Please Explain Battery Output and Motor Torque

Anything to do with Propulsion.
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rehoward
Posts: 49
Joined: Dec 15th, 2015, 5:41 am
Location: Everett, WA

Please Explain Battery Output and Motor Torque

Post by rehoward »

Any battery and motor guru's out there?

I recently bought from HobbyKing a hexTronik DT700 Brushless Outrunner 700kv motor to experiment with as a thruster drive. HobbyKing recommended a Lipo battery for the motor and I asked for amp/time curves and voltage drop graphs but they could not help me. So now I have at least dozen questions on brushless DC motors and Lipo batteries. I also would like to know the performance specs of the motor regarding torque/amps so that I can find a "sweet spot" to operate my thruster at. Knowing battery performance and motor performance would allow me to make a good decision on what prop to use. I can find a good prop by doing experimentation but I need some groundwork figures for the motor and battery first.

So, in general, how do Lipo batteries deliver their power over time? And typically do brushless DC motors have a sweet spot for best efficiency?

Randy
rossrov
Posts: 383
Joined: Feb 28th, 2013, 5:01 pm
Location: Australia

Re: Please Explain Battery Output and Motor Torque

Post by rossrov »

Many variables. For starters, how big is the ROV? How fast? What is the intended environment? My approach would be to do some basic homework (refer "draw, thrust and components" thread on this forum) and take an educated guess at prop pitch and diameter and use that as a starting point. Buy 2 or 3 different pitch props of same diameter and test them. Maybe buy a couple of different diameter props of the same pitch and try those. Most of the ROVs on this forum will not need anywhere near maximum power for most of the time in the water. Diving could likely be the biggest peak power demand.
rehoward
Posts: 49
Joined: Dec 15th, 2015, 5:41 am
Location: Everett, WA

Re: Please Explain Battery Output and Motor Torque

Post by rehoward »

rossrov wrote:Many variables. For starters, how big is the ROV? How fast? What is the intended environment? My approach would be to do some basic homework (refer "draw, thrust and components" thread on this forum) and take an educated guess at prop pitch and diameter and use that as a starting point. Buy 2 or 3 different pitch props of same diameter and test them. Maybe buy a couple of different diameter props of the same pitch and try those. Most of the ROVs on this forum will not need anywhere near maximum power for most of the time in the water. Diving could likely be the biggest peak power demand.
Thanks for your reply rossrov. One thing you said caught my attention. Why would diving be a high energy drain? Isn't the ROV supposed to be neutral or slightly negative in buoyancy? I probably need some practical info regarding this as I know that what is buoyant on the surface won't necessarily be so at depth.

Randy
rossrov
Posts: 383
Joined: Feb 28th, 2013, 5:01 pm
Location: Australia

Re: Please Explain Battery Output and Motor Torque

Post by rossrov »

Likely because diving implies getting down to a specific depth, ie, where your subject of interest is. Once at depth, power usage should be minimal.
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