RC BASICS: What is KV?
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- čas přidán 28. 08. 2024
- When using electric motors to power RC model aircraft you'll often hear the term KV - so what is it?
In this video, I explain what KV is and how it can be used to calculate the RPMs of a motor when powered from a battery of a known voltage. I also demonstrate how to measure the KV of a motor when it is unknown -- although you can buy a KV meter which will do the job much more simply if you want.
I'm sorry that this video is a bit long -- I may re-edit it later and shorten it up a bit.
I'm an electrical engineer who has worked with all sorts of industrial variable frequency drives, Pwm limited stepper motor controllers, 90 and 180vdc DC speed controls etc. for nearly 40 years.
I've been baffled by all the techno-babble garbage supposed experts have been pouring onto helpless RC builders who don't know what to think. The RC forums are full of "experts" expounding on meaningless formulas which are either mis-applied or totally ficticious. It's infuriating, but I, being a gentleman, have resisted the temptation to point this out to any of them (so far).
THANK YOU FOR CUTTING THROUGH THE BS!!
You are the 1st person I've seen who really knows what they are talking about. Great demo and explanation! This video should be required reading for anyone buying a motor. Well done sir.
Best wishes for your continued sucess.
Scott Marshall
Elbridge NY USA
My uncle is machine engineer
There is no way you're getting paid for how much useful knowledge you're providing to the modern RC community. I guarantee your videos and explanations have helped SO Many modern component manufacturers to figure out how to design and build their products thatve changed the whole game in RC flight. You're like the Wikipedia for RC model designers and production companies. You're like the printed Encyclopedia for the newer generation to reference and design their next generation RC products. Without ppl like you, our generation of engineers would be totally clueless. Thanks for your service of distilling a generation of electronics and RC knowledge for us to use!
I just wanted to take a moment to say Thank You for the educational videos. I'm just getting back into RC after last flying glow plug engines some 30 years ago. To say things have changed would be an understatement, to say the least. There is no local flying club so the only education I get is from a few select forums and CZcams. I've made a few videos myself and can definitely appreciate the time it takes. Thanks......
Thanks, Bruce. In order to stabilize the tachometer reading, you should avoid florescent lighting. I use an LED flashlight to override the noise from ambient lights.
Thanks Bruce. When I was a commissioning engineer on Shell Stanlow oil refinery, a 6.6kV motor was the size of a Ford Transit van. You'd never get one to fly or if you did, you'd need one big battery and an inverter from the planet Krypton. So it's revs/minute/Volt (ish). Brilliant!
After some years involved in small RCs, I now understand what KV means. Thanks Bruce!
What a helpful video! I hope you still read these older comments. I'm in the U.S. when I was a kid, in th 50's, we used to have a tv program called Mr. Wizard. He would explain concepts in a similar manner. He would place it in real world applications, explain the instruments and tools, and provide great visual examples. Unlike some, when it comes to electronics I am dumb as dog food. I know how to google formulas but it's the understanding and application to RC that's the hard part. Your series is right on target for a newbie who wants to understand these things as they are used in his hobby. Please keep up the great work. You bring value. Richard t.
I learn something new every time I watch one of your videos. You explain everything in a way that is very easy to understand. Thank you for your extraordinary contribution to this hobby.
The car gearing to KV rating on a motor finally made that clear for me. Thank you.
Hello again!
For some reason (...) I decided to watch all your videos chronologically.
So your trade is electronics 🙂. That explains a lot...
Thanks for posting!
And on we go... we are still way back in the past...
I noticed you say "Rule of thumb" a fair bit. For those who do not realize this term was coined in the before time when people would brew beer and without thermometers would use there sensitive thumb to test if the temperature was right to add the yeast for the fermentation process hence rule of thumb. Great channel, I always thought of Kv as Kilo volts also before watching this :)
In the movie Boondock Saints a huge man-hating feminist lesbian pointed out that saying meant back in the medieval times or earlier a man was legally able to beat his wife with a stick no bigger than his thumb.
One of the Irish twin brothers who are the lead characters who was training the huge woman with on-the-job training in some meat warehouse then jokingly replies( knowing the woman hates men by the "Never touched by man" tattoo under her chin) , "Rule of thumb? Well what's that gonna do? Perhaps it shouldve been rule of wrist?". Which of course led to a fight where the boys convinced the woman to keep her comments to herself with a right hook to her chin.
Damn good movie.
