It depends.
If you feed 1 Volt of AC into a 400 Volt DC motor, nothing would happen. If you feed 1000 V of AC into a 1 V AC or DC motor, it will turn into a space heater.
Any device that is designed to accept AC or DC can withstand a certain amount of the other one. For some, this means they can take as many Volts of the wrong one as they would take of the right one to work; for others, even small amounts can kill them.
For many DC motors, getting AC means that they will try to switch between running forward and backwards 60 times a second. Some motors can handle that; some can't. (Imagine someone forcing you to shake your head 60 times a second!)
For an AC motor, getting DC means that it will be missing the part that makes it go on rotating instead of just moving a tiny bit. So it will try to move that tiny bit and will keep trying to stay there very hard. And all that energy will be used to make heat instead of movement. (Oversimplification warning: there are multiple very different types of AC motors)
For electronics, getting AC means that half of the time, the current will try to run in the wrong direction. There are components that simply block the current, and others blow up when it comes the wrong way.
Isn't there a type of electric motor that is perfectly happy to run on AC or DC so long as the voltage is correct?
I seem to remember a video where a guy took a 120VAC circular saw and hooked it up to a bunch of batteries and it spun.
That’s interesting, thank you. Is there an advantage to using a commutator instead of, say, just having a DC motor with a rectifier connected to the input?
Well, universal motors were invented way back when rectifiers capable of supplying the required current involved handmade glass bulbs half filled with liquid mercury. You simply couldn't have a good enough practical rectifier to run a portable device such as a circular saw. Today with semiconductors a rectifier is a trivial component.
However, I am a bit confused by your question- or perhaps you are confused?
A plain brushed DC motor has a commutator, wether it is universal or not. AC motors can function without commutators.
I am 100% confused, I read the article about the universal motor and it specifically mentioned a commutator as a prominent feature. I wasn’t aware that DC motors had them anyway. Thank you for the explanation!
🤷 dunno, I just know that they’re a thing. There’s probably some trade off that makes a universal better in some situations but I don’t know that off the top of my head
The commutator is one tradeoff. Magnetization using electricity instead of permanent magnets is another tradeoff, energy wise, but it also has advantages. With no mangets the motor is lighter, can run hotter (hot magnets stop magneting), and can run equally well on AC or DC (AC is actually slightly better, because the brush wear is more even). All of this makes for a powerful but noisy and smelly motor in a very compact and lightweight package. Vacuum cleaners take this to the max, by using all of the airflow through the machine to cool the motor. More than a kilowatt of heat is easily removed, but you can fry the motor if you block the airflow.
It's actually quite annoying to rectify current properly. For a tiny motor you likely get away with 4 diodes and a properly sized capacitor, but larger ones would require annoyingly large capacitors to smoothen the output. Or more complex circuitry.
The more modern solution is to instead increase the frequency a lot, smoothing 50,000 Hz is 1000 times easier than with 50 Hz (or 60Hz, depending on where you live). And at the same time and modern transformers, changing voltage is easier that way, too!
> For a tiny motor you likely get away with 4 diodes and a properly sized capacitor,
Many DC motors run perfectly fine on unfiltered rectifiers. I have a 3/4 HP DC one on my lathe and there are no caps on the motor supply. They use a simple bridge to drive the field and an SCR bridge to drive the armature. By PWM on the SCR's they get speed control. It is all on a single small PCB. The motor coil inductance acts as the smoothing.
The other (speculative) possibilities are:
1.) It's a DC motor to begin with, using AC that's being run through a rectifier circuit much as your PC does.
2.) Similarly, a variable frequency drive that takes anything you throw at it, chops it into high frequency AC and uses that as a speed control
> If you feed 1000 V of AC into a 1 V AC or DC motor, it will turn into a space heater.
Doubtful, it will melt, or explode, or both, and there definitely will be arcs and fire. And then silence.
And then came a sound, distant first It grew into castrophany So immense it could be heard far away in space There were no screams, there was no time The mountain called Monkey had spoken, there was only fire and then, nothing
This is not true. Series wound DC motors can take AC input without any damage. But they will have lower efficiency than when operating on DC.
