Model Boat Mayhem
Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: tobyker on April 30, 2019, 07:42:40 pm
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I've made a stepped hull freelance MTB with a 540 buggy motor, 7.2 v buggy NiMH pack, 15amp ESC, x35 prop and 10amp fuse. When I gave it some urge, the fuse blew. Is this to be expected?
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It rather depends on the characteristics of the motor. Some 540s are wired to draw very large currents.Colin
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Hi, you will find buggy motors are power hungry and with a 35mm prop you can bet your motor
Is pulling more than 10 amps. Hence the blowing fuse.
Have you or any of your club mates got a watts up meter you can put in the circuit to read the
Current draw. You may have to change your esc 15 amp seems a little low, but without reading
The motor spec I can only guess.
John
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I used to race a club boat, 540 motor with an X35 prop and used to flatten a 2.2A/hr pack in under 6 minutes so what was that pulling...2.2A in 1/10 of the hourly rate? 20 amps?
No surprise blowing a 10 amp fuse.
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First thing would be to change the X pitch prop to a standard p35, and work on from there.
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Thanks, folks. I'll try changing the prop first, as I have a few in stock. I also have a 50amp ESC, but I think that might be overdoing things a bit!
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A 50amp esc would not bean overkill, just gives more protection from burning out.
Then you can try 15, 20, 25 amp fuses to see which one is right for your boat.
I only use 50amp esc,s in my boats and fused to suit.
John
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As Chas says, reduce the prop loading until you maintain acceptable performance. Chances are that it will be pulling far less amps by giving a better balance between motor and prop.
Colin
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Tried a P40 3 blade, popped the fuse at less than full revs. Now fitted 50amp ESC and 25 amp fuse, and will test again.
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I used to race electric buggies. Back when we were using brushed 540 motors, the fastest guys could almost flatten a 3600mAH pack in a 5 minute (plus 1 lap) race. Not totally flat, but a noticeable drop in performance by the last lap - over 3000mAH to recharge. That's an average current of over 30 amps, so the peak could've been around 50A (twisty track).
I guess a boat will be a little less power-hungry - easier for the motor to spin up turning a prop than some grippy tyres.
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Motors intended for buggys are usually rated by their turn count. How few turns is this one? "Normal" is 24 or more. "Sport" is lower. Low turn count motors spin very fast at the cost of much reduced torque and much higher current. Overloading them puts them into a virtual stall condition, pulling a lot more current for very little extra output power, and causing them to produce heat rather than turning force.
The normal rules of can type motors is "not less poles than prop blades, prop should not be of greater diameter than the motor can". With very high revving low turn motors, too much prop will push the motor into permanent semi stall mode and cause it to pull a lot more current than it would if it was correctly loaded. A 2 blade 35mm prop was suggested earlier. That is definately the best place to start, probably going to smaller props if more speed in wanted - its the distance that the water is pushed per second, not the volume, that counts.
At very high currents, the chemical reaction going on inside the battery becomes less efficient at turning stored energy into electric energy. The "lost" energy becomes heat, hence the warm battery in that kind of use.
It sounds like the fuse is doing an excellent job of protecting everything.
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Now tried 50 amp ESC, 25 amp fuse, and as this worked with the P40, put the X35 back on. now going well but needs spray rails! Nice level plane on the stepped hull.