AC vs DC Switching Demonstration with Arcs
ฝัง
- เผยแพร่เมื่อ 13 มิ.ย. 2024
- The difference between switching AC and DC.
With DC, the arc formed as the switch contacts open is far more difficult to extinguish, and can cause significant damage.
Part 2 with normal switch: • 10 amp AC Switch used ...
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For me at least, this is the best demonstration and explanation of AC and DC switching I have seen - superb, thank you.
A good demonstration. Not just switchgear, though; I've got a solar PV setup in my house, and on the inverter there is a warning label: "DO NOT disconnect DC plugs and sockets under load - turn off AC supply first." There is a pair of switches for disconnecting both ends, but it's wise to switch out the AC end first (then the inverter shuts down internally). Of course, those who are used to DC electrified railways will be familiar with what happens from time to time!
Explains why relays have different voltage rating for AC and DC.
Double pole relay
240 volts AC 100A
12 volt DC 13 amps
so I have a ryobi compound miter saw that was given to me. was told the switch is bad. it runs of a 120 volt 15 amp DC brushed electric motor. I took the switch out after a failed continuity test and cleaned the contacts along with removing the brushes and sprucing them up as well. then I noticed the armature assembly needed a good cleaning so I did that as well. put everything back together and sure enough ran great. long story short my brother said he had the correct new brushes and a new switch so I let him take it and put them in. he did not have the brushes and his DPST 6 pin toggle switch...
blew a fusible link with the field. my question is can I wire in a DPST 4 pin that I have now that I have the field back to working order and if so can you help me out with the correct wiring schematic if I was to send pictures and or video. I really need this saw have 4" baseboard to cut and install l. thanks for the video as well very informative
I've only ever seen one other video like this before and this one does a better job of displaying the difference. Great video and thank you for showing everyone this important switching concept!
Can you dismantle a a.c. breaker and dc breaker next to each other to show the diffrence
Thanks for this. As a kid I used to play around with old switch gear that had “AC only” stamped on it. This really baffled me until now 👍
Would be interesting to repeat the experiment using 12v DC at different current ratings. I am using standard mains 10a light switches for a 12v DC solar installation running some 12v 10w LED floodlights.
probably explains why a dc inverter arc welder is much easier to strike than the older ac welders
Jusb1066 I got an old buzzbox, what a pain in the ass with aged rods
me too pdgeman, really annoying, sticking etc! mine is ancient big and heavy and a load of rods probably older than me, but it was cheap (free from a friend actually)
do you mean stick tig or mig? All of those are arc and sometimes Ac and other times dc depending on material.
stick weld is commonly referred to as arc weld, mig or tig, although use an arc , are not referred to that way
Well I have hardly ever heard it referred to as anything other than stick over det hundreds of welding videos I have watched. But in the age of brunel I guess it was. JK
I learned a lot, i never realized it is that much of a difference, thank you
Excellent demo. Thank you
Thanks for the demonstration
I have learned this the hard way when I used solar panels to power my water heater directly. Had to replace the thermostat because of burnt contacts.
Excellent demonstration. Thankyou
It would be interesting to show how the arcing can be reduced using different types of snubber circuits...
You can't really use a "snubber" here, because there isn't much of a large change in current or voltage, but rather a continuation of conduction through air instead of wire, so "arc-quenching" techniques need to be used.
Thank you again JW.. I learn something every video of yours
Spectacular video. Nothing better than a visual to learn
Great video there John. Thanks for the demonstration.
Brilliant demo as usual thank you. I wish other electronic experts were more approachable I’ve been trying to ask electronics user groups etc for a solution to my problem but it seems too simple for them to even reply? Thanks for sharing your considerable knowledge, I’m sure many appreciate it as I do.
BigClive needs to pick up on this and step up the game until it all goes horrible wrong 😈😈😈
Yes!
Either Clive or Photonicinduction
For anyone still watching this, I just posted a video of that 🤣
Great demonstration, thanks John!
I've always known the difference between AC & DC, but never made the connection (no pun intended) as to why a spark with AC is much smaller. Brilliant explanation, thank you for appearing randomly in my recommended feed!
Excellent learning demo. Thanks for your great effort
Thank you sir! As an electrician in Europe I found this very informative and helpful as well very well presented. It is very easy to understand. I have some advice or a challenge for you. Please do the same or similar video with name and tags about photovoltaic, solar cells so it will reach more people and bring you many new views also. There were 6 roof fires in my near area just this year, just because of photovoltaic. As I heard news from investigators recently, there was probably combustion because of an arc.