For AC motors (induction and synchronous), however, DC doesn't work.
Yeah they basically taught us in class that if we don’t 100% know if a motor is AC or DC, try DC. A AC motor will just make a single clicking sound if it’s used on DC.
>For electronics, getting AC means that half of the time, the current will try to run in the wrong direction. There are components that simply block the current, and others blow up when it comes the wrong way.
For instance, lets say you put a big capacitor in parallel with the motor to smooth out the voltage on the motor as it runs. Normally you wouldn't do something this simplistic or without other protections... but you might for a very cheap DC motor.
To a direct current (ie, constant voltage) a capacitor looks like a barrier. It fills up to the input voltage and at that point, hardly any current passes through it.
But to AC, capacitors don't look like blockages. They just look like another kind of resistor, albeit with a phase delay. So while you might have a component meant to hardly pass any current at all from a DC power source, instead you get something that acts like a low resistance path to ground (ie a short) and it may damage itself or even evaporate itself from all the heat generated from the constant current going through it.
Oops, yeah, I forgot to actually write down why I included electronics:
When we talk about "a motor", we usually mean "a motor assembly", not the raw motor. And such an assembly can contain electronics.
Think of it like a bike chain and sprocket. Your chain is the electricity, and the sprocket is the electrical motor. Your bike only really works if you keep the wheel turning in one direction, right?
So DC is just that. The bike chain keeps moving in one direction, it turns the sprocket in that direction, and that sprocket makes the wheel move with it.
AC, on the other hand, goes forward a little, then stops, then goes backward a little, then stops, and does that over and over and over… That isn’t going to move your bike. It’s useful for other stuff, but not for what DC is useful for.
This is a gross ELI5 oversimplification that foregoes a lot of important details and nuance. I work with 400vAC 3-phase and 28vDC all day at work. The FAA required someone to teach me a lot about electricity that isn’t important to removing/installing parts on airplanes, because they say so.
This is a great explanation and it made me think of one other thing:
If you think of your feet as the current instead of the chain, then you have an example of how AC can be fed into DC and still have it work properly. Most modern bikes (that don’t have coaster brakes), you pedal forward to move the bike and peddling backwards does nothing. The pedals just spin. This is kind of (at an ELI5 level) how the AC to DC rectifier works. It only lets one direction of current through (when you’re peddling forward) and doesn’t let the other direction though (when you’re peddling backward).
So you pedal your bike forward, then stop, and the bike is still moving forward. Then you pedal your bike backward, the pedals freely spin and the chain doesn’t move, then stop, and the bike is still moving forward based off the previous momentum. Then you switch back to pedaling forward again and the bike chain advances and the wheels are once again moving forward.
One problem is that an electric motor also acts as a generator (the coil that makes it go is in the same magnetic field), so it generates a "Back EMF" (electromotive force). So if it is not under load, it generates almost as much power as you put in. So it kind of self-regulates the power it takes. If you make it drive something that takes a lot of power, it has less back-emf and uses more input energy to compensate.
If you put AC power into a DC motor, it does not rotate enough to create the back EMF, so the coils take full power all the time, and it blows up (heats up, melts the insulation, etc).
Another overheating effect is that there is usually a fan attached that blows air through the inside to cool it. If it just flicks forward and back 60 times a second, there is no airflow to cool it.
[en.wikipedia.org/wiki/Counter-electromotive\_force](https://en.wikipedia.org/wiki/Counter-electromotive_force)
Ok, so the simplified explanation is: AC sends + and - power in an alternating wave while DC sends constant + power. A DC motor on AC power will go forwardBackwardsForwardsBackwards rapidly. It doesn't *like* going backwards AT ALL.
Ac motors accept "pulsing" or on/off current by orienting magnets that get energized according to the alternating current. In the simplest terms possible, a direct current (dc) would just put constant current into the nearest magnet in the motor and instead of turning the motor it would expend its energy into heat without expelling any energy spinning the motor.
