motor/esc guide

[00kitty]

im new to brushless and as i read throught the threads and see what people are running for motors and speedos all i see is numbers and none of them make any sense. is there a list or chart that anyone knows of to help me figure this stuff out. or can someone explain it to me. im assuming that the higher kv rating is a faster motor but we all know what happens when we assume things. and how do the motor ratings apply to batteries and esc's.

[jamespl]

From what I havd learnt on here the kv is a reflection of how many cells and a lower kv motor will have more tourque than a higher kv. So for example if you was to run 4s (14.4v) you want a motor around 2000kv 5s around 1600kv and so on. Plus the lower the kv the bigger the prop you can use. Someone please feel free to correct me if I'm wrong.

[cyberhoops]

For what it is worth ... as a general guideline... this is what I think about for motors/esc/battery/prop combinations.

First I think about what shaft RPM I want on the boat, where sport running is maybe 25K to 30K RPM and full out racing is 30K to 35K RPM. If I am running 4S (14.8volt) power system then 1600kV to 2200kV puts me in the general RPM range. If I were running a 6S boat (22.2v) then I would be looking for a 1100kv to 1600kv motor. Larger or longer motors will generally be able to handle more total power than small motors and motors will usually list either a maximum current value or a maximum power rating in watts. The you have a watt rating then divide the watts by the voltage you will be running will give you an idea of the maximum number of amps you can draw. A 1000 watt motor on 4S (14.4v) would yield a maximum amp draw of 70amps.

So, the next step is about amp draw. The larger the prop you use in terms of diameter and pitch the more amps you will draw. The faster you try to spin any given prop the more amps it will draw. The goal here is to select a prop that will not exceeed the motor rating. Start small and get bigger is the general guideline for prop selection, but picking a good starting prop is the hardest part of the equation.

As far as the ESC goes you need a speed controller that will at least be able to handle the number of amps the motor can handle.

Now comes the batteries which need to be able to supply the number of amps requested by the motor/prop combination. If you multiply the battery amp hour rating by the C rating you will get the number of amps the battery can supply. (assuming you can believe the C rating) A 5000mah battery (5 amp hour) with a 30C rating means it can supply 5*30= 150amps. The larger the capacity of the battery, for example 4000mah or 5000mah, will determine the run time for your boat.

As a side note: A battery under high load or as the battery is drained it will experience a voltage drop. Since the prop rpm is porportional to the voltage, you will get an RPM drop and speed drop. I strongly suspect that a 4000mah 20C battery drawing 80 amps will suffer a larger voltage loss than a 5000mah 30C battery that is also drawing 80 amps, so having some "head room" between the amp draw that a battery can supply and what is needed for the motor/prop combo would seem to be a good thing.

As a sanity check take the UL-1 motor/esc combo. I think the motor is rated for around 600 watts and 4s (14.8v) power - which would be around 40 amps. The speed controller supplied is rated for 60amps which should handle the motor amp draw. The motor is 2030 kv, which at 14.8 volts means 30K rpm which is right in the sport/racing range. All good. Of course people regularly push a UL-1 motor way past 40amps... but hey... it is racing!

For really high power setups it is easier to increase the voltage than the amp draw. If you want a 3000 watt power ( 4 horsepower) system and want to do that with 4s batteries - you would need to pull over 200 amps which is serious current. On the other hand to do a 3000 watt system with a high voltage 10s (37 volt) system you would only need 80 amps. In the case of 10s you would be in the market for a 700 to 950kv motor to stay in that 25 to 35k rpm range.

So, that is how I think about motor/esc/batteries ... probably room for improvement, but that is how I look at it.

[Griff]

cyberhoops great post, I learned a lot from that thanks, I have actually printed it out for future reference :)

Tom

[detox]

4s Lipo and 1800-2200kv motor configuaration is by far the most popular. If building a 28-34" boat try to build around that. Here's Castle's chart in link:http://www.castlecreations.com/suppo..._boat_apps.pdf

Here's kv chart

[stadiumyamaha]

Kv is how many rpm's per volt. So a 1000kv at 1 volt will be running 1000rpm and so on. 4s x 4.2= max 16.8v x what ever kv rating the motor is equals total unloaded rpm.

[00kitty]

this is sweet info. when im looking at the target RPM is that loaded or unloaded. and great posts everyone, thanks!

[detox]

Those figures are unloaded.