Cool project Jonh, thank for your examination
SWA Cleet used as a switch, genius :)
Great video
Καλημέρα φίλε μου. Είσαι συνάδελφος καταπληκτικός και κάνεις πολύ καλή δουλειά. Μπράβο!! Σε παρακολουθώ... I'm Kostas From Greece!!
Great demonstration! Thanks
Very educational demo...Thanks.
Nice demo of how conductive air becomes once it turns to a plasma.
Fixing small powerful magnets in the correct orientation close to the contacts can "throw" the arc out in a curve and help to snub it for DC. It works for AC too but the arc goes in two directions so positioning can be tricky.
Heavy duty DC contactors sometimes have them built in.
Viewers, don't make the mistake of choosing "gold contacts" thinking they are better. Silver is best for DC even at the small scale. Choose gas envelope enclosed contact relays if possible, the ones opened by a rotating armature are faster, if you can get them big enough.
I have long been puzzled by the regs quoting higher safe voltages for DC. Crunching your muscles in one direction rather than making them "ripple" they are a lot more painful, or so I assumed, and seeing it happen to others that conclusion seems safe.
You are a great teacher. Thank you JW :-)
Very interesting experiment!
Thanks for information you shared .
Fantastic explanation!
Good example. Thanks
I'll use my 3500V series mosfet switch to show the difference between positive and negative DC sparks.
difference in heat, color, and sensitivity to magnetic fields.
there are a few major differences...
Very informative, thanks!!!
John, another very instructive video. How about a video on relay switching of AC induction electrical motors. This could cover inrush currents and inductive spikes when switching inductive loads and explain how these are dealt, I.e. by relay current rating and an RC snubber, for example. Perhaps show a typical relay data sheet and explain about life of relay varies with kW (or horsepower) of motor. Perhaps cover concepts on LRA and FLA.
This very concept presents problems for designers of switchgear. Until just very recently high voltage DC circuit breakers were developed by ABB. Because of the extreme stress by the DC current, it has taken years to develop them for practical use. DC has always been difficult to design switches with. Frequently DC voltage is used for protective relaying in substations since it is easily obtained by batteries. However, this convention has forced the use of very high rated contacts in the substation to be able to cope with the DC voltage. Here in the US, 125VDC is very common for substation relaying power. The switches and relay contacts need to be heavily rated, and many break 2 sets of contacts in series to achieve the ratings.
Very surprising, never knew this before.
Be nice to see some differing types of anti spark and quenching methods used over the years in relays and fuses and then force a mosfet switch to spark.
Brilliant. Thanks very much
Wicked video, cheers John. You said that rectifier was rated for 150A, (on Facebook) what was the uF of your caps you where using?
60uF each, 120uF total - although they are manky old ones so actual capacitance may be less.
John Ward fair enough cheers for the response. Could I ask you maybe in an upcoming video to do this experiment with your Welder on the high current setting on a 6A AC MCB?
I think that would wicked. Reason I ask is that I was once fixing an installation were by a 50A AC MCB was used on a battery disconnect for a solar installation.... lol.
I’d like to see how the MCB reacts as all I saw was a burnt out thing that I then changed for the correct device. :)
Kyle
Try this test through a MCB with DC. It would.be interesting to see when you disconnect the circuit if it arcs the same.
Thanks (from a mature evening student) for the end summery about the 0o part of the wave form. To just ask, would the possibility of the arcing be increased (in terms of AC) when it's 3 phase with the 120o separation? Hope you don't mind me picking your brains,
JW, thank you for this nice demonstration. I wonder, how much current did the heater draw both when supplied with AC and DC? With AC, there could be significantly less current due to the inductance of the load. The voltage should be set to achieve the same amount of current in both cases, I think.
with 60hz there will not be significant inductance ,i think
Cheers John
+John Ward
What about wiring the DC//AC 240v heater through the CLIX 15A socket without back, and a suitable plug, if clamped down in place should be easy to press the big white button etc... Both contacts clearly visible on camera would be good!.
Will do - that particular switch is stored away somewhere so may take a while to locate it.
Thanks John Like your Videos
Very nice video.
+John Ward
A particular DCAC case I'm interested in is SELV 12v fittings and wiring. A useful upgrade in some situations is just to replace the 12vAC halogen electronic or magenetic transformer, with a 12v DC power supply (typically of reduced rating) to feed small MR16 12v LED bulbs free of 100hz flicker and minimum-loading and other related annoyances.
