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Electric Motor Primer
Where's the primer article or website to figure all this electric stuff out. I need to learn more. I want to get a shocky but I need to understand how you tell one electric motor is more powerful than another. Or, do I need 2 cell LiPoly or 3 cell.
I need a basic primer to understanding electrics How do you tell what kind of electric motor to get? What size and number of batteries? Does the radio RX run off the single battery and how does it draw that current...through the speed controller? Take this one example I posted this on another forum...but I think it belongs here. Everyone is telling me to get an AXI for the Shocky...but WHY? I'm looking out there and there is a boat load of motors. Take [link=http://www.aeromicro.com/Catalog/himax_brushless_motor__2015-2800_3298458.htm]Himax Brushless Motor [/link] No one even talks about this one...weighs about the same, $20 cheaper. How do you compare the output of these electric motors....whats the secret, why is one better than another? Is this just a general feeling that folks have gotten from trying these particular motors or is there any hard science for saying that AXI is better. This is so much harder than Glow! :( |
RE: Electric Motor Primer
Part of the reason you're so confused is because you're asking the wrong questions, based on your experience with glow.
There is no such thing as a "more powerful motor." Motors don't make power, they transfer it. You can make any motor have any amount of power by increasing the voltage and/or increasing the load on the motor. Of course, motors will have their limits, and if you push them too hard, they will burn out. You may think of the battery as just a "fuel tank" but in fact, it's your combustion chamber too. With electrics, you start by figuring out how much power you need to fly the plane in the manner you want to fly it. This is completely different from glow, where you crowbar the biggest engine you can into the cowl :) YOU tell the motor how much power to make by adjusting the voltage (cell count) and prop size. With glow, the engine tells you how much power it can make, and you have to work around that limitation. Now, people tell you to get the AXi for your Shocky because it has been proven to work well on that particular airplane. Its characteristics are that it produces large amounts of torque at a low RPM, which is ideal for swinging a large propeller, which in turn is ideal for 3D flying. In contrast, the Himaxx motors, due to their design, are low-torque, high-RPM motors that either need gearboxes, or are limited to spinning tiny props at high RPM. There's absolutely no reason you couldn't put the right geared Himaxx on your Shocky and get performance comparable to the AXi. That's the beauty of electrics; there are literally hundreds, if not thousands of different ways to arrive at the same place, and they're all perfectly fine. If you want reading material, pick up an issue of Quiet Flyer. Also check the FAQ thread at the top of the Electric General Discussion forum. There is also www.rcbatteryclinic.com to check out. Be warned, though. This stuff takes time to learn. If you try to digest too much, you'll only make yourself more overwhelmed than you already are. I suggest that the best learning is achieved by doing. Go ahead and get that Shocky fitted out with the recommended equipment; don't take any poetic license just yet. See what goes into a successful power system, learn about Volts and Amps. You won't be sorry. |
RE: Electric Motor Primer
ORIGINAL: Matt Kirsch Now, people tell you to get the AXi for your Shocky because it has been proven to work well on that particular airplane. Its characteristics are that it produces large amounts of torque at a low RPM, which is ideal for swinging a large propeller, which in turn is ideal for 3D flying. In contrast, the Himaxx motors, due to their design, are low-torque, high-RPM motors that either need gearboxes, or are limited to spinning tiny props at high RPM. Thats what I'm saying though. Is there a way to look at an AXI or a Himaxx stat sheet before you buy it and know that the Himaxx is designed for high RPM's and that the AXI is designed for high torque/lower RPM's.??? You can look at HP numbers and see that one glow engine puts out relatively more power than another. These eletric engines will fry at certain current levels and that information seems to be hidden or not even discussed when you look at the stats for an electric motor. I looked at your links and there is some information in there that I will read. But, I'm surprised there is not a 20,000 foot view article or thread on this. How do I learn for example the answer to your supposition ---> With electrics, you start by figuring out how much power you need to fly the plane in the manner you want to fly it. YOU tell the motor how much power to make by adjusting the voltage (cell count) and prop size. I understand that V=IR and yet how do I even apply this. Is it all just trial and error in electric flying? |
RE: Electric Motor Primer
The Axi 2208/34 has just about become the standard for the ShockFlyer. They are small and light enough to fit on the ShockFlyer without getting the weight too high. AFAIK, Himax doesn't make a motor that I would consider light enough for a ShockFlyer, especially when you add the weight of a gearbox. The Himax 2015 XXXX series of motors are more suited to a flat foam plane a little larger that can handle the added weight and power of this size motor. The ShockFlyer is a light and delicate plane, it needs to be kept light to perform well. Not only will the Himax weigh too much (IMHO) for the airframe, but it would need heavier batteries to support it, so weight goes up again. A Razor 350 is lighter than the Himax, but it will need the heavier batteries as well and would stress the airframe with the power it develops.
