|
Originally Posted by Hoese37
(Post 14000214)
Temp will drop at top speed because there is less of a load on the electronics. It takes more energy to change speed than it does to maintain speed. Same reason a car gets better mpg cruising on the highway than it does in stop and go traffic. I see what youre trying to do, but you seem to be ignoring facts based on real experience for how you think they should work.
|
Originally Posted by wingracer
(Post 14000156)
I know nothing about RX8s but if it's set-up for two pole and he's using a 4 pole, it will read 2x the rpm. Since that's what your calculations come up with, I wonder if that's what is going on?
New formula? ((Poles)*V*KV)/2 = RPM Working backwards from the max of 65,000rpm, this newly derived formula indicates that Tekin determines their KV rating based on 14.4 volts power supply. Ha hah! Looks like we worked out a couple interesting facts! :) |
Originally Posted by Hoese37
(Post 14000137)
You've stated that you broke a rear gear due to your gearing, and you went up to compensate. Since a brushless motor is capable of producing max TQ@0rpm, you have just increased the overall load on the motor. It does sound like you are over geared. Now rather than being able to turn the driveline, you have effectively made the motor have to work harder to drive the wheels. The more the motor has to work to overcome the gearing, the more current it pulls.
But to the best of my knowledge, a brushless motor behaves more like a servo than a brushed motor. A servo seeks to hold a specific position, and it has sensors telling it to move one way or another to maintain that position. In the same way, a brushless motor tries to maintain a specific rpm, with sensors that tell the ESC to supply current for longer or shorter intervals. The PWM ESC used for brushless controls the motor by controlling how long the motor can draw current, as opposed to the brushed motor where the ESC controls how much) voltage to send to the motor. A brushless motor does not only draw 10 amps under a light load; the brushless motor draws 100A for 10% of the time, as the current is switched on and off. If the load increases, it will draw 100A for 15%, 20%, or however much it needs to maintain that RPM. Peak torque occurs when the ESC is 100% on, which can happen at any RPM. With that in mind, I realized that the smaller pinion actually increases the force applied to drive train! And the question of gearing was about the force applied to the drive train under acceleration. (This torque on the drivetrain relates to gearing and current draw, but current draw is not determined by gearing. A brushless motor is always drawing it's maximum current, which is determined by it's physical characteristics. Again, this current is switched on and off. The battery sees a short 100A draw 2,000 times per second, even though we tell ourselves that it is only a 5A draw....when more torque is needed, the duration of that 100A draw increases. So I am figuring out that the only effective way to limit current is to get a smaller motor.) With a 1:1 gear ration, the motor torque and force applied to the drive train are the same. But at 1:2 (pinion to spur), the force on the drive train is double the motor's torque; 1:3 is triple. Yeah, the motor is turning 2-3 times faster than the drive shaft, but the brushless motor doesn't care about rpm. So a bigger pinion reduces stress on the drive train, but increases stress on the motor. And as Randy Pike said to the person who "tattled" on me about being over geared, "I see no problem with that as long as temps are okay." And temps are totally fine. They're well beyond fine. The T8 gen2 can withstand over 200 degrees, but I've never gotten mine to over 160 after 20 minutes of constant laps. |
Originally Posted by fredygump
(Post 14000233)
Temp will drop at top speed because the ESC for a brushless motor is nothing more than a switch. At 100% throttle, the switch is basically just "on", instead of switching on and off at something like 2,000hertz.
But the motor must be considered as well, it's a pretty significant part of the system. |
Take a look at a typical brushless motor dyno chart. Note amps tend to drop off with RPM. The motor windings and interaction with the magnetic field create a complex impedance with a lot of inductance, it is not a simple resistance load. Increasing back EMF is another way to describe, essentially just another way to explain the same effect.
This becomes increasingly important as RPMs increase. The inductance means it takes time for the current to develop, at higher RPMs there is not enough time for the current to reach high values before switching to the next phase occurs. As a result torque also drops off with RPM. The chart is based on the single load of the dyno, differing loads will move the current around, but the RPM trend will be significant in most all practical uses. In many respects not much different than a brushed motor. The place where the commutation/switching occurs is different yes, but both have inductance/back EMF and phase switching that become increasingly important as RPMs increase. A significant aspect of what is going on in the system. Dyno chart credit goes to RCT member John Stranahan. http://www.rctech.net/forum/attachme...ss-17-5001.jpg |
Originally Posted by fredygump
(Post 14000289)
I think brushless are very different from brushed motors, and what you say sounds like a brushed motor.
