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Author Topic: Brushless motor advice needed  (Read 3221 times)

BournemouthBoater

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Brushless motor advice needed
« on: December 18, 2011, 08:01:49 pm »

Hi,

I've only just got my Perkasa brushed setup working well, after a lot of help and advice from this forum and from club mates, but clearly I'm a sucker for punishment, and I'm now considering a brushless setup for a new approx 46" torpedo boat.

I've found the brushless gear even more difficult to make sense of, so hopefully I can get some advice from someone with experience of a similar setup.

I have the following limitations/requirements:
1. Limited budget, I'm not looking for the performance of an electrified greased weasel, my choice to go brushless is more about trying to save some weight and simply to try it out
2. It will have to run on 8.4v or 9.6v Nimh batteries, one pack per motor
3. I want to use two motors, but I'm open to using 1 or 2 speed controllers, I guess the choice will come down to which option works out cheaper
4. I'd prefer not to use water cooling - but I have no idea if that's feasible
5. It would be great to get a recommendation of prop size too

I would appreciate any advice or guidance.

Thanks
Gloves
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MikeA

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Re: Brushless motor advice needed
« Reply #1 on: December 18, 2011, 08:04:42 pm »

have you got a pic of the boat?
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BournemouthBoater

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Re: Brushless motor advice needed
« Reply #2 on: December 18, 2011, 08:20:12 pm »

Hi Mike,

Sorry, no, it's still in the planning stage. I'm currently building my second boat which will be powered by brushed motors and is not a high performance craft anyway, but I've been keen to build something high performance, incorporating some of the lessons learned in my Perkasa - which was my first boat. I haven't decided on what exactly it will be yet, except that it will be in the 40-46" size range and a fast attack/torpedo type boat.

Thanks
Gloves
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nick_75au

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Re: Brushless motor advice needed
« Reply #3 on: December 19, 2011, 09:45:32 am »

Im assuming a fast planing hull style boat here ;)

Two motors definitely requires 2 controllers for Brush-less motors, they can be run off one channel with a Y servo cable, you cannot save here. A 30 amp unit per motor should be enough but I would probably go larger just so you are not pushing the edge.

As you have specified the voltage it gives a fair Idea of the Kv or RPM per volt the motor needs to be and for a fast boat 10,000 RPM is a good starting point, lower if you use a larger prop or want to sacrifice performance for run time.

On 8.4 volt an 1200 Kv motor to achieve 10,000 RPM or 1050 Kv for 9.6 give or take 10% and this gives you one important number, the size of the prop will determine the can size, and for anything close to 40-50 mm
(which a 46 inch boat would probably use) a 36 mm diameter out runner will prove sufficient.

The motor will probably not need cooling if you keep the motors to what I recommend, a lower Kv motor will reduce the possibility even more. For the relatively short runs the Nimh will give, a fan cooled Car ESC will work. If you run parallel packs then using a water cooled ESC would be better due to the longer run time adding more heat.

As for prices local members can point you out to good UK sources, I buy my gear from Hobby King out of Hong Kong.

Cheers
Nick

Another option to consider is run the 2 packs in series (both packs supply both motors and double voltage) and halve the Kv of the motors to 5-600 Kv, this increases efficiency sllightly but ads a little extra complexity to the wiring.
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dodgy geezer

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Re: Brushless motor advice needed
« Reply #4 on: December 19, 2011, 10:57:47 am »


2. It will have to run on 8.4v or 9.6v Nimh batteries, one pack per motor

3. I want to use two motors, but I'm open to using 1 or 2 speed controllers, I guess the choice will come down to which option works out cheaper



Because brushless motors do not have a commutator which limits the amount of power they can absorb, they are capable of drawing huge currents and producing a lot of push. But only if the battery can provide that current. LiPo batteries can do this, and are commonly used with brushless motors for this reason - have you thought of using them?

As was mentioned earlier, you will need two ESCs. The reason for this is that, unlike a brushed motor, the ESC is an integral part of a brushless setup. A brushless will not run without an ESC - it determines the proper coil timings for the motor - so it interacts with the coils on that one motor alone. If you connect two motors to a single brushed ESC neither will work and they may be damaged.

I find that Giant Cod have a good selection of low-cost brushless motors - http://www.giantcod.co.uk/brushless-motor-c-25.html?max=50


 
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BournemouthBoater

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Re: Brushless motor advice needed
« Reply #5 on: December 19, 2011, 09:20:29 pm »

Thanks for the advice everyone, it has been very helpful.

This is what I'm considering buying:

Motor:
http://www.rcmodelcentre.co.uk/Thumper-3536/prod_560.html

Esc:
http://www.rcmodelcentre.co.uk/Alturn-Brushless-Speed-Controller-40A-with-Heat-Sink-P-ACS40/prod_22463.html
Doesn't have a fan, but does have a heatsink, it would of course be easy to install a small fan if needed.

Thanks
Gloves
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dodgy geezer

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Re: Brushless motor advice needed
« Reply #6 on: December 19, 2011, 10:25:24 pm »

Thanks for the advice everyone, it has been very helpful.

This is what I'm considering buying:

Motor:
http://www.rcmodelcentre.co.uk/Thumper-3536/prod_560.html

Esc:
http://www.rcmodelcentre.co.uk/Alturn-Brushless-Speed-Controller-40A-with-Heat-Sink-P-ACS40/prod_22463.html
Doesn't have a fan, but does have a heatsink, it would of course be easy to install a small fan if needed.

Thanks
Gloves

Hmm...

You can get a more powerful motor for £8.50 less from here: http://www.giantcod.co.uk/xyh4240-1020kv-brushless-outrunner-p-403282.html

And the ESC has not got reverse. Is that ok? I would usually like to overspec the ESC and get a 60A or 80A - particularly if I have no cooling. GC offer a 60A watercooled version of this for £5 less - here: http://www.giantcod.co.uk/60amp-boat-p-404813.html . It hasn't got all good reviews, but they're not all bad either. GC do a brushless fan-cooled reversing ESC for £35...
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