Excellent video - many thanks for helping me to understand. We need more people like you to help newbies like myself.
Kv is the reciprocal of the back-emf constant.
A 500 Kv motor will generate a 1 V back-emf when the motor is rotating at 500 RPM. At 3700 RPM that motor will generate 7.4 V.
Applying it the other way around ("7.4V will make the motor rotate at 3700 RPM") is only an approximation and technically wrong.
This video is actually full of misinformation because RCModelReviews relies on the latter understanding of Kv which is wrong.
The motor RPM is NOT calculated by V*Kv. The actual RPM = (V - R*Io) * Kv = (Voltage - Motor Resistance * No Load Current) * Kv.
So given the above motor with R=0.5, Io=1, we get 3450 RPM at 7.4V.
Also, Kv tells you nothing about how powerful or efficient the motor is. That's another point where RCModelReviews is wrong. High Kv does not mean the motor has low power.
You can get high Kv motors with higher power than low Kv motors....
very interesting. Thank you for this video, subscribed and liked this is the only clear informational RC source on the internet.
After a nearly 14 minute video, in a nutshell, kv is a motor's no-load RPM-per-volt rating. The number of RPM a motor will run at with one volt applied and no load on the motor. He could have told everyone that right away :)
At 2 volts, the no-load RPM will be twice the kv, at 3 volts, three times, so on and so forth.
So, a 1000kv motor at 7.4V (2 lipo cells ~1/2 charged) will run at 7400 RPM without a load on the shaft and the ESC at full throttle.
+BasementEngineer No
Kv = "Generator Constant, Dynamo Constant"
astrobobb.com/electric_motor_handbook.pdf
zann fox Generator constant is more accurate but few of us use it for RC applicationss. There's an offset due to the motor's windage & magnetic losses when used as a motor, but few of us use RC grade brushless motors as alternators. They're actually rather good for that! Assuming kv is a motor's unloaded rpm per volt is close enough for RC use.
You're forgetting some people learn visually
I learned by touching the video
Would it really be worth watching a 30s video where a guy just reads a definition from a text book. Yes it longer then it needs to be but it by design.
Tom Reed,
An electric motor will never spin any faster than the Kv value multiplied by the applied voltage. You do not ever have to worry about an electric motor over-speeding with no prop installed.
Thanks for putting this series together. I have just recently started converting from glow and this is going to be a big help.
Hey jfan4reva!
I am down here in Colorado Springs, and you do in fact have to go larger in prop to compensate for air density. I end up just going a prop size larger or pitch depending on the model type. Having too small of a prop will not "over rev" a brushless motor as you see by the bench test, it maxes out and that is it. Running a small prop means less wattage.. that simple. Just keep your manufacturers specs in mind and match a prop accordingly (which I am sure we will see next!)
Clear and easy to follow .You and 360 painless are the best videos on u tube .Thank you
You sir, are a great teacher!!! Very gifted teacher!!! Thanks!
I agree with Jon B. What a great teacher you are. Respectfully.
Thank you very much for clarifying the KV vs. kV thing. I used to work with high voltage stuff and the term "KV" used to confuse the heck out of me. Not any more, thanks to you.
+DrBleck35
They should have used "KRV" ... then again, as you say you used to work with high voltage stuff, I'm guessing it was something to do with power distribution? Then you'd probably confuse "KRV" with "kvar" XD
Nope about power distribution. Capillary Electrophoresis. Also, "KRV" would always remind of the time I met Stevie Ray Vaughn's illegitimate (and fictitious) brother Kevin.
I’m an rc car enthusiast. 3 years ago I made the switch to battery and these small dc motors. I’m trying to learn the theory, terminology and practical from using chemicals to generate electricity (to more clearly understand the battery) to the use of electromagnetically spinning a rotor to power my rc trucks. Please continue your series and do you have a book you could recommend to a beginner to teach the basics. Thanks
Big thanks Bruce! My background is in radar and I couldn't understand why a tiny motor would need several hundred to thousands of KV (kilovolts) to run and how that voltage was being developed off such tiny batteries. RPM X 1K per volt makes a lot more sense.
I love it when you start looking for things... :D
I know most of this stuff, but this series is still really enjoyable to watch. Looking forward to the next parts in the series. . .
Thanks a million for telling us what a KV stand for ..Regards from California
Great introduction to KV. Sure it will set off the EE guys, but this is a very practical explanation that is clear and easy to follow. Good job.
