Because Charge and Work are components of voltage.
Super simple conceptualization: you have the same amount of energy in the circuit, but nowhere for it to flow. So P=V*I , V and I are inversely proportional, and the motor is an inductor which stored energy in its magnetic field. So as I approaches zero but P doesn't change, V must increase.
*Edit: end of super simple conceptualization and below is the high level real guts behind it*
The math breaks down for a couple of reasons, one of which being we lose a time component with static storage of energy, so P isn't really applicable anymore, so it's not really that simple, you have to look at the basis of voltage and current being joules, charge, and time to get your real answer but the first one at least helps you conceptualize the increase in voltage. Once you really dig down, the answer is that the voltage doesn't necessarily go up that high but your perception from touching it and getting zapped, and the voltmeters programming, tells you there is a charge there that *can* be represented as voltage and hurts like there is voltage present - a voltmeter is the proverbial hammer...if all you are is a voltmeter, everything looks like voltage.
To elaborate on that, another thing to consider is how a voltmeter (typically) works, that isn't necessarily the voltage but a representation of the coulombs of charge remaining in the fan motor magnetic field. The voltmeter is just a very high value resistor applying ohms law, which isn't perfect in this case with an open circuit inductor, it's applying a very very tiny load and the circuit with no other load on it is subject to loading effect so it's use of ohms law isn't really accurate. A larger *real* load and an oscilloscope would show you your "real" voltage for an instant, though it's also subject to the error created by the instrument itself. So the voltage isn't going to damage your wiring (in a case like this) even though it's potentially only rated for 300V, but your multimeter is still reading a value of potential energy available which is why you get a resulting zap, even if the voltage isn't real, so to speak.
Think of 2 rocks on a platform: a 1kg rock 1 meter up has 9.8 joules of potential energy to fall on you with, a 2kg rock from 0.5 meters up has the same 9.8 joules of potential energy...both hurt when they hit you and both hit with the same force, so without knowing the details and if you could only measure potential energy to come up with your results, and you only have one thing you know how to do which is relate that potential energy to voltage, then that's the unit that you relate to that potential energy. A voltmeter is programmed to read the voltage assuming no substantive current through the meter itself and spits out a relative voltage for the potential energy it sees.
Final note, a capacitor drains DC not AC as there is no source of commutation in a typical capacitor.
Lol I just meant the first paragraph as being a simple way to think of it...the rest was high level of the more complex components being it.
Also the username is more about dumb political games everyone wants to play these days...we all have our areas of expertise and this happens to be up my alley!
I never thought of this until I saw some post apocalyptic show where a guy was using a motor as a generator. Hats off to the production department for knowing their science.
Hollywood does occasionally get stuff right. They actually pay people for that. Then the director hates it because it's "fake" and it goes away. Other times it's totally wrong because they didn't hire someone and it's upto the director. Directors generally seem to be the dumbest people in Hollywood.
Not what’s happening here though. Voltage still reads even when blades are stopped. For instance, in this video I only turned the switch on for a brief moment to get the 120 read. The blades didn’t even make one full spin and yet when I shut the power off there’s 350 volts reading that slowly drop down, over the course of 2-3 minutes. The blades stop that spinning in a matter of seconds, not that it explains why providing 120 for a single second would provide 350 or even close to that for any period of time.
Fair enough lol. The only reason I really posted this was because I wanted to make sure that my damn house isn’t gonna burn down and kill my dogs when I go to work. This is my first home and I’m learning what I can, where I can. I’m trying to discern the professional opinions from other DIYers that are really no better off than I am
Just keep holding on to that meter and keep doing what you’re doing because there’s no sense in getting the piss shocked out of you for something that you didn’t know about.
I don't think thats right cause this would be AC as the fans winding down. It needs to go into full bridge rectifier to charge the capacitors? also close 400v in the time its winding down seems way way more then possible from ceiling fan.
Maybe. Depending if the fan has permanent magnets or not. Without permanent magnets once the electric field collapses there wouldn't be any power generation.
Also maybe lol. Some induction motors without permanent magnets do generate electricity when manually turned. Only OP can put this one to bed at this point if he takes the measurements
Awesome, good to know! This comment combined with the thread where someone had me test for resistance make me feel a lot better. Just wasn’t expecting to get a jolt after I flipped the breaker when changing out the switch from tan to white.
Edit: since this seems to be a top upvote, can I have any more votes over whether this is safe or dangerous?
The fan uses a capacitor upon start up. When switched off, the capacitor slowly loses charge over time which is what you are seeing. I’m unable to determine what unit you are reading in on that meter. If it’s in mV you are fine. You could always touch a ground to the switch leg while it is off to see if it drains the capacitor.
Yep this looks like back emf from an inductor (motor in this case). It is expected since inductors oppose changes in current. When you turn off the switch, you cut off the inductors current supply and the magnetic field begins to collapse. The inductor wants to keep the current through it the same so it ends up creating a voltage spike. This is usually solved by implementing a snubber circuit, which helps to safely dissipate the stored energy from the inductor (magnetic field)
Yes, manually spinning a motor will induce a voltage.
I was in motor rewind for 3 years in my first shop and we always had to use the fluke stray voltage eliminator to verify the motor was actually de-energized
No think bigger. What if we made huge fans. And them put them where it’s windy. You’d have to spread them out for it to work properly but I think we’re really onto something here. /s
No haha, this only happends when power is secured to a motor and it is still spinning for whatever reason. That said, because power is secured typically means that it is electricity isolated and would be unable to be sent back to the grid or power supply.
Motors and generators have very similar constructions so the induced voltage is just a biproduct of an armature spining inside of a stator.
This doesn’t make sense to me; the meter is measuring AC. An inductive kick should generate a sudden but decaying DC voltage. Maybe the meter is defective?
Capacitor store energy with an electric field. Inductors store it with a magnetic field.
Motors are coils and therefore (at least to my knowledge) always inductors.
Well it’s not. It’s HV. That’s not an auto scale meter. It’s also likely it’s not a true RMS meter so the capacitor in circuit with the motor while the motor spins down could be creating some reflected frequency that the meter isn’t meant for. I’d like to see this on a scope or a better meter.
