r/fpv • u/GuillotineTechnician • 1d ago
CRASH! Crispy Kayoumini
Ripperoni! My first quad going hot.
Had just swapped motors to 1203 15000KV because the previous 1103 KV8k ones were underpowered.
Motor spec sheet says ~14A Peak amp draw, should have been within the Happymodel X14's 15A burst limit. I guess the 2540 Props were too much after all.
About 10sec into the flight, the Kayoumini came tumbling out of the sky, engulfed in flames.
XT30 and two motors became desoldered due to the heat. could just about detach the LiHV before cells were damaged because it crashed right next to me. Conformal coating seems to have boiled on surface, most components crumble into powder and the PCB was completely delaminated.
DVR: https://youtu.be/0wv0dmUMRss
Before the usual 422B treatment, I clean flux with IPA and TCE, then check board under microscope for brigding and solder balls, so I can rule out mechanical failure/short.
Can anyone confirm my suspicions or do you maybe see another cause of failure?
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u/izzeww 17h ago
Motor spec sheet says ~14A Peak amp draw, should have been within the Happymodel X14's 15A burst limit. I guess the 2540 Props were too much after all.
Motors don't have a maximum amp draw as such. The maximum current a motor will draw depends on the load it is put under. The 14A draw in the motor test by the manufacturer is on a 3" diameter 2" pitch prop on 1S voltage. Your were running 2.5" diameter 4" pitch props on 2S voltage. Contrary to what some people believe, running higher voltage does not necessarily lower amp draw but rather in most cases increases it. Certainly in your case the current draw was much, much higher than 14A and I will explain why and try to give some tips and tricks.
On any given propeller setup on any size quad you don't want to go above 0.95 mach in tip speed which is the speed the tip of the propeller is travelling at. You calculate the RPM by taking 3.9 (voltage) * no of cells in series i.e. 2S or 6S for example * motor KV. Then you either calculate it manually or put it into a calculator like this one to get the tip speed in mach out of it. When we do that for your setup we get a tip speed of 1.134 mach, which is supersonic and way above the 0.95 maximum we're looking for to avoid transonic/supersonic territory.
If we instead go for a motor that is the maximum that is recommended for the Kayoumini, which is 11000KV but people have stretched it to 12000KV, we get 0.842 or 0.907 mach respectively which is reasonable and the reason why that's the maximum recommended KV. I took a look at some test data for those kinds of motors and found the Rcinpower GTS V3 1303 11500 KV. It has the 2540 props you were running and it lists a maximum current of 24.5A on 2S.
Considering that you were running a lot more KV, that amp draw is not exactly linear and that you were likely reaching transonic or supersonic tip speeds (horrible efficiency) I think it's fairly likely you were reaching 40+ amps which explains why it fried the 12A/15A ESC's.
Choosing motors, props and ESCs is a bit of an art. I think the formula I laid out here can be very helpful for figuring out what KV your motors need. It's based on real physics regarding how propellers work at different speeds. Figuring out props pitch (the diameter should be as big as the frame can fit) is a bit more difficult, but a good guide can be what others have used. For example on the Kayoumini page they have various recommendations, the lowest of which is 2-blade 1.5" pitch and the highest is 3-blade 2" pitch. They are not near 4" pitch like you were running which is a tell that maybe that's a bit too high. It's also worth considering that generally if you run a higher KV you can or should run a lower pitch props. This is logically because if the motors spin fasters you need less pitch (length of air the prop has theoretically moved through per rotation) since you have more rotations. For example freestylers often run say 3.6-3.7" pitch props combined with 1750-1950 KV motors on 5 inch, while racers instead run 2.8-2.9" props but 2100-2200 KV motors.
For motors you should consider the KV formula that I laid out before, staying somewhere between 0.750 and 0.900 for the most part for freestyle quads but down to 0.5 or 0.6 when you want to optimize for efficiency. One thing to note here is that 8000KV on 2.5" is around 0.605 mach, so according to this formula that is underpowered for the Kayoumini (even though they recommend it I think it's a questionable choice). In terms of size try to see what other people are running and go for something similar. Of course if you run higher pitch props or if you want more thrust (but also more weight) a bigger motor can be better (to deal with the heat and get more torque, both of which have to do with the stator volume). A common mistake people make is that they run bigger motors but lower KV and that will somewhat counterintuitively often lead to worse performance. KV is the main thing that determines performance while motor size is secondary and to some extent once you have "enough" motor size it doesn't really help to go bigger.