Just for you'r info. Your video helped me to learn even throught i'm not interested in RC and more on building a electrical skateboard! Greetings from Spain!
Came here for the exact same reason.
Great shop teacher! I feel like I'm back in technical school.
Ooooh.....nicely explained and continues to be a shinny forum of info for us new or somewhat new to the hobby. Much thanks.
For all those wondering what was used to control the esc it was a servo tester.
Bruce, when an engine specs would say 600w for an engine on 3s so 54 amps is it safely able to increase power through voltage instead of current? Such as a 4s battery running 50amps?
Thanks for keeping it simple, I'm sure there's more to this subject and tons of variables.
Be careful running a motor without a prop, it can spin at too high an rpm for the ESC and jam it. And you won't get the magic smoke back inside when it comes out.
Only one smoke charge
Tom Reed this is nonsense. Good esc will not jam like that. If it happens then esc is bad and should not be used with the given motor (it happens due to software bug in the esc that does not reduce rpm when it can no longer keep up with the motor)
It's not nonsense. I've designed and built BLDC motor controllers before, and the combination of the phase lag between the electrical and physical angle of the motor, and the finite amount of time that the interrupt handling routine takes to execute do indeed limit the maximum speed. In this situation, a poorly designed ESC may drive a phase at the wrong time, with no back EMF to work against, thus potentially exceeding the maximum current for the motor or ESC. It's not a software "bug", but rather poor implementation.
let the magic smoke out.. watching too much AvE channel lol
Actually it is a ESC design flaw, your program is in charge of the current output and frequency based on feedback. If the feedback is not in range you don't have the program increase the power you either have it maintained or drop it at a reasonable rate until it is with in range; a full power drop can produce back emf which is why you back off at a reasonable rate. Unless the motor itself is fried, it will catch up or slow down to the output; VFD's are pretty much the same thing except on a much larger scale. I have never seen a VFD drive or motor malfunction because it lost water to the pump which is the exact same as running these little motors without a prop; of course VFD's are $10k to $1,000,000 price range, they tend to spend a bit more time on the engineering side.
Lol, I wish this vid was available 5 years ago :)
Thanks for sharing Bruce!
J
Brushless motors are 3 Phase DC, BLDC motors are not exactly the same as AC induction motors
Yes and no. Brushed motors use brushes to convert Dc to AC, after a fashion. A brushless motor needs a means to do the switching that the brushes do. An ESC is a misnomer because it's MORE than an electronic speed control. It does do that, but it also does the switching necessary for the motor to turn.
I went through your WHOLE video history but never found anything about Prop size or 2 vs 3 blade or how to pick a prop for a motor, or a motor for a specific mm size quad. Your videos are the best on YT for RC, but have you done a follow up for your RC Basics series on motors / KV / ESC's - you mentioned next would be about props and how to combine it all? I'm interested for QuadCopters mostly but fixed wing is something that will interest me in the future certainly.
Dusan: The small box with a knob is a servo tester. You can get them everywhere. Hobbyking has several models.
I love your channel, thank you for the time you invest in sharing. You do a wonderful job of explaining complex topics, it's a gift.
Bruce, I've a couple questions.
1)Why 2 marks on the tape. That seems like it would give you 2x the rpm.
2)Does the thickness of your marker lines affect the accuracy of the RPM reading?
I really enjoy the techy videos and all the excellent detail which I consider to be something very particular to your posts.
Thank you for consistently making quality videos of this type and using language that all can understand.
-Jim
Whenever you lost something, I find my self looking for it (not even sure what to look at) on the video and help you.
Finally this is explained, thank you! No to go find the prop video!
Don´t you just love this guy?
Great series. I hope in the end you can make a guide on how to select a powertrain for any model.
Like, how does it fit together, how can I apply all those basics to buy the right motor/prop/ESC/battery the first time round for a new model.
I tend to go with, what did others use for similar planes or what does the ARF version use. But I never feel like I know really why this combination is right. I just improvise.
Thank you. Simple, straightforward and amazing to a beginner. Keep it this way, thank you again :)
Crystal clear now thnx. Waiting for the next installment.
thank you so much for these videos. you really helped us in preparing for SAE competition
Thanks again Bruce for offering up such great essential information for so long!
This was so so informative. To the point. thank you.
So instead of calling KV we should be calling it K/V, KoverV, KPV, KrPV or something to that effect.
Yes?
