


Joined Mar 2007
261 Posts

Discussion
Calculating "power" for a brushless motor
I'm comparing 2 motors that list different specs.
I know Power=Volts x Amps but not sure which amp or volt spec can be used (if any) for brushless motors. Do these look like comparable motors? Are enough specs given to even compare them? Motor 1: Quote:
Quote:






For you first motor
around APC 7x5, GWS 8x4, 15,000 rpm +/ 2025A 3s. and around 7072% effy. Only suposition. For you 2d. motor same props, around 11000 rpm 12A 3s. effy clouse to 70 max. Dont use Slow Fly props . Manuel v. 


Joined Jul 2006
22,990 Posts

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I think it would be easier if you'd include the name of the vendor. I can't figure out why you omitted that information... Take care, Chuck 





Staffs, UK
Joined Nov 2003
12,301 Posts

There isn't enough information to accurately asses the first motor but there is enough to tell that they're very different motors. Every single characteristic given is different, type, size, weight, Kv, power etc. Just the fact that one weighs well over twice as much as the other should suggest they're not all that similar .
Steve 



#1 is an 86g 250W300W 2000Kv inrunner, #2 is a 36g 80W100W 1500Kv outrunner... how much more different can you get! As my grandfather would have said, "as different as chalk and cheese".




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My guess: Motor 1 is a Himax 28152000. Motor 2 is a No Name Chinese Outrunner, similar to a Suppo A2208/14. The specs given are obviously bogus (Max. Efficiency 98%  NOT!). All else being equal, the amount of power that a motor can handle is proportional to its size and weight. The reason for this is simply that a bigger motor can absorb and dissipate more heat. Other factors which may contribute include: 1. Electromechanical Efficiency. The higher the efficiency, the less waste heat the motor has to get rid of. Efficiency is not a constant however, but usually peaks at a power level below the motor's rated current (for maximum power output the motor could be designed to achieve peak efficiency at its rated current, but this would reduce efficiency at lower power levels). Another reason for wanting higher efficiency is that less input power is required for the same output, so you don't need to push as much power into it. 2. Temperature Ratings. Neodym magnets may become demagnetized at 100ºc, glue may soften at 60ºc, winding insulation may break down at 150ºc. Using materials that can tolerate higher temperatures allows the motor to run hotter and dissipate more heat. 3. Cooling Design. A motor which has fins, air vents and/or a fan should be able to dissipate more heat than one which is in a sealed can. Outrunners often achieve better cooling because the rotating case acts as a fan, and the magnets are close to the case surface. Inrunners usually have to rely on external airflow for cooling (some inrunners also include an internal cooling fan). 




The neverending stream of questions, similar to post #1, shows that there is a crying need for a simple way to present meaningful motor data to the average modeller.
Here is an example of meaningful data. 70% NLS at 11.1volts Motor ABCD, 50 gram 100watts out.... 148watts in.... 10,000 rpm.... 8 x 4 "70% NLS" indicates that the data is not sales hype. It means that the data relates to a realistic, nearoptimum, operating condition. It means that with the prop indicated, the motor speed is 70% of the noload speed [at WOT]. 70% NLS should be the standard operating speed for data presentation. Voltages should be standardised at 7.411.114.8etc. "100watts out" is the motor shaft output, that is, the power put in to the propeller. This value is generally not known to the user, despite its prime importance. "148watts in" is the power transferred from the battery to the motor. It is the "volts times amps" indicated by your wattmeter. Frequently referred to, incorrectly, as motor output. 


Staffs, UK
Joined Nov 2003
12,301 Posts

That might be meaningful to you but I have no idea what 70% NLS is intended to tell me (and I've been playing with electrics for 12+ years so what the "average modeller" would make of it I dread to think). Nor do I really want to know the "watts out" because something like 10000rpm on a specific 8x4 is far easier to understand and much more meaningful and the output power will only ever be calculated from the prop speed anyway.
Which really just demonstrates the problem. We don't all have the same experience or education so one person's "Finally, useful meaningful data !" is anothers "What on earth is all that rubbish supposed to mean" . Steve 



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Joined Jul 2006
22,990 Posts

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LOL.... I was actually going to say I needed three pieces of info. The first being that I can actually trust the specs. This is actually the sole reason I use EFlite Park motors exclusively. It always seems if I go with their recommended props and batteries and prop them for a hair under constant current... they run cool as a cucumber. You beat me to the punch... Chuck 






Using a little experience and data from the forum, to develop this tool in Excel to get a first impression of a motor.
I believe that the data in the second post I was very close to those obtained with the engines that publish Bruce. Manuel V. 