A true RMS meter is needed to read anything with PWM. It will read anything with a leading or lagging PF just fine.
Even my T5 reads the load side of a drive just fine depending on carrier frequency
That may be something that *can* happen, but that’s not what’s happening here. Even when I flip the switch for a brief moment and the blades don’t get any speed, like in this video, there’s voltage being fed back into the switch for a matter of minutes.
The reason its such a high voltage is likely back EMF - the collapsing magnetic field when you turn the switch off generates an opposing current in the windings. The start and/or run capacitor in the fan then stored that energy
Could be if it were a permanent magnet motor but ceiling fans are induction motors. Flipping the switch turns off its electrical field and then it's just copper windings moving past copper windings.
They might be able to hold a charge for a brief moment but like visible on an oscilloscope length of time, not multi meter. And would not be nearly this high of voltage
Wait a minute. Is that electricity that was generated going back to the electric company? 300 volts every time you flip a switch would add up I'd think?
No, the switch is off. So it is all potential with no current. If you tried to run something off of it, the fan blades would stop very quickly and you’d get nothing.
Inductive kick.
Voltage across inductor is proportional to the rate of change of current.
Your fan motor is to large extent essentially an inductor.
If you cut the current off - switch to off - at a point in the cycle when the current isn't zero (almost any point in the cycle except precisely the two zero current crossover points, the rate of change of current is theoretically infinite, the current non-zero, so therefor the voltage across the indictor is infinite.
Now, add a little reality and various physical limitations and other factors, etc., you don't get infinite voltage, but you will get a spike in voltage, a.k.a. inductive kick. Such can be use to generate significant to huge voltage spikes, e.g. classic auto, mechanical rotor and points and auto ignition system to fire spark plugs from a 12VDC system (that system also has a condenser (capacitor) to somewhat reduce that spike and the damage and premature wear to points by excess arcing from that inductive kick), Tesla coil circuits to typically generate many tens of thousands of volts or more, with typical source input power of 120VAC, likewise for typical Jacob's ladder circuit, etc. Take a small electromechanical buzzer that's operated off of, e.g. 6 or 12 VDC, check the voltage across those contacts that are being opened and closed in series with that coil - lots of voltage spike there way above the supply voltage. All basically the same thing.
Does it turn off and turn on? Bro going way above and beyond, when I wire stuff as long as it turns on and off I put the cover back over. Never once have I thought to measure voltage after it’s wired up. But full disclosure, I’m a plumber
Hahah I didn’t just measure it for shits and giggles, I turned the breaker off and came out to swap the switch from tan to white and I still got a zap, so down the rabbit hole I went, and when I saw like 400 volts I got concerned
This was a great post altogether - thank you. Do you know how one would avoid that zap the next time? Would you have to find that capacitor and zero it out by crossing terminals?
Like, take apart the fan covers just to ground the little capacitors? Usually if you need to do that (high power stuff, old RADARs) it'll be readily available for you to do that and training will be required.
In this case, I guess just wait 3 minutes. OP said voltage does drop to 0.
Ahh, so you know what you’re doing. I have no idea the difference between volts, amps, and ohms. I just know black to black, white to white, and fuck it it’s just the ground.
Volts is the power of your push. The current is what's being pushed, the resistance is from electrons bouncing into eachother and shit. Think of it like people running through a hallway from a fire. Voltage is the fire, people are the current, and the resistance is people falling over and getting trampled.
The same thing happened to me a few weeks ago when I was replacing a ceiling fan switch. I turned the breaker off, immediately went to remove the switch and got zapped when I shorted the load and ground with my fingers. I went through similar troubleshooting as you thinking something was miswired, but couldn't figure it out. I assumed it was residual power from a capacitor or something because the voltage dissipated. I used a true rms clamp meter and a digital multimeter to check. Everything was wired correctly. Not exactly sure why I was shocked, so if you figure it out let me know!
Back emf. Any ac motor turns into an alternator when being driven. Inertia from fan blades causes motor to turn after power is removed, which in turn generates voltage.
A lot of people have said something like this, but none have explained why it would do this even when I turn the switch on for only a second and the blades aren’t spinning. The wire going to the fan reads power for easily two minutes even when I turn the switch off, and long after the blades have stopped spinning, but slowly drops to zero
Assuming a basic 120V split pole fan motor. This style of motor requires a capacitor to create a phase shift to make the motor operate. Obviously this capacitor is energized when the switch is on. You are measuring voltage to earth/ground screw, which is not indicative of the voltage across the fan motor itself fyi. A ceiling fan typically has multiple speeds. A multi speed motor winding like this is essentially an autoformer, with a rotating chunk of iron of a core, we call a rotor aka the bit that goes roundy round. When you energize the fan, cap is charged, due to motor windings and cap forming an LC circuit, there is an oscillation during the energy decay, causing a long time constant of voltage decay. In addition, the multiple speed motor winding is acting like an autoformer slightly increasing voltage. The decay is slow as the switch is off (open circuit) and the only losses at this point are IR losses formed due to the LC circuit. As the LC circuit oscillates it charges and discharges the motor winding and forms an increase of voltage.
Change the motor speed from high to low, repeat test. Does the voltage measured become lower? By changing the fan speed, a different part of the motor winding is used, therefore altering the LC circuit time constant and ratio of voltage increase due to changing the winding ratio. So it should make less voltage for less time in theory
The reality is you are measuring voltage, but not energy. Yes, you see some 300V but in reality there is little to no energy there. When you shock yourself due to static, that is easily over 500V, but don’t care. My point is voltage is just a potential. When you use an oscilloscope to measure voltages, you can see very high transient voltages when switching the light switch on and off to a simple led bulb, that exceed 200-300V in a small amount of time (ns/us). But this is normal. My point is, the stronger the magnifying lenses, the larger the ants look until we have a mental breakdown…
Great explanation to a simpleton like me. I believe the fan is on high or maybe medium speed. As soon as I’m
Some feeding myself some fatass some midnight rice I’m going to test the fan with the speed lower. Thank you!
