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Author Topic: Blown fuse  (Read 2486 times)

gerard

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Blown fuse
« on: June 14, 2014, 02:46:21 PM »

Can anyone enlighten me as to why I would blow a 25amp main fuse?
I am using 2 x 5000 nimh 12v batteries wired through Actions P103 to a P92 dist board driving 2 Graupner 700 BB turbo motors, on returning home replaced the fuse and all worked OK. Electrics are not my thing but I would like to know why it may have blown.
Regards
Gerard
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Time Bandit

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Re: Blown fuse
« Reply #1 on: June 14, 2014, 02:58:24 PM »

Seems not to be physically impossible to blow a 25A fuse with 2 of Graupners 700 motors.
We were running the TurboC at 25A each in the good old times.

Some more information would be helpful.
What kind of boat is this (fast?)
What props do you have installed?


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regards

Tobias

gerard

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Re: Blown fuse
« Reply #2 on: June 14, 2014, 03:05:38 PM »

Its in a 1/16 Speedline Severn ,the props are a 4 blade brass about 50mm
gerard
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sparkey

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Re: Blown fuse
« Reply #3 on: June 14, 2014, 03:25:25 PM »

Question is how old is the fuse,what load is on the fuse,how often that load is applied to the fuse,fuses degrade over time with the load,fuses get hot then cold every time they are used the metal degrades and so does the rating,if the electric's in the boat are now working OK with new fuse on load then it might be this, have you picked up some weed or rubbish on the props thus increase the load on the motors it would not take much if the fuse is tired,Ray.
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gerard

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Re: Blown fuse
« Reply #4 on: June 14, 2014, 03:31:50 PM »

Thanks Ray,
I did pick up some weed a couple of sails ago and the fuse has been in for about a year so it may be as you suggest
Gerard
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inertia

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Re: Blown fuse
« Reply #5 on: June 14, 2014, 03:42:59 PM »

Careful. P103 is only rated to 15A!  :o The props sound a tad on the large side for Speed 700 motors. 40mm x 4-bl or so would be kinder on the motors and the batteries (and the 25A fuse  8) )
Dave M
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john44

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Re: Blown fuse
« Reply #6 on: June 14, 2014, 03:47:34 PM »

Hi Gerard, Which motors are you using, G6317 12v, or G3308 8.4v?
The G6317 12v have a current drain of 12.5 amps @ max efficiency
Stall current 45 amps.
The G3308 8.4v have a current drain of 15 amps@ max efficiency
Stall current 75 amps.
If you are running both motors on 1, 25 amp fuse it will keep blowing.
You need to find out the peak drain on your set-up, if you or a fellow
club member has a watts -up meter put that in circuit and run the boat
the info stored will show you the way to go.
Ideally fuse both motors and use a suitable larger main battery fuse.

 hope this has helped

john
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Time Bandit

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Re: Blown fuse
« Reply #7 on: June 14, 2014, 04:53:12 PM »

At this prop size both motor and fuse are about to blow up soon  :}

It seems to me that the general setup is wrong for that style of boat, normally Id guess you need like 100-150W for scale speed. So a 25A fuse should not blow up at any time.

Motors generally have too much rpm for brass propellers with a pitch rate of around 1.
With 30mm props maybe perfekt, but the boat wont accelerate anymore.
50mm is definetly too big for a size 700 motor in direct drive, fully submerged I would stop at 40mm.

That would be the first change to make sure the fuses dont blow up.
If you want a real economic drive train you should consider using the Robbe Power 755/40 motors (Im sure somewhere available in the UK as well).
Im using these in all my SAR ships around 1m length since years with brass props from 40-50mm and mainly 4S Lifepo4 batteries.




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regards

Tobias

gerard

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Re: Blown fuse
« Reply #8 on: June 14, 2014, 05:16:29 PM »

Dave,
The wiring I used is from the wiring diagram of yours for !/16speedline for Erik Dijkstra
Gerard
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gerard

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Re: Blown fuse
« Reply #9 on: June 14, 2014, 05:17:59 PM »

Thanks for all your info
Will reduce the prop size and see how that goes.
Gerard
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malcolmfrary

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Re: Blown fuse
« Reply #10 on: June 14, 2014, 05:21:46 PM »

An extra factor is that it is now goose feather season at Fairhaven.  Finding one of those will put an extra drain on the motor with resultant extra current demand.  If its running a bit near the limit, that might just tip it.
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inertia

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Re: Blown fuse
« Reply #11 on: June 14, 2014, 06:02:34 PM »

Dave,
The wiring I used is from the wiring diagram of yours for 1/16speedline for Erik Dijkstra
Gerard
Not quite, m'duck. That diagram shows 12v Speed 600BB motors.
The problem isn't in the wiring; as others have said it's the prop size relative to the motors. Excess load on the motors by the props will cause higher currents which result in all sorts of things getting hot and bothered. Feedback from several customers told us that the Speed 700BB 12v motors were happy with nothing larger than 45mm 4-blade Raboesch Type A props in the Model Slipway Tamar and Trent - which are pretty much the same size as the Severn. I don't know what size props Erik used with his 600 motors but there was no adverse feedback from him. If you want the model to fly around the lake then brushless is the inevitable way to go.
Hope you get it sorted,
Dave M
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gerard

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Re: Blown fuse
« Reply #12 on: June 14, 2014, 09:27:24 PM »

Thanks Dave
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grendel

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Re: Blown fuse
« Reply #13 on: June 15, 2014, 07:33:25 AM »

having recently done a course at work on fuse characteristics, fuses can blow at a lower current if it is sustained for a longer time, so while a 25A fuse is designed to blow as soon as 25A passes, at 24A it will eventually blow if the current is sustained for a period of time, likewise it could also blow at 20A if sustained for even longer, then you have the accuracy of the fuse, if the accuracy is plus or minus 5%, then it could blow at 23.75A or may not blow until 26.25A, these are all reasons a fuse might blow with no apparent fault needing to be present.

Grendel
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NFMike

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Re: Blown fuse
« Reply #14 on: June 15, 2014, 09:39:37 AM »

I've never heard of any 'normal' fuse that behaves like that, so I think either this was a special application or your trainer is just wrong.

Marked fuse ratings are for continuous current at normal ambient temperatures. A high temperature will cause earlier operation as can aging, but for most general purposes those effects are not worth bothering with. Most gp fuses will carry 50% or more of their rated current for many seconds, even into minutes, before blowing.

malcolmfrary

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Re: Blown fuse
« Reply #15 on: June 15, 2014, 10:18:42 AM »

Yes, the fuse action described sounds very much like a "quick-blow" type.  Most "normal" fuses will survive a brief excursion to 150% the number on the label, "slow blow" types are useful where a circuit might need a pulse of high current, but would rapidly incinerate itself if left on.  It is sad that there are still instructors out there who are good at instructing, but know little about the subject that they are instructing on.
The blade type fuses for cars are commonly the "normal" type, and should only be rendered more sensitive if a high rating is used in a mount for a lower rating, when the current through the contacts might start warming things up over time.  This is the main reason why I went off the in-line glass tube types.  At anything like their rated current, they start to melt the plastic holder.
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grendel

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Re: Blown fuse
« Reply #16 on: June 15, 2014, 01:42:29 PM »

this was for electricity supply fuses up to 300A, but all fuses have a characteristic curve that they operate to.
Grendel
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