Joined Mar 2007
261 Posts

Im new brushless motor selection so excuse my lack of this subjects knowledge. My first goal is to learn some ways to roughly estimate a motors performance. Rough as in "will this motor lift a 4lb plane or only a 1lb plane?"
Take Chucks example: Quote:
By the way, does "constant current" = "free load current"? Do brushless motors generally hit the rated kV with a prop installed and draw what ever current is necessary to do this (or burn up trying) or is this a no load rating that varies greatly depending on prop and the motors current specs? Thanks for all the replies, Martin 




Can you take kV x constant current = Power to give you some sort of comparison with other motors or how do you correlate these two values to be meaningful?
Take battery voltage (for 3s, say, 10.5v under load) x constant current = wattsin (that is about the max you could expect). By the way, does "constant current" = "free load current"? Yes. Do brushless motors generally hit the rated kV with a prop installed and draw what ever current is necessary to do this (or burn up trying) or is this a no load rating that varies greatly depending on prop and the motors current specs? Ideally (re: peterangus) a motor should spin at ~70% of Kv x V when loaded with a suitably sized prop. If you get a lot more than 70% (say, 95%) you don't have a big enough prop on there... if you get a lot less than 70% you have too big a prop on (bogging down the motor), you'll draw a LOT of amps, and you'll generate a LOT of excess heat. 