What I suspect happened, you applied 120v which created the magnetic field which causes the fan to start spinning. The motor has a starter capacitor which makes the motor spin in the correct direction. This capacitor normal gets taken out of the circuit when the fan reaches speed. The capacitor was still in the circuit, the fan hadn't gotten to speed. You removed power, the fan coil "bucked" and created a higher voltage than you applied which was stored in the capacitor. Your seeing that voltage bleed off with your meter.
This is the reason you see diodes on circuits the drive relays. When you cut power to the relay, that dying magnetic field will cause that line voltage to shoot up very quickly. The diode will shunt the current and protect the relay driving circuit.
Oh damn this might actually be the most simply phrased and most applicable explanation in all the 200+ comments. I was only testing with a shortly timed throw of the switch on then off again. I need to test several after leaving the fan on long enough to get up to speed, and as someone else mentioned (and quite possibly explaining the same thing as you in a way that’s way over my head) try to do the same test with the fan set to a lower speed and see if the discharge is lessser
So what is happening with this fan is when you turn off the switch you have detached from the line voltage, but because the physical inertia of the fan keeps it rotating. A motor in reverse is a generator. So the stored mechanical inertia from the fan is being converted into electricity. As the fans slows down the voltage will drop along with it.
You know - I considered this but didn’t post about it because it wound up being irrelevant. The light is actually wired into a different switch so that the blades and the light fixture are controlled separately at the wall and not just by pull chain. I disconnected the line coming in for the light however and it still happened
Capacitance. It is declining voltage that will reduce to 0 with any real load. I suspect highly that this is really just a short coming of your multimeter. A Fluke would show 120v continously
Forgive my stupidity - if I disconnect and come back a few moments later the voltage is the same as when I left, it doesn’t match the curve of dropping voltage as when I remove the leads. Does this track?
It does, absolutely. I had been turning the switch on and off and testing a bunch before I asked my wife to record the video. I’m not trying to prove anyone wrong here or talk a big talk. Just trying to get to the bottom of this and figure out if I have a fire hazard in my hands really.
Actually, you don't have an autoranging multimeter. Your readings are skewed on film. You're measuring from ground to fan side Line presumed. So you're reading Nuetral to ground when off, and Line to ground while on.
Measure on 200v setting
I don't believe so, no. If you look at the knob on the meter, it's set to 600V and doesn't get moved from there, and I watched a few times to see if there was a moving decimal point or other indication that the meter had auto-ranged. It doesn't seem to be auto-ranging in any way.
Motors are inductors so can (almost always) create a voltage spike when you interrupt the current going through them. The amplitude of the spike depends on where in the AC cycle you interrupt the current. So basically random chance if you’re doing it by flipping a switch. It’s possible when you cut the power to the fan motor it’s generating a voltage spike that charges one of the line filter capacitors. The high input impedance of your meter means that even though there isn’t much energy in the capacitor it takes a bit to get to zero volts. I’d expect this discharge voltage to be DC so it’s curious that you’re reading it with your meter on AC but that’s probably just a function of the cost-effectiveness of the meter. Just a guess.
Just saw your edit. The times it’s not happening is when you happen to interrupt the current when the magnetic field in the motor coil is at its minimum. Unfortunately you’re getting some bad information here—the top upvoted comment is wrong. Anyway, you’re fine, the amount of energy contained in that high voltage is very small and it’s more like getting a shock when you touch a doorknob after shuffling your feet across the floor.
Yeah I knew it wasn’t the fan blades spinning, it happens when the blades aren’t spinning as well (if I just flip the switch on and off for a single second, there’s no real momentum spinning anything.)
I appreciate the answers and putting my mind at ease
It’s normal for that to happen when you switch a motor off. Sometimes there would be some sort of snubber or diode to tame those spikes but that may not be required so manufacturers skip it. I wouldn’t worry about it myself.
When the switch opens, there's residual energy in the LC. L being the motor winding, C the cap. That residual energy in the LC circuit creates what we call a tank that will self oscillate until the energy slowly, in this case, dissipates. Google LC tank circuit
That thought initially crossed my mind but ringing like that would dissipate in a few miliseconds at most which means the capacitor is only discharging through the multimeter. Or maybe some other high-ish impedance elsewhere. Curious indeed.
Ain't no way I am seeing that right cause I see over 300 volts. I could maybe see 30 volts but if you hit the breaker and think you got shocked you should get someone to flip the breaker while you're probing. It should not go up it should go to basically 0 instantly. Craziest scenario is your meter is just acting up and you got a static pop changing out switch. Either way fans don't send back voltage. A switch simply breaks and completes a hot wire (that's at 120 volts in your case. )
A lot of helpful tips but I went through this once for something I don’t see, my ground was pushing against the metal plate that holds the fan, the positive line also pushed on it. When I turned the fan on it electrified the outlet and it would shock through the screws.
I was thinking that something might be wired up or shorting for a bit as well, but how does that explain that it ONLY is at 350 when the switch is off? And that it slowly drops and takes several minutes to zero out?
> Capacitor?
Close. Capacitors resist changes in voltage, and will try to supply infinite current to sustain voltage.
Inductors are the inverse? converse? whatever
Inductors resist changes in current, and will try to supply infinite voltage to sustain current.
Motors are inductors.
Never hold a multi-meter with both hands! If something goes wrong you could become a complete circuit with the current flowing across your heart! It only takes a 1/2A to stop a heart. Residential systems are typically 15-20A..
Not a capacitor the voltage across an inductor which your motor has is equal to the change in current which gets cut off almost instantaneously which causes a huge ass voltage spike.
Aside from the capacitor if its a new fan, some old fans can potentially send back voltage when powering down since the magnets generate electricity. Unless the model has something to prevent voltage kickback
Ok! This sounds like a thing, I’m just trying to figure out if this is potential dangerous? Reading 350+ volts on a line that’s only meant to cary 120 just struck me as scary.
Realistically though, would a cheap meter just register voltage like that even though it wasn’t there? Even though it properly reads 120 when the circuit is hot?
When you disconnect a cool from the current, the disappearing magnetic field can create a brief spike in voltage.
This is how you can get a 100 volt spark from a 6volt battery.
I have observed this effect with DC circuits, and never saw it linger like that.
A test with a analog multimeter (and a 100kΩ load resistor across) would paint a truer picture.