From what I can see, no real practical problem comes from this, and in any case the secondary side of 12vDC is not being switched in this case, seems likely not to arc anyway.
Any views on this?
sbusweb Are you asking if it is okay to switch the ac input to the power supply? If yes, then there should be no problem with this.
No problem with that.
No, I'm more asking if it is okay to re-utilize fittings intended for 12vAC, to hold small 12v DC led-bulbs, by changing both the power-supply and the lamps, but leaving the light tracks/wiring... Everything tells me this is fine, though in (some) cases the fittings SAY "ac only" on the MR16 holders.
sbusweb Well, if the fittings and the contacts in the fittings are in good condition, there should not be any arcing. So I don't think there would be a problem. Especially if the LED lamps have a lower power rating compared to the originally fitted lamps. However, I must make the point that this is general advice as it's not possible to give specific advice.
I've experimented a bit myself with arcs in a magnetic field. A decently strong magnetic field changes the way an arc behaves - makes it much harder to strike, shorter-lived and shorter in length. It also melted my copper wires with ease once inside the field, so I suspect hotter.
Vyl Bird Special DC contactor relays used to be made which had magnetic blowouts to suppress the DC arc.
Thanks a bunch
It would be great if you covered the construction of DC switches and showed them in operation. I noticed that some people insist that DC is an overall better system because of the efficiency you get with UHV transmission. I won't get into a debate on whether AC or DC is superior, but I will say that it looks like AC is more practical in the home so you can use higher voltages to use cheaper, thinner wires to distribute power while using more cheaply made switches. It doesn't look like you could use a 120-240 volt thermostat very reliably on DC without the added complexity of arc suppression. Certainly it would seem that you would want AC during a malfunction so the arc could much more dependably snuff itself out to help prevent a house fire. On a side note, I notice that your voice seems to drop in intensity in the middle of your sentences, and it becomes harder to hear you. Usually, I have the auto volume on with my television to help deal with the large variances in volume level on TH-cam between channels. I prefer to turn in off when I play music. Anyway, most people likely don't have a volume compressor on their audio. Perhaps you need a lapel microphone? You can get them rather cheaply on eBay if your camera can use an external microphone. Many people are just using their smartphones these days also so I don't know what you are using. Anyway, it should help boost your subscriber count to help normalize your audio pickup.
John could you do a video on overloading a 13 amp wall socket with a plugtop in it?
Thanks that was great ❤
Good demonstration, but what was missing (for me) was an explanation for why the arcing condition is so destructive..? Would it be something to do with oxygen burning thus acting in a similar way to a blow torch..?
Very good!
Super awesome demo of DC vs AC! This is probably the best demo of this i've ever seen. Not jokin'!
I've always wondered. Why do we use 50/60hz? Why isn't a higher frequency better? Isnt it true that higher frequency requires smallers cores on transformers?
I'm really curious.
/Dennis
The generators would need to run at 20x the speed they currently run at. Either that or add more poles to the generators. It's not cost effective. I suspect people would also be annoyed at hearing a continuous 1kHz tone
Also: If the frequency is high enough, the power lines would act like antennas, radiationg your energy into space instead of transporting it to the consumer. At 50 Hz, the wave length is 6000 km. Only extremely long transmission lines over a quarter of that wavelength (1500 km) will have issues. Which is part of why we use HVDC for such long distances. But at 1 kHz, the wavelength is only 300 km, which means any line longer than 75 km will start radiating a significant part of the power.
There's trade offs. Yes, a higher frequency would allow for smaller transformers. But as someone else mentioned, there's the skin effect. For 60Hz, the depth is about 0.4 inches. But for 400 Hz, the depth is 0.16 inches. Also power lines have capacitance and inductance. So some part of the power being transmitted over the power line is spent charging and discharging lines which reduces the effective power available at the end of the transmission lines. In a nutshell, DC is superior to AC for almost any purpose except for a few specialized instances.
1. It's easier to change voltages using AC cheaply.
2. As demonstrated in this video, it's easier to switch off AC.
But for almost everything else, DC is easier to handle. The choice of 50 or 60 Hz is a historical compromise between the efficiency of using DC with the ease of changing voltage for AC so that higher voltages could be used for long distance power transmission and lower voltages used locally for safety.
Check out Roobert33's version of this from August 2012, where he uses a big knife switch to demonstrate the difference.