Marti V.
Surely you interested in reading this article MR. Lucien M. scorpion on Motor. Post 35. http://www.rcgroups.com/forums/showt...=660918&page=3 The Scorpion motors are are made on the most modern CNC equipment available, so from a purely mechanical standpoint, they are manufactured as well as a Hacker or AXI motor. The fit and finish is second to none, and there a few nice touches like the laser engraved part number and company logo, and the angle cut cooling holes on the front housing that act as an air pump while the motor is running to pull cooling air across the stator and motor windings. The biggest difference in Scorpion Motors is the magnets that are used in the construction of the motor. Before I go into the details, I think it would be best to give a little bit of background information about the magnets used in brushless motors in general, so you will understand how important this is. This will take a little bit of time, but once you get to the end, you should have a very complete understanding of how a motor works and how power, heat and magnets interact with one another. The magnets used in our brushless motors are a ceramic material that is comprised of Neodymium, Iron and Boron, and are often referred to by their chemical symbols from the periodic table as NdFeB magnets. Since Boron is a fairly toxic substance, the magnets are chrome plated after manufacturer to seal them up. This serves 3 purposes, one, to protect the users from Boron exposure, and two, to seal the magnets so they do not absorb moisture, which can cause the magnets to break down over time, and three, to strengthen the magnets, since the ceramic material is fairly brittle. The magnets have 2 major properties, their magnetic strength, usually expressed in the units of MegaGaussOerteds, and the temperature rating, which is expressed as a letter suffix to the strength value. The strength value of NdFeB magnets varies from a low of around 28 MGO to a high of 50 MGO. The majority of highend namebrand brushless motor use magnets in the 4550 MGO range. The one downside to NdFeB magnets, is that when compared to other types of magnets, such as Samarium Cobalt, they have a relatively low operating temperature. The majority of magnets used motor construction have a maximum operating temperature of 100 C or 212 F. Operation above this value will cause permanent and irreversable loss of magnetic strength in the magnet. The amount of magnatism lost increases as the temperature increases, and once you reach the Curie temperature of the material, which is around 310 C, all magnatism is lost. NdFeB magnets are available in several different compositions with varying max temperature ranges. These temperature ranges are signified by a letter code after the strength value. Here is a list of the ones that are currently available, along with the max temp. 80C  No suffix 100C  M 120C  H 150C  SH 180C  UH 200C  EH The entire part number of a magnet normally started with a letter "N" to signify that it is a NdFeB magnet, which is then followed by the magnet strength, and finally the temperature rating. So common magnet part numbers would be N48, N45M, N42H, N35UH N30EH and so on. Typically, as the magnet strength goes up, the max operating temperature goes down, so you end up with the following magnet strengthtemp ranges being commonly available. The "No Suffix" magnets, which are rated for 80C (176F) are available in the N48 to N50 range. The "M" magnets, rated at 100C (212F) are available in the N45N48 range. The "H" magnets, rated at 120C (248F) are available in the N42N48 range. The "SH" magnets, rated at 150C (302F) are available in the N38N42 range. The "UH" magnets, rated at 180C (356F) are available in the N30N35 range And the "EH" magnets, rated at 200C (392F) are available in the N28N33 range. Now with that understanding about the magnets, I can move on to explaining the magnets used in our motors. As I stated earlier, most of the better motor manufacturere are using N50 or N48M magnets in their motors. Using an N48M magnet give you a motor that can withstand 20C more temperature, ann still have 96% of the magnetic strength of an N50 magnet, so it is a good tradeoff. For cost reasons, the majority of the cheap Chinese Noname import motors use N35 or N38 magnets in either 80C or 100C temperature ratings. Scorpion wanted to build a motor that in normal use, was virtually impossible to burn up, so they went to the best magnet manufacturer on the planet and looked at magnet options. After going over all the options, Scorpion discovered that it is possible to create an N50 magnet with an EH rating, that would be good for 200C operation. Currently there are only two magnet manufacturing companies on the planet that have the equipment and the technical knowhow to produce such a magnet. To produce these magnets it takes some extremely rare trace elements to mix in with the Neodymium, Iron and Boron, and in order t make it economically feasible, a LOT of them need to be made in the production run. Since Scorpion wanted to make the best motor available, they decided to go ahead and have N50EH magnets custom made for them, and they are the ONLY motor manufacturer that has them. Because of this, the Scorpion motors are the only motor made that can operate at temperatures exceeding 150C, and sustain no damage whatsoever. To compliment the incredible magnets used, Scorpion uses wire to wind their stators that has an insulation which is rated for 180C. These two components combine to produce a motor that can handle at least 50% more power than any comparable motor without burning up. Most people will tell you that, "I never burned up my magnets, it was the wire that fried". What they don't understand is the reason the wire fried is because the magnets DID start to demagnatize. When a motor fails due to the windings burning up, in most cases it is due the the magnets getting too hot. Let me explain. When a motor is running under power, it not only functions as a motor using up the electricity being sent to it, it also functions as a generator, supplying power back to the source. This is why a motor draws less current when it is running than when it is stopped. Here is an example to explain this. Let us assume for a moment that we have a motor that is running on 10 volts, that has a Kv of 1000, and has an Rm value of 0.1 ohms. In an ideal situation, if you put 10 volts on this motor , it would spin at 10,000 RPM. We all know that it will spin a little slower than that due to the drag of the bearings and the air on the motor and other factors. Let's say that in this case it spins at 9,800 RPM. Now since the motor can act as a generator just as well as a motor, a