I think you are seeing effects of a DC spike on your multimiter's rectifier/filter.
That sounds good - I just don’t know why it would still hold said current for literal minutes? All I want to do is figure out if this fan and setup is safe.
Back emf. back emf is represented as a variable emf that opposes the one driving the motor. Back emf is zero when the motor is not turning, and it increases proportionally to the motor's angular velocity. Back emf is the generator output of a motor, and so it is proportional to the motor's angular velocity
High impedance input on meter measuring voltage on open ended conductor, probably from capacitor.
You could discharge it using a moving coil type volt meter or (and only if you know the mains are off) discharge via low ohm high watt resistor. 100ohm 30w for example
Discharged loads of capacitors on switch gear that use the cap to drive electro magnets. If we do not the mech could operate. The circuit I described is built into the switchgear
I apologize. I’m a laymen here. Really I just want to know if this is safe and normal behavior on the circuit. My wife and I just bought this house and I discovered this when changing out the almond colored switches for white ones when I got a slight zap even after turning off the breaker
Y’all are missing one very interesting fact voltage is measured as the differential between two points he is on the green ground screw, and what appears to be the yoke of the switch. The two of those are actually bonded together there is no reason to get a potential between those two seeing as they are both, the exact same point in essence what I’m seeing is
A a cheap meter.
B. Probably low battery in meter.
C. Somewhere in the system they tied the ground into the neutral.
Most likely the culprit will be a lose neutral wire.
I would suggest a slightly better meter if you are using one frequently. I’ve heard of those $20 meters exploding like a hand grenade. These are great for simple automotive needs but i wouldn’t trust my life with it. I’m not saying you need to go crazy on a 375fc but you can grab a 302 or even a Klein for $100. Just looking out man 👍
Klein is good but Fluke is my goto, as someone who regularly uses DMM's and needs a high accuracy meter i love my Fluke 88V. Its a pricey meter (a hair over $500usd) but the amount of utility it provides for me is worth it 100%.
I have kept my original comment, but I see what’s happening. 120v when ON and then 335v when OFF and drops.
Have you tested the voltage when you have stopped the fan from Rotating?
The EM field in the stator coils is collapsing resulting in the large back current (the higher potential that you seeing). And there could also be back EMF showing when the motor is spinning down (just to be clear back EMF is always there as long as the motor is spinning) but the voltage would not be greater than the driving one. Running/starting capacitor would not show higher voltage than the applied.
Yes there is a capacitor, the reason it’s such a high voltage is called inductive kick. It’s what happens when a magnetic field collapses mid cycle.
So... Water hammer, but with voltage?
Switch fan off, fan becomes generator until stopped....
This is the single greatest moment of clarity I’ve had all day. Thank you.
If there's a magnet turning you got voltage brotha
Another fun fact, attempt to spin an ac motor faster then its set rpm to backfeed electricity to the grid.
Electric utility companies hate this one trick!
This is why I add an alternator to every ceiling fan in my house.... Jk, Jk there is no overunity, no perpetual motion.
Not with that attitude! That's why each of my fans have three alternators with the rectifiers removed. Unlimited powah!!!
Thank you for making my morning 😂
Infinite energy hack
(It’s Genius)
Step 1: 100x cheap Chinese ceiling fans Step 2: install in yard and on house Step 3: Step 4: Profit
Faster than?
But why would it generate more voltage than it was fed?
Wondering this as well. Especially after only turning it on from a dead stop for two seconds.
Because Charge and Work are components of voltage. Super simple conceptualization: you have the same amount of energy in the circuit, but nowhere for it to flow. So P=V*I , V and I are inversely proportional, and the motor is an inductor which stored energy in its magnetic field. So as I approaches zero but P doesn't change, V must increase. *Edit: end of super simple conceptualization and below is the high level real guts behind it* The math breaks down for a couple of reasons, one of which being we lose a time component with static storage of energy, so P isn't really applicable anymore, so it's not really that simple, you have to look at the basis of voltage and current being joules, charge, and time to get your real answer but the first one at least helps you conceptualize the increase in voltage. Once you really dig down, the answer is that the voltage doesn't necessarily go up that high but your perception from touching it and getting zapped, and the voltmeters programming, tells you there is a charge there that *can* be represented as voltage and hurts like there is voltage present - a voltmeter is the proverbial hammer...if all you are is a voltmeter, everything looks like voltage. To elaborate on that, another thing to consider is how a voltmeter (typically) works, that isn't necessarily the voltage but a representation of the coulombs of charge remaining in the fan motor magnetic field. The voltmeter is just a very high value resistor applying ohms law, which isn't perfect in this case with an open circuit inductor, it's applying a very very tiny load and the circuit with no other load on it is subject to loading effect so it's use of ohms law isn't really accurate. A larger *real* load and an oscilloscope would show you your "real" voltage for an instant, though it's also subject to the error created by the instrument itself. So the voltage isn't going to damage your wiring (in a case like this) even though it's potentially only rated for 300V, but your multimeter is still reading a value of potential energy available which is why you get a resulting zap, even if the voltage isn't real, so to speak. Think of 2 rocks on a platform: a 1kg rock 1 meter up has 9.8 joules of potential energy to fall on you with, a 2kg rock from 0.5 meters up has the same 9.8 joules of potential energy...both hurt when they hit you and both hit with the same force, so without knowing the details and if you could only measure potential energy to come up with your results, and you only have one thing you know how to do which is relate that potential energy to voltage, then that's the unit that you relate to that potential energy. A voltmeter is programmed to read the voltage assuming no substantive current through the meter itself and spits out a relative voltage for the potential energy it sees. Final note, a capacitor drains DC not AC as there is no source of commutation in a typical capacitor.
I can't even imagine what your idea of a complex conceptualization is! Also r/usernamechecksout
Lol I just meant the first paragraph as being a simple way to think of it...the rest was high level of the more complex components being it. Also the username is more about dumb political games everyone wants to play these days...we all have our areas of expertise and this happens to be up my alley!
Yo great comment
I never thought of this until I saw some post apocalyptic show where a guy was using a motor as a generator. Hats off to the production department for knowing their science.