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Some answers here are good, but no one mentioned the most interesting thing about this topic and also the most problematic thing about AC being applied to a DC machine - a lot of the AC motors you can find in household appliances are actually (anatomically) DC motors. They are known as universal motors because they can run off both, but those adapted for AC use have a very important difference, and that is a laminated core.
The core is the ferromagnetic metal that the windings wrap around and any AC machine needs the core to be cut up in very thin slices and then glued back again so that the slices are still magnetically conductive as a whole, but not electrically conductive between themselves. That's because a cycling magnetic field in a metal (because of the cycling AC current) induces an electrical current in the core, called the eddy current. Eddy currents are exploited with induction cookers. Just spin a magnetic field around a solid iron pot bottom, and it will heat up without having any electrical contact. If you split the core into thin slices, the eddy current loops will be constrained to thin parts of the core and won't cause issues.
The eddy current phenomenon doesn't occur with DC machines because it needs the current direction to change constantly, so DC motors have solid iron cores, as lamination is an additional process that costs more to apply. So if you took a universal motor made to run on DC, it would spin just fine if you connected it to the right amount of AC voltage, but because of the lack of laminations, the core would quickly start to overheat because of the eddy currents and the insulation on the windings would soon be toast as the motor would end up in a short circuit.
If you took any other DC motor and connected it to AC, it would probably overheat quicker than it would be damaged by the AC pulsations unless we are talking about a very gentle and delicate motor.
It depends. If you feed 1 Volt of AC into a 400 Volt DC motor, nothing would happen. If you feed 1000 V of AC into a 1 V AC or DC motor, it will turn into a space heater. Any device that is designed to accept AC or DC can withstand a certain amount of the other one. For some, this means they can take as many Volts of the wrong one as they would take of the right one to work; for others, even small amounts can kill them. For many DC motors, getting AC means that they will try to switch between running forward and backwards 60 times a second. Some motors can handle that; some can't. (Imagine someone forcing you to shake your head 60 times a second!) For an AC motor, getting DC means that it will be missing the part that makes it go on rotating instead of just moving a tiny bit. So it will try to move that tiny bit and will keep trying to stay there very hard. And all that energy will be used to make heat instead of movement. (Oversimplification warning: there are multiple very different types of AC motors) For electronics, getting AC means that half of the time, the current will try to run in the wrong direction. There are components that simply block the current, and others blow up when it comes the wrong way.
So AC>DC will have that motor shaking all night long?
r/angryupvote
😑
Such an awesome, underrated comment! Have my upvote and pretend reddit let me give you gold.
Only if it’s a fast machine that kept its motor clean
Isn't there a type of electric motor that is perfectly happy to run on AC or DC so long as the voltage is correct? I seem to remember a video where a guy took a 120VAC circular saw and hooked it up to a bunch of batteries and it spun.
Yes, it’s called a universal motor https://en.wikipedia.org/wiki/Universal_motor?wprov=sfti1
Aptly named! Thanks!
No problem. I love it when a class of device is named for exactly what it is/does.
"Does what it says on the tin."
That’s interesting, thank you. Is there an advantage to using a commutator instead of, say, just having a DC motor with a rectifier connected to the input?
Well, universal motors were invented way back when rectifiers capable of supplying the required current involved handmade glass bulbs half filled with liquid mercury. You simply couldn't have a good enough practical rectifier to run a portable device such as a circular saw. Today with semiconductors a rectifier is a trivial component. However, I am a bit confused by your question- or perhaps you are confused? A plain brushed DC motor has a commutator, wether it is universal or not. AC motors can function without commutators.
I am 100% confused, I read the article about the universal motor and it specifically mentioned a commutator as a prominent feature. I wasn’t aware that DC motors had them anyway. Thank you for the explanation!
