snubber 3 different 230V AC motors in a record cleaning machine - Page 1
Forum section: electronics / projects

author: Peter_O
author_rank: Frequent Contributor
timestamp: December 20, 2021, 11:05:24 am
content: Unsure if this is more abeginner'stopic or aproject.My project is the automation of a Loricraft Record cleaning machine.The machine is sucking cleaning fluid with a nozzle at the tip of an actively driven arm.The record platter can be turned cw and ccw.The pump sits inside the big black box.The original schematics are as attached.M1 - arm drive, line synchronous clock drive motorM2 - pump motor, 80WM3 - platter motor, 2-directional(Sorry, I found no 4 pin AC motor symbol and used a stepper motor one.)The capacitors are 250V X ones.I'm about to replace the switches with relais and of course I could just trust the three original snubber designs to be good enough.But I'd like to understand what's going on in more detail.M1: In my understanding, C1 is kind of an X-capacitor and the 330K resistor might be for decharging the capacitor to prevent shock from users who pull the line connector and touch the pins. 0.1µF is just "standard".M2: I understand this to be a classic snubber design, 0.1µF "standard", but 470K seems way to high.M3: I do not understand, what's the idea behind connecting the capacitor to the two live pins of the motor.Questions I have:1 - Do you have any tipps for good tutorials about simple snubber designs?2 - I'd like to understand the effects of the snubbers by doing some measurements. I have an active differential probe (Micsig 10007) limited to 700V peak to peak. Will it be fine, or should I think about getting e.g. a Micsig 20003, limited to 5kV peak to peak.3 - Any direct comment on the three snubbers will help.4 - Where can I find more appropriate motor symbols for Kicad?Any insights, tipps and hints appreciated!

author: themadhippy
author_rank: Super Contributor
timestamp: December 20, 2021, 11:41:08 am
content: Quoteit puts one coil out of phase with the other,its how it sets the direction.

author: Peter_O
author_rank: Frequent Contributor
timestamp: December 20, 2021, 12:01:02 pm
content: Ah. Ok. So for M3 it's not a snubber but just that phase shift. Thx!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2021, 12:29:30 pm
content: Presumably the line terminals all act in parallel? (They're not drawn as a bus, so... But it would be weird if they're actually like separate cords, right?)Then the 0.1 should act to bypass everything, and the high value resistor is just a bleeder so you don't get shocked when unplugging the cord and touching the blades.That helps attenuate the switching noise, but doesn't prevent it.What happens is:First, the motor current needs to be significant, near a peak. This might be mostly real current (coincident with a voltage peak), or inductive (near voltage zero crossing). Those motors are likely quite inductive, so probably more the latter case, but it doesn't really matter. What does matter, is that there is also significant (leakage) inductance, of the winding with respect to any work it's doing, as this is the part that plays in the following situation.**That is, there is effectively a series inductance between the terminals, and whatever internal motor-physics-operation-stuff the motor is actually doing. The motor can be modeled as a Thevenin voltage source, where the voltage is the EMF associated with applied field and rotor motion, and the series impedance is winding resistance and leakage inductance. Which also means these limit how fast the motor can accelerate, max torque, that sort of thing, and indeed they do.When current is near peak (or "significant" in any case), then when the contact is opened, the inductance discharges, causing voltage to flyback up to a peak determined by air breakdown voltage, and it rises at a rate determined by capacitance (stray capacitance in the wiring and winding).Consider the contact for a moment. As it opens, microscopic points of contact gradually move apart and separate (and probably slide and drag on the surface for a bit, as well). When the last point separates, in the first, whatever, microseconds, where it's only a few nanometers away -- the voltage drop rises, current no longer flowing through the contact but now charging into stray capacitance instead.As the contacts get further apart, gradually the breakdown voltage rises. You can imagine, if they separated fast enough, perhaps the breakdown voltage could rise fast enough as well, that breakdown never occurs, and a single flyback pulse and ringdown is experienced by the motor winding, all by itself.This is actually designed to be the case, in archaic (points) ignition systems: the "condenser" slows the voltage rise, enough that the contact can open far enough to avoid breakdown. The ignition coil develops a high voltage (some hundreds on the primary, 10k's on the secondary), then finds a path through the spark plug. At least, when the system was working right... points were notoriously bad, subject to continuous mechanical and electrical wear.We might not have that luxury here. We could add such a capacitor across the motor winding, and repeat the function here; but it might turn out that the switch doesn't open far enough, or fast enough, to avoid breakdown, or that the ringdown (even without sparking) is still enough noise to be annoying. Note that ringing is coupled through the motor's casing into nearby ground (or if ungrounded, then electrostatic fields to anything nearby), so we still get a common mode (flows through neutral alone, even without the hot wire connected) noise current, albeit much less than the direct route.Ringdown is given by the high-frequency losses of the motor winding, its inductance, and the capacitor value. It's more-or-less a simple RLC parallel resonant circuit. You'd need to measure the R and L at the frequency, to solve for the waveform (peak amplitude, resonant frequency, damping rate), but this can be done with basic test equipment ("basic" as in scope and signal generator).So that's kind of, your top example: simply slowing it enough that the switch doesn't spark (or as much). The capacitor is evidently on the wrong side, so I'm imagining it across the motor for sake of argument. (Across the line, it acts to reduce line impedance; more on that later.)The middle example is snubbing the switch, typically with an R+C. Though the value is wrong; is it really 470k?So, what happens when the switch breaks down?The switch is full of air, which has the effect that, when breakdown occurs, the voltage drop suddenly (within <1ns) goes to maybe 10V (give or take spark length; we're talking quite short sparks here, so the voltage drop will be low), and remains conductive as long as sufficient current flows (holding current).We're probably at less than that current, so we can assume the spark goes cold after a few microseconds, goes out, then we're back where we started: voltage is rising (into stray capacitances), and the motor inductance has discharged only incrementally (proportional to the area under the flyback pulse).The effect is a relaxation oscillator, where the bias current is supplied from the motor inductance, the breakdown element is the switch airgap, and the capacitance being discharged is the strays of wiring and winding. (Note that the capacitance supplies holding current for some ns to µs, but eventually runs out.) In this case, the return path to ground, is the mains system itself -- discharging that capacitance into the supply transmits an electromagnetic pulse (EMP) of significant amplitude. Note that the spark risetime is extremely fast: under 1ns, so we really can speak in terms of a wavefront: mere 10s cm wide, propagating up the mains cable at a modest fraction of the speed of light (waves travel in cables somewhat slower, due to the dielectric). This is fast stuff!This waveform is important enough, that it is included in EMC standards, such as IEC 61000-4-4 electrical fast transients / burst transients. The peak voltage can be several kV, the risetime a few ns, and the pulse energy comparable to ESD -- it can be destructive in its own right. Add to that, the fact that it repeats rapid-fire until the motor inductance has discharged