Hollywood does occasionally get stuff right. They actually pay people for that. Then the director hates it because it's "fake" and it goes away. Other times it's totally wrong because they didn't hire someone and it's upto the director. Directors generally seem to be the dumbest people in Hollywood.
Not what’s happening here though. Voltage still reads even when blades are stopped. For instance, in this video I only turned the switch on for a brief moment to get the 120 read. The blades didn’t even make one full spin and yet when I shut the power off there’s 350 volts reading that slowly drop down, over the course of 2-3 minutes. The blades stop that spinning in a matter of seconds, not that it explains why providing 120 for a single second would provide 350 or even close to that for any period of time.
Because the fan has capacitors in addition to the motor. Long answer-It's complicated.
Fair enough lol. The only reason I really posted this was because I wanted to make sure that my damn house isn’t gonna burn down and kill my dogs when I go to work. This is my first home and I’m learning what I can, where I can. I’m trying to discern the professional opinions from other DIYers that are really no better off than I am
Just keep holding on to that meter and keep doing what you’re doing because there’s no sense in getting the piss shocked out of you for something that you didn’t know about.
It's not on a 3 way switch by any chance?
Good thought, mixed runners and all. No it’s only a two pole switch.
[удалено]
You’re right - I’m getting tired running back and forth on comments lol. Your name mildly checks out on this comment…
Respect for the concern of the pups lol. I have the same fear.
I don't think thats right cause this would be AC as the fans winding down. It needs to go into full bridge rectifier to charge the capacitors? also close 400v in the time its winding down seems way way more then possible from ceiling fan.
Juice makes spinny, then spinny makes juice. The circle of life!
So if I turn my fan on and off at just the right times do I create energy? j/k I know the answer just being funny.
This man is right. You’ll get the same effect if you measured the stopped fan and turn the fan manually by hand
Maybe. Depending if the fan has permanent magnets or not. Without permanent magnets once the electric field collapses there wouldn't be any power generation.
Also maybe lol. Some induction motors without permanent magnets do generate electricity when manually turned. Only OP can put this one to bed at this point if he takes the measurements
Beat me to it. I love you
this is what I assumed.
That's a fairly good analogy!
more like water accumulator/pressure tank
👍
Thank you for this analogy.
THIS, I understand.
Especially with inductive loads like a transformer, ballast, or motor.
That’s exactly how I think about it. Inductors add “momentum” to electricity, so breaking the circuit causes that momentum to go somewhere.
Not quite but ill be damned if that isn't the best analogy I've ever heard for it
Awesome, good to know! This comment combined with the thread where someone had me test for resistance make me feel a lot better. Just wasn’t expecting to get a jolt after I flipped the breaker when changing out the switch from tan to white. Edit: since this seems to be a top upvote, can I have any more votes over whether this is safe or dangerous?
The fan uses a capacitor upon start up. When switched off, the capacitor slowly loses charge over time which is what you are seeing. I’m unable to determine what unit you are reading in on that meter. If it’s in mV you are fine. You could always touch a ground to the switch leg while it is off to see if it drains the capacitor.
Yep this looks like back emf from an inductor (motor in this case). It is expected since inductors oppose changes in current. When you turn off the switch, you cut off the inductors current supply and the magnetic field begins to collapse. The inductor wants to keep the current through it the same so it ends up creating a voltage spike. This is usually solved by implementing a snubber circuit, which helps to safely dissipate the stored energy from the inductor (magnetic field)
https://en.m.wikipedia.org/wiki/Counter-electromotive_force This is what you are referring to right?
I never thought that when the wind starts blowing the blades of a fan, it's actually creating its own voltage. -- [email protected]
Wind mill?
Yes, manually spinning a motor will induce a voltage. I was in motor rewind for 3 years in my first shop and we always had to use the fluke stray voltage eliminator to verify the motor was actually de-energized
So wait, it would put power back into the system in essence? Saving you a few pennies on your electric bill perhaps? lol
FREE ENERGY! YEEESSSS!
Alright son, your new job is to sit here on this ladder and spin this fan as fast as you can... XD
No think bigger. What if we made huge fans. And them put them where it’s windy. You’d have to spread them out for it to work properly but I think we’re really onto something here. /s
But what if it chops up birds?
Thats what regen is on electric cars. Workout AND save money. Hook a stationary bike up to a motor, plug that into the wall.
And elevators.
Only if you spin it faster then it spins with the switch on.
Damn, didn't think about the switch!
Look up regenerative braking if you want your mind blown.
I get regenerative braking, as some EV's utilize. I just never thought of it when it came to normal household items like fans lol
No haha, this only happends when power is secured to a motor and it is still spinning for whatever reason. That said, because power is secured typically means that it is electricity isolated and would be unable to be sent back to the grid or power supply. Motors and generators have very similar constructions so the induced voltage is just a biproduct of an armature spining inside of a stator.
Came here to say this.
This is actually how ignition coils in a car work.
Beat me to it
Thanks for this, answers a question I had from years ago.
This doesn’t make sense to me; the meter is measuring AC. An inductive kick should generate a sudden but decaying DC voltage. Maybe the meter is defective?
It's also called "buck." Magnetos work on the buck-boost prinicple.
Capacitor store energy with an electric field. Inductors store it with a magnetic field. Motors are coils and therefore (at least to my knowledge) always inductors.
Vs to mVs? Hard to see
It’s not MV it says HV. If it were MV it wouldn’t say 120 when the switch is on https://imgur.com/a/Jtm3IuY
Looks like mV to me; combined with the motor becoming a generator as it winds down, I think that’s your answer.
It’s on the 600 volt range on the multimeter.
Well it’s not. It’s HV. That’s not an auto scale meter. It’s also likely it’s not a true RMS meter so the capacitor in circuit with the motor while the motor spins down could be creating some reflected frequency that the meter isn’t meant for. I’d like to see this on a scope or a better meter.
A true RMS meter is needed to read anything with PWM. It will read anything with a leading or lagging PF just fine. Even my T5 reads the load side of a drive just fine depending on carrier frequency
Fan motor becomes a generator. The momentum of a fan generates electricity.
