Model Boat Mayhem - Forum

Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: tobyker on April 30, 2019, 07:42:40 PM

Title: blowing fuse
Post by: tobyker on April 30, 2019, 07:42:40 PM
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?
Title: Re: blowing fuse
Post by: Colin Bishop on April 30, 2019, 07:53:22 PM
It rather depends on the characteristics of the motor. Some 540s are wired to draw very large currents.Colin
Title: Re: blowing fuse
Post by: john44 on April 30, 2019, 07:59:44 PM
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.

Title: Re: blowing fuse
Post by: grasshopper on April 30, 2019, 09:18:32 PM
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.
Title: Re: blowing fuse
Post by: chas on April 30, 2019, 11:05:24 PM
First thing would be to change the X pitch prop to a standard p35, and work on from there.

Title: Re: blowing fuse
Post by: tobyker on May 01, 2019, 10:24:21 AM
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! 
Title: Re: blowing fuse
Post by: john44 on May 01, 2019, 06:24:51 PM
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.


Title: Re: blowing fuse
Post by: Colin Bishop on May 01, 2019, 06:53:53 PM
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.
Title: Re: blowing fuse
Post by: tobyker on May 14, 2019, 11:23:02 PM
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.
Title: Re: blowing fuse
Post by: clockworks on May 15, 2019, 07:47:53 AM
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.
Title: Re: blowing fuse
Post by: malcolmfrary on May 15, 2019, 09:42:27 AM
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.