Brushless motor power / voltage

[Dave ReedParticipant @davereed72029]

Hi all,

Here's a question for the brushless motor experts out there.

Because of the KV rating we know that the relationship between voltage and RPM (no load) is linear, ie every additional volt applied gives the same increase in RPM.

My question is: Does the same apply to power output under load?

Example: A motor is rated at a max voltage of 4S(14.4v). With the same load would the power at 2S(7.2v) be half that at 4S(14.4v)?
To put it another way, if I plan to use 2S and need 100w, would a motor rated at 4S/200w be a close match?

Thanks
Dave

[Dave ReedParticipant @davereed72029]

[Richard SimpsonParticipant @richardsimpson88330]

Dave, I'm no way an expert on brushless, in fact I haven't got one so haven't got a clue, however the old equations still apply.

Power, i.e. watts, is current x voltage so 5A going through a circuit at 12V is 60 watts. You cannot really consider one without the other so if you halve the voltage and the power remains the same you must double the current. This can be a concern then with cable sizes.

It is also precisely why large distribution networks use very large voltages so the current is reduced and consequently cable sizes can be smaller.

So in your example above if you halve the voltage for the same power the current would double.

[Dave ReedParticipant @davereed72029]

Hi Richard,

Thanks for your response.
I do get the P=IV issue and understand what is needed to keep it safe. If I want 200w output at 7.2v, I need to allow for 28A + a few to allow for losses/inefficiency.
I was trying to ask a generic question to get a general feel for how the power output of a motor changes with applied voltage.

Let me give a specific example that might better explain what I'm trying to ask.

Consider the Vic Smeed Vosper RTTL plan (1/24). It's drawn with a D.C. Rapier motor which I believe is about 200w.
It could be powered with (for example) the 210w Overlander Tornado 2830/09 using a 4S battery pack and suitable ESC. However 4S Marine ESC's seem quite rare/expensive. I also like the idea of a twin shaft build.

An alternative would be a twin shaft arrangement using 2 of the same motor as above and twin ESC's running at 2S.

So back to the question:- Would 2 motors running at half max voltage produce the same power as 1 motor running at max voltage? Efficiency at different loads and voltages are likely to change and I have no idea if the relationship of voltage/power output is linear for these motors.

I know there are plenty of examples on the forum of suitable power options for various size and type of hulls, but I prefer to know why I'm choosing a particular option rather than just copying someone else's.

Dave

[harry smith 1Participant @harrysmith1]

Hi Dave

A bit of a 64 dollar question because brushless motors are generally more power !!!

The unloaded rpm is easy, KV x Volts = RPM.

Also the longer the motor to more powerfully they are.

Harry

[ashley needhamParticipant @ashleyneedham69188]

Dave.my gut feeling says half voltage does not equal half the useable power output. It may give you half the unloaded rpm.

using a tiny 28mm outrunner will of course save a lot of weight, by about 2/3 that of a brushed motor of equivalent power. The downside is that you heed two esc for two brushless motors.

ALSO, as brushless motors are more efficient, you get more bang for your buck (watt)…by this I mean that 200 measured watts in a brushed motor produces less actual useful turning power than 200 measured watts in a brushless motor.

You could further say that the transmission losses of having two shafts is…wait for it….double that of using a single shaft, further decreasing useable power.

To sum up, if Vic used some old 200Watt can motor and a bunch of Nicads, and you used a 200w brushless on Lipos, you would be way ahead in the useable power stakes…AND have a lighter boat to start with, in itself a speed booster.

Ashley

[Ray Wood 3Participant @raywood3]

Hi Dave,

Having just dug out the R.T.T.L plans I wish I hadn't she is a good looking craft I may have to build one

I think it's down to personnel choice over expense at the end of the day , Iv'e built quite a few twin screw boats following the fullsize, but makes very little difference once it's in the water.

