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Author Topic: Dont understand fuse blowing  (Read 1847 times)

lesfac

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Dont understand fuse blowing
« on: July 11, 2015, 06:07:21 PM »

I was out the other day running my Perkasa happily blasting up and down the lake at full throttle for about half an hour until the battery cells went low and the ESC cut the power. As I was advised and since proved if left for a minute the battery comes back up again and I can usually tootle back to the side. I waited and sure enough got some headway but probably rushed it a little and the cells went low again and it stopped before I got it to the bank. I then found it wouldn't move after waiting for the battery voltage to rise and I had to wait for it to drift to the side.
Upon investigating I found that the two 30 amp fuses in parallel had blown.
I can't understand how the fuses are ok blasting up and down but blow in a low voltage and low throttle setting. I replaced the fuses and everything works as it should. The only thing that occurs to me is if the ESC actually sinks power to prevent movement but that seems unlikely. The ESC is a 160 amp Aquastar

Can anyone explain this?
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sparkey

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Re: Dont understand fuse blowing
« Reply #1 on: July 11, 2015, 08:41:09 PM »

 :-)) Like us fuses get tired,don't forget when you are blasting up and down the lake they get very hot and cool off when there is no load on them this weakens the metal that they are made of,this happens if the fuse rating is on or close to the amount of current the motor is using and any surge will break the fuse, vibration can also do this to a weak fuse,try with new fuses and see how long they last if it is quite a few sessions at the lake then I would change anything,but if it happens again uprate the fuses slightly(remember the current rating of the cable).Also noticed that you have 2 x30 amp fuses in parallel is this to get 60 amps overhaul? if so 1 fuse blows and you have all the current on 1x 30 amp fuse which will blow at a lot less speed,if this is your set up I would ditch the 2 x 30amp setup and get 1x 60 amp fuse and carrier ,hope this of some help,Ray. :-))       
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sparkey

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Re: Dont understand fuse blowing
« Reply #2 on: July 11, 2015, 08:58:16 PM »

 ;) Forgot to add you can get a 60amp fuse and carrier can be obtained from flea bay for a fiver,I use these on many of my fast boats,and must add fuses in parallel is not a good idea and can cause a lot of problems,Ray. %) 
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lesfac

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Re: Dont understand fuse blowing
« Reply #3 on: July 11, 2015, 09:25:01 PM »

Thanks for taking the trouble to reply.
Yes the fuses were in parallel just to get roughly 60 amps. Although there has been no calculation in arriving at that. Its far less than the ESC rating of 160 amps but it has been fine for many sessions.
I am puzzling over it because my common sense says that full throttle running will pull the most amps.
After the battery ran down and the boat stopped it still worked OK when the battery picked itself back up a little and the cells rose over the cut off voltage. To get back to the shore I only used light throttle yet the fuses blew.
It just seems odd
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wombat

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Re: Dont understand fuse blowing
« Reply #4 on: July 11, 2015, 09:46:18 PM »

Actally, the point where most current is drawn is when power is applied but before the motor has spun up to speed. This current can be 10 times normal full load current. Blipping the throttle can be worse, or at low throttle speeds where there isn't enough go to spin the motor over.

Fuses in parallel is not a good move - better to have a single fuse of the right rating. What probably happened is one of yours failed and then the other

Wom
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inertia

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Re: Dont understand fuse blowing
« Reply #5 on: July 11, 2015, 11:25:51 PM »

Hi, Wombat
Good to know you're still there watching this stuff. Bringing electronics to the masses was never going to be a short-term, single-handed job.
Dave M
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malcolmfrary

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Re: Dont understand fuse blowing
« Reply #6 on: July 12, 2015, 09:44:11 AM »

With a brushed motor set up with a PWM ESC, starting is relatively gentle, the stream of pulses allows the inductive components (ie the motor windings) to produce current limiting back emfs until the pulses get wide enough to move the motor, when it starts to get sensible. 
I suspect that the opposite applies with brushless motors.  At low speed, a very wide (switched on for a long time electronically speaking) pulse must be applied to get it moving.  This leaves the motor coil acting as a pure DC resistor, and a high power motor has a DC resistance of sweet very little.  If there isn't quite enough voltage to provide the power to move the motor, it will act as a dead short. 
This will result in high start-up currents.  If they are over in a short time, the fuse doesn't have time to overheat.  Fuses have ratings, normal, slow and quick blow.  These give a general idea of how quickly a fuse will react to a current slightly above its current rating.  Give a "normal" fuse a few percent more current than its rating, it will take a long time before it blows.  Give it twice that, and it blows almost instantly.  Put such a fuse through a lot of abuse cycles of heating and cooling, expansion and contraction, and it will weaken, either reducing its current carrying ability, or slowly converting itself into a quick blow, or both.


Just a thought about two in parallel - it is the way of the world that they will not be identical, so one will die off first, probably quietly and gently, leaving the survivor doing the work.  You might not notice the first one going, but you will notice the second, because that's when everything stops.
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lesfac

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Re: Dont understand fuse blowing
« Reply #7 on: July 12, 2015, 10:02:26 AM »

Thanks for your interesting reply. That may well be the reason.

I would have installed a single fuse but readily available automotive fuses seem to be 40 amp maximum and I want to be able to pop in and buy replacements easily. (Pound shop shopper  :-)  )  I know bigger fuses are available on line. The parallel fuse set up serves its purpose of protection and in the main runs without problem
Les
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