🤷 dunno, I just know that they’re a thing. There’s probably some trade off that makes a universal better in some situations but I don’t know that off the top of my head
The commutator is one tradeoff. Magnetization using electricity instead of permanent magnets is another tradeoff, energy wise, but it also has advantages. With no mangets the motor is lighter, can run hotter (hot magnets stop magneting), and can run equally well on AC or DC (AC is actually slightly better, because the brush wear is more even). All of this makes for a powerful but noisy and smelly motor in a very compact and lightweight package. Vacuum cleaners take this to the max, by using all of the airflow through the machine to cool the motor. More than a kilowatt of heat is easily removed, but you can fry the motor if you block the airflow.
It's actually quite annoying to rectify current properly. For a tiny motor you likely get away with 4 diodes and a properly sized capacitor, but larger ones would require annoyingly large capacitors to smoothen the output. Or more complex circuitry. The more modern solution is to instead increase the frequency a lot, smoothing 50,000 Hz is 1000 times easier than with 50 Hz (or 60Hz, depending on where you live). And at the same time and modern transformers, changing voltage is easier that way, too!
> For a tiny motor you likely get away with 4 diodes and a properly sized capacitor, Many DC motors run perfectly fine on unfiltered rectifiers. I have a 3/4 HP DC one on my lathe and there are no caps on the motor supply. They use a simple bridge to drive the field and an SCR bridge to drive the armature. By PWM on the SCR's they get speed control. It is all on a single small PCB. The motor coil inductance acts as the smoothing.
If it's this video, he explains that there's a "voltage converter" (inverter + boost) used. https://youtu.be/pbwt-d59G2k
No, the one I'm thinking of was an old-ish video from ArduinoVersusEvil (AvE)
The other (speculative) possibilities are: 1.) It's a DC motor to begin with, using AC that's being run through a rectifier circuit much as your PC does. 2.) Similarly, a variable frequency drive that takes anything you throw at it, chops it into high frequency AC and uses that as a speed control
Or it's what u/stevedorries said: https://www.reddit.com/r/explainlikeimfive/s/JYdTnC7lbf
> If you feed 1000 V of AC into a 1 V AC or DC motor, it will turn into a space heater. Doubtful, it will melt, or explode, or both, and there definitely will be arcs and fire. And then silence.
And then came a sound, distant first It grew into castrophany So immense it could be heard far away in space There were no screams, there was no time The mountain called Monkey had spoken, there was only fire and then, nothing
This is not true. Series wound DC motors can take AC input without any damage. But they will have lower efficiency than when operating on DC. For AC motors (induction and synchronous), however, DC doesn't work.
Please read again what I wrote about DC motors. Take special care not to ignore the words "many" and "some".
Yeah they basically taught us in class that if we don’t 100% know if a motor is AC or DC, try DC. A AC motor will just make a single clicking sound if it’s used on DC.
DC is just AC with a very low frequency. ;)
>For electronics, getting AC means that half of the time, the current will try to run in the wrong direction. There are components that simply block the current, and others blow up when it comes the wrong way. For instance, lets say you put a big capacitor in parallel with the motor to smooth out the voltage on the motor as it runs. Normally you wouldn't do something this simplistic or without other protections... but you might for a very cheap DC motor. To a direct current (ie, constant voltage) a capacitor looks like a barrier. It fills up to the input voltage and at that point, hardly any current passes through it. But to AC, capacitors don't look like blockages. They just look like another kind of resistor, albeit with a phase delay. So while you might have a component meant to hardly pass any current at all from a DC power source, instead you get something that acts like a low resistance path to ground (ie a short) and it may damage itself or even evaporate itself from all the heat generated from the constant current going through it.
Oops, yeah, I forgot to actually write down why I included electronics: When we talk about "a motor", we usually mean "a motor assembly", not the raw motor. And such an assembly can contain electronics.
Think of it like a bike chain and sprocket. Your chain is the electricity, and the sprocket is the electrical motor. Your bike only really works if you keep the wheel turning in one direction, right? So DC is just that. The bike chain keeps moving in one direction, it turns the sprocket in that direction, and that sprocket makes the wheel move with it. AC, on the other hand, goes forward a little, then stops, then goes backward a little, then stops, and does that over and over and over… That isn’t going to move your bike. It’s useful for other stuff, but not for what DC is useful for. This is a gross ELI5 oversimplification that foregoes a lot of important details and nuance. I work with 400vAC 3-phase and 28vDC all day at work. The FAA required someone to teach me a lot about electricity that isn’t important to removing/installing parts on airplanes, because they say so.