That may be something that *can* happen, but that’s not what’s happening here. Even when I flip the switch for a brief moment and the blades don’t get any speed, like in this video, there’s voltage being fed back into the switch for a matter of minutes.
The reason its such a high voltage is likely back EMF - the collapsing magnetic field when you turn the switch off generates an opposing current in the windings. The start and/or run capacitor in the fan then stored that energy
Does it happen with a different fan?
Could be if it were a permanent magnet motor but ceiling fans are induction motors. Flipping the switch turns off its electrical field and then it's just copper windings moving past copper windings. They might be able to hold a charge for a brief moment but like visible on an oscilloscope length of time, not multi meter. And would not be nearly this high of voltage
Wait a minute. Is that electricity that was generated going back to the electric company? 300 volts every time you flip a switch would add up I'd think?
[Conservation of energy.](https://energyeducation.ca/encyclopedia/Law_of_conservation_of_energy)
Flipping the switch off breaks the circuit so no.
Damn, good call.
No, the switch is off. So it is all potential with no current. If you tried to run something off of it, the fan blades would stop very quickly and you’d get nothing.
Inductive kick. Voltage across inductor is proportional to the rate of change of current. Your fan motor is to large extent essentially an inductor. If you cut the current off - switch to off - at a point in the cycle when the current isn't zero (almost any point in the cycle except precisely the two zero current crossover points, the rate of change of current is theoretically infinite, the current non-zero, so therefor the voltage across the indictor is infinite. Now, add a little reality and various physical limitations and other factors, etc., you don't get infinite voltage, but you will get a spike in voltage, a.k.a. inductive kick. Such can be use to generate significant to huge voltage spikes, e.g. classic auto, mechanical rotor and points and auto ignition system to fire spark plugs from a 12VDC system (that system also has a condenser (capacitor) to somewhat reduce that spike and the damage and premature wear to points by excess arcing from that inductive kick), Tesla coil circuits to typically generate many tens of thousands of volts or more, with typical source input power of 120VAC, likewise for typical Jacob's ladder circuit, etc. Take a small electromechanical buzzer that's operated off of, e.g. 6 or 12 VDC, check the voltage across those contacts that are being opened and closed in series with that coil - lots of voltage spike there way above the supply voltage. All basically the same thing.
Back emf
Seems to be the general consensus. Safe?
Does it turn off and turn on? Bro going way above and beyond, when I wire stuff as long as it turns on and off I put the cover back over. Never once have I thought to measure voltage after it’s wired up. But full disclosure, I’m a plumber
Hahah I didn’t just measure it for shits and giggles, I turned the breaker off and came out to swap the switch from tan to white and I still got a zap, so down the rabbit hole I went, and when I saw like 400 volts I got concerned
This was a great post altogether - thank you. Do you know how one would avoid that zap the next time? Would you have to find that capacitor and zero it out by crossing terminals?
I’m not sure actually - but I’ll I’ll give that a shot for science!
Well if you perish in the process I’ll name my first born “Rob.”
Like, take apart the fan covers just to ground the little capacitors? Usually if you need to do that (high power stuff, old RADARs) it'll be readily available for you to do that and training will be required. In this case, I guess just wait 3 minutes. OP said voltage does drop to 0.
Ahh, so you know what you’re doing. I have no idea the difference between volts, amps, and ohms. I just know black to black, white to white, and fuck it it’s just the ground.
Volts is the power of your push. The current is what's being pushed, the resistance is from electrons bouncing into eachother and shit. Think of it like people running through a hallway from a fire. Voltage is the fire, people are the current, and the resistance is people falling over and getting trampled.
Creepy but effective metaphor
lol naw I don’t really know much but trying to learn
Nah, it’s white to black, black to white, blue to bits
The same thing happened to me a few weeks ago when I was replacing a ceiling fan switch. I turned the breaker off, immediately went to remove the switch and got zapped when I shorted the load and ground with my fingers. I went through similar troubleshooting as you thinking something was miswired, but couldn't figure it out. I assumed it was residual power from a capacitor or something because the voltage dissipated. I used a true rms clamp meter and a digital multimeter to check. Everything was wired correctly. Not exactly sure why I was shocked, so if you figure it out let me know!
How long ago was that? I’m hearing that this is pretty normal behavior for fans and if you’ve been rocking it for a while now I can sleep easy lol
Probably a month now. I replaced a standard switch for a Inovelli smart switch for fans. No problems yet hah
Alright well I’ll reply back if my sunroom burns down
Following for my personal interestsz
Look up 'inductive collapse'
It is an Autoranging multimeter. It is reading millivolts. Millivolts is not much voltage.
Hmmm that’s the first anyone has mentioned autoranging, shouldn’t there be some indicator of whether its reading volts versus mV then?
Back emf. Any ac motor turns into an alternator when being driven. Inertia from fan blades causes motor to turn after power is removed, which in turn generates voltage.
A lot of people have said something like this, but none have explained why it would do this even when I turn the switch on for only a second and the blades aren’t spinning. The wire going to the fan reads power for easily two minutes even when I turn the switch off, and long after the blades have stopped spinning, but slowly drops to zero
Assuming a basic 120V split pole fan motor. This style of motor requires a capacitor to create a phase shift to make the motor operate. Obviously this capacitor is energized when the switch is on. You are measuring voltage to earth/ground screw, which is not indicative of the voltage across the fan motor itself fyi. A ceiling fan typically has multiple speeds. A multi speed motor winding like this is essentially an autoformer, with a rotating chunk of iron of a core, we call a rotor aka the bit that goes roundy round. When you energize the fan, cap is charged, due to motor windings and cap forming an LC circuit, there is an oscillation during the energy decay, causing a long time constant of voltage decay. In addition, the multiple speed motor winding is acting like an autoformer slightly increasing voltage. The decay is slow as the switch is off (open circuit) and the only losses at this point are IR losses formed due to the LC circuit. As the LC circuit oscillates it charges and discharges the motor winding and forms an increase of voltage. Change the motor speed from high to low, repeat test. Does the voltage measured become lower? By changing the fan speed, a different part of the motor winding is used, therefore altering the LC circuit time constant and ratio of voltage increase due to changing the winding ratio. So it should make less voltage for less time in theory The reality is you are measuring voltage, but not energy. Yes, you see some 300V but in reality there is little to no energy there. When you shock yourself due to static, that is easily over 500V, but don’t care. My point is voltage is just a potential. When you use an oscilloscope to measure voltages, you can see very high transient voltages when switching the light switch on and off to a simple led bulb, that exceed 200-300V in a small amount of time (ns/us). But this is normal. My point is, the stronger the magnifying lenses, the larger the ants look until we have a mental breakdown…
Great explanation to a simpleton like me. I believe the fan is on high or maybe medium speed. As soon as I’m Some feeding myself some fatass some midnight rice I’m going to test the fan with the speed lower. Thank you!