to zero current: the sustained noise can disrupt communications between equipment (e.g. knocking out a USB, Ethernet, etc. packet in transit, or causing garbled data on less error-tolerant networks).So that's what we're ultimately protecting against.Note that, mains filtering is ineffective for two reasons:1. If we simply put a shunt capacitor across the line (as C1), this reduces the line impedance for high frequencies, shunting the EFT -- instead of ~kV, maybe it's only 10s or 100s of V. But we cannot eliminate it this way, not without using a more complicated filter at least.2. There is significant capacitance to ground, or whatever nearby metal there is -- and again, this is a very fast pulse, so it literally radiates out from the switch at light speed, washing around anything metallic nearby; and anything metallic provides a partial ground-return path for that wave to launch down the mains wires in parallel (common mode), so that any (differential mode) filter has no effect at all.So we really are quite interested in preventing that noise in the first place!If we put an R+C (that is, R in series with C) across the switch, with R dimensioned so that, even at peak maximum load current, the voltage drop is never more than switch breakdown voltage; and choose C = L / R^2, then: the resonant circuit is the series path between mains, snubber R+C, and motor R+L. (We can likely ignore stray (parallel) C here, because the snubber C will be much larger; or we can include it in a more complete model of the circuit, that's fine too.) Note that to solve for C, technically we should use the total loop R, not just the snubber's. (But if we don't have the R, L and C of the motor/wiring, it's safe enough to assume the snubber R is dominant. This isn't rocket surgery, being within a factor of 2 is more than good enough.)Note that, for DPST switches, we don't in general know which contact will open first, so we need a snubber across each. If we did know, we could snub just the first, and by the time the other opens (probably some ~ms later -- mechanics are slow!), current is already ~zero so we don't care. But yeh, we don't know. Or it might even vary with wear, maybe one contact wears faster so opens sooner after a lot of use. So, safest to do both.For small motors like these, likely the motor current isn't much over say 100mA, and if we want a peak voltage under 200V, that's 200V / 100mA = 2kohm. If the inductance is ballpark say 1H, then C ~ 0.25uF will do. Likely anything from 0.047 to 1uF will do. You often see pre-packaged components with 100R + (0.22 or 0.47uF) for exactly this purpose, and that will do a fine job as well.Or for the 80W motor, that's maybe a few 100 mA peak, so will need a somewhat lower resistance, and more capacitance. That's fine too.Interestingly, your middle example is right, in that the switch has an R+C across it; but wrong, in that 470k is wholly ineffective. Is the 'k' a misread? 470R would be in the right order of magnitude. Note that if it's an old type, it might have failed, measuring very different from its rating.Finally, about the bottom example, the reversible motor -- it's not obvious that the capacitor is serving as a snubber, really. It could be for phasing, so that, actually your choice of a stepper may be much more accurate than you expected -- namely, the capacitor causes a phase shift between windings, so whichever winding is powered is reference, and the capacitor causes a lagging phase shift to the other. Thus creating a rotating magnetic field that spins the rotor, and that spin is CW / CCW depending on which winding is energized. Cool, huh?Reflecting on the above analysis, if we consider each winding as an ideal motor in series with winding impedance, then we can realize, regardless of what each motor is doing internally (and, generally speaking, they'll be doing similar things, they're part of the same motor after all), we'll still have impedance facing the outside world -- basically, the winding R+L act in parallel with respect to the switch, assuming the capacitor between them is large enough to ignore for this purpose (which I'm guessing it is). So, we will still need the R+C snubber here. The effect of C3 is to couple both windings together at high frequencies -- one snubber should do the job for both windings.TL;DR:- Mechanical contacts open and close extremely quickly. Sub-ns quickly. Easily creating EMP that can travel along wires and disrupt nearby devices.- Use the RLC equivalent circuit to solve for damping: R^2 = L/C. Add R to the loop as needed, to ensure damping. If you don't have exact values, crude assumptions are okay.- Particularly for the snubber, place it across the contacts -- where the EMP is emitted -- and dimension R to avoid contact breakdown at peak current, if possible. R = Vbd / Ipk. Assume Ipk from sqrt(2) * Irms(max) of the motor.Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: December 20, 2021, 01:48:56 pm
content: Tim, thx a lot for taking the time to write up this comprehensive answer.At first reading it adresses my "reception level" extremely well and explains a lot to me.I will need some time to think about it in detail and I will come back here later these days.For now to your questions:Quote from: T3sl4co1l on December 20, 2021, 12:29:30 pmYou're right, of course.QuoteI've updated the schemactics acc. to that.QuoteYea. I was in doubt too and measured it multiple times with two different meters.Did it again. Measures near 470k, but from the colour code it's a 47 Ohm 1% one.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 20, 2021, 03:38:07 pm
content: Little 47 Ohm is probably burnt early in its life due to the transient charging current when the switch opens.47 R, 0.1 uF, Could be E = 230 * 1.414. not considering motor winding: 6.9 Amp peak I_sq * t is approx 0.223 mA_sq.sec.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2021, 05:10:25 pm
content: Ah yep there you go. Would recommend something bigger like 1/2W, or pulse rated, for a replacement.As for relays, consider using SSRs -- TRIAC based switches. The snubber may not be strictly needed (a TRIAC turns off at ~zero current; actually, like a spark, it has a minimum holding current too, but it's much better behaved than a spark, and easier to trigger), but improves operation. There are "snubberless" TRIACs with more robustness than the garden-variety kind.Mechanical relays are of course mechanical contact, so will need the snubber. The advantage is lower voltage drop, especially helpful at high currents (so, not at all important here, but handy to know). The turn-on is still very fast so emits a noise impulse, though not as much as turn-off without a snubber. (Ideally, an R || L would be in series with the contact to dampen this as well. R || L is the inverse of R + C, the symmetry covers for both edges. This is rarely done because it's rarely worthwhile, but handy to know.)Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: December 20, 2021, 08:35:34 pm
content: Short update:I've measured the motors with my small component tester.First I checked the tester with small known inductors. 68µH measured OK as '0.07mH', 450mH measured OK as '0.45mH', so it seems to be suitable for the job.M1 (small line synchronous arm motor): 7127 Ohm, no inductivity detectedM2 (80W pump motor): 110,5 Ohm, 118 mHM3 (two coil cw/ccw motor): 3536 Ohm and 3534 Ohm, no inductivity detectedThis seems to illustrate, why Loricraft put a snubber at the pump motor switch only only.Coming from Tim's calculation:Quote from: T3sl4co1l on December 20, 2021, 12:29:30 pmfor the pump motor:Type label says: 80W and 0.5A, 80W equals I=80W/230V=0.35A.If we stay with 200V peak voltage, R=200V/0.35A=570Ohm, plus - as we do know it now - motor resistance of 110 Ohm: R=680OhmC=C = L / R^2 = 0,118H/680²Ohm² = 0,255µFSo 560 Ohm and 270nF should do it, right?For the platter motor it would be 11.5W/230V=50mA; R=200V/0,05A=4kOhm plus 3.5KOhm = 7.5 kOhmWith an assumtion of 1H as upper estimate: C=1H/7.500²Ohm²=0,017µFSo 4.2KOhm and 18nF would be the choice,right?For the arm motor I'd estmate 5W ^= 20mA; R=10k+7.1k=17.1kOhm; With 1H C=3.4nFSo 10k and 3,3nF it would be.Are these values sensible, or are there more practical values to choose?