KV Meters require you to count stators etc.. and then inline with one of the phases he reads pulse widths, you can mis count easy.. but you are right you get a much better reading. RPM in this creative way works better to explain.
Thanks Bruce, this video has been more informative than the past dozen hobby shops in my area. Looking forward to pt. 3 of the XG8 review.
The number of "winds" of wire on the stator is a hint whether it is high or low KV. If there is extremely fine (small guage) wire on the stator, it will like have a lot of winds and be a low KV. Heavy guage wire (less winds) will be a high KV motor. so if you get a motor with a number like 35-48-10, 10 is the winds. It will have a higher KV than a 35-48-16 for example
I think you might find the opposite, less larger windings mean more current and more torque so lower KV
Excellent job Bruce.
core blimmy rating. these videos ill become the core literacy for the general population. i love learning what would have facinated Tesla and Edison in the USA as AC and DC dueled out for supremicy. well done bruce.
if you're talking about the first motor, watch again from 5:30 he does rpm/voltage. If you're talking about the second motor he does kv * voltage = rpm. He is putting around 7.45 volts because that is the voltage under load. If you see the screen it is a little higher 7.67 volts when the motor is not running.
Awesome, thanks very much for clearing this up!
I studied for a year at Massey University when I was in school, won an exchange student scholarship, that was in 1983. Things CHANGED a lot a year or two after I left. I liked the country a LOT like California but from the past, I guess that has changed to, now its HOLLYWOOD!
Thank you so much for clarifying this! I have always known 'kv' to represent kilovolts and just could not grok what it meant in reference to rc model motors!
I love your use of the word grok, I have used it for years, from when I read stranger in a strange land. This is the first time I have seen it used anywhere else.
@@eclipseonline3852 You have excellent taste in reading materials! :)
Great info ! Thank You. And the best is there were no baby's killed in the making of this lesson !
Many thanks for this well done video.
You've guided me from ignorance to knowledge !
Thanks for sharing your wealth of knoledge with us and taking the time to simply explain so everyone understands i love your vidios
This is what I was looking for. I know this is an old post, but was your tachometer set up for two bladed props? Wouldn't that mean that your calculations should be half? This is not a criticism, but me making sure I understand KV. This is a good video, and I learned alot out of it.
Fantastic explanation by referring to gears in a car. Just made it instantly sink in for me.
Hi, great video! I'm just wondering what "speed controller" you are using in this video? and where can you buy this simple turn-dial variable speed control? Thanks. Dave.
I had the same question.
I think I found this... It looks like it is just a servo tester... Like:
www.hobbyking.com/hobbyking/store/__17143__Simple_Servo_Tester.html
but I think this one looks better:
www.hobbyking.com/hobbyking/store/__8296__Turnigy_Servo_Tester.html
Found this on Yahoo, it basically says that c is already being used for "constant offset": "Because from time to time you'll end up with a variation that includes a constant offset. Constant offsets are indicated by c, by convention. Here, "convention" means "there's no real reason for it, but everyone does it the same way". To avoid confusion with the constant offset c, we typically use k for the constant of variation. Again, it's conventional."
These are incredible helpful! Please keep going! I'm going to share these around to make sure that people just starting like me get your awesome help.
Hi. A great explanation, thank you. Since kV is quoted at no-load how can I know what the rpm will be when a prop is fitted ? Regards John McClean
Looking forward to the next video. Motor nomenclature has always had me befuddled.
Thanks for the very informative videos Bruce.
Awesome videos and explanations. Been watching your vids for a while now, just only signed up with CZcams. And fairly new to the quad scene, been an electric off road car hobbyist for years. Hopefully you have your "wings" back by now officially.
Thanks Bruce you answered a few questions,what was the device you used to replace the receiver ?? and where did u get it from??
Great video - so clear - thank a lot. BTW, did you make a video about choosing the right prop for e-motor?
Nice work.. the brushless motor, esc, and Lipo.. it's re-invented the hobby..
awesome video! I wish u could explain about thrust as well n how to choose the right motor for our model n how to match it with ESC n battery
Great videos!
Btw, can you tell me where did you get that little speed control that connects to the ESC? I need one just like it! Thanks !
very true i totally agree with you xpresa even newbies can't do wrong using kv meter and it very accurate
Thanks Bruce. That cleared up a lot of my questions. The analogy you did with the car gears brought it into focus for me very well. I get it!
Cheers.
@RCModelReviews, did you ever put up the power video you said was coming next in this video?? I looked in your videos list and didn't see it. I'd *really* love to watch it.
