Protection for ESC

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9 March 2016 at 17:50 #63821
Robin Lee
Participant
@robinlee31938
This is really a question for Dave Milbourn. Dave, I am converting two model Lifeboats bought by a benefactor as toys from the RNLI. The Surface Warship association would have 12 such boats to be used to raise funds for the RNLI at shows where children can use them (for a fee).

For the two boats I am converting I have been given Action P78 Condor 2A Autoreset Speed Controllers. The ESC will drive two type 280 motors in parallel using a 7.2 volt battery. I have measured the current drawn by the two motors combined – about 1.5 amps in a bucket of water. 6 amps stalled. I'm a bit concerned about the scenario of lifeboat jammed in weed with full welly being applied.

I would always fit a fuse (hopefully just a single fuse in the battery output) but what rating? I note the P78 has "2 amps maximum motor current" in the data sheets but the power FET seems to have a rating much higher than that.

You view would be invaluable.

Regards

Robin
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9 March 2016 at 17:50 #5408
Robin Lee
Participant
@robinlee31938
9 March 2016 at 23:09 #63826
Dave Milbourn
Participant
@davemilbourn48782
Robin

It's the relay contacts which is/are the limiting factor. That said, there is no current flowing when you change from ahead to astern, so it ought to be able to carry more than 2A once the motors start running. Unfortunately there is no available data on how much more because the manufacturers wouldn't want to imply that they somehow approved of its "abuse".
I'd use a P81 or fuse the P78 at maybe 3A – but don't quote me!

Dave M
10 March 2016 at 11:00 #63830
Malcolm Frary
Participant
@malcolmfrary95515
The fuse needs to be able to carry the normal current plus a bit for luck and then use the next available value up provided that that value is less than what the ESC can handle. If its more than that, for comfort, a bigger ESC is wanted.

In an installation with two motors off one ESC, it makes sense to fuse each motor separately. The motor fuses should be a lower value than the one protecting the ESC, although "protecting the ESC" is putting it the wrong way. It is actually protecting the wiring and battery from misfortunes within the ESC, and, eventually, protects the rest of the boat. Separate fuses for the motors ensures that should one motor short out, the other is still available.

If jammed in weed, the protecting fuse will leave the boat intact, but obviously just as stuck. If the weed is heavier on one prop than the other, you still have the other motor to try to get back with and you still have power to the radio to help with steering.
10 March 2016 at 18:58 #63841
Robin Lee
Participant
@robinlee31938
Dave – thank you for your information Dave. Your suggestion on fuse rating sounds sensible.

Another question for you. We are also using an Action BECC. Is it acceptable to use just the BECC on/off switch to control the boat ie the battery is connected direct to the ESC all the time or do we need a switch for the battery as well? I suppose we could use one on/off switch for everything?

Its all a bit influenced by what the original toy as sold had installed. It has a rather dubious slide switch under a water proofing cover accessible from the deck. This switch just switched the battery supply to the electronic unit which comprised an integrated receiver and independent motor drive circuit board for steering (no rudder(s)). Pretty sure this switch would not be up to switch the main drive current in the new arrangement as the original drive motors were under hull pods of pretty low power (and low ability to stop water getting into the pod hence this modification).

Problems Problems…

Malcolm – thank you for your input. In this instance the ECSs have already been purchased and I'm sure we cannot justify 3 fuses. In this case the single fuse would be protecting every thing, including the ESC.

Robin
10 March 2016 at 19:19 #63842
Dave Milbourn
Participant
@davemilbourn48782
Robin

I'm of the belt-and-braces persuasion, so I would put switches in both the +ve wire from the battery to the separate BEC (P19?) and most definitely in the main wire to the ESCs. These are kiddies' boats we're talking about and you don't want a sudden unexpected burst of power with little fingers about. Switch on the Rx power first and off last, every time – but you knew that anyway.

The main power switch doesn't have to be large – you're only dealing with low single amps, but a toggle switch is preferable to a slide switch every time. If you use a 5A DPDT toggle then you can isolate both ESCs from the battery with one switch.

Dave M
11 March 2016 at 10:07 #63846
Dave Milbourn
Participant
@davemilbourn48782
Second thoughts – this should do the job. Note that there is only one system switch to isolate the battery and there are alternatives for fusing either the whole thing or each ESC individually.

Suit yourself!

Edited By Dave Milbourn on 11/03/2016 10:09:12
11 March 2016 at 10:43 #63848
Robin Lee
Participant
@robinlee31938
Many thanks Dave – I will submit your diagram to the committee and take it from there. Actually there is only a single P78 but I can work out what that would mean…..

Regards

Robin
11 March 2016 at 11:13 #63849
Dodgy Geezer 1
Participant
@dodgygeezer1
Posted by Dave Milbourn on 10/03/2016 19:19:37:

….. Switch on the Rx power first and off last, every time – but you knew that anyway…

Are we sure about that?
11 March 2016 at 11:39 #63851
Dave Milbourn
Participant
@davemilbourn48782
DG

Nope; you're right.

I should have put "Tx". Mea culpa – blame the fact that T and R are immediately adjacent on the keyboard (a real bummer, that!).

Looks like Hara Kiri time again…but before I go, here's a revised one for Robin:

DM

Edited By Dave Milbourn on 11/03/2016 11:54:37
11 March 2016 at 17:16 #63852
Robin Lee
Participant
@robinlee31938
Thanks Dave. Just the ticket. I guessed it was a typo. Hope the diagram passes the scrutiny of the committee and director of operations in Switzerland.

Robin
11 March 2016 at 19:33 #63855
Dave Milbourn
Participant
@davemilbourn48782
DM
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