author: james_s
author_rank: Super Contributor
timestamp: December 20, 2021, 10:30:47 pm
content: Quote from: Peter_O on December 20, 2021, 12:01:02 pmYes, this is referred to (at least in English) as a PSC or Permanent Split Capacitor motor. It is called that because the capacitor is in the circuit full time instead of being removed after the motor gets up to speed as in a capacitor start motor. Often the start and run windings are interchangeable and swapping which one is fed through the capacitor reverses the motor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 21, 2021, 01:29:45 am
content: Huh, doesn't say what frequency it tests inductance at? (Can you scope it in progress..?)Guessing the frequency is just too low for it to register the inductive component of the two. Relevant test frequency is, well, whatever the characteristic frequency of the resulting RLC would be -- some kHz to MHz most likely.Quote from: Peter_O on December 20, 2021, 08:35:34 pmIf true (and I mean, it may be) -- indeed!QuoteYeah, that sounds quite reasonable. And that's the limiting values, so smaller R and larger C are acceptable (it can be overdamped just fine).You don't want to go too high in C, as it draws some leakage at mains frequency. You certainly don't want it series resonant with the motor inductance; but we've already ensured that in a sense (not really, but it would be a very peculiar motor design to have so much inductance in normal operation), so that just leaves leakage. If you say an upper limit of say 1mA, that's 13nF. Well, that blows out the spec for the 80W motor, but, well, it's ~350mA, so we can presumably allow a higher "off" current for it. And this figure is pretty reasonable for the small motors, so something in the 10s nF seems reasonable.QuoteI'd stick to a lower value, say a few kohm, and 10nF or so should be fine.Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: December 21, 2021, 10:17:39 am
content: Quote from: james_s on December 20, 2021, 10:30:47 pmJames. thx for the keyword that leads to the relevant tutorials.I just had a look. The capacity of 0.22µF is written on the label in fact.Its a Crouzet 82540031.https://soda.crouzet.com/pn/?i=82540031Unfortunately they do not provide a datasheet. But on the other hand everything necessary is on the label.

author: Peter_O
author_rank: Frequent Contributor
timestamp: December 21, 2021, 06:06:36 pm
content: Quote from: T3sl4co1l on December 21, 2021, 01:29:45 amNo. (Yes)Seems, this little guy decides it's a resistor before it starts any signal for an inductor measurement.But I rescued a HP4260 Universal Bridge some months ago and took it into action. And I doublechecked the resitance values with a lab multimeter.80W pump motor: 129mH, 110Ohm11W platter motor: 7.3H, 3500Ohm per windingarm motor: 9.1H, 7.1kOhmRecalculating with 200V voltage limit:80W pump motor: I=0.35A, R=570+110=680Ohm. C=0,129H/(680Ohm)² = 0,28µF560R/.27µF it would be. And following your last comments, Tim, I'll start with 420R/0.47µF11W platter motor: I=50mA, R=4k+3.5k=7.5k, C=7.3H/(7.5kOhm)²=0,13µF... 4.2k/18nFArm motor: I=20mA; R=10k+7.1k=17.1kOhm; C=9.1H/(17kOhm)²=31nF... 4.2k/47nF, this one for each of both switches, acc. to Tim's first comment: not knowing which of the switches will open first.I've ordered some "X2" class film capacitors with different values. 1W metal film resistors I've here at hand.After arrival of the capacitors we will see, if the Atmega runs stable, or if there's some switching noise left, that gets it out of track.Ah. While surfing I found this snubber module beeing everywhere in the arudino cosmos:https://de.aliexpress.com/item/1005001418499710.htmlThe blue component is an MOV-10D471K varistor. More stuff to read and understand. :-)

author: Peter_O
author_rank: Frequent Contributor
timestamp: December 21, 2021, 06:47:16 pm
content: Forgot to add a screenshot of the scope measurement of the component tester. Added above.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 21, 2021, 07:58:59 pm
content: Oh and speaking of MOV -- that's another good option, as it has some capacitance while also clamping the voltage. So it can serve the role of the RC in a different way. There's still ringdown after the spike is clamped, but the fast risetime and high voltage are dealt with. (Breakdown occurs like a soft zener diode, rather than snapping like a spark or gas discharge.)MOVs have rather crude clamping specs, so may not be suitable for solid state switching, but are effective for mechanical contacts and general mains application.Ideally you'd put it across the switch, but that may not be a great idea as it allows mains transients through the switch; it may also wear over time. (Snubbing is well into the "infinite" lifetime curve, should be even for the smallest suitable MOV you can find; I add this, as some will tell you they always wear.) The next best option is putting it across the motor, then bypassing neutral to hot before the switch, so there's still a tight loop around the switch, bypassing the contact. (Which, with the H-N bypass already in there, isn't a bad idea, and the same principle applies to the RC snubber as well.)Tim

author: james_s
author_rank: Super Contributor
timestamp: December 21, 2021, 09:37:54 pm
content: I'm not one to question your expertise, however I'd certainly be hesitant to use a MOV in a snubber application if that's what is being proposed here. They wear out with use and they fail silently, or in some cases they fail as greatly increased leakage which would cause current to bypass the switch in this case.