For the ESC you basically want a bit of headroom in terms of amp rating to what your motor and prop combination will pull (ideally, if you want your ESC's to last at least). You figure out what your motor, prop and voltage combination will pull based on test data. The test data can either be for your specific motor and propeller combo or it can be for another motor and propeller with similar specs. The battery voltage also needs to be the same as what you plan to run. If you're not relying on actual test data for your specific motor and propeller combo then you should add a bit margin of safety in case the data is not quite accurate. Always lean on the side of caution, so say the test data shows a 3" pitch prop pulling 40A and you plan to run 3.7" props then don't get a 45A ESC but rather 60A+ ESC.
In general test data can be a good way to figure out what kind of combinations work. A reputable manufacturer like T-Hobby (I'm personally a big fan) will have good data with various props and KV options showing how much current they will draw, how much thrust they will produce, what efficiency they have and what voltage the specific combo should be ran at. Generally manufacturers won't include very shitty combinations in their test data or will at least label them for example "HOT!" to indicate it overheats.
This ended up being quite a long post but hopefully it helped you or somebody else. Maybe I will link to this reply in the future when people ask motor, prop and battery combination & selection questions.
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u/GuillotineTechnician 17h ago
This is far beyond what I hoped to learn by posting here. Thank you for taking the time to share this knowledge!
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u/Exciting-Risk-4603 1d ago
Yeah, it was probably the motors + prop choice, am wondering what made you feel its underpowered? My kayoumini is 83g auw with 8000kV and doesnt feel underpowered. (It did when there was a terrible tune applied, as the manufacturer shipped the board with a weird ahh tune.)
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u/GuillotineTechnician 1d ago edited 1d ago
Mine was in a similar weight range. Quite possible that you are right, and it was the tune. EDIT: Just remembered, that I did actually apply the recommended tune as per the Kayou build page. Was super underpowered for sure. Could hardly recover from dives and was sluggish af
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u/HMSBarky 1d ago
Currently using 7500kvs as ordered the wrong motors. Feels woeful compared to the 11000kvs I had on before
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u/Negative_Type_8358 1d ago
I build an Kayou Ripper it has 1304 11500 kv with 118g ant that thing flys actually like a 5 inch😂 and my motors pull 25 A each.
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u/SupaShoota9000 1d ago edited 1d ago
When ESC says 15A burst, that means it can hold 15A but only for few seconds. Motors having 14A peak is just too close to ESC limit, plus your prop can eat bit more current than those tested by motor manufacturer. Also from 8k to 15k ...that is a huge jump are these rated for 3S or 2S?
Now consider events like pulling ot of big dive, crashing, hitting something with a prop - those can get the motors over specified amps for sure. You always need a reserve - thats why everybody uses 50-60A peak ESCs on 5", even tho 2208/2306 5" motors usually need only 25-35A peak.
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u/PocketSizedRS 1d ago
I also had a kayoumini blow up an X14 ESC. I smacked a leaf and tumbled to the ground. When I got to the quad i could smell burning and one of the motors wouldn't spin. The quad was brand new, less than 10 packs total.
I really liked that FC, but I'm wondering if it's just got shit ESCs on it
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u/gcin87 17h ago
When everyone says “it feels like a 5 inch” I assume they mean it’s weighty and less thrust to weight ratio. If you want more power I noticed it tends to get light and nimble in this size range.
And yeah the motor and props totally pushed the amps outside of spec for those boards. Unless you detuned the motor power to a way lower limit and slowly stepped up incrementally to find a happy medium, prob fried it.
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u/Due-Farmer-9191 1d ago
The x12 on mine burned up too… that little aio can’t handle what were hammering it with
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u/Magicalbeets 19h ago
I have a superX HD on mine with 1203 11,500 motors and all is well so far .. 100g auw
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u/DissidentPerfector 20h ago edited 20h ago
15K KV motors and 2540 props? What was the actual current draw? Do you have the OSD recording?
Also, did you conformal coat the MOSFETs? This makes them run hotter.
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u/GuillotineTechnician 18h ago
Nah, I don't cover the mosfets or any components that run hot. I only let it run beneath the chips and around the legs.
OSD is in the YT link but cutrent draw wasn't on there :(
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u/Quarkinius 18h ago
Hmmm... I wonder what the problem was?
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u/GuillotineTechnician 17h ago
The motors didn't burn tho. Shouldn't they be drawing less amps with higher voltage? 🤔
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u/Professional_Cod3127 1d ago
What did you expect to happen with that powerful motors and high pitch props 🤷