The Vic Smeed design is built like a "Brick Outhouse" for the rigours of starting the 3.5cc Davies Charlton Diesel engine, so she will be heavy, I could build her to be half that weight for electric with lighter bulkheads and skins as she really deserves to be double diagonal planked and will look better.

The Diesel would have reved at around 9k rpm so not easy to guess the power output and also has a brass flywheel 1 5/8" diameter so plenty of torque.

Regards Ray

[harry smith 1Participant @harrysmith1]

Hi Ray

Have the 68 foot plan, dam nice boat.

Twin screw/motors with tank steering.

Turning would be great.

As for going astern at slow speed super great.

A Osprey and Cig from your plans are getting build my one of the club members.

Harry

[Ray Wood 3Participant @raywood3]

Hi Harry,

Yes I must admit to being tempted

I have PM'd you

Regards Ray

[Dave ReedParticipant @davereed72029]

Thanks to everyone for your comments.

Ashley wins the prize for getting closest to answering the actual question with:
"my gut feeling says half voltage does not equal half the useable power output"
That was what I thought as well and why I threw it out there for comment.

The post seems to have morphed into discussion of Vic's RTTL, my fault I suppose and OK anyway.

Ray, I hear you will have more time for boat building, so why not build one?
I came across a review of the aero version of the diesel (http://sceptreflight.com/Model%20Engine%20Tests/Davis-Charlton%20350%20MkII.html) which gives it a power of 0.281bhp – Google tells me that is about 210w (hence the 200w in my original question).

I'm new to boat building so I need to get more experience before building twin shaft models anyway, just thinking ahead of the game.

Anyway, I hope to get a chance to visit the RAF museum in London next week and have a look at RTTL 2757 and take a few photos. Unfortunately as she's located in the car park, there's no opportunity to view from above the waterline. May also not be in great condition after several years outside.

Dave

[Kevin BeallParticipant @kevinbeall91525]

If it is any help I'm getting near to the end of a Seaplane Tender build, 36" with a weight of 2.9KG. I've fitted twin Overlander Tornado 2836/08 brushless outrunner motors, twin esc, 2S Lipo and 40mm 3 blade prop. I'm running a pair of 60A waterproof buggy speed controllers, the only disadvantage is that reverse is engaged using a "double tap" method.

 

I did a bath trial last night and to say the model is over powered is an under statement! These motors are very powerful and didn't even get warm (they can run on 4S batteries). I think just one of these would be enough.

They are rated at 270 watts on 4S batteries, I don't think they are half on 2S but I wouldn't be surprised if they were putting out 100 watts each. Don't be fooled by their small size, they really do look tiny next to a brushed 600.

I went for 2S Lipo as they tend to be cheaper due this being the most common car battery and 2S hard pack batteries are cheap on Amazon.

 

I'm not too sure if they are too much, but I'll give them a go…what's the worst that can happen! I'll remember to take a black rubbish sack along to the first outing just in case..

Edited By Kevin Beall on 06/03/2023 08:51:02

Edited By Kevin Beall on 06/03/2023 08:52:48

[harry smith 1Participant @harrysmith1]

HI Kevin

The brush 600's would be about 200 grams each against the overland's 97 grams each.

What 2S Lipo rating are you running??

Harry

[Kevin BeallParticipant @kevinbeall91525]

Hi Harry, ZEEE hard case 5200mAH 80C 7.4V. £37.00 ish for 2, same day delivery on Amazon.

80C is way over what we will be using them for but as just £2 more than a 50C I thought I might as well them..they're probably all the same but just a different sticker!

[Dave ReedParticipant @davereed72029]

Hi Kevin,

Thanks for your comments, sounds like you've already gone down a path similar to the one I'm considering. Good to know I'm in the right ball park. A bit too much power is better than not enough!

Would be nice to hear how it goes once you get it on the pond.

Dave

[harry smith 1Participant @harrysmith1]

Hi Kevin

You are not going to run out of battery power with those !!!

You could run the boat with one battery and a Y connector to the ESCs.