Hi Mr. John I had a question. Why do DC breakers have three poles? What’s the third pole for? Isn’t DC just positive (+) and negative (-)?
Very good,thank you
John I have a gsm switch thats rated to switch 240v ac at 2 amps.
Would it be ok to get it to switch 12v dc, if I used an automotive 12v relay? Current to switch the relay would be probably be something like 300 milliamps.
Excellent and useful video, as usual. btw whats with the purple 'bell-end' behind you?
th-cam.com/video/5iwGzXCwC1s/w-d-xo.html
I remember when vehicles still used ignition "points" before electronic ignition took over, there was a "condenser" I. e. capacitor connected across the ignition points to spare it somewhat. Can a capacitor across the contact not serve the same purpose?
A capacitor along with a resistor. The vales must be correctly calculated based on load and voltage though.
Would the arc be the same if the caps attached to the rectifier were a different size or even removed ?
Would be interesting thing to look at with one of those super FPS slow-motion cameras.
I heared somewhere, if you want to switch DC with an AC switch, you basically divide the rating by 10, and it can switch it fine.
Why is the voltage drop so much more significant in the DC setup? During the AC test at about 4:20 it fell by just a couple of volts, but during the DC test at around 5:40 it was around 50.
With no load, the capacitors are fully charged 100% of the time and the voltage is the same as the peak of the AC input. With a load connected, the capacitors charge and discharge on each cycle and the voltage is nearer to the RMS value of the AC input.
Not to mention 240v is RMS voltage isn't it? the peaks are wayyy higher. Is that correct?
Yes, something like 340V. Well spotted - I shouldn't have said "voltage drop" which has a more technical meaning. What I meant was the amount (or, better still, proportion) that the voltage decreased.
@@Madsstuff The peak is square root of 2 * 240V = 339.4V peak .
@@Hrostbjergsorensen Interesting. Is it always the Square Root of the working voltage?
Very very good John well done hey how about doing experiments with capacitive loads, inductive loads as well as the resistive you've just shown
Kind regards Warren
Warren Gray this is an inductive load. Huge voltage spikes on disconnect indicate that, but JW hasn't got his meter across the heater he's got it across the supply so it's not shown. The heater he's used is basically a huge coil of thin gauge wire, which has inductance as well as resistance.
burgersnchips Hi there,
I am not sure that the fact that the load is a coiled wire and therefore inductive, I think it will play a part in the energy discharge, but I reckon the huge reservoir capacitors on John's test rig would cause it regardless of load as they have a low internal resistance and therefore can supply massive amounts of Q in one burst,
Load is not the main factor in what was shown,
But it is a good point that you have raised!
This wouldn't be inductive, otherwise it would completely destroy everything it's connected to. It is probably coiled, but the wire is folded in half before coiling so that any magnetic field would cancel itself out instantly, destroying the chance of being an inductor even worth worrying about
Yes, I learned this as an undergraduate trainee trying to break a 60Vdc supply with a relay. It melted! Eventually got away with a bigger relay and snubber across it, but these days prefer solid state solutions.
Billy Sugger trouble is solid state relays tend to fail closed.
Lee Wheatley They can, as well as Mosfets, which is what I’d use these days. But both are substantially more reliable than electromechanical relays.
Very informative video, but still missing one important aspect - the case when DC is just rectified AC i.e. current flows in one direction but voltage reaching zero at double AC frequency. Will it be sufficient to suppress excessive arcing or not?
also filtered
Very good
Very interesting demo, I didn't think it would ark that bad. Question though, would it be fine to use a normal house switch (AC 120v) on a 12v DC system?
Bille747 - my advice would be to derate the current rating of the switch by 50% minimum. So for a 5A switch, don't switch a 12V load greater than 2.5A. The lifetime of the switch will be even greater with even less current flow.
Mike EL - i don't know if i agree. this demonstration utilized high voltage dc, and thats the significant difference. high current dc loads (30-50 amps) at low voltage are very easily handled by standard 5 pin relays, take one apart and you will see the gap between the contacts is not very large. there is a point at which dc becomes dangerous to touch (as in touch ur 12 or 24 volt car battery terminals and nothing happens) i think its about 60 volts or so and that is probably the point where using an ac switch for dc becomes a liability.
Mark 1024MAK - generic 5 pin automotive relays rated at 50 amps in either 24 or 12 volt varieties have contacts whose size is less than 4 mm and their gap is around 3 mm when open so ur recommendation is wrong.
