforms the ceramic C + R combo because the ceramic RC snubber has too little capacitance, only equaling the low-ESR C; you need to have more.)

I know it is more difficult to find an appnote specifically discussing battery-driven BLDC inverters, but the principle (and the remedy) is the same - especially since hot-plugs do happen a lot in the RC world. Another remedy is to use precharging connectors which have started appearing lately.

[T3sl4co1l]

Keep in mind that we're talking very low impedances to begin with, so that stray inductances probably need to be swamped with quite large capacitors, with quite low ESR, to have any benefit.  Likewise -- the capacitors will have essentially no impact on hold-up time, or mechanical performance, because their voltage equalizes in tens of microseconds, while the mechanics move in tens of milliseconds, and the battery state of charge moves in hundreds of seconds.  As long as the battery has low ESR (as it must in this application), it's dominant at long time scales.

The confusion, then, comes down to a lack of technical understanding (as it so often does), that circuits depend on consistent power every microsecond, a time scale where electrolytics and wiring inductance matters, but not where mechanical energy matters.

In particular, the fact that the capacitor stores a minuscule fraction of the total energy, is a hint that energy storage really isn't needed in this situation at all, and that's why the tranzorb works.  Indeed, once one realizes this, and calculates how much energy is needed to be dissipated, and what the capabilities of tranzorbs are -- it comes out very well indeed, taking a fraction of the size of the caps you'd need otherwise.

In short, there is a powerful, general theory underlying all of these wishy-washy yet overly absolute, common-language statements that people make.  It's unfortunate that that's how language is, but... it is what it is.  In any case, the theory is there for those who wish to seek it out!

Tim

[IDEngineer]

In particular, the fact that the capacitor stores a minuscule fraction of the total energy, is a hint that energy storage really isn't needed in this situation at all, and that's why the tranzorb works.

Precisely. This isn't a traditional "inverter" application. We're not trying to damp out transients originating from normal operation. Instead, we have a great deal of energy stored in the magnetic field of (one of) the motor(s) and then an external event suddenly freezes the motor. The issue is about dealing with the resulting energy coming back out of the magnetic field, which is a relatively large amount of energy over a very short amount of time. Exactly the sort of application tranzorbs - not electrolytic capacitors - are for.

[Richard Crowley]

EDIT: Had a washing machine that went through four controller boards in less than five years

I got my washer and dryer used 20 years ago and they are still working flawlessly.  Of course, they use the old-school clock-motor driven cams with microswitches. No active components at all.

[Siwastaja]

OK, let's continue with the RC BLDC inverter offtopic. For the possible motor inductance discharge, the problem with a TVS diode is that it's so imprecise that it imposes an additional maybe 40-60% Vds overrating spec, hence an active crowbar circuit may make sense over it for a tight, economical design, if you can afford someone's time designing it. (Electrolytics almost always impose the same kind of additional overrating requirement since you can't store much energy in them unless you allow some dV. You can calculate it!)

(By additional requirement, I mean, for a 25.6V battery pack, you end up with actual TVS clamping at around 35-40V, and at that point, you haven't added any voltage robustness to the system yet; you still need to overrate this like you would normally do for very local layout parasitics, safety margins, etc.

For example, I'm just now designing in a SMC5K28A TVS in a 6s-li-ion driven 2kW BLDC system - it's not allowable to let it leak during normal operation, so this is the closest part at 28V working voltage. A 26V part could maybe possible, but as these working voltages are defined at room temp only, I did say no to it.

Now, it breaks down at max 34.4V, but only at 1mA - actual max clamping voltage is as high as 45.4V at 110A surge. In reality, if the motor current is less than this 110A, the clamping voltage is going to be something between these two numbers, let's say 40V. So now we are 59% over our nominal maximum DC bus voltage, and still need to account for normal switch node ringing, whatever derating we do; well let's say 30% if you know how to do layouts. Of course, such an overvoltage event is not commonplace, so we may be able to depend on additional avalancing in the FETs.

Now, let's see how the elcap-only solution would work. Let's estimate the total motor inductance at 200uH, magnetized by 40A. The energy stored is 160mJ, then. Let's assume some not-over-the-top electrolytic capacitance, like 3000uF (still small fraction of the BOM cost, surface area and volume of the total product; I hope. This also satisfies Celectrolytic > 3...4*Cceramic for good damping). Finally, we can calculate the voltage rise from demagnetizing the motor as:
V = sqrt(E / (0.5C)) = 10.3V

So the discharge peak would be limited to nominal max Vdd + 10.3V = 35.5V. So, this does a bit better than the TVS example (around 40V), but not by much.

Now, the numbers are somewhat pulled from thin air. I didn't go back to modify them to prove any point, and happened to end at a boring "they are almost the same" result, but I think this isn't too boring after all because it shows the interesting nature of the design: "it depends". Had I chosen much higher current, or limited the size of capacitance due to space constraints, using a TVS or an active crowbar would become mandatory.