I understand your quest for knowledge, but for starters let the guys who have already busted trail do the work this one time. My advice: Get a Shocky, an Axi 2208/34, a CC P10, a 2S 830 or 850 lipo and start reading. By the time you wear the Shocky out, you will start to understand more about electrics. Matt is loading you up with broad theories, and that is fine. It can all be just so overwhelming when you are starting out that to take the easy route and use proven power options while learning is about the best way to keep sane! |
RE: Electric Motor Primer
ORIGINAL: ChuckAuger The Axi 2208/34 has just about become the standard for the ShockFlyer. They are small and light enough to fit on the ShockFlyer without getting the weight too high. AFAIK, Himax doesn't make a motor that I would consider light enough for a ShockFlyer, especially when you add the weight of a gearbox. The Himax 2015 XXXX series of motors are more suited to a flat foam plane a little larger that can handle the added weight and power of this size motor. The ShockFlyer is a light and delicate plane, it needs to be kept light to perform well. Not only will the Himax weigh too much (IMHO) for the airframe, but it would need heavier batteries to support it, so weight goes up again. A Razor 350 is lighter than the Himax, but it will need the heavier batteries as well and would stress the airframe with the power it develops. I understand your quest for knowledge, but for starters let the guys who have already busted trail do the work this one time. My advice: Get a Shocky, an Axi 2208/34, a CC P10, a 2S 830 or 850 lipo and start reading. By the time you wear the Shocky out, you will start to understand more about electrics. Matt is loading you up with broad theories, and that is fine. It can all be just so overwhelming when you are starting out that to take the easy route and use proven power options while learning is about the best way to keep sane! |
RE: Electric Motor Primer
OK, on the batteries.
Hobby Lobby sells Kokam brand lipos. The 340Mah Kokam can deliver 20C, or 20 times it's rated capacity, in discharge. This is 6.8A that they can put out. Other brands of batteries have different sizes. These are, for instance, 830Mah for Tanics. These packs are rated at 12C, or 12 times their rated capacity at discharge..or about 10 amps. I think Thunderpower makes a similar 850Mah pack. So you try to pick out a pack that will deliver the amps you need and yet not weigh too much for your plane. For the motor. Here are the specs for the 2208/34. Notice that it will turn a 10X5 on 2S lipo..just right for a Shocky. Notice it has maximum efficiency at 4-7A, so the little 340 pack will be taxed at 7A, and the max load is 8A. So the 830 Tanic will deliver 8A with some overhead. So you need a ESC (speed controller) that will handle 8A and have a little overhead. The Castle Creations Phoenix 10 Brushless ESC (commonly referred to as CC P10) will fit the bill nicely. Another choice is the Jeti 8-3P, it is an 8A brushless ESC. See that the motor weighs 45g. Now for the Himax. If you run 2S lipos, then the 2015-5400 would be a nice choice. Here are the specs for the 2015-XXXX Himax motors. This motor weighs 48G bare, but it will need a gearbox. on This Page it gives the weight of the 2015 5466 (motor and gearbox) as 62g, so 17g more than the bare Axi. But this motor will draw more current than either the 340 or 830 can deliver reliably..15A max it says..so you will need larger, heavier, batteries, and possibly a larger ESC, or just have to run the motor at a reduced power level. If you wanted reduced power, you could get a smaller motor..but there are no smaller Himax and you still need the gearbox. So that is the reason the 2208/34 is so popular for the Shocky. It is light enough and powerful enough at the same time, and uses lighter batteries that a heavier Himax. After you get into a slightly larger model, the shine of the Axi wears off...but they are about perfect for the Shocky. |
RE: Electric Motor Primer
I really appreciate this....it is very helpful. If there is a website that has all this written down that I could be pointed to, I would gladly read it on my own time.