motor with a KV of 1000 will also generate 1 volt for every 1000 RPM that it is spun by an external source, even if that external source is the motor itself. In the case just described, the motor has 10 volts applied, and since it is spinning at 9,800 RPM, it generated 9.8 volts. When you take the difference between these 2 voltages you get 0.2 volts, and if you divide that value by the Rm of 0.1 ohms, this yields a noload current (Io) of 2 amps. This is a simplified verrsion of the Io calculation, but it explains the point. Now, as you put a prop on this motor, and load it down, to motor turns slower, and as a result, generates less internal voltage. Let's say that you prop the motor so it now turns 9,000 RPM with 10 volts applied. In this case, the motor will only generate 9 volts, and when this is subtracted from the 10 volt supply, you have a difference of 1 volt. Now, if you divide this 1 volt by the Rm of 0.1 ohms again, you will see that the motor will now draw 10 amps of current. If you put on a bigger prop, and slow the motor down to 8,000 RPM, it will draw 20 amps of current and so on. So know you know why the motors draw more current as the prop load increases and the motor RPM decreases, so how does this all pertain to burning up magnets? Well, I will tell you! The heat that is generated inside the motor is proportional to the square of the current, since the formula for power can be expressed as P = I x I x R, where P is power, or in our case heat, I is the current flowing through the motor, and R is the resistance of the motor, or Rm in this case. Every motor has a mass of metal inside it, namely the stator, and this makes up a large portion of the total weight of the motor. Since the stator is in direct contact with the wire motor windings, it takes the brunt of the heat that is generated by the motor windings. In most of our motors, the magnets are only a few thousandths of an inch away from the stator, so as the stator heats, a large portion of this heat is passed on to the magnets since they are so close. The amount of heat a motor can take varies with the size of the motor, but a generally accepted rule of thumb is that a motor can safely handle 100 watts of power per ounce of motor weight. Some can take more, and some can take less, but this is a good middleoftheroad value. For our example that we have been using so far, let's say that the motor we have been using weighs 3 ounces, so it can safely handle 300 watts of power. With 300 watts of power in at 10 volts, the motor would be drawing 30 amps of current. In this condition, using the numbers we derived earlier, to pull 30 amps of current, the motor would be generating 7 volts. Since 7 volts is 3 volts less than the supply voltage of 10 volts, and the Rm value is 0.1 ohms, 3 volts divided by 0.1 ohms is 30 amps of current, and since the motor has a Kv of 1000, then it must be turning at 7,000 RPM. I know that this is a lot of numbers, but if you can follow along, you can clearly see how Voltage, Kv, RPM and Current all interact in a motor as the prop load changes. Now with all that said, we can finally address the heat issue of the magnets! With our motor now spinning at 7,000 RPM, drawing 30 amps of current, we are putting in 300 watts of power. So how much of this power is going to heat? Since the power is equal to I x I x R, in this case it would be 30 x 30 x 0.1 which is 90 watts. This means that for the 300 watts we are putting in, 90 watts are going up as heat, and the remaining 210 watts is available to spin the prop. This means that the efficiency of the motor in this condition is equal to 210/300 or 70%. Now that we have established the maximum operating condition for this motor, we can look at how the generated heat can effect the magnets. We have determined that with our motor is using 300 watts of power, and 90 watts of that is turning into heat. To get a understanding for how much heat 90 watts is, grab hold of a 100 watt light bulb that has been running for a while. Don't actually do it, but I think you get the picture. Light bulbs are only about 10% efficient in converting electricity into light, so a 100 watt bulb makes 10 watts of light and 90 watts of heat. This is how much heat the hunk of metal called your stator has to dissipate while running at full power. Fortunately for us, our motors spend most of their life strapped to the front of an airplane that is going through the air at 5060 MPH or more while the motor is producing this power, so most of the heat blows out and everything stays cool enough to operate. Let us assume that this motor is using N50 magnets, which are rated at 80C. And for the sake of this example, let's say that when the motor is running at 300 watts, the stator temperature is 70 C. At this temperature, the magnets are still happy, since it take 80C before they begin to get damaged. Since normal room temperature is 25 C (77F), with 300 watts of power going into the motor, it is at 70C, so the motor is now 45C hotter than it was at the beginning of the flight, and since there is 90 watts of heat being dissipated into the stator, this means that for every 2 watts of power the stator has to dissipate, the temperature will go up 1 degree C. With this value, we can calculate the stator temperature for any current draw of the motor. I know, it has been quite a journey since we started this post, but now we can finally see how all this comes together. Lets say that you have been flying your plane with your 10x6 prop for a couple flights, and you land, flip the plane over and break your last 10x6 prop. Bummer! You go to your tool box, and discover that you do not have any more 10x6 props, but you do have an 11x6 prop. So you figure, Oh well, close enough, and bolt it on and go flying again. Now what you have done is put into motion a chain of events that will lead to the destruction of your motor, so like an episode of CSI, we will go through all the steps that will lead to the demise of your favorite motor. The amount of drag that a prop puts on a motor is what I like to call the Prop Load Factor. The load factor of a prop is proportonal to diameter cubed times the pitch, so the load factor of a 10x6 prop is 10 x 10 x 10 x 6 or 6,000. An 11x6 prop has a load factor of 11 x 11 x 11 x 6 or 7986, which is 33% grater than the load factor of the 10x6 prop. Since the current draw on an electric motor is proportional to the load factor of the prop, we can estimate the current draw based on the load factor. The 10x6 prop, with its load factor of 6000 pulled 30 amps from the motor. This means that for every 200 units of load factor, the motor will draw 1 amp of current. Using the same ratio, the 11x6 prop, with