I agree that a TVS probably results in a smaller and cheaper solution overall, especially at very high motor currents; then it would double as the input hot plug inrush limiter as well, although for that purpose, it's not as effective as a large enough elcap bank, but it doesn't matter when the components are dimensioned to survive the clamping voltage anyway.

But, in any case, it helps to understand all the complexities of designing such a circuit - this may explain why, in some cases, someone has seen a real benefit adding a retrofit elcap on an cheap ESC. Maybe it had stopped them blowing up, for instance, during inrush or during motor inductance demagnetization. Maybe a TVS would have been an even better choice, or maybe not. But, if you go and state that people who say that elcaps are used to "suppress transients" are wrong and stupid, it's you who are wrong, because that's exactly what they can be - and are actually used for!

So, to be actually helpful, you'd do better if you could show that maybe the value of electrolytic used cannot provide enough transient suppression, or there is something better. Or, you would do best if you showed how complex the matter is; it's something even an experienced designer would need to carefully calculate, simulate and test, and not something that should be left for the end-user of the RC modules to fix as an afterthought. Finally, the use cases of the ESCs vary. Not all are always wired up using very short wires, and the battery isn't always just centimeters away - sometimes there can be significant inductance which will require damping.

Sorry if I appeared condescending. I think we both know what we are talking about.

[IDEngineer]

So, to be actually helpful, you'd do better if you could show that maybe the value of electrolytic used cannot provide enough transient suppression, or there is something better. Or, you would do best if you showed how complex the matter is; it's something even an experienced designer would need to carefully calculate, simulate and test, and not something that should be left for the end-user of the RC modules to fix as an afterthought.

But as you implied and I outright stated: Many in the RC community (and other communities too, hello audiophiles) aren't interested in hard science. When I try to give concrete explanations for things, their eyes glaze over and I'm waved away with condescending looks. It's hilarious to engage them in a discussion of antennas, for example. Even moreso when my 17YO son weighs in, particularly on RF topics. He's not a degreed Engineer (yet), but he got his first ham radio license at 6YO and was calculating, building, and using my equipment to evaluate the performance of his own antennas since about that same age. He knows more about actual, real-world RF behavior of antennas than most highly technical people I know, yet the adults in the RC community dismiss him - until they get to know him! He and I sit around and laugh our @$$es off after some meets based on what some folks pontificate as reality.

Another cap-related example: Who needs bypass caps on each PCB when there's already a big fat electrolytic right at the battery? What possible good is another 0.1uF here and there when they have a 1000uF just a few inches away?    Offer to show them the difference on a scope (I have a portable scope that we take to some meets) and the response is "I don't need to see your scope, point-one is way smaller than a thousand. It's not complicated, it's simple math." 

So while you, I, and others on this forum enjoy getting into the nitty gritty details, you're exactly correct that others are satisfied to "fix as an afterthought". And the sad part is, they're not necessarily interested in anything more concrete. Rumors, old-wives tales, and tribal lore are good enough for them and they're not interested in "how complex the matter is" no matter what we say.

[coppercone2]

not having local bypass capacitors is bad practice

[IDEngineer]

not having local bypass capacitors is bad practice

Agreed. Your point?

[ebastler]

not having local bypass capacitors is bad practice

Agreed. Your point?

I guess coppercone2 was just proving your point: 

Rumors, old-wives tales, and tribal lore are good enough for them and they're not interested in "how complex the matter is" no matter what we say.

Edit: Of course he is factually correct, but he also nicely demonstrates being too lazy to dig into the complexities. Beautiful!

[T3sl4co1l]

OK, let's continue with the RC BLDC inverter offtopic. For the possible motor inductance discharge, the problem with a TVS diode is that it's so imprecise that it imposes an additional maybe 40-60% Vds overrating spec, hence an active crowbar circuit may make sense over it for a tight, economical design, if you can afford someone's time designing it. (Electrolytics almost always impose the same kind of additional overrating requirement since you can't store much energy in them unless you allow some dV. You can calculate it!)

Yes, a crowbar could be much more effective; but it would require design.

Given that the TVS and capacitor are pretty evenly matched, as you note, it basically comes down to physical size.  There is some contribution from the TVS's nonlinearity, where once you're up at breakdown voltage, it takes a whole shitload more current to push it higher, whereas the capacitor remains linear, so it always must be dimensioned for worst case conditions.  Hopefully the motor drive is current mode controlled, so its current draw/return is within known bounds, so these conditions can be known at design time -- if not, that could be even more in favor of the TVS.

Tim

[coppercone2]

do you over rate TVS ?

is there a power electronics rule of thumb like for electrolytic capacitors? (for its current handling). Is it different then choosing it for protecting a signal line from possibilities (but not regularities)?

I think with a MOV you can put pretty much whatever so long the leakage is acceptable?