As Dave stated and showed, the curves for brushed and brushless motors look the same. To a first approximation, current and torque drop linearly with increasing RPM. Maximum current occurs at stall. Maximum power occurs at about 1/2 of the free-running RPM. |
Originally Posted by howardcano
(Post 14000365)
This is incorrect. The operation and behavior of a sensored brushless motor is conceptually the same as a brushed motor. The only difference (of course) is that the brushes and commutator of the brushed motor are replaced by position sensors and MOSFETs that do the same thing (but with less resistance, less friction, and no mechanical wear). Other differences are in materials and construction-- rare earth magnets, Y vs. delta windings, etc.-- which are not necessarily unique to brushless motors.
As Dave stated and showed, the curves for brushed and brushless motors look the same. To a first approximation, current and torque drop linearly with increasing RPM. Maximum current occurs at stall. Maximum power occurs at about 1/2 of the free-running RPM. thanks for that. I will consider that in the future. This comment about torque is interesting in relation to the previous comments in the thread about over-gearing. If max power is 1/2 the motor rpm (I believe you...), then this is the metric for determining gearing...set the gearing so you are using this range, since you lose power when you get too high in the rpm range. This makes sense now that I think about it. My SC felt very "weak" when it was geared low...I was trying to reduce motor temp, but it appears I went the wrong way with gearing. In that case I heard the RPMs race, but it just didn't perform. So I went to a bigger gear, which worked much better. But I still am stupidly under-geared most of the time. I'm working on that. But sometimes it is hard when everybody is so quick to blame any and every problem on too high of gearing. But too low of gearing can be just as bad. If you look at the chart above, it is clear that if you are spending much time at max rpm, your engine will get very hot--at max rpm, efficiency drops to 0%, meaning all the power to the engine is converted to heat. This is all good then...I just jumped up to a 22t pinion on my truggy, after being told that 18T was too big... I might put in a 23T, depends on how things go. The guy I bought it from had a 25T in it with a Castle 2200kv... Currently my ESC shows rpm peaks around the 50% range, occasionally jumping up toward max rpm. It's nice to have those extra rpm's, especially if I want my truggy to fly upside-down for a change, on those occasions when I get bored of it flying flat and level... :) |
For most high-power (low turn count) motors, the motor will overheat when geared for maximum power. In this case, the gearing must be shorter (smaller pinion and/or larger spur) to keep the motor within its temperature limits. This is more of a problem in high-traction conditions (like onroad carpet racing) than in low-traction conditions (like a dusty off-road track).
Yes, efficiency does drop at very light loads, but the electrical power input and the motor's power output also approach zero, so the total heat dissipated is small. Unless the motor is set for insanely high timing, it will run cooler when under-geared. |
Please be aware there are limitations to the posted dyno chart, it was one I grabbed to just show some basic trends. For one the behavior at the ends of that chart are suspect, all tests have some error. The real curves don't collapse to zero at high rpm. And the motor doesn't really generate maximum torque with 0 current. I've attached another one that is cleaner at the upper rpm, but still has the 0 current glitch at the start.
Better yet feel free to take a look at John Stranahan's thread where these graphs and many others were posted, a lot of interesting work: http://www.rctech.net/forum/electric...highlight=dyno While I agree undergearing a powerful motor can generate excessive heat, note at lower RPMs the efficiency tends to be lower and the current higher. This can easily generate a lot of heat if overgeared, even more in many if not most cases. Also note this chart represents one load condition. Selecting an appropriate load is discussed in the dyno thread. Gearing changes the load on the motor and thus the curves. Motor designers will talk about the importance of the "slip angle", has a big effect as well. Again really only posted to display trends. http://www.rctech.net/forum/attachme...locity-3-5.jpg |
3 Attachment(s)
I built a rig to test my battery IR via Ohms law. It's a little crude, but it works.
I am using 12v light bulbs as resistors. I have a small bulb (4w) that is on all the time, and draws .25 amps. Then I have an "instant on" type switch that connects two larger 12v bulbs. (The circuit is open until I press the button, and it only stays open while I hold the switch.) When the switch is off, current is .25A. When I press the switch, the current jumps to ~7A. The formula is resistance = change in voltage / change in current. This gives you Ohms, so multiple by 1000 to get mili-Ohms, or just push the decimal over 3 places to the right. I saw a video of a guy who took the voltage at no load (in the formula, current = 0A). But I think starting with a small load should be more accurate. The same video shows the voltage and current measurement being taken after 10 seconds. This seems to be too long. I think the faster the new voltage and current readings can be taken, the more accurate the IR number is. (Note that the resistance of the light bulb changes with temperature, so when the light bulb is cold, amps will be higher. I have seen an initial reading of 9A, then dropping down to 7A after ~2 seconds and holding steady.) Now the good part. NUMBERS!!! I am getting IR measurements of 15-19 mili-ohms on the battery packs I have been concerned about. The measurements are repeatable. I re-measured the one ~ 19 after a quick run in my truggy, and I got almost exactly the same reading (.0197 vs .0191). I still do not know if this reading I am getting is the same reading a professional, caliberated, device would give. But I'm thinking it is pretty close. What I need now are a few brand new packs to test against. If the new packs test <6 mili-ohm range, then I'll feel pretty confident in my method. I am using the "Watts Up" meter to measure both current and voltage. (You can buy essentially the same meter under different brand names.) This is not the most accurate way, but it is the best way I have at the moment. The meter itself is accurate to .1 volts, and the total voltage differences in this test are often less than .1 volts, like .08 volts. So if I really want more accurate numbers, I need more sensitive measuring equipment. The light bulbs are really annoying--the terminals do not take normal solder! So I made my own sockets with ABS plastic, aluminum, and some bolts I had laying around. |
How much current can a 10AWG cable carry without heating up? A lot less than I thought.
