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Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
DerAlbi wrote ...
I cant see how a 30A device in a "FlyingThing" can be maxed out... The absolute most unrepeatable current peaky may not exceed 30A... so the actual load should be way way lower on average. If not, its the buyer who decided wrong and again not the engineer nor his boss I think there is a problem with the philosophy between engineer and consumer... Engineer discribes maximum rating, consumer takes it as average... doesnt work that way
You and I know that the 30A is a maximum, however it doesn't make it that clear does it? I mean, the only way we know is because Patrick took a non-functioning one apart and found out what MOSFETs it used.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Heres a problem, i just made a short flight in moderate wind.
the data logger, known to be good, recorded 12.66 amps peak. i put the machine through a climb, pitch forward, pitch back, side skid/slip and brought it down in about one minute.
The heat sink (not in windstream mind you) was getting almost to hot to touch. il use a IR thermometer next time. This is totally baffleing unless the forced air really does matter so much. but at 15 amps i didnt have this problem on other machines.
The others dont seem to be this hot through the heatshirnk. and ill make a current measurement on them next.
Im wondering at this point, if this one came from the factory, with a setting not quite right? but thats unlikley, twice i think.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
As McConner suggested earlier, Im starting to think its a motor timing issue. the motors kv is effectivly to high for the natural rotating speed of the prop. and this cuases the ESC to chop the three phase with current and voltage across the silicon at the same time.
the low kv motors are all $100 or more, (vs $15) and are fat pancake ones. As opposed to "Can" type motors. so them being special means hard to get and $100+ each. needing 4 or 6 becomes a bank account problem.
Im thinking of a 30 inch prop (as Ash and i are attempting) but using a two or more gear reduction train. for good capablity with low cost. the extra friction loss and latency in the PID loop will be penalties though.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
As McConner suggested earlier, Im starting to think its a motor timing issue. the motors kv is effectivly to high for the natural rotating speed of the prop. and this cuases the ESC to chop the three phase with current and voltage across the silicon at the same time.
the low kv motors are all $100 or more, (vs $15) and are fat pancake ones. As opposed to "Can" type motors. so them being special means hard to get and $100+ each. needing 4 or 6 becomes a bank account problem.
Im thinking of a 30 inch prop (as Ash and i are attempting) but using a two or more gear reduction train. for good capablity with low cost. the extra friction loss and latency in the PID loop will be penalties though.
The 'ideal solution' here is to build your own motors, but that's not practical in most cases.
I've calculated that Mach 1 tip speed for a 30" prop is 8,500 RPM, but maths isn't my strong point, please check this.
Gearing may be the way to go if we want to get close to Mach 1 tip speed, I don't know enough about the available motors.
The simplest form of gearing is pulleys and drivebelts. This can also keep the mass of the motors close to the C of G
EDIT: It will depend whether the CH 'thin airfoil' section has sufficient strength to withstand the constraints associated with ~8,000 RPM.
I'm sure it's possible, though. Angle of attack will be minimal. Chord length will be whatever is required for sufficient strength.
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Three Phase Electronic Speed Control In-Flight Blow-Out.