author: Peter_O
author_rank: Frequent Contributor
timestamp: December 22, 2021, 09:49:49 am
content: The linked arduino cosmos module combines the MOV with an RC circuit.But when those MOVs do not live a long time, I think, I stick to the RC solution and find a way to check their effectiveness.Simple check would be the processor running stable during switching.Is it possible to measure the inductive voltage with a differential probe and scope?What maximum voltage the probe should be specified for at least?Or would you consider that approach as to risky for the scope?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 22, 2021, 01:34:52 pm
content: Use an isolation transformer. Regular 10x probes will read that kind of voltage range just fine.Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 23, 2021, 03:01:42 am
content: Quote from: james_s on December 21, 2021, 09:37:54 pmHi j_s,I agree with all except the wear-out comment.Can you provide a currently applicable ref? Littelfuse for example website data does not presently mention that for the LA series assuming within the Max Volt & Joule ratings.And I agree that no semiconductor should ever be used to bypass a mechanical safety switch, and that would be in violation of many standards.In my experience varistors would/should always be used downstream of isolation and overcurrent protection.

author: james_s
author_rank: Super Contributor
timestamp: December 23, 2021, 04:58:55 am
content: Quote from: mag_therm on December 23, 2021, 03:01:42 amI don't know where I first heard it, I thought it was just common knowledge? I found a few references.https://www.nmmi.edu/wp-content/uploads/2017/09/NMMI-What-to-Know-About-Surge-Protectors.pdfhttps://www.researchgate.net/publication/312112062_Deducing_metal_oxide_varistor_life_span_from_pulse_rating_curves_for_surges_of_different_magnitudesPerhaps somebody on this forum will have some direct knowledge.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 23, 2021, 06:06:59 am
content: I recall using, as young engineer, the varistor first in about 1973 to replace the selenium voltage-surge suppressors on industrial 600V class switchgear.The selenium cells emitted toxic gas upon failure and they were discontinued .I recall using D.O.L., off load, and emergency break rotary switches on transformer primaries from Santon, Kraus & Naimer, and moulded case breakers from Westinghouse. Usually the load was a rectifier transformer followed by a scr bridge or double star.The varistors were mounted in parallel gangs under. I don't remember if fused. I recall the varistors were from GE and Semikron mostly, at that time.They were listed world-wide and detailed application sheets were provided.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2021, 02:17:56 pm
content: Quote from: mag_therm on December 23, 2021, 03:01:42 amThey do -- check the datasheet:https://m.littelfuse.com/~/media/electronics/datasheets/varistors/littelfuse_varistor_la_datasheet.pdf.pdfThree ratings are given:Absolute maximum energy, 10/1000usAbsolute maximum current, 8/20usPeak clamping V, I, 8/20us(I don't know why they call it "8 x 20 us" and etc., seems to imply a repetition or multiplication. It should be read as specifying two dimensions, not that they should be implicitly multiplied. Actually the figures are additive if anything. Anyway, the waveform is given Fig.2, where they also use the more common slash notation, go figure.)Now compare to the life curves. Pick V320LA40BP for example. This has 150J, 6.5kA, and 810V @ 100A ratings, respectively. Its life curve is Fig.18. The "1" curve ends at about 8kA / 20us, so we can assume the abs. max. rating is a one-time event, at best. For the energy figure, we need to know voltage, then divide to get surge current; if we assume 810V is representative then 150J/810V = 0.185As, or 185A for a 1ms surge (ballpark -- presumably they integrate energy well into the tail of the pulse, where V will be noticeably different, and total energy higher than the 50% cutoff point at 1ms). This is conveniently close to the "1" curve's ~150A, 1ms intercept, so maybe it's not that far off after all.Note that the slope for single events is close to 1:1, meaning constant energy (assuming constant voltage drop). Apparently the 6.5kA 8/20us surge is as destructive as the 150A 1ms surge.Further down the plot, we see an "INDEFINITE" curve, and various other numbers of cycles. Presumably you'd want to choose a device for induced lightning protection, or residual surge (category A?), somewhere in the thousands perhaps, for long life, and less would be acceptable for shorter lived consumer equipment.Note that the figures quickly go away from meaningful surge values, for any kind of repetition -- 10^5 cycles at 100A peak, 8/20us, corresponds to a 2-ohm generator of merely 200V above clamping voltage; most devices are expected to handle 1.5kV or more (total). It seems likely this is where the claim "MOVs always wear" comes from -- they do, in any kind of useful mains-surge regime.It seems they can indeed offer unlimited life, at least so the figure claims -- when limited to much lower values, like 7A / 1ms. You could use this for more casual voltage limiting applications, like, well -- inductive flyback clamping for one.(In contrast, avalanche/zener/TVS diodes evidently don't wear, they either survive, or overheat and fail shorted, or open if failed real bad. Across-the-line rated TVS are in fact available -- they're just terribly expensive, because a whole load of them must be stacked to get the energy rating!)Anyways, in this particular application, I think the contrast between RC and MOV is interesting. You have obvious cautions like this:Quote from: mag_therm on December 23, 2021, 03:01:42 am(although, note no one brought upsafetyswitches here, so this is peculiar)Quote from: james_s on December 21, 2021, 09:37:54 pmBut then, any surge will be let through in full, at whatever current the resistor sets it to, for the RC case, and the motor isn't about to do anything from such a short transient, whether reduced by a series MOV or attenuated slightly by the RC. In contrast, the RC may well leak more mains current than the MOV -- particularly notable in this situation as the load resistance is so high, every mA of leakage may matter. (This is especially true of certain types of loads, like LED lamps that tend to charge up under leakage, then turn on briefly once in a while -- flashing!)As for general application, best is to preserve the off-state insulation of the switch -- perhaps snubbers across the contacts should only be employed in certain situations, when it is known to be safe to do so. For example, with more than some minimum load, guaranteed, so that the off-state line voltage is acceptably low. Whereas for any general load including none, it may float up as if the switch is already closed, making a hazardous situation. It would seem, then, the best practice is to snub from hot to neutral, namely using a bypass cap from hot (inlet) to neutral, and the R+C snubber from hot (after the switch) to neutral.Still another option, is to just not care about contact bounce and EFT -- industrial environments are noisy in so many ways that you might not notice the noise from one particular contact. It's the responsibility of everything else, then, to tolerate such noise levels. For OP's situation, dealing with the contacts is easier, but only solves the immediate situation, and the device could presumably be exposed to external sources of the same sort of noise -- so, it's not a fix for the underlying issue, that the MCU board and wiring are so sensitive. But solving this is a bigger task as well.Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: December 25, 2021, 10:03:45 am
content: Thx for the educating discussion MOVs and other TVS devices. Inspired by your posts I found another nice overview in "Art of Electronics - The x-Chapters".As you guys already stated, they emphasized that these components might fail and short cut the circuit. they recommend to always have a fuse in series. From that the small chinese snubber board I linked above seems to disqualify.Referring to your discussion about snubbers and safety switches I shoud add, that I've already planned for a 2 pole general manual power switch in front of the relay stuff. In Germany we have those power plugs where you can easily interchange L and N. As I'm aware of beeing an amateur, I always have such a switch in front of all my DIY line handling botches.For a first step I will come back after having installed RC turn off snubbers.I'm organizing for a differential high voltage probe and plan to fiddle around a little with the three motors after arrival to gain some hands on understanding in addition.In case there are very high spikes the RC snubbers can't handle I might get into MOVs and fuses.

