Voltage Question

[Amdaylight]

Greetings,

This is a strange question so here it goes, when a supplier list batteries for a particular motor like the specs below for max power,

Specs:
Model: NTM Prop Drive Series 28-30A 750kv
Kv: 750rpm/v
Max current: 20A
Max Power: 120W @ 12v (3S) / 140W @ 15v (4S)
Shaft: 3mm
Weight: 67.1g
ESC: 25A
Cell count: 3s~4s Lipoly
Bolt holes: 16mm & 19mm
Bolt thread: M3
Connection: 3.5mm Bullet-connector
Suggested Prop: Prop test data coming soon!

if I run it with say a 2S battery will I be doing any harm to the motor? Can a brushless motor be run at half throttle with out any harm? A little back ground on what I am trying to do, I am building a model of a WW1 Subchaser and the model comes set up for one motor on the center line where the full size boat had three props. One on the center line and two outboard and a little forward of the center prop. I want to install three props and decided to try and use an out-runner motor / lipo combination due to size and weight. I tried to pick a motor with a low Kv or should I get one with a wattage rating? I would ask these questions on a boat page but they would mostly be lost like myseld.

Andre

 

[Craftydan]

you can run 2S or fractional throttle and it won't hurt anything, but you might be running in an inefficent region for that motor -- hard to say.

I'm assuming the limit your designing up against is prop RPM. you're near the bottom of what a typical motor that weight will do for RPM -- weight will increase as you move the KV further down. If you can get a gearbox in-line and reduce the RPM at the shaft, that'll open up the options for many other motors in that power/weight range, and get you a motor running in a more efficient region.

 

[stay-fun]

Those motors will run just fine on 2s lipos. Be aware that the rpm that the motor makes is dependent on voltage, as well as kV rating. Higher kV (=rpm per volt) is higher rpms, and since you're going to run only 2s packs, the props will spin slower than when on 3s or 4s.

So in fact, when you're going to use only 2s, you may consider getting a higher kV motor (which has the same power-rating (watts)) to compensate the lower rpms you'll get from a 2s lipo.

 

[pgerts]

You want a motor for a boat.
Isnt a chaser a fast boat? Why do you want low rpms?
It is normal to use inrunners in boats as they are more easy to cool with a water tube wound around the motor. An outrunner wont get a lot of cooling in a boat.
Your setup with low rpms and large prop sounds more lika a tug boat.

 

[Ron B]

get the weight of your plane and use 100 watts of power for every 16 oz. the less voltage the less watts.
as you can see by your specs 3s = 120 watts 4s = 140 watts.

 

[Amdaylight]

You want a motor for a boat.
Isnt a chaser a fast boat? Why do you want low rpms?
It is normal to use inrunners in boats as they are more easy to cool with a water tube wound around the motor. An outrunner wont get a lot of cooling in a boat.
Your setup with low rpms and large prop sounds more lika a tug boat.

The top speed for the full size chasers in WW1 was about 17 knots. It is kind of like the little story about the gazelles and the lions, if you are a gazelle you have to be faster than the slowest one in the herd, and if you are the lion you don't have to faster than all of the gazelles just faster than the slowest one. The subs that they were chasing only had a top speed on the surface of about 10 or 11 knots and 3 knots submerged. The reason I did not look at the traditional inrunners is the high Kv ratings and yes I am trying to keep the shaft speed down.

Thanks
Andre

 

[Amdaylight]

get the weight of your plane and use 100 watts of power for every 16 oz. the less voltage the less watts.
as you can see by your specs 3s = 120 watts 4s = 140 watts.

Ron thanks for the info on an airplane I had wondered about that for a large scale airplane I am building but this question is regards to a model boat that I am building.

Thanks Craftydan and Stayfun, that is what I was kind of thinking was to get a gear box to bring the rpm's on the shaft while letting the motor spin at something closer to best speed.

Andre

 

[stay-fun]

You could also consider re-winding a motor yourself, more windings give you lower kV and thus lower speed (and more torque). That would eliminate the need of a gearbox.

You would have to look into that yourself though, I have no experience at all in re-winding a motor. And maybe it is way easier to just go with a gearbox, I don't know. Just wanted to throw it out there

 

[quorneng]

A 750kV on a 2S will have a nominal full power no load speed of about 6000rpm, lets say 5000 under load.
It all comes down to what maximum shaft speed you are after.
All motors have a maximum continuous current which must not be exceeded. Obviously on a 2s rather than a 4s at its current limit it will only generate half the power which for the motor you specified would be 70W.