Happy first run with the boat.

Good Luck

Harry

[Kevin BeallParticipant @kevinbeall91525]

I just thought if I want to water ski behind it a bit of extra weight would do no harm..

I'm making the battery mounts so I can run 1 or 2. Good thing about hard case ones they are made to be strapped down firmly so easy to to do.

Just got to find a local club as my nearest was Broomfield Park…but the pond there is looking a little empty .

[Dave ReedParticipant @davereed72029]

Hi Kevin,

I'm not far from there myself, looked like this last time I visited

Once I get something that floats, thought I'd visit Bushy Park – bit of a trek but shouldn't be too bad on a Sunday morning. There's a few people on this forum who meet there, check out Ashley's "Today's Boating" thread in the "All things floating" area.

 

Edited By Dave Reed on 07/03/2023 18:43:31

[harry smith 1Participant @harrysmith1]

Hi Kevin

For a ski-er you would require PT73 from McHale's Navy.

McHale did that in Season 1 Episode 1 !!!!

Harry

[ashley needhamParticipant @ashleyneedham69188]

Hi Dave, be nice to see you at Bushy Park! Not sure why more people do not visit, there must be loads of people within a 30 min drive of us.

Kevin, as I recall you live that bit further away to make a visit easy?

Ashley

[Kevin BeallParticipant @kevinbeall91525]

Hi Ashley, yes I'm a bit of a distance but not too far. Might have to pop over once the weather gets a bit nicer, what is the best day to visit?

Get the rescue boat/waders/fire extinguishers ready..

[ashley needhamParticipant @ashleyneedham69188]

We boat Sunday but you really have to be there by 1000 or the car park fills up.

However…ok during weekdays, if you fancy it I am free mon tue fri……oh no….u are still working ?.?

Ash

[John W EParticipant @johnwe]

Hi there

to really throw the pigeon to the cat Vic Smeed's RTTL plan (the real vessel) was originally triple screw. Then, in her lifetime she was converted to a twin screw with higher output engines – so why dont you try and put 3 brushless motors in – and class it as a low flying airplane I can hear Ray's brain working right now.

John

[Ray Wood 3Participant @raywood3]

Hi John,

A good try but just 3 times more expensive !! I'm all for single's from now on

Regards Ray

[Kevin BeallParticipant @kevinbeall91525]

Just out of interest I thought I would do some runs in the test tank (bath) today to get an idea of how much current my motors were using. Each run was done with freshly charged 2 cell Lipo (about 8.4v).

Graupner Speed 600 ECO on 2 blade brass 30mm prop, looks fairly aggressive, flat out 11 amps! The best I ever seen it do on the 6 cell Nicads was 8 amps. looks like the extra ooomph of the Lipo has pushed it up a bit. So getting on for 90 watts consumption, quoted 72% efficient, about 65 watts at the shaft.

Tornado Thumper 2836/08 1120kv on 3 blade plastic 40mm prop, standard cheap plastic not very aggressive, flat out 7.5 amps. This motor is rated at 20 amps on 4 cell Lipo, so halving the number of cells reduces the consumption by more than half. So if we say the motor 85% efficient, about 52 watts at the shaft.

Last check was with the prop of the first test, 30mm brass, on the brushless Tornado 2836/08. This should give a like for like same prop load etc. This time the current draw was up to 8.5 amps. Maths done and about 60 watts at the shaft.

So the brushed Speed 600 ECO has about the same output as the brushless Tornado 2836/08 on a like for like propeller using 2S Lipo. The difference is the brushless is about 25% of the size, and running at less than 50% of it's max output (so running a lot cooler and last longer).

None of this is really relevant on a slower model, but if you are building a faster style design then it might be of use. All the test showed 4 amps on what seemed like half power and more normal speeds rather than flat out.

[ashley needhamParticipant @ashleyneedham69188]

….AND a 28mm brushless is probably1/3 the weight of a 600.

Ashley

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