Its true that DC tends to be lower voltage but doesn't electric cars uses batteries that supply somewhere around 300 + volts?
Bille747 - dc is lik ac it can be whatever voltage it needs to be i have a few low voltage (12 volt) ac power adaptors its the same as low voltage dc if you touch the output it won't do anything to you. electric cars as far as i know are high voltage dc. they just put the batteries in series.
I read that DC discharge is not harmful like ac what is right and what is wrong please answer me cause I got confused
very good demo, Thank you.
What if I used a SPST AC Toggle switch to switch on and off a 12v DC Circuit that carries no load while switch is thrown? lets say I have a 12v dc cigar male one end and female other end with the SPST AC toggle switch (10amp at 220V AC) between the male and female point (Im using a cigar lighter extension cable with the AC toggle switch as a simple on off switch, If I only switch the toggle on and off with no load or very little like a 2 amp usb phone charger, and digital volt meter, would it still be as bad? I mean I can always throw the switch first and then only plug in the load, then you may ask what the point of the switch... the point of the switch is that always connected to the other end of the cigar cable extension (Female side) is a little digital volt meter to show the volts and amp drawn. I do want to switch this off, the vehicle that it is in, only gets used on weekends and the Digital Volt and Amp meters lit up display will drain power during the week, unnecessary
It won't be damaged with no load. Many AC switches have DC ratings anyway, the DC rating will be much lower current.
looks good for higher volts but what about 12/24/48/60??
John, Thanks for the video. Would it be a good idea to have a capacitor across the contacts when switching DC like on old car contact breakers in a distributor?
Nuts n Proud they are used for noise suppression. Radio's and TVs used to pick up that noise as they went by your house..
Nuts n Proud - A capacitor would not help with DC switching to reduce the damage by the arc.
can you use a capacitor to suppress the DC arc ?
On a related note, isnt there something called the skin effect which means the higher the frequency the more the current 'clings' to the outer part of the conductor, therefore for AC you need a thicker guage for any given current than you do for DC?
Skin effect is a thing, but it's not significant in normal AC circuits - at 50Hz in copper, skin depth is about 10mm, so is not a factor until you have copper conductors over about 300mm².
Would modulating the potential DC current reduce the arc?
Try the same experiment with an inductive load.
Looking forward to the follow up with real switches
You know... I just read the title and was thinking for a moment ~aww... shit~ but then I realized my switch will be just fine. :))
I'm using a 20A 240Vac switch on a 48V battery that powers an ebike. But for me it's all well and dandy as when I operate the switch there is never a load on the battery, there is no current flowing so there is no arching.
That’s interesting, i never knew that, but what exactly causes that problem like fundamentally? When you get specific, what is it about dc voltage that is different? And im not referring to the obvious here where 60 hz cycles on and off 60 times a second, im saying there must be more to it than that, right?
It's the obvious thing - AC voltage goes to zero 120 times every second so arcs are easy to break.
DC is on at full voltage 100% of the time.
Great demonstration. Nicely explained. Not sure about the caravan you are in.
It's a shed. No caravans here now, or ever.
i had no idea 245 volts DC was even capable of producing a sustainable arc. The lowest voltage arc i've even been able to make was while monkeying around with a microwave oven transformer and I could stretch the arc out to about 5 inches before it breaks (about 2100 volts but then it is AC).
How AC 50Hz voltage goes 100times a second thru 0V ?
how meany sw Amperes on AC and DC voltage?
Halogen heater or fireglow bulb creating the orange glow, either way it was dimmer on DC.
Voltage was lower - it was set with no load which was nearer the peak of the AC, and voltage dropped when the load was applied. You can see the voltage drop on the multimeter when the switch is closed.
A great demo.
Would I be correct to assume that the reason the arc is bigger while disconnecting (as opposed to connecting) the heater from the DC supply is due to back EMF from the inductive load of the heater?
There's not much, if any, back EMF from a resistive load, so no, the arc that's formed when the contact is closed is quickly quenched by the fact that the current prefers to take the easiest path from metal straight to metal. On contact opening however, the arc ionises the air it's flowing through, which makes that air highly conductive, so the arc grows, gets more powerful, ionising even more air as the contacts continue to open, as so the current can continue to cross the increasing gap, ionising more air as it does.
Ken Phillips I was imagining the heater element was a big coil of wire, kind of like a big air core inductor but I guess even in that case it’s inductance would be pretty low. So I guess like you said the load is mostly resistive. Thanks for the explanation!