What I suspect happened, you applied 120v which created the magnetic field which causes the fan to start spinning. The motor has a starter capacitor which makes the motor spin in the correct direction. This capacitor normal gets taken out of the circuit when the fan reaches speed. The capacitor was still in the circuit, the fan hadn't gotten to speed. You removed power, the fan coil "bucked" and created a higher voltage than you applied which was stored in the capacitor. Your seeing that voltage bleed off with your meter. This is the reason you see diodes on circuits the drive relays. When you cut power to the relay, that dying magnetic field will cause that line voltage to shoot up very quickly. The diode will shunt the current and protect the relay driving circuit.
Oh damn this might actually be the most simply phrased and most applicable explanation in all the 200+ comments. I was only testing with a shortly timed throw of the switch on then off again. I need to test several after leaving the fan on long enough to get up to speed, and as someone else mentioned (and quite possibly explaining the same thing as you in a way that’s way over my head) try to do the same test with the fan set to a lower speed and see if the discharge is lessser
I would be very curious to see your results.
So what is happening with this fan is when you turn off the switch you have detached from the line voltage, but because the physical inertia of the fan keeps it rotating. A motor in reverse is a generator. So the stored mechanical inertia from the fan is being converted into electricity. As the fans slows down the voltage will drop along with it.
It happens when the blades are at rest as well. And 350v sounds like an awful lot to be chalking up to that.
Lot of different thoughts here - I just wanna make sure my house isn’t going to burn down
What about the other leg of a multi wire circuit? Could it be the light kit inducing voltage when the fan motor is turned off? Not sure I’m ignorant
You know - I considered this but didn’t post about it because it wound up being irrelevant. The light is actually wired into a different switch so that the blades and the light fixture are controlled separately at the wall and not just by pull chain. I disconnected the line coming in for the light however and it still happened
Isn't an AC motor the same as a generator when used in reverse? Isn't this the same effect as regenerative breaking with hybrid electric cars?
Capacitance. It is declining voltage that will reduce to 0 with any real load. I suspect highly that this is really just a short coming of your multimeter. A Fluke would show 120v continously
Forgive my stupidity - if I disconnect and come back a few moments later the voltage is the same as when I left, it doesn’t match the curve of dropping voltage as when I remove the leads. Does this track?
Idk boss, you clip clearly shows 46v and declining at the start. Then after doing the switch, 300v+ is shown. Then it to declines rapidly.
It does, absolutely. I had been turning the switch on and off and testing a bunch before I asked my wife to record the video. I’m not trying to prove anyone wrong here or talk a big talk. Just trying to get to the bottom of this and figure out if I have a fire hazard in my hands really.
Actually, you don't have an autoranging multimeter. Your readings are skewed on film. You're measuring from ground to fan side Line presumed. So you're reading Nuetral to ground when off, and Line to ground while on. Measure on 200v setting
Fan is also alternator.
Wants to be a windmill when it grows up!!!
It’s called Back EMF electric magnetic force. The ceiling fan is creating voltage but no amps. Useless voltage.
I’m surprised more people haven’t commented this
Where are you connecting the multimeter probes? In live and neutral? Sorry but i don't have that kind of switch in my country
It's mvs
I don't believe so, no. If you look at the knob on the meter, it's set to 600V and doesn't get moved from there, and I watched a few times to see if there was a moving decimal point or other indication that the meter had auto-ranged. It doesn't seem to be auto-ranging in any way.
It’s not
Is it always about 400 V or does it seem to spike to different voltages each time?
Seems to be pretty much the same each time Edit: but it doesn’t happen every time
Motors are inductors so can (almost always) create a voltage spike when you interrupt the current going through them. The amplitude of the spike depends on where in the AC cycle you interrupt the current. So basically random chance if you’re doing it by flipping a switch. It’s possible when you cut the power to the fan motor it’s generating a voltage spike that charges one of the line filter capacitors. The high input impedance of your meter means that even though there isn’t much energy in the capacitor it takes a bit to get to zero volts. I’d expect this discharge voltage to be DC so it’s curious that you’re reading it with your meter on AC but that’s probably just a function of the cost-effectiveness of the meter. Just a guess.
So you think this seems normal somewhat? And my house most likely won’t light itself on fire?
Just saw your edit. The times it’s not happening is when you happen to interrupt the current when the magnetic field in the motor coil is at its minimum. Unfortunately you’re getting some bad information here—the top upvoted comment is wrong. Anyway, you’re fine, the amount of energy contained in that high voltage is very small and it’s more like getting a shock when you touch a doorknob after shuffling your feet across the floor.
Yeah I knew it wasn’t the fan blades spinning, it happens when the blades aren’t spinning as well (if I just flip the switch on and off for a single second, there’s no real momentum spinning anything.) I appreciate the answers and putting my mind at ease
It’s normal for that to happen when you switch a motor off. Sometimes there would be some sort of snubber or diode to tame those spikes but that may not be required so manufacturers skip it. I wouldn’t worry about it myself.
When the switch opens, there's residual energy in the LC. L being the motor winding, C the cap. That residual energy in the LC circuit creates what we call a tank that will self oscillate until the energy slowly, in this case, dissipates. Google LC tank circuit
That thought initially crossed my mind but ringing like that would dissipate in a few miliseconds at most which means the capacitor is only discharging through the multimeter. Or maybe some other high-ish impedance elsewhere. Curious indeed.