author: Peter_O
author_rank: Frequent Contributor
timestamp: January 03, 2022, 06:09:31 pm
content: Hi guys, hope you made it into 2022 smoothly.(sorry, I tried to reduce the width of the inline images, read all the topics about it, ... ..., and it doesn't work for me.)Edit Jan 4th: Was a lack of coffee, fixed.Meanwhile I managed to get a high voltage 200x/2000x differential probe in addition tot he 10x/100x I had already.I swapped in the ATmega and a relay board for the manual switches.And I put in some snubbers as caculated or suggested by you and did some probing.Here’s are the new schematics.(In the old schematics above there was an error: I confused the 1 and 2 pole switches for M1 and M2.)Motor 1:This is the line synchronous clock drive motor that drags the suction arm across the record with L=9.1H.In the original setup oft he machine it was this one that had the 47R / 0.1µF RC snubber across the switch.Tried to measure the max current with an 1Ohm shunt but it was covered by noise.So 50mA ist the best guess and R calculated to 4k, C 550nF.I put in 3k3 and 470nF. This got the motor running without having closed the switch.So I took the RC off again.The following scope shots show the voltage over the switch (C3, green) and the +5V line of the ATmega PCB (C2, magenta) when switching off the arm motor.Trigger is on C2 with trigger level some 5.xV.10ms/div:0.5ms/div:I understand the burst as fast transients / burst transients, T3sl4co1l, described in reply #3 above, which „sends“ into the 5V line heavily.I’m not that clear about the negative 1.4kV spike following 1ms later. Is it just the inductance voltage of the motor after the relais contacts are wide open and there is no more breakdown inside the relais? Why is it, that it takes 1ms before that Voltage builds up.As a next step,I will try the original values of 47 Ohm and 0.1µF and see what I get.Motor 2:This is the 80W vacuum pump motor.The manual switch was a 2 pole to switch L and N simultanously. Now it’s two relays, the processor switching them on N first then L, off L first then N.In the original setup oft the machine it had no snubber across the switch.Here’s the current (1V/div = 1A/div) with an 1Ohm shunt:Max current at the beginning is about 1A, reducing to the 0.5A noted on the motor label.R calculate to 200Ohm, C 3.22µF for 1A and 400Ohm, 806nF for 0.5A. Because the Pump will not be switched off during the first second, 0.5A is a good assumption and I take 360 Ohm and 470nF later, which I have available.But first here are some scope shots without any RCs to compare with the arm motor 1:5ms/div0.1ms/divSo here’s a burst too and no kV spike, which I understand because L=0.13H is a factor 70 lower that with M1.The burst on the 5V line might be coming from the subsequent switching oft the N relays. I will put in a delay between N and L switches to sort that out.Here’s the subsequent switch off of N relays, generating a proper bust too.5ms/div0.1ms/divWith 360 Ohm and 470nF on each of both (L, N) relays:Switching L off does not even trigger.Switching N off shortly after having switched L off:5ms0.5msMotor 3 will follow in the next post (picture limit)

author: Peter_O
author_rank: Frequent Contributor
timestamp: January 03, 2022, 06:47:28 pm
content: Motor 3:This is the two directional permanent 10W split capacitor motor that drives the platter of the record cleaning machine. L=7.3H.In the original setup there was no snubber installed.Here the current is too low to measure with an 1Ohm shunt.So 100mA ist the best guess and R calculated to 2k2, C 1.5µF.I put in 2k2 and 470nF. This got the motor running without having closed the switch like motor 1.So I took the RC off again.(To do: Check with original RC values.)Swithing off with platter in backward direction:10ms2ms0.1msSo this ones needs some snubber too.But because the calculated values let the motor spin in „off“ state, which is the same as fort he arm motor 1, I will first test that one with the original values being smaller that the calculated ones, and then come back to this motor 3.As above: Any comment highly appreciated!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2022, 05:36:23 am
content: Hmm, I have no explanation for that surge:It sure looks like EFT, but then sitting flat for a while -- with current flowing into what? But then that <what> breaks, and a slow transient resumes. Well, the contacts seem to be far enough apart by then that breakdown does not occur, so at least you get the big tall "whump" without more machine gun fire.Looks like most of the C's need to be reduced then, to reduce leakage current; that, or moved to shunt, which is fine because of the filter cap at the input there.Oh, I take it you won't be reversing the one motor while running? The SPDT will be open in the middle (well, unless you get a make-before-break type; most devices are break-before) so should be snubbed if it will be switched under power.The others show behavior just as predicted though:Notice two things happening at once: the spark amplitude goes up over time, because the contact spacing and thus breakdown voltage goes up. It doesn't go up smoothly, because the contacts are wiggling as they move away.Meanwhile, the green trace has a sort of constant slope, which is the magnetizing current decaying -- the slope decreases as that energy is dissipated. Eventually there's one last spark at just the peak breakdown voltage, and you can see how much time there was between that and the previous spark -- it's really curved, low energy, just barely enough to break down. And after that, it just kinda "aight I'm going home now" and rings down as whatever RLC equivalent the winding is.And with the snubber, yeah, just goes thud, exactly as predicted.This one is particularly nice:The amplitude grows quite smoothly, implying less bounce/wiggle, either on this contact, or the wiggle is inconsistent and this was just a lucky event.Note also, after the very last spike, it almost comes back to enough voltage to break down again -- aaaand just barely doesn't, and turns back, and so on as it rings down.So, seems you just need smaller C -- I should refine my suggestion, as the RLC damping condition is a safe assumption, but less may do, also. The critical factor is getting the voltage rise slow enough that the contact doesn't break down. This could be done with C alone (relying on winding resistance to dampen the ringing -- as you can see from these un-snubbed waveforms, it turns out they don't ring very long on their own!), and maybe some ~nF will do the job. You can then adjust the resistor to get best damping; and should start with some resistance (100 ohms would be fine) just so the turn-on current is also limited.Can also try a MOV -- this would be in shunt across the motor, since you don't want it potentially failing and bypassing the switch all the time. This has the effect of clamping the peak voltage, so that you get a waveform, well, very much like the first one above -- it's held flat for a while, but an actual MOV will not have that weird spike at the end, it'll just slink away and do its ringdown, having spent all the energy dumping into the MOV rather than sparking. (It may still spark some initially, it's not necessarily going to do everything by itself -- but what RC is needed to handle that, will be much smaller, too.)A MOV rated for nominal-max AC voltage (probably 250VAC/330VDC) and, it doesn't need to be big, you can get SMT chips that will handle more than enough energy really -- but a 7mm disc is probably the smallest thru-hole version you'll find, and thus cheapest and easiest to use.The MOV itself has some capacitance, so can serve somewhat as a snubber too; interesting possibility is putting a resistor in series with it, to provide extra RC damping, while increasing the clamping voltage only modestly. Interesting, because it defeats the purpose of a MOV (surge clamping), but we don't really need that feature here, indeed preventing it from clamping surges will have the effect of increasing its lifetime (MOVs accumulate wear from the aggressive magnitude of mains surges), downside is that resistor needs to be rated to handle such surges instead (prefer wirewound).Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: January 04, 2022, 10:00:53 am