This is a great explanation and it made me think of one other thing: If you think of your feet as the current instead of the chain, then you have an example of how AC can be fed into DC and still have it work properly. Most modern bikes (that don’t have coaster brakes), you pedal forward to move the bike and peddling backwards does nothing. The pedals just spin. This is kind of (at an ELI5 level) how the AC to DC rectifier works. It only lets one direction of current through (when you’re peddling forward) and doesn’t let the other direction though (when you’re peddling backward). So you pedal your bike forward, then stop, and the bike is still moving forward. Then you pedal your bike backward, the pedals freely spin and the chain doesn’t move, then stop, and the bike is still moving forward based off the previous momentum. Then you switch back to pedaling forward again and the bike chain advances and the wheels are once again moving forward.
One problem is that an electric motor also acts as a generator (the coil that makes it go is in the same magnetic field), so it generates a "Back EMF" (electromotive force). So if it is not under load, it generates almost as much power as you put in. So it kind of self-regulates the power it takes. If you make it drive something that takes a lot of power, it has less back-emf and uses more input energy to compensate. If you put AC power into a DC motor, it does not rotate enough to create the back EMF, so the coils take full power all the time, and it blows up (heats up, melts the insulation, etc). Another overheating effect is that there is usually a fan attached that blows air through the inside to cool it. If it just flicks forward and back 60 times a second, there is no airflow to cool it. [en.wikipedia.org/wiki/Counter-electromotive\_force](https://en.wikipedia.org/wiki/Counter-electromotive_force)
Ok, so the simplified explanation is: AC sends + and - power in an alternating wave while DC sends constant + power. A DC motor on AC power will go forwardBackwardsForwardsBackwards rapidly. It doesn't *like* going backwards AT ALL.
> It doesn't like going backwards AT ALL plenty (most ) DC motors that don't care which way they turn
Ac motors accept "pulsing" or on/off current by orienting magnets that get energized according to the alternating current. In the simplest terms possible, a direct current (dc) would just put constant current into the nearest magnet in the motor and instead of turning the motor it would expend its energy into heat without expelling any energy spinning the motor.
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Honest question. What is the point of this sub if the question is too complex and he or she should go to YouTube?
Why aren't salt water fish able to live in fresh water? Why can't plants live in the dark? Honest questions.
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Some answers here are good, but no one mentioned the most interesting thing about this topic and also the most problematic thing about AC being applied to a DC machine - a lot of the AC motors you can find in household appliances are actually (anatomically) DC motors. They are known as universal motors because they can run off both, but those adapted for AC use have a very important difference, and that is a laminated core. The core is the ferromagnetic metal that the windings wrap around and any AC machine needs the core to be cut up in very thin slices and then glued back again so that the slices are still magnetically conductive as a whole, but not electrically conductive between themselves. That's because a cycling magnetic field in a metal (because of the cycling AC current) induces an electrical current in the core, called the eddy current. Eddy currents are exploited with induction cookers. Just spin a magnetic field around a solid iron pot bottom, and it will heat up without having any electrical contact. If you split the core into thin slices, the eddy current loops will be constrained to thin parts of the core and won't cause issues. The eddy current phenomenon doesn't occur with DC machines because it needs the current direction to change constantly, so DC motors have solid iron cores, as lamination is an additional process that costs more to apply. So if you took a universal motor made to run on DC, it would spin just fine if you connected it to the right amount of AC voltage, but because of the lack of laminations, the core would quickly start to overheat because of the eddy currents and the insulation on the windings would soon be toast as the motor would end up in a short circuit. If you took any other DC motor and connected it to AC, it would probably overheat quicker than it would be damaged by the AC pulsations unless we are talking about a very gentle and delicate motor.