I found out that at 20A continuous, the 10AWG wire on my batteries/wiring harness starts getting warm. I measured almost 10degree increase above ambient. Heat increases resistance, which reduces voltage, which reduces motor RPM and max current.... . (Another lesson learned--the importance of an ACCURATE temp gun! I had been using a cheap infrared temp gun, but turns out it is wildly inaccurate. But now I paid good money for an accurate one, so now I don't have to wonder if 160 degrees = 130 degrees.) In my detective work, I re-wired my truggy and upgraded my connectors. Now I have 10AWG throughout (used to have 12AWG), and am using the Amass XT90-S connector (4.5mm bullet). I did a max current/ acceleration test with my truggy. Before the upgrade, I got a max current of ~105A average over several runs and several batteries. After the upgrade, with the 10AWG and XT90-s connectors, I got a max current of 115 amps! I gots myself a 10 "A" more power! And for the record, I am running a tekin 2250kv/RX8 combo. This second test was just today, and the road was wet and puddled--it was still drizzling a little when I made the run. As one might guess, instant 100% throttle resulted in all 4 wheels breaking loose...so I had to gradually apply the throttle. This means it is almost certain my system can now draw more than 115A. So what did I learn? With high current comes high temperature, which reduces current and voltage, which reduces power. The 12awg cable was limiting power, and the 10awg, while it handles more current, is also getting warm even at "reasonable" current loads. Ironically, at first I was worried about drawing too much current. But now I'm drawing even more! But at the same time, I found out that the actual max amperage of 115A from my 7,2000 mAh packs equates to 16C, which is well below the claimed limits of virtually every pack on the market today. I still don't know what the actual "C" rate of my batteries is....so I am still plagued with unanswered questions. |
Accurate temp gun, what are you using for the emissivity value? Same for both guns being compared? Without an accurate emissivity setting, which is difficult for the typical user to determine, claims of accuracy are suspect. Shiny metals are some of the more challenging surfaces to measure accurately with an infrared gun. Temp guns are handy tools no doubt, but best to consider the readings as only a reference, not real accurate.
Yes good connectors and proper soldering are important to keep heat and losses low. Not unusual for most of the heat to come from less than ideal soldering and connectors. Wouldn't be surprised if much of the improvement came from re soldering and new connectors. Although I don't see many electric 1/8 scales lacking punch and power. In general they don't need great batteries as you are discovering, although some headroom/extra over measured values is always wise. Generally good/great packs are more important for 4x4 short course and competitive stock classes. |
Originally Posted by Dave H
(Post 14031767)
Accurate temp gun, what are you using for the emissivity value? Same for both guns being compared? Without an accurate emissivity setting, which is difficult for the typical user to determine, claims of accuracy are suspect. Shiny metals are some of the more challenging surfaces to measure accurately with an infrared gun. Temp guns are handy tools no doubt, but best to consider the readings as only a reference, not real accurate.
Yes good connectors and proper soldering are important to keep heat and losses low. Not unusual for most of the heat to come from less than ideal soldering and connectors. Wouldn't be surprised if much of the improvement came from re soldering and new connectors. Although I don't see many electric 1/8 scales lacking punch and power. In general they don't need great batteries as you are discovering, although some headroom/extra over measured values is always wise. Generally good/great packs are more important for 4x4 short course and competitive stock classes. the main feature of my new temp gun is that it actually has an emissivity setting! :) I saw a recommended emissivity of .77 for anodized aluminum (i.e. motor temp)..another chart said .60-.95. But I haven't had a chance to shoot motor temps with it yet...and I like my tekin motors for their internal temp data via hotwire, which will still be my reference. The meter is a dewalt 12v max...matches my driver. It has emissivity and a 12:1 spot. The old one is a Ryobi I found in the grilling section... The Ryobi is a 6:1 spot, no emissivity, and the readings wander quite badly. Shooting a flat painted wall, the ryobi is 4 degrees higher (@70 degrees f). If I shoot my hand (.99 "E"), the Ryobi is ~5-6 degrees higher. The higher the temp, the further off the Ryobi is. The Ryobi makes me think things are hotter than they actually are. Regarding my batteries, it turns out that they aren't getting as hot as I thought. I was consistently getting temps around 130 degrees, but now I believe that measurement translates to an actual temp of 110 degrees f (to be tested next time at the track). And 110 happens to be the temp that hybrid lipos are said to be most efficient at. |
Originally Posted by howardcano
(Post 14009433)
For most high-power (low turn count) motors, the motor will overheat when geared for maximum power. In this case, the gearing must be shorter (smaller pinion and/or larger spur) to keep the motor within its temperature limits. This is more of a problem in high-traction conditions (like onroad carpet racing) than in low-traction conditions (like a dusty off-road track).