content: 10:30 in the morning here in Hamburg.Tim, thx a lot again for your time and your very informative and educating comments e.g. on what we can see in the charts. You're closing a gap within the information I found online so far. Hope there are and will be some others following your explanations.Quote from: T3sl4co1l on January 04, 2022, 05:36:23 amThe manual switch was a 3 postion one: dir1-off-dir2.I swapped in two Relais on-on in series like in the schematics: 1st is on-off, 2nd is dir1-dir2. Before change of direction I switch off the motor and allow for spin down:digitalWrite(PIN_PLATTER_POWER, HIGH); //switch platter offdelay (2000);digitalWrite(PIN_PLATTER_DIRECTION, HIGH); //switch directiondelay (500);digitalWrite(PIN_PLATTER_POWER, LOW); //switch platter on againI will do some more repetitive measuremens later to doublecheck for the delayed surge with the arm motor 1.And some tests with smaller caps as suggested and come back.I wonder, if I should work into some simulation. What would be the appropriate no or low cost SW? LTspice?Would it be worth the effort?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2022, 02:27:11 pm
content: Ah nice, then that handles direction.Modeling is hard, because sparks are hard to model -- but basic models can be found, and you can easily set up the EFT scenario for example. Likewise, the motor is hard to model unless you have complete measurements of it -- also its minimum spinning voltage/current, to know what threshold to set for leakage. But there too, a basic RLC model will do the job, just to show off how it works, if not necessarily the particulars.Which in turn gets into matters of RLC equivalents, damping factor, stuff like that -- the voltage and current aren't so important, as the ratios between them (impedance), and other things in turn (e.g., note that sqrt(L/C) has units of impedance).Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: January 09, 2022, 01:16:12 pm
content: Here are some more results withMotor 1This is the line synchronous clock drive motor that drags the suction arm across the record with L=9.1H.In the original setup of the machine it was this one that had the 47R / 0.1µF RC snubber across the switch.The resistor seemed damaged. Here’s a better picture, and damaged it is indeed.Here‘s this 47R/0.1µF snubber with new R and C components.Channels/Colors are:• C4, green: Voltage over the switch and snubber• C1, yellow: relais control line of the ATmega (+5V off, GND on) for trigger and noise pulses• C2, magenta: Voltage over RSwitching on …… there’s 400V over the resistor at first, which would be 160.000/47 = 3.4kW.I found a Vishay paper on that topic.https://www.vishay.com/docs/48516/_ms9702509-2003-vishaychecklistpulseload.pdfWe see an avarage of about 1000W over 15µs. Vishay suggest to average for 3ms, which would be 5W only. That would be OK acc. to the paper but was obviously not acc. to the picture above.Switching off:Generally the snubber seems to work fine:But the capacitor ist too big and the motor is rattling against the gear in off state.Rattling does not vanish with 47nF, but with 22nF it finally does.Here’s switching on with 47R/22nF:There’s 300V over R or about 2kW.Switching offshows, the snubbing works OK. Here are two different switch off cycles:To limit the power pulse into R I’d settle with 200R/22nF:Switching on:Voltage over R is 300V, initial Power 450W.The sharp drop down period is strange. It’s more visible here, but we can see it for 47R/22nF above too.Snubbing the switch off is not as good as with 47nF but still OK. Here are two different cycles:Overall 200 Ohm / 22nF to me seems to be a good compromise for this motor:No 'off'-state rattling, acceptable snubbing and acceptable 'on'-pulse into the resistor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 09, 2022, 01:34:14 pm
content: Nice!Got a nice capture of contact bounce too...The pulse width BTW is about the RC time constant, so take peak power over that time scale and that's about what rating you need. Probably a metal oxide or wirewound resistor would be ideal. Peak power doesn't depend on capacitance, but gives a shorter duration and less energy --> more pulse capacity for the same resistor. And yep, increasing the resistance looks to be a good idea, had some ringing with the lower value and reduced C, more R dampens that out nicely.As for the common mode -- that's what you're seeing on the ATmega trace. Most likely it's a probing artifact, because your other probes are connected to the line that's doing the transient. This will be reduced with better isolation, and then for whatever's left that still reaches the board -- well, the board really ought to be designed to withstand such things, with adequate shielding (ground plane construction), filtering at all inputs/outputs, etc. (One downside: programming cables are rarely shielded, so can be difficult to program/debug in the presence of noise. Probably, your mains isn't noisy enough for this to be much of a problem.)Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: January 18, 2022, 01:30:28 pm
content: Tim, thx a lot again for your comments and ideas!Meanwhile I did some more tests with the remainingMotor 3:This is the two directional permanent 10W split capacitor motor that drives the platter of the record cleaning machine.All tests are done with the same motor direction, only the on/off relais is used.In the original setup there was no snubber installed.I showed some shot already, but anyhowhere are two more shots switching off without any snubber:C1 yellow: relais signal (active low)C4 green: Voltage across the switchC2 magenta useless (forgot to deactivate):Following the calculation done above, the snubber should be R 2k2, C 1.5µF.But with 2k2R/470nF, 4k3R/270nF and 100R/270nF the Motor runs in full action with the relais open.With 200R / 47nF the motor hums but does not rotate, with 200R / 22nF it hums not that loud, even with 4k3 R / 22nF there’s some hum left. (I doublechecked to switch life not neutral.)This is ‚off‘ with 4k3R/22nF:C1 yellow: relais signal (active low)C4 green: Voltage across the switch or RCC2 magenta: Voltage across RAnd 200R: / 22nF:Here, there are near to no spikes on c1.So this might be the RC to go with, if we ignore the humming of the motor.To address the humming it might be an option to e.g. half a second later switch off neutral too with a spare relays I have on the relais module.I also had a look on the ‚on‘ switching, which shows some contact bounce, but maybe tolarable …… compared with 4k3R / 22nF ‚on‘:As a next step I will implment the switching of neutral, which shouldn’d have any effect beside stopping the hum. But maybe the hum is a live to earth ground effect. An earth ground of the motor was missing, but I had added an earth connection again.And I will have some tests with an china clone Omron G3MB-202P SSR module that just arrived.https://www.codrey.com/electronic-circuits/solid-state-relay-ssr-primer/As always, any comment is highly appreciated.