Ok, I'm trying to follow... 1. On the battery, what calculation is allowing you to go from 340 Milli amps per hour (which is what I thin Mah is) and 20C give you an output of 6.8A....seems critical I understand this math. 2. I thought the 20C meant that it could be recharged quicker than a 10C...what I've read so far was describing that. Maybe a lipoly that can be discharged quicker can also be charged quicker?...making a supposition there. I have no idea what C stands for except temperature. 3. If a Tanic can deliver 10A + and the 2208/34 max load is only 8A....doesn't that mean the AXI burns up because your overdriving the capacity of the motor and thus its going to fry? 3A. Who sells Tanic...I found their [link=http://www.tanicpacks.com/catalog/product_info.php?products_id=366]website & that product [/link] but they don't list distributors. Also that pack is 1.6 ounces...seems awfully heavy to me. thats nearly 20% of the weight of the plane. The Kokam I believe weighs less than half that much. 4. Ok, on the Himax...why do you decide on the 2015-5400? Whats your thinking process there? What I mean is why not the HA2015-2800. The HA2015-2800 has an 8Amp rating so you would not need a different ESC or battery...? I assume your answer is that it wouldn't provide enough power or its too small or something...and so my follow-on is how do you know that? Is it from experience or is there something on that chart that lets you know that? 5. Why does the Himax need a gearbox and the AXI doesn't? The chart actually says na (not applicable) for the gear ratio so they must make the motors to run without a gearbox since they have rated that particular motor in that fashion. Is this experience again or is there something in that chart like the kv that lets you know the torque is too low and will need to be geared up? Why don't the manufacturers put max RPM and torque at max RPM numbers...? I could compare any Electric motor easily. |
RE: Electric Motor Primer
1. The battery manufacturer sets the 20C rating. 20 X .340A = 6.8A
2. C is the Capacity of the pack. A pack that can be discharged at 20C can be discharged at 20X it's capacity. The discharge rate has no bearing on the charge rate..all lipos must be charged at no higher than 1C. So the 340Mah can be discharged at 6.8A (20C), but must be recharged at 340 milliamps. 3. No, the motor will only draw as much as the load (prop) and voltage determine it will. Having a pack rated for more than the motor can draw is good, having a motor draw more than a pack can deliver is bad, and will ruin the pack. You would burn the Axi up if you put too much load (too big of a prop) and tried to run it at 10A, but the 10A capable battery would be fine. 3A. You can order direct, or PaulSwany in the ProBro section is a dealer. 4. The 2015-5400 has a Kv rating of 5400 RPM per volt. This gives a good speed at 7.4V (2S lipo). 39960 RPM (theoretically..) divided by the popular 6.6:1 gearbox gives about 6000 RPM. Of course the prop won't turn all of that, maybe 4500. Take the 2015-2800 and it has (I'm guessing..) a 2800 Kv or 2800 RPM per volt. at 7.4V, this is only 20720 RPM. Now you could run say 4:1 ratio and get a theoretical prop RPM of 5180, or you could run 11.1V and get more RPM, but you now have a 3S battery to tote. Generally speaking, the higher Kv motors are happier on low voltage and low Kv motors on higher voltage so things even out. There are exceptions, I'm just talking about general 3D type flying. 5. The Axi is an outrunner, a special kind of motor where the outside of the motor turns and the Himax is an in-runner, the shaft turns but the can is stationary. The Axi has a Kv of about 1020 seems like, so at 7.4V it only turns 7500 or so RPM...pretty close to what you want to turn your prop, so no need to mount it in a gearbox. Of course the RPM will come down when the prop is mounted, and the amps will go up. And the outrunners, being mounted 1:1 with the prop, are not as flexible as a geared motor....go up just a little too much in size and the amps can come WAY up, you don't have the reduction of the gearbox to help minimize the effect of overpropping. Now it's just a matter of selecting a prop that flys the plane like you want, and does not make the motor draw more amps than it or the battery can handle. |
RE: Electric Motor Primer
ORIGINAL: ChuckAuger 1. The battery manufacturer sets the 20C rating. 20 X .340A = 6.8A Not everyone seems to list their C... 2. C is the Capacity of the pack. A pack that can be discharged at 20C can be discharged at 20X it's capacity. The discharge rate has no bearing on the charge rate..all lipos must be charged at no higher than 1C. So the 340Mah can be discharged at 6.8A (20C), but must be recharged at 340 milliamps. So how long with it take to recharge these? And, does it take longer to charge a 2 cell or a 3 cell or a larger milliamp pack? 3. No, the motor will only draw as much as the load (prop) and voltage determine it will. Having a pack rated for more than the motor can draw is good, having a motor draw more than a pack can deliver is bad, and will ruin the pack. You would burn the Axi up if you put too much load (too big of a prop) and tried to run it at 10A, but the 10A capable battery would be fine. Ok, what about the weight, isn't it too heavy? 