As the supply voltage is fairly low, at 240v DC, it's incapable of jumping across the gap of the contacts until they're very close, so only a very small spark at switch on. However, when opening, the current is already flowing so as the contacts start to open a tiny bit the current can initially jump across the gap and then as the gap increases, as Ken Phillips said, due to the ionising effect of the initial spark on the surrounding air the spark grows and continues to do so until the gap is much wider.
Good point, that I wasn't specific enough about saying, so cheers for the clarification. I only learned about stuff like this properly when I witnessed a cheap incandescent lightbulb fail, one that apparently didn't have any internal fuses. It popped slightly, flickered, then lit up like a small blue white sun, before exploding and taking out the fuse (a fuse wire one!) in the distribution board. So yeah, that's when learned how conductive that plasma's are :-)
As the heating element is probably a coil, albeit probably a loosely wound one, there would actually be some inductance because it'd be creating a magnetic field, but it'd be negligible compared to the resistive load. High power wire-wound resistors can sometimes cause issues in electronics, because their coiling can be tight enough to start causing significant enough amounts of inductance, which can unexpectedly upset circuits. I love stuff like this because it really does show why electromagnetism is called such. Even how the iron filings and stuff align around the return cable of a welder is fascinating, because it highlights just how powerful the field around even a straightish wire can be if the current is high enough.
Why is the heater humming with DC (for ex @4:57-5:00 and @5:22-5:29) and not with AC?
foufou33g Maybe its because the resistance changed because DC doesnt has to push against the magnetic field. That could end with more power and the Variation in the voltage(because its not pure DC) would make the sound. Its only a theory. It all depends on what kind of resistance this heater is. But that would make this test useless. I mean 1A 240VAC vs. 10A 240VDC wouldn't show the difference between AC and DC.
How do the Noark 63A DC breakers "pull it off" ?(No pun directly intended.)
DC breakers and switches are designed with much larger contact gaps, and usually a very fast spring loaded opening mechanism.
what was the Amp on DC ?
Is it a fair comparison? 240VAC RMS carries less energy than 240V DC so of course the DC arc will be brighter.
a frend from "elektronik mal einfach " make a similar vid! With a 1000 watt halite lamp a 25A relais manuell switcht is after first try with dc melt together! but you realy had to say that the same problem with fuse is!
i found a street light in malaysia and this is so common where they still dont use circuit breakers (some of them does) but a lot of light poles still uses fuses or some of them are just like bypassed with a wire. well come to malaysia and look at the street poles you can see a lot of street lights cover opened with some black overhead cable.... some of then were taken action by the goverment and they put some metal plates to cover it but most of them are real crap. i touched one of the black overhead cables near the "protected wiring circuit". and most of our street lights are 260+ because the voltage on these lights arent balanced. the reason why they use those overhead black cables on the light poles because the armoured cable inside the road is not working anymore. now if you touched those cables going to the light poles most of the time they will spark if youre really lucky it wouldnt spark at all. well one day i was at a clothing shop there was this not turned on street light with its black overhead cables coming to provide power to light because the inroad cables are broken i touched the live wire and "BZZZZZZZ SPARKSSSSSS BZZZZZZZZZZZ *and then light turns on*" theres some people in malaysia who's almost killed by a lamppost because at some state theres a problem where the live has leaked to the metal body, and most of the times when you ground it, theres not gonna be a RCD nearby
I think Tesla's way was right given the choice,
A.C. has it's issues.
But D.C. issues tend to be more destructive, as
Well as the arcing john has shown, electrolytic action can effectively plate contact areas effectively forming dissimilar materials at junctions,
The whole basis for metal plating, but sometimes it's an effect that will not be required and then it becomes a maintenance issue, it just pays to do research on these matters!
If AC electricity is switching between negative and positive flow why don't appliance motors change direction with the polarity change?
Twig depends on the motor, for standard squirrel cage and universal motors their stator magnetic field is also generated using the line voltage, so it flips exactly as does the rotor field allowing it to continue to rotate. Universal motors can be used on both AC and DC and are actually more efficient on DC I believe
If you connect a DC motor which has magnets to AC voltage, then it does as you say, it will vibrate better than any whore's vibrator.
How will a AFDD react to this?
Just out of curiosity, why are capacitors called capacitors most of the time but, when they're associated with engines, they're called 'condensers'?
Condenser is the older name, they were all called that once.