author: Peter_O
author_rank: Frequent Contributor
timestamp: January 21, 2022, 02:08:26 pm
content: Here’s the test with the sameMotor 3… of the last post but with the china cloneOmron G3MB-202P SSR module instead of the mechanical relay module. The G3MB-202P is designed to switch at zero crossing. Which it seems to do.SSR, no snubber, switching off:Of course there is no bouncing any more and no noise on the relay control wire (yellow), but still a huge induction voltage peak of -800V over the SSR. (I needed to know and was lucky: The SSR survived.)With 200R/22nF snubber, switching off:And to get the complete picture:With 200R/22nF snubber, switching on:Finally everything is fine. 😊…except that the motor ist humming with the snubber in place.In case I go for the SSRs I will stick with an mechanical switch für L and N for the whole machine for security reason. Or better go with an contactor that could be swiched off completely by the Atmega and a mechanical relay after some idle time.To prevent the motors from humming I had the idea to switch off N for all motors e.g. 500ms after all motors are L-switched off, and switch on N before switching on the first motor.Now I’m thinking about two points:1I did some research and all sources agree that it is not OK to switch neutral alone. You shall always swich N together with L. Would it be OK to switch N alone, when you know L is switched off before N, or is it important to have the L and N switch ‚mechanicly‘ coupled?2Is it possible to use an SSR to cut N after L is ‚off‘ already, or does it need a voltage difference at the switch contacts?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2022, 11:12:07 pm
content: Going back to an earlier idea, you can put the snubber across the load as well, and this will not cause leakage current to flow. Prerequisites are having a bypass cap across the input (check) and short enough lead length between cap, switch and snubber (some feet will be okay).SSRs are much slower than sparking contacts, with risetime in the 100ns range, and turnoff delayed until load current drops to zero (so the amount of induction remaining in the motor will be small).Note there are two kinds of SSR: AC-only and AC/DC. The former uses a TRIAC, which latches on until load current drops to zero. The voltage drop is modest (1-2V), not varying much with load current. The latter uses back-to-back MOSFETs, with a stack of solar cells opposite the LED to generate gate drive voltage (typically something like 6-8V at a few µA); due to the tiny current into the large gate capacitance, switching is very slow (a few ms), and capacity is not usually very much, especially at line voltages; they do however excel at low voltages, where 10s of A ratings are easy enough, and of course being suitable for DC.The G3MB is a TRIAC type.TRIACs also have the property that, when exposed to overvoltage (breakdown), they latch on. Or they usually do. I'm surprised to see what looks like a flat-topped spike there, and 800V is plausibly the breakdown voltage of that device (the actual breakdown is always somewhat more than the rating (output PIV >= 600V) so this makes sense). I wonder if it just wasn't enough current to make it turn on? Certain types of TRIACs may be more robust to this stimulus, though I haven't seen any white papers documenting it yet.It also claims to be a snubber-integrated type, though clearly not too much (value) compared to your load, and they don't document what that value is, unfortunately.TRIACs can be destroyed under certain conditions, which I'm not real clear on, honestly; because they tend to latch on under breakdown, they tend to be quite robust. The most common failures I think, are overheating (too much current flow, die overheats, melts, whatever), and dI/dt (too fast turn-on causes current flow to be uneven across the die, causing local hot-spots and rapid failure). The waveform shown, should be okay: if it's not enough avalanche (breakdown) to cause damage outright (and I think they would latch on before that point?), it's fine. Most semiconductors act like a zener/avalanche diode in breakdown, reliably handling some current flow that way; typically just not a whole lot more than that, unless rated specifically so (like common rectifiers failing at a few to 10s of mA, or avalanche-rated MOSFETs handling up to some amperes but not more than that).Overall, an SSR will do a fine job here, free from the sparking of mechanical contacts, and (optionally, it would seem?) with a snubber to dampen the load.Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: January 22, 2022, 10:04:37 am
content: Tim, thx a lot again.The question what exactly happens inisde the SSR exists somewhat cloudy in the back of my mind and you've already anwered it.Overall:Quote from: T3sl4co1l on January 21, 2022, 11:12:07 pmAfter these first measurements I plan to go with SSRs. I will check, if these 2A SSRs are able to switch the 0.5A pump motor without cooling, otherwise I have some chinese "Fotec 40A" block ssrs somewhere, overkill, but definitely able to handle the motor's start current.Even if not strictly necessary, I'd like to stick with snubbers, now that I've spent some time and beginning to understand what's going on.That spike inspired me to think about a MOV again to get some security in addition to the snubbers.Put snubbers on the load, not on the swich:Quote from: T3sl4co1l on January 21, 2022, 11:12:07 pmI'm happy to follow your advice to put the snubber on the load. So I can stop thinking about weird N switching.Distance to the small motors is less than 1 ft, to the 80W pump motor it's 10 feet. It would be nice to have that snubber with the others, but it's possible to mount it on the pump motor also.From my basic understanding of physics I could imagine, that design and calculation might stay the same for R and C, so that the same values would be right?How do I size a bypass cap? Are the 0.1µF sufficient that are in place now?MOV:Thx for your ideas regarding the 800V spike.There was no stutter or a motor running at that time, nevertheless I like the idea to cut spikes with a MOV.I found a nice design paper, aplication note and datasheet:https://https://www.tdk-electronics.tdk.com/download/531266/5897f2786eaab0be1e043f85ba4ca55d/siov-selection.pdfhttps://www.tdk-electronics.tdk.com/download/185762/8cd06030e8ddf20bd381c1e8917f0e9c/siov-applications.pdfhttps://cdn-reichelt.de/documents/datenblatt/B300/YVLEADED-VARISTORS_EN.pdfI thougth about oversizing and choosing EPC B72220-S051 510V type, that covers up to 190J over 2ms. 510V would give some leeway above 325V for snbber overshoot to avoid stressing the MOV every cycle. Some useless cent spent due to oversizing would cause no harm, it's no mass production.When I put the snubber to the load, would you clamp the MOV on the load too or would it be better on the switch?Ah, and when I order that MOV I can spent some cent on oversized 5W resistors now that I know the values required.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2022, 10:26:10 am