[Siwastaja]

TVS datasheets tend to have quite good curves for pulse handling. I just try to follow those (at least when I have time to design properly...). OTOH, I have never had a TVS blow up on me, they are quite robust. I had one 1kW center tapped boost converter with quite a bit of excess leakage inductance where I used a TVS in the RCD snubber (instead or in parallel to the R), to the degree that the PCB epoxy material was smoking while operating, and operated it for hours total while emitting smoke all the time, no problem (except the smoke)! It was a converter equipped with a SED (Smoke Emitting Diode) power-on indicator. Later I dimensioned the R properly to take most of the load away from the TVS, and added fan cooling.

For electrolytics, the current handling is specified in the datasheet. Single pulse is typically not defined, but they are really robust against single pulses; many, many applications do apply full DC working voltage over the elcaps without any precharging or current limiting and this seldom causes any damage to the capacitors, unless the capacitors are really old and need reformation of the passivation layer.

[T3sl4co1l]

I've seen a one of those big ass 15kW (~10mm dia.) TVSs go up in flames.  You really do want a different method to handle automotive load dump.   (That was a bit of a lark, just to try; we had already settled on a combination of MOV, high voltage handling and load disable on that project.)

Same sort of thing goes for mains transients, unless you get the really big ones (literally $100 each) that are rated for it.  Otherwise, just... don't put TVSs on mains.  Just don't.

This is all pretty redundant because, yeah, they give curves, or should, and you can work from there.

Along the lines of load dump, I did design a power limiter box, that drops up to 30V at 20A for 150ms, and dumps the excess power either into a ground return (if connected), or into a stack of TVSs.  Only needs three SMDJ type diodes to handle it.  Thermal protection prevents you from hitting "start" too many times, which evidently would ultimately be limited by the board smoking then, anyway.

Tim

[ocset]

its interesting to hear about storing or handling the energy of a motor load dump.
This is one of the most common severe transients that  needs handling.
And as we know, motors driven from the mains are often after a diode bridge and PFC stage and then an isolated converter, so any motor overvoltage would be  unlikely to ever  find its way back out into the mains electricity grid. It might blow up the isolated converter's secondary  etc but wouldnt be able to get out into the mains and wouldnt cause a mains transient.

[dmills]

You think motors are bad? Meet my 1960s Oxford stick welding set.

Incidentally, Treez turns out that those NSG200 series interference simulators that Dave did the video on are available cheaply on the surplus market, given your perpetual problems with failing mains powered LED drivers it might be a few hundred quid very well spent?

Regards, Dan.

[ocset]

Thanks, yes i enjoyed watching Dave's vid on the mains transient simulator.

https://youtu.be/fp3eJkH8llI

From the link in the top post, you'd think you dont need a specialist bit of kit to give you a mains transient...from that, it seems that any electric shower or vacuum cleaner will have us
riddled with spikey mains transients. -Somehow i doubt it.
I left our scope on the mains all day today.......on sample and hold, hoping to "catch" a mains transient...but didnt get lucky.
I am wondering if anyone has caught a mains transient like this?

Meet my 1960s Oxford stick welding set.

Does it give good mains transients?
How much do you want for it?

NSG200 series interference simulators that Dave did the video on are available cheaply on the surplus market

Thanks ill take a look but Dave seemed to suggest that this was specialist stuff thats hard to come by.

Also Dave's NSG200 gave very fast transients...but is there such thing as a slower mains transient?...eg 100us/1000us instead of the usual 1.2us/50us?

I mean, the capacitor coupled transient management circuit of pg 28 of the lytswitch 4 datasheet wouldnt be much good with a really slow transient, as the capacitor just wouldnt couple  it through well  enough.
So i wonder, why does power.com not  say that their circuit is only for fast transients?

LYTswitch 4 datasheet...
https://led-driver.power.com/sites/default/files/product-docs/lytswitch-4_family_datasheet.pdf

Also, really high voltages can occur on the mains from "disconnected neutral" faults...i wonder if  this regularly happens on certain remote areas of the mains but the power company dont 'fess up about it?

[Nusa]

You need to define the extreme transients you desire protection from. If you can't do that, the rest of the exercise is pointless.

If your product is intended for an international audience, then you may not be able to make any assumptions about grid quality or standards adherence. Over-engineered is always better, but of course that costs more.

Note that transient causes often don't originate in your house and are not under your control. You still have to tolerate them.

[tooki]

"Tumble dryer" is the term of art in the appliance industry. And yes, that's why the labels on clothing refer to "Do (or "Do not") tumble dry" with some sort of heat setting recommendation (usually low or medium).

The alternatives are line drying and flat drying, neither of which requires a machine, so it does seem a bit redundant to say "tumble" dryer when referring to the appliance. But that's what they call them. 

There are also heated mangles, used mostly in commercial laundries, for pressing and drying things like sheets in a single pass:

And of course the manual equivalent: hand ironing.

Additionally, in some places, like here in Switzerland, there are also laundry room dehumidifiers (for accelerating line-drying), which are also marketed as clothes dryers (in German): http://www.secomat.ch/secomat

Of course, none of this changes the fact that 99.9% of the time, when someone says "clothes dryer", a tumble dryer is meant.