Gear reduction ratio vs Amps consumption?


#1

Hello,

Naive question : All other parameters being equal, would a 4:1 reducer gear consume more amps / hour, less amps / hour, or just the same as a 5:1 reducer gear ?

I would tend to think that this would be the same as 4:1 would turn faster but would have less torque compare to 5:1 so total energy would remain constant. But… I could be wrong.

Thank you


#2

the way i see it (and i might be wrong so please feel free to add)

not easy to answer but:

  • same propeller ,same motor ,same rpm:
    a lower reduction gearbox will mean low voltage by the esc, higher amp (for constant torque), more heat less efficent

  • same propeller, same rpm, different kv motor to match the gearbox, same voltage
    the higher reduction gearbox will match the lower kv motor, IR motor lower, less heat from the motor from a “higher torque potentiel”

so the highest voltage, the highest reduction, the lowest kv is the way to go (IR, KC, torque, heat…)


#3

But wouldn’t higher reduction plus lower kv mean lower rpm, and since for obvious safety reasons we do not wish to go over 12S, we would then need a larger prop, i.e. more energy consumption to force it to turn i.e. more amps ?

I guess that there must be some optimal point somewhere…

It sounds like a 5 variable equation (voltage / amps / prop / gear reducer / kv) and we only get it the right balance by pure luck :slight_smile:


#4

pure luck no really ,more “educated guess” ,trial and error, first i got a 780kv in my set up , it did the job but i wanted more rpm so i got a 840kv, my mistake, i should have taken a 720kv and look more less amp and not more speed

as far prop i don’t known yet, i don’t have the skills to make one, but it is a sweet spot between diameter , pitch and area of the blades as far i have tried, you will need some power to turn a “small” prop with high pitch


#5

14S would be the highest voltage to go for. That’s what Lift and Fliteboard use. In terms of the motor-gearbox combination it makes sense to get them as low as possible as you will use a lower amount of power.
I went for a 5684 750Kv motor and coupled it with a 7:1 gearbox which made my unit smaller that the standard 500Kv + Neugart 5:1. Ultimately it will have a similar prop RPM, but will actually allow me a bit more power if I want. I haven’t pushed it to full speed yet. But I will do it shortly.


#6

Hi @philgib

I just want to share the thoughts I had on this topic.

I took a 6:1 4P Motorchief gearbox, TP4070 7Y motor (630Kv / 104A / 79V), Alu prop (have to look up the spcs.),Gong Allvator L foil .
My thinking behind this decision was:

• I have 12s Battery (so, the max voltage is fixed)

• I need between (rough number for my big prop) 2500 and 4000 rpm at the prop. (thats also fixed)(20-30%drop)

• I need as much power as possible. ( -> because voltage is fixed by battery, I need the possibility to draw some amps)

• High Kv motor is (normally) rated for more amps. (less windings -> thicker wire -> stronger for amps) (example TP motor: 440Kv 75A and 910Kv 150A)

• Reisenauer Motorchief is rated for max. 50000rpm and 5.5kW

So my thinking: -> take a gearbox with maximum reduction so you can take a motor with higher Kv rating which can handle more amps.

And after some calculations with Voltage/Kv/Amps/rpm(prop)/rpm(motor)/rpm(drop)/Power/reduction/Powerrating(gearbox), I made my decision.

Voltage is fixed by the battery so for more power you need more amps and as a higher Kv motor can handle more amps you need a higer gear reduction to take a motor with more Kv. ( but I guess that’s only a problem with the smaller motors)

That was my thinking. It may be wrong but my setup works and has lots of power. :wink: (I just need a better board)

Feel free to correct me :wink:

PatRick


#7

yes but at some point with small motor you will not be able to get more rpm, and a higher kv has a lower A:NM, a higher internal resistance (volatge drop so rpm drop)…
the voltage is not “fixed”, the esc at 80% throttle, “applies” 80% of the battery voltage so for the same mecanical power needed, the amps will be higher (more heat…)
higher kv has more “pontentiel” electrical power but less mecanical power at a given rpm

so i figured it is better to run “lower KV” at 100% throttle, and by lower kv i mean the higher of the lowest to match the rpm needed including losses


#8

maybe LIMITED would have been the better word… :wink:

YES… you want a motor with high Kv (for the higher current rating) . But yes, it has to match your gearbox and your needed/wanted max rpm.(prop) :+1: you have to calculate what is better… :sunglasses:

all you win with higher reduction is a motor with a bit higher Current rating.
only necessary if you really reach the limits of your motor.
in my case the difference between my motor (630Kv) and the next lower kv motor (550kv) is about 600W of calculated Power that I won.

cheers PatRick.


#9

@mr_magic, absolutely perfect path of thinking, thank you for sharing :slight_smile:
Your TP motor… 630KV 104A… Sounds like the Aquastar 620KV 105A… Except your TP is twice the price so I imagineit is much better.

Virus, I think, has a TP motor too, and a Chief too, so you may have nearly the same set-up ? Except Virus is making his own carbon props if I am not mistaking. You mentioned an alu prop, I guess that this is the typical cheap one many of us have. Have you mentioned your ESC ?

My trouble taking a Super Chief is that the SSS56104 has an 8mm shaft, and the only 8mm pinion Reisenauer would sell is for a 4:1 gearbox. I would like to avoid lathing the SSS shaft down to 5mm, out of my league


#10

are you sure you gain 600w? ,higher IR… it is the “same” motor after all with the same max power, with a small motor, at continous rpm with torque at max, the potentiel amp gain will not be transfert to mecanical power i think, but we all want more speed and more power so we choose a little more than less, and that was my mistake… :wink:


#11

@philgib I can’t tell if the TP is that much better because I don’t have the AquaStar to compare them.

Yes I have one of those cheap Alu props.(7 1/4 x 5)
300A AlienPower Boat ESC watercooled (heats up to 30-35°C)

I had the same problem when choosing motor and gearbox. (Superchief only rated for 3000W afaik) :thinking:

@Alexandre
I just recalculated what I did when choosing the motor and gearbox.
TP4070 9Y 490KV 83A 104V GB 1:5 vs 7Y 630KV 104A 79V GB 1:6
result: +945W for the 1:6 version.
BUT: I calculated very simplified!!! NO losses ect. included.!! Calculated like RC motor. simple P=UxI
so don’t count on those 945W… probably more like a random number… :see_no_evil: