Hi Steven,
Wattage is V x A, if you lower the Voltage, the Amps go up under load.
As an example of how a continuous load heats things up:
The testruns with my MAS powered by a 1000W 800KV outrunner were made without watercooling, running on 14,8V, this meant a peak current of over 60A, so I used a 150A ESC.
I walked next to the boat along the canal, so the speed was a brisk walking pace, with the occasional burst of full throttle, but 90% of the runtime was at partial throttle.
After over an hour (it's a long canal), the boat stopped.
When it has floated back to me the motor was boiling hot as was the ESC, both items smelled slightly 'electric', I burnt my fingers on the motormount, so I didn't try to touch the motor...
Fortunately the ESC had shut down thermally, otherwise it would have burnt up.
Granted, the weather was warm, the unfinished hull (no deck yet) was exposed to direct sunlight and the runtime is rather extreme, compared to pottering around on the pond, but that's how I use my boats, as our town has a lot of canals, but no suitable model pond.
So, even with an ESC with an headroom of 100% compared to the max motorcurrent, it shut down eventually.
After watercooling was installed, both motor and ESC never exceeded 45°C, currently the boat runs on a watercooled 100A ESC and it stays cool.
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Assuming you got this motor for your boat:
http://www.graupner.de/en/products/1794/product.aspx I think I know why the motor got hot.
In the video the motor didn't sound like it was revving at 15000 rpm, half that is more likely, meaning you're bogging down the motor severely.
It'll still do it's best to get up to the rpm dictated by the Voltage applied and will draw quite a lot of current, heating things up.
As no brushtab or cancooling was installed, the heat build up has nowhere to go.
Another point (literally) is breaking in a brushed motor; new brushes have just two tiny contactpoints to the commutor.
When you apply full throttle, all current has to flow through these minute points, the brushes will arch badly and possible burn into the copper of the commutor, damaging it for the rest of its working life.
The arching also causes a lot of heat.
A new brushed motor should be broken in, running submerged in a glass of water, on an as low Voltage as possible (1,5 - 3V) to avoid arching.
When the water starts to get dark, check if the brushes have assumed the curve of the commutor, if so, dry out the motor (compressed air work well) in a warm place, put a tiny(!) drop of oil on the bushings and you're good to go.
Brushless thoughts:
For an underwater prop, rule of thumb is about 10.000 rpm under load.
Running on 7,2V, the math is simple and would mean a motor of about 1400 - 1500KV.
In-runners with such a low KV are often a Ø36 mm size, so fitting that into your boat would mean major surgery.
Also the low end performance of an in-runner is often non existant and the motor will burst into life at one third throttle, making crawling along an illusion.
As most outrunners have more poles, the low end response is much smoother (albeit brushed motors still do a better job in direct drive).
As cooling an outrunner has it's limits, it's wise to choose a bigger motor to have more headroom.
Something like this would work nicely, make sure it'll fit and include space for a watercooled motormount (bought, or DIY):
http://www.hobbyking.com/hobbyking/store/__16234__NTM_Prop_Drive_Series_35_30A_1400kv_560w.htmlSmaller and still plenty of power:
http://www.hobbyking.com/hobbyking/store/__25078__NTM_Prop_Drive_28_36_1400KV_560W.htmlTo control it, a car ESC gives you a smoother low end than a boat ESC, something like this:
http://www.hobbyking.com/hobbyking/store/__17232__Hobby_King_60A_Sensored_Sensorless_Car_ESC_1_10_1_12_.htmlThe fan cooled heatsink isn't very effective in a closed hull, but taking off the fan and watercooling the ESC by making a watercooler, fitting on the same boltpattern as the heatsink is relatively easy to do.
If you don't want to go that route, Hobbyking also has the watercooled version available, but I have no experience with these (yet) so I can't say how well the low end response will be:
http://www.hobbyking.com/hobbyking/store/__28223__HobbyKing_90A_Boat_ESC_4A_SBEC.htmlRegards, Jan.
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Topic: Vosper RTTL (Read 19478 times)