Fuses in electric drive circuits

[HaigParticipant @haig]

Hi Gents,

I was wondering where the best place in an electric drive circuit to put a fuse (or fuses) and more importantly why.  I have seen a few circuits including those on the Action-Electronics website and some of these have two fuses.

 

My circuits comprise of 2 sets of the following

 

Graupner 700 (12v) motor with two bladed 35 mm prop

40 amp rated ESC (with BEC disconnected)

Battery consisting of 8 x D cells(9ah)

 

Although I have been modelling RC boats for a very long time this is the first time I am going to use ESCs.

 

From experience the most likely problem would be fouling of the prop by weeds or plastic bag causing a stall etc. etc.

 

Regards

Haig

[HaigParticipant @haig]

[Robin LeeParticipant @robinlee31938]

Hi Haig

The usual place for a fuse would be at the output of the battery. This would prevent a short circuit in the ESC or wiring or motor resulting in a very high current from the battery which would cause damage to your boat/ESC. The rating of the fuse should be matched to the ESC’s maximum output thus protecting your ESC. You have not quoted the stalled current drawn by the motor. If this is lower than the 40amps of your ESC all will be well but if it is greater than 40 amps there could be a problem. Ideally the motor stalled current should be lower than the ESCs maximum rating. You need to protect your ESC in the circumstances of a jammed prop. If the fuse rating is say 40amps and the stalled motor current is greater than this, then if you get caught in weed your fuse will blow and your boat will need to be rescued. In your setup I would have two fuses – one per ESC. That way if you have a problem on one side the other will still be OK and get you home. 

 

I have just looked at your 2 photos. Nice model. One question though – The bright blue gun tubs and green TTs – is that temporary or is that an authentic colour scheme?

Robin

Edited By Robin Lee on 23/02/2010 18:00:57

[Colin BishopModerator @colinbishop34627]

My standard practice on a twin screw boat using a single  battery is to have three fuses. One rated at 150% of the stall current of one motor in the positive line to the battery and one fuse each between the ESC and its motor which approximates to the stall current of the motor.

 

The idea is that the main fuse protects against short circuits and the individial motor fuse will blow if the prop is stalled due to weed or something round the prop.

 

Fuses don’t cost much and can save you a great deal of money so it is a no brainer to fit them.

 

Colin

 

 

 

[HaigParticipant @haig]

Hi Robin, Colin,

Thanks for you helpful advice.

I have got a bit more info.

The 2 x ESCs are actually rated at 50A forward or 20A backward.

Thr Graupner 700BB s have a nominal 12V Voltage, drain at max efficiency 12.5A and stall current of approx 43A.

 

There are two independent banks of 8 x D cells

 

I had in mind one fuse of 10A in each circuit (to be installed near the battery as advised), but I think 15A may be better.  This is well below the 40A which is probably the recommended max.  Would this be OK.

 

Test idling on 6V the motors were humming along using 1.2A and I could still feel the power of these motors.

 

I have updated the photos in the album.

 

Regards

Haig

[Robin LeeParticipant @robinlee31938]

Hi Haig

The stalled motor  current of 43 amps is very high and unless you intend wiring your battery and motor circuit with pretty heavyweight wiring it would seem it would be better if your fuses blew if your motors were stalled (particularly if the boat was reversing at the time). Your suggestion of using a 15 amp fuse in each battery output is very close to your optimum current for your motors so you may get nuisance blowing of that fuse. Flexible wiring size 32/.2 will take 10 amps continuous so you might get away with that but should really use wiring a bit heavier if you intend running at full throttle for extended periods. Maplin catalogue has a flexible wire size 50/0.25 will carry 30amps max but this would be awkward to install as it is 3.81mm diameter but something like this would be the best engineering solution.

 

If it was my boat I would use 20 amp fuses and use 2 off 32/.02 cables wired in parallel or equivalent single cable of 20 amp rating throughout the high power circuitry. Using these fuses hopefully the normal operating current will not cause nuisance blowing and the reverse maximum rating of the ESCs would also be protected. A jammed prop would of course blow the fuse/fuses so better keep away from weed Haig.

 

Alternatively you could install a high value fuse to permit stalling but it would be imperative that if your boat got stuck in weed that you only use full throttle for a few seconds to try and free it.

 

 I once had a boat that occasionally had a situation where I lost control of it with the ESC stuck in full reverse of forward not responding to the transmitter. Under those circumstances getting stuck in weed with inadeqate protection of the wiring would have been nasty! Bit of an extreme situation perhaps but you never know…..

 

i have looked at your new photos – looks very good (I assume the bright colours was before final painting?)

Robin

 

 

[HaigParticipant @haig]

Thanks Robin for your time and very helpful advice.

 

I have 2 in-line mini blade fuse holders that I will install into the circuits.  I will insert 20 amp fuses.  The fuse holders are rated for 30 amps and the circuit cabling is rated for at least 20 amps (heavy duty auto cable).  I will take a picture or two of the inside of the boat over the weekend once I have completed the circuits.

 

The earlier photos had some unpainted/unfinished items.  I have just completed the Vickers 0.50 machine gun tubs.  All the guns traverse assisted by ‘servo-morph’ units.

 

The best feature will be the operating front torpedo tubes.  I still need to build the second torpedo, but the prototype works.  More on them once they are proven.

 

So far the model has taken two and a half years.

 

Regards

Haig

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