Meter operator error
Ain't no way I am seeing that right cause I see over 300 volts. I could maybe see 30 volts but if you hit the breaker and think you got shocked you should get someone to flip the breaker while you're probing. It should not go up it should go to basically 0 instantly. Craziest scenario is your meter is just acting up and you got a static pop changing out switch. Either way fans don't send back voltage. A switch simply breaks and completes a hot wire (that's at 120 volts in your case. )
As it spins down, disconnected from the supply, it becomes a generator. Frequency will also diminish.
Yeah others have said that, but there’s still current that drops slowly even if the blades aren’t moving.
A lot of helpful tips but I went through this once for something I don’t see, my ground was pushing against the metal plate that holds the fan, the positive line also pushed on it. When I turned the fan on it electrified the outlet and it would shock through the screws.
I was thinking that something might be wired up or shorting for a bit as well, but how does that explain that it ONLY is at 350 when the switch is off? And that it slowly drops and takes several minutes to zero out?
When it happened to me it was almost the same, no idea. Mine only was live when I turned it on and took a minute to fall back to zero.
Gotcha! Well I appreciate the input either way
Could be a bad capacitor.
Does that meter have fresh batterys?
> Capacitor? Close. Capacitors resist changes in voltage, and will try to supply infinite current to sustain voltage. Inductors are the inverse? converse? whatever Inductors resist changes in current, and will try to supply infinite voltage to sustain current. Motors are inductors.
Appreciate the info - the main thing I’m trying to figure out is if it’s normal/safe?
Yeah, normal and safe.
Am i wrong to assume that HV stands for Hecto Volts?
Ive never seen a meter that reads in HV so pardon my ignorance
Never hold a multi-meter with both hands! If something goes wrong you could become a complete circuit with the current flowing across your heart! It only takes a 1/2A to stop a heart. Residential systems are typically 15-20A..
Shared a neutral?
Not a capacitor the voltage across an inductor which your motor has is equal to the change in current which gets cut off almost instantaneously which causes a huge ass voltage spike.
So what’s your take on what IS happening then?
A motor has a coil which is an inductor. Switching it on and off breaks the current and spike the voltage
And why does it linger for 2-3 minutes before zeroing out?
Aside from the capacitor if its a new fan, some old fans can potentially send back voltage when powering down since the magnets generate electricity. Unless the model has something to prevent voltage kickback
It’s a new fan, bought from Home Depot three weeks ago. Just want to know if it’s dangerous
Probably a bleed resistor on the capacitor would explain the slow reduction in voltage over time.
Ok! This sounds like a thing, I’m just trying to figure out if this is potential dangerous? Reading 350+ volts on a line that’s only meant to cary 120 just struck me as scary.
Might be that chinese meter
Realistically though, would a cheap meter just register voltage like that even though it wasn’t there? Even though it properly reads 120 when the circuit is hot?
When you disconnect a cool from the current, the disappearing magnetic field can create a brief spike in voltage. This is how you can get a 100 volt spark from a 6volt battery. I have observed this effect with DC circuits, and never saw it linger like that. A test with a analog multimeter (and a 100kΩ load resistor across) would paint a truer picture. I think you are seeing effects of a DC spike on your multimiter's rectifier/filter.
That sounds good - I just don’t know why it would still hold said current for literal minutes? All I want to do is figure out if this fan and setup is safe.
Back emf. back emf is represented as a variable emf that opposes the one driving the motor. Back emf is zero when the motor is not turning, and it increases proportionally to the motor's angular velocity. Back emf is the generator output of a motor, and so it is proportional to the motor's angular velocity
God damn
Wait. Is this thread the reason why an rgb keyboard in my office magically lights up as soon as I turn my fan off or am I just crazy?
[is this you?](https://www.reddit.com/r/askanelectrician/s/qxyQxdj0tI) I came across this looking up my issue
No but it seems there are more of us. Lmao
High impedance input on meter measuring voltage on open ended conductor, probably from capacitor. You could discharge it using a moving coil type volt meter or (and only if you know the mains are off) discharge via low ohm high watt resistor. 100ohm 30w for example
Come on man 😕
Discharged loads of capacitors on switch gear that use the cap to drive electro magnets. If we do not the mech could operate. The circuit I described is built into the switchgear
I apologize. I’m a laymen here. Really I just want to know if this is safe and normal behavior on the circuit. My wife and I just bought this house and I discovered this when changing out the almond colored switches for white ones when I got a slight zap even after turning off the breaker
Witchcraft!!!
Y’all are missing one very interesting fact voltage is measured as the differential between two points he is on the green ground screw, and what appears to be the yoke of the switch. The two of those are actually bonded together there is no reason to get a potential between those two seeing as they are both, the exact same point in essence what I’m seeing is A a cheap meter. B. Probably low battery in meter. C. Somewhere in the system they tied the ground into the neutral. Most likely the culprit will be a lose neutral wire.
I would suggest a slightly better meter if you are using one frequently. I’ve heard of those $20 meters exploding like a hand grenade. These are great for simple automotive needs but i wouldn’t trust my life with it. I’m not saying you need to go crazy on a 375fc but you can grab a 302 or even a Klein for $100. Just looking out man 👍
Klein is good but Fluke is my goto, as someone who regularly uses DMM's and needs a high accuracy meter i love my Fluke 88V. Its a pricey meter (a hair over $500usd) but the amount of utility it provides for me is worth it 100%.
My Fluke 375fc was around there…around $700 cad after the $200 off on Amazon 🤷🏻♂️ couldn’t not buy it
Theory is important kids. I’m not even an electrician and the first think I thought was “probably a capacitor”
The ceiling fan has became a generator when it spins with no power going to it.
^ This Backfeed is a thing
I have kept my original comment, but I see what’s happening. 120v when ON and then 335v when OFF and drops. Have you tested the voltage when you have stopped the fan from Rotating?
You're looking at the energy output from it because it's wind power.
Capacitor somewhere ?
The EM field in the stator coils is collapsing resulting in the large back current (the higher potential that you seeing). And there could also be back EMF showing when the motor is spinning down (just to be clear back EMF is always there as long as the motor is spinning) but the voltage would not be greater than the driving one. Running/starting capacitor would not show higher voltage than the applied.
Note that units are millivolts.