content: I'd be worried actually that a 40A SSR might not even stay turned on, with as little load as you have...MOVs are kind of dubious, because the clamping voltage is fairly high compared to nominal. For example, the 275Vrms family in the first link (Fig.7 p.11) has a nominal rating around 350VDC, 430V breakdown (at 1mA), and 580V at 1A. Which is acceptable for a 600V switch (that's been observed to break down at 800V, even), but it's not much of a margin -- so this solution isn't always possible/reasonable. So, it does seem to be reasonable in this case, which is nice.Evidently -- knowing that your switch breaks down where it does -- a 385Vrms MOV might even do. A 510V definitely will not however.You don't need much MOV here. A little SMT chip type would even be fine, but a 5 or 7mm disc will probably be easier to wire in, heh.Mind that a smaller MOV will have a higher clamp voltage at the same current; checking the curve at the worst-case point, say 1A, should be fine.You can put the MOV across the switch. If you find that causes too much leakage current, then I would recommend putting it across the load, with some ohms (10 say?) in series with the MOV -- this limits surge current through the switch so it's not just hammering the poor thing when a line transient does come along. The resistor should be a fusible / flameproof type.Regarding the motor on a long wire -- note I didn't say anything about the load, only the switch, snubber and bypass! This is exactly such a situation where this arrangement is ideal: the stray wiring inductance acts in series with the motor, so we don't care about how much length is out there. Just that the switch and snubber are nearby.Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: January 22, 2022, 10:57:24 am
content: Tim, it must be in the middle of the night over there in Wisconsin?! 04:30, something like that.That a fasts answer was not expected at all.I will take your comments on MOVs and read the papers in more detail. Obviously I've got RMS and cut voltages confused.If I may ask again:- Is a bypass cap of 100nF sized fine?- Is it correct to just swap the same RCs across the loads, or is some recalculation needed?Any tip for a nice textbook to study the basics on snubbing etc.? I found some sources online to calculate R C, but nothing really comprehensive for educated beginners. :-)I'm starting to feel somewhat guilty to take advantage of your generous explanations.I mean, it's just great to get this one to one coaching. Appreciated!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2022, 01:13:57 pm
content: It's a popular misconception that I don't sleep; rather, I just sleep when I want to.Yes, 100nF is fine; basically, it should just be bigger than the largest snubber cap. That might not even matter (the exact answer depends on mains impedance, which depends on frequency; so, at the load ringdown frequency, if the mains impedance in parallel with the cap is less than the snubber impedance, it's fine..) but should be pretty safe. Anyway, you don't need much snub C here so that checks out regardless.Same RCs, yes.Theory comes down to: you're making a damped series resonant circuit.Tim

author: Peter_O
author_rank: Frequent Contributor
timestamp: February 20, 2022, 07:06:23 pm
content: Finally I’ve settled with a ‚preliminary final‘ setup.It integrates all three approaches:- snubber across the motors,- MOVs,- and SSR zero crossing switchingwhich might be some overkill.There is one china clone module with4 Omron G3MB-202P SSRsswitching the motors at zero crossing, one for each direction of the platter motor. They stay cool with the ‚bigger‘ 0.5A pump motor and seem to have no problems switching the motors.I put all of thesnubbers and MOVs in parallel with the loadsnow as described below in more detail. Because place is no issue, I put all of them on a breadboard pcb.Unfortunately, taking the snubbers from the switches does not fix the hum issue. Because theSSRs do not switch off completely but leave 7V RMS on the load, especially the platter motor is humming and you can feel that hum mechanically when touching the motor.Voltage across the platter motor when SSR is switched ‚off‘:So I stuck to the idea to have anadditional mechanical relayswitching off L for all motors with 1s delay after all SSRs are ‚off‘ (see schematics) and switching on with the start of the first motor. This with a delay to allow the mechanical contact to settle in ‚on‘ position before the SSR starts the motor. It works fine.For security reasons the complete deivce is switched (L and N) with a small but serious major brandcontactor(see schematics). A second mechanical relay (not in schematics) sits normally connected in the contactor‘s coil circuit and engages to switch off the machine completely after an idle time of 15 minutes.Also not in the schematics are fourfuses, one in the power connection for the complete machine as well as one for the 12 and 5V PSUs, one (1A slow) for the motors, MOVs and snubbers, and one for the contactor coil circuit.In more detail:All snubber capacitors are Wima X2 rated ones. The snubber resistors are 5W metal oxide resistors now.The MOVs are the 275V ones discussed above. I did some measurements to verify the effect but was not able to get any significant differences with versus without MOV. As Tim stated above, because of the zero Volt switching of the SSRs there seems to be no overvoltage to engage the MOVs. I decided to leave them in place anyhow. But - as they are oversized and seem not to be triggered into action normally, I did not put a e.g. 10 Ohm resistor in series. In case they end shorted, there is the 1A motor circuit fuse in place.Motor 1This is theline synchronous clock drive motorthat drags the suction arm across the record with L=9.1H.It's the motor that had killed the original snubber resistor. From the last test setup, the capacitor now is increased to 100nF, R decreased to 100 Ohm, because the snubber no longer bridges the switch, which in the last setup led to motor rattling.Switching off:C2/magenta:SSR input signal (active high now with the new four SSR module)C4/green:voltage across the SSR switch/load side.Motor 2This is the 80W vacuum pump motor, dragging 0.5A.The snubber here is 470nF/330 Ohm now.Switching off:There is some +/-450V at the beginning. I assume it’s somehow caused by the motor inertia, maybe modulated by pressure inside the three pistons. But honestly I have no idea. Anyhow, the 450V are not enough to trigger the MOV into action.Motor 3:This is the two directional permanent 10W split capacitor motor that drives the platter of the record cleaning machine.I did not chage the values. Originally the calculated ones were higher, but as 22nF/200 Ohm worked in the last setup, it’s 22nF/220 Ohm (new 5W resistor) now.Switching off for both directions:There is a small overshoot of 370V, but even less critcal than the 450V at the pump motor.One question left:Does the mechanical relay see any kickback when switching off the contactor’s coil?Obviously not. There is some bouncing, I’d say, but nothing to worry about. Even when switching exactly at max voltage as it is the case here.During switching,the ATmega shows no problems at allwhich was the major objective of this endeavor. I had it counting tenth of seconds on a I2C-display and there were no hickups at all.Tim and all, thanks for your input.It’s far from being a professional solution now, but for me it’s a status to work with, and I do now understand parts of what’s going on at least.I will now print a case for the display unit and start to write some more software to make the appropriate LEDs blink. 😊As always, any comment appreciated.