Yes, efficiency does drop at very light loads, but the electrical power input and the motor's power output also approach zero, so the total heat dissipated is small. Unless the motor is set for insanely high timing, it will run cooler when under-geared. Last weekend at the indoor oval, I witnessed a guy burn up a motor. He only ran the motor 3 times. On the second run his temp was 150 degrees, so he went down 1 tooth. He was going to make his motor run cooler, right? But on the third and final run, he released the "magic smoke". He completed the race, but his motor ended around 250 degrees. This event confused a lot of people. It doesn't fit the assumptions everybody makes about rc brushless. What happened is the exact opposite to what people expect. The reason might have something to do with motor timing, and that is one thing I didn't think about at the time. Guys seem to think more timing is always better (for indoor), but it seems that the timing made the motor extremely inefficient at high rpms, which produced a lot of heat. |
My question is if you have a meter. What voltage drops are you seeing at the different amps? Maybe you just have a bad battery or "weak" one. If you have nothing wrong with your truck and have a friend do the exact same test with his battery. You should get a similar voltage drop. If your is much higher your battery just needs to be replaced. IR under load is also a good way to test. There are all sorts of videos of computing IR under a load. If your IR has doubled under load on those batteries they may not be good for truggy. Move them to another life. Keep the newest in truggy. A new set of batteries is cheap once a year compared to Nitro fuel.
What seems to be hell on my batteries. Time, Overdischarge, Overcharging, Improper storage. You can easily fix 3 of them. Never charge completely to 4.2 per cell. try 4.18 or something where if the charger is a bit off you are not overcharging. LVC between 3.2 and 3.5 per cell. Store them half charged. I am not Ryan so I don't miss a little of the top and the bottom. When I get that good I will get free batteries. lol
Originally Posted by fredygump
(Post 13998583)
Back to the "lipo" part of this post..
I purchased a "Watts Up" current meter and did a couple brief tests today before it rained. This is with my dex408T, 4S, Tekin 2250kv On gravel, driving similar to how I would drive on the local track, I got a peak current draw of 45 amps. Driving through freshly mowed grass at full throttle resulted in a peak of 75 amps. And full speed run on asphalt resulted in a peak of 80 amps. These numbers are quite low compared to the ratings of Lipo batteries. My 70C 7,200mAh batteries technically have a 500amp continuous discharge rate, with up to 1000amp peak draw. But my motor's peak current was measured at 80 amps, which is only 11.1C draw from my batteries. And the thing is that running at the track should be quite close to that 45amp peak--lets call it 50amps. That translates to only "7C"! So my initial conclusion is that the "truth" is that my batteries are being pushed (or rather, being pulled) to their limit. And that limit is ~7C. This is 10% of the pack rating, and it is only 25% of the theoretical 25-30C limit of lipo technology, as stated by Danny of SMC. I am a little surprised that the numbers are so different than all the claims. I knew the claims were grossly in error, but I'm surprised to find out how far off they really are. I'm surprised that a 7C load is too much for my batteries, which are the no longer available SMC "race spec" 70C 7,200mAh batteries. Now, I know that these batteries are as good or better than the others, but... I'm a little disappointed right now. My second conclusion is that it appears that yes, my motor is too big for the batteries. (Also worthy of noting is that with this 50-80 amp load, my ESC is operating near it's maximum. Data logging shows that it's internal temp gets up to and holds steady at 180 degrees, with the fan running continuously. So the ESC is also over-rated. I did a little research, and what did I find? It turns out that the 220amp (per phase) spec is true but deceptive. What most people don't realize is that there are actually two current measurements for brushless motors, battery current and motor current. Motor current is significantly higher than battery current for some complicated physics reasons I don't understand. So the 220A "per phase" is a trick. Advice I have found suggests that it is relatively safe infer an ESC's battery current capacity of 50% of the rated motor current. This would make the Tekin RX8 a 110 amp ESC... But it does not seem likely that it is designed to run continuously at 110 amps. Maybe the 'fets can handle the current, but the heat sink is too small to dissipate the heat that kind of continuous load would generate....) |
| All times are GMT -7. It is currently 02:10 PM. |
Powered By: vBulletin v3.9.3.9 Patch Level 3
Copyright © 2026 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.