Bruce, great video as always.
Can you please do a video explaining the effect of pole count and star vs delta windings on output RPM and torque
I really like this RC basics series!
Thanks Bruce. I've been flying for donkey years, but never understood the motor sizes... This video is very helpful.
I'm looking forward to your next video about props in relation to motors. Cheers.
Hi! What is the name of the manual controller you use to control the speed? Thanks!
Beginner's questions:
I guess that before doing the ratio RPM to Volt, you did put the engine to full speed, right?
In that case, how can you be sure that it is at full speed?
What happens if you change ESC or potentiometer?
Where did you get that little controller from? Is that just 5V PWM with a knob on it?
A question for your next video (I apologize for jumping the gun): does field altitude (air density) affect prop selection?
I live at 1500 feet ASL (about 460 meters). Denver Colorado - 5280 feet ASL (1600 meters) - is a 7 hour drive from my house. If I fly there, would I need to change props to avoid over revving the motor and overheating the battery?
what's that little device you used to control the speed controller? I get fed up of ripping my RX out of my planes in order to test new motors using my 9x
Thanks for the Vid! It has helped me to understand the basics. Top Job!
It's explained near the start of the video at 00:19 . He said KV = RPM(K) per volt(V).
but wouldn't upping the voltage saturate the motor's core? or are they prepared so that they can withstand a little overload for a short period? Because i cant see the rpms having a linear relation to the voltage :(
Well done! Makes it easier to understand for sure!
How about a vid on ESC TIMING ISSUES AND MULTICOPTER ESC TIMING
If you have an unknown bldc motor, how does one determine the proper controller.
I built a bldc motor and bought a cheap controller that sort of worked with the motor but would sometimes have trouble starting the motor and after about 3 dozen test, blew 2 mosfets on the controller.
I can answer any question about the motor's design if asked. A couple things about the motor..... 1) the motor is wired wye.
2) At about 216 rpm, the motor generates 0.126vac across any two coils.
3) The rotor has 12 poles.
4) The stator has 3 coils wound in a 4 turn serpentine or rather falharbor winding pattern for the 12 rotor poles.
5) The copper resistance across any two coils is a tiny fraction of an ohm. Each of the three coils is made from about 80 inches of 16 gauge wire. that should be about 0.027 ohms to a coil.
6) I made my own laminated slotless steel stator to fix the stator coils on.
Do i need to buy a meter and measure the inductance?
hello
I'm looking to build a model for a project.Problem is,the project I'm trying to do implies an empty motor shaft.Do you know if there are any such models of motors or if I can let's say drill inside the shaft myself,without causing major structural damage.I haven't had any luck searching for similar projects on google
Whats the device you have hooked to the power elimination module of the ESC with which you are actually controlling the speed?
excellent video bruce!
Hello i have got question.
I am building an rc airplane. it is not too smal cuz it have about 1,2 m long and 0,9+0,9 wings long.
what kind of motor should i use?? how much KV? ( i want to use only one motor)
Bruce I have an issue with your calculation of KV, at least as how you described it. You used the volt measurement from your battery, but that is a DC voltage. The ESC is creating an AC voltage which is not going to be the same as the DC voltage from the battery. The AC voltage will have some RMS value that would be roughly equivalent to a DC voltage. Your calculation gets within the ballpark, but I bet your lack of accuracy is due to difference between AC RMS and DC volts.
What is the name of the analog variator you´re using connected to the ESC? Thanks!
I know this is a bit old but I found it quite interesting. Correct me if I'm wrong but light detecting tac's are be affected by electric lights. In the US are lights cycle at 60 hertz.
This could introduce a large error and thus you should always do measurements in the day light.
Hi Bruce, I'm guessing you have built or flown the HK Bixler 3? Couple of questions for you. What do think of the standard supplied motor (2630 1400 Kv), and I've heard they are rubbish. Can you recommend a replacement motor? Some people have used a 2200kv (D2826-6) or the HK AXN motor. Another motor that was used was the turnigy aerodrive 2836 1500 Kv. Hoping you can help out.
Bruce, You mention the trial and error approach to motor/prop selection. Have you tried using Webocalc?
Hi there Bruce who makes the speed controller you are using in this informative demonstration? By the way after a mind numbing couple of late night knowledge quests I have found all that I need to know about motors.As a rank newcomer I just ordered a good cross section in the trainer range. I will do the tests myself :) Thanks