it's load factor of 7986, will cause the motor to draw 39.93 amps, which is a 33% increase in current. Now let's see what this does to the motor. Since the power dissipated in the stator is equal to the current squared times the resistance, or I x I x Rm, the new power dissipated in the stator is equal to 39.93 x 39.93 x .1 which is equal to 159.4 watts. So now, instead of the motor pulling 300 watts, with 90 watts going to heat, we are now pulling 399.3 watts of power into the motor with 159.4 watts of it going up as heat. This leaves 239.9 watts to spin the prop and yields an efficiency of 239.9/399.3 which is 60.08%. So the efficiency of our motor has gone down from 70% to 60%, and the internal heat has gone up from 90 watts, to over 159 watts. If you remember earlier, we calculated that for this motor, every 2 watts of heat energy in the stator would cause a 1 degree C temperature rise, and with 159.4 watts, this means a 79.7 C temperature rise. Since the day we were flying, it was 25C, if we add the new temperature rise, we see that the stator temp has climbed to 104.7C! So now, the simple act of changing from a 10x6 prop to an 11x6 prop has caused your stator temp to rise from a safe value of 70C to the now dangerous level of 104.7C! At this point you are probably asking yourself, why would that hurt the wire? it is most likely rated for at least 120C if not 150C, surely the wire will not burn up. As I said earlier, the problem is not the wire, although it will die in the end as well, it is the magnets that will start the ball rolling. So now your magnets are no longer happy campers, since they have a max operating temperature of 80C, and the stator that they are only a few thousadths of an inch from is at over 104C. Now the permanent and irreversable damage is starting to take place. The magnets begin to lose some of their strength because they heat up beyond their operating temperature. This has 2 detrimental effects, first, it raises the Kv of the motor slightly, and second, it reduces the ability of the motor to function as a generator. If you continue operating the motor in this condition, a runaway thermal condition will take over and the motor will quickly burn up. What happens is that the weaker magnets can no longer generate as much voltage. From our earlier calculations, we know that if our motor was pulling 39.93 amps, then it was turning just a hair over 6,000 RPM, and was generating 6.007 volts. With the weakened magnets, let's assume that it is now only generating 5.5 volts. This means that now the motor will pull 45 amps, which will cause the heat level to go up to over 200 watts, and this will raise the temp of the motor to 125C, and then the magnets will demagnatize some more and the motor will only generate 5 volts, and the current will go up to 50 amps. Now the heat generated will go up to 250 watts and the stator temp will go up to 150C. Now the magnets are really hurt and the motor can only generate 4 volts, so the current goes up to 60 amps and heat generated goes up to 360 watts and the temperature of the stator rises to 205C and, Whoops, all the insulation just melted off the wire and the motor windings short out and the big trail of blue smoke starts pouring out of the back of your motor. Now since your ESC is working into a dead short, it more than likely let out it's magic smoke as well, and if the failure mode was a shorted FET, your battery will follow along behind the other two rather quickly! And that is exactly how a motor burns up when you push it too far. In the end, the wires burning up caused the ultimate death of the motor, but the root cause was the magnets losing their strength because they got too hot. Now, having said all that, let's go back to the Scorpion motors. With magnets rated at 200C, if you had the same scenario of switching the 10x6 prop to an 11x6 prop, the stator would still be at over 104C, but the magnets couldn't care less, because they can happily exist at 200C. And since the wire is rated at 180C, it is happy to run at 104C as well. In fact, in order to cause the catastrophic chain of events to occur as with the earlier motor, the stator core would have to heat up to 180C, and the wire would fail first, leaving the magnets intact. Working the numbers backwards, this would require a power level in the stator of 310 watts, which would require a current level of 55.7 amps and a power input level of 557 watts. Now comparing the 2 motors, the other Name Brand motor with 80C magnets versus the Scorpion motor with it's 200C magnets, increasing the load of the prop by only 2530% can quickly cause a thermal runaway condition that results in the complete destruction of the motor with 80C magnets. On the other hand, the Scorpion motor, with it's 200C rated magnets, can take an overload of 86% and the only thing that happens is that the wire burns out without affecting the magnets at all. If you pushed the motor that hard, you could rewind the stator and the motor would be good as new. With the other motor, there would be nothing salvagable other than the metal parts. The motor would need to have all the magnets replaced, and be rewound to get it going again, which is so much work that it would not be worth it. So there you have it! I know that it was a VERY long answer, but I think that it was needed to explain the advantage the Scorpion Motors have over every single other motor out there. They truly are a revolutionary product, and the best part of all is that they are about 2/3 the cost of a comparable Hacker motor, and only 1/2 the cost of a comparable AXI motor. When you put all that together, and add in the 2year manufacturers warranty that Scorpion has, it is pretty much a nobrainer as to which motor to choose. To answer your other question, the Scorpion ESC's are just about the same in function to any of the name brand ESC's out there. They are very well built, and also come with a 2year warranty. With the precision stamped and gold anodized heatsink plates, they do look a little nicer than most other brands, and when you figure that they include a standalone programming card at no extra cost, AND sell for about 2/3 of what other controllers cost, it is again an easy choice to make. Scorpion is working on a series of ESC's that have builtin Switching BEC Regulators, and they too, will be about 2/3 the cost of other comparable products. Plus, they will have some very interesting features that no other controllers out there will have, so it will be very interesting when they finally do come to the market! So take a few minutes to digest all of that and let me know if you have any other questions. Lucien 
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