3A. You can order direct, or PaulSwany in the ProBro section is a dealer. 4. The 2015-5400 has a Kv rating of 5400 RPM per volt. This gives a good speed at 7.4V (2S lipo). 39960 RPM (theoretically..) divided by the popular 6.6:1 gearbox gives about 6000 RPM. Of course the prop won't turn all of that, maybe 4500. Take the 2015-2800 and it has (I'm guessing..) a 2800 Kv or 2800 RPM per volt. at 7.4V, this is only 20720 RPM. Now you could run say 4:1 ratio and get a theoretical prop RPM of 5180, or you could run 11.1V and get more RPM, but you now have a 3S battery to tote. Generally speaking, the higher Kv motors are happier on low voltage and low Kv motors on higher voltage so things even out. There are exceptions, I'm just talking about general 3D type flying. Ok 5. The Axi is an outrunner, a special kind of motor where the outside of the motor turns and the Himax is an in-runner, the shaft turns but the can is stationary. The Axi has a Kv of about 1020 seems like, so at 7.4V it only turns 7500 or so RPM...pretty close to what you want to turn your prop, so no need to mount it in a gearbox. Of course the RPM will come down when the prop is mounted, and the amps will go up. And the outrunners, being mounted 1:1 with the prop, are not as flexible as a geared motor....go up just a little too much in size and the amps can come WAY up, you don't have the reduction of the gearbox to help minimize the effect of overpropping. Now it's just a matter of selecting a prop that flys the plane like you want, and does not make the motor draw more amps than it or the battery can handle. |
RE: Electric Motor Primer
"Not everyone seems to list their C... "
Well, I would only buy a pack from a mfg that does, but most of the current generation lipos will do 10C. The Kokams have some 20C packs other than the 340's, but they are kinda heavy. "So how long with it take to recharge these? And, does it take longer to charge a 2 cell or a 3 cell or a larger milliamp pack?" 2 Cell, 3 Cell..it does not matter, they all charge at 1C. Since 1C is the capacity of the pack, it takes one hour (theoretically) to recharge any of them. That said, some chargers quit a little early and don't put 100% back in, some chargers have a very slow last little bit that takes longer than an hour to fully charge a pack. It will always be close to an hour. But, if you put 2 packs in parallel, you can charge 2 packs in an hour at twice the current, Still takes an hour, but you have 2 packs. This can be touchy if the packs are not at the same state of discharge when you start, and a built up parallel pack will most likely be at the same level so it's OK...still gonna take an hour. "Ok, what about the weight, isn't it too heavy? " That's where you have to make careful choices, especially on a very small plane like a Shocky. A little weight can make quite a difference. Once you get a little larger, like a 36" flat foam that the Himax 2015 XXXX would fly, the pack weight doesn't matter as much. |
RE: Electric Motor Primer
ORIGINAL: ChuckAuger "Ok, what about the weight, isn't it too heavy? " That's where you have to make careful choices, especially on a very small plane like a Shocky. A little weight can make quite a difference. Once you get a little larger, like a 36" flat foam that the Himax 2015 XXXX would fly, the pack weight doesn't matter as much. The LiPoly technology and pack offerings seems to be moving so fast that anything you buy this month is obsolete next month anyways. |
RE: Electric Motor Primer
ORIGINAL: ChuckAuger 4. The 2015-5400 has a Kv rating of 5400 RPM per volt. This gives a good speed at 7.4V (2S lipo). 39960 RPM (theoretically..) divided by the popular 6.6:1 gearbox gives about 6000 RPM. Of course the prop won't turn all of that, maybe 4500. Take the 2015-2800 and it has (I'm guessing..) a 2800 Kv or 2800 RPM per volt. at 7.4V, this is only 20720 RPM. Now you could run say 4:1 ratio and get a theoretical prop RPM of 5180, or you could run 11.1V and get more RPM, but you now have a 3S battery to tote. Generally speaking, the higher Kv motors are happier on low voltage and low Kv motors on higher voltage so things even out. There are exceptions, I'm just talking about general 3D type flying. 5. The Axi is an outrunner, a special kind of motor where the outside of the motor turns and the Himax is an in-runner, the shaft turns but the can is stationary. The Axi has a Kv of about 1020 seems like, so at 7.4V it only turns 7500 or so RPM...pretty close to what you want to turn your prop, so no need to mount it in a gearbox. Of course the RPM will come down when the prop is mounted, and the amps will go up. And the outrunners, being mounted 1:1 with the prop, are not as flexible as a geared motor....go up just a little too much in size and the amps can come WAY up, you don't have the reduction of the gearbox to help minimize the effect of overpropping. Now it's just a matter of selecting a prop that flys the plane like you want, and does not make the motor draw more amps than it or the battery can handle. John |
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