Brushless mtor help

[tiqueman]

I remember the good ol days of running cars and trucks and even boats on brushed motors. It seemed that it was quite easy to follow the ratings of a motor. 12T doubles, triples etc. Manufacturer would usually tell you right on the package what rpm it was and the turn was usually stamped at the output shaft end of the can.

Im trying to understand and learn more about brushless and what it all means.. more importantly, how to apply it.

For example, am I correct on this... First, KV multiplied by voltage = rpm??? So a 1500 kv motor on 4S (14.8 volts) will turn 22200 rpm unloaded? ALSO, if you had a 2000kv and ran it on 11.1V, 3S, your still turning 22200 rpm, BUT, your pulling more amps? Am I doing this correctly.

And if I understand all that, and Im correct, without the charts and what not, how do you apply it? I guess what Im saying is, I want to know why? Why would a 1500 KV be great in one hull and not so great in another? Or better yet, I have a 32" cat that came with a 2045KV and have been told by many that, that is the wrong motor for that hull... why?

[Fluid]

I remember the good ol days of running cars and trucks and even boats on brushed motors. It seemed that it was quite easy to follow the ratings of a motor. 12T doubles, triples etc. Manufacturer would usually tell you right on the package what rpm it was and the turn was usually stamped at the output shaft end of the can.

I remember those sad days too. The rpm on the literature was meaningless, some makers used 6 volts, some used 7.2 volts to calculate it. Makers (EPIC was famous for this) changed can/magnet/stator designs so often that a 12T from one year had little performance resemblance to the 12T of the next year. Some motor winders used heavier wire on their 15T triple than another maker, so the motors were only "close" in performance. Then as the brushes and comm wore the performance characteristics of the motor changed, often over the course of a single heat. Brushed motor makers never used Kv; this is a big advantage of BL motors, although it is still goofy because some makers use a different amp loading when calculating Kv.....

First, KV multiplied by voltage = rpm??? So a 1500 kv motor on 4S (14.8 volts) will turn 22200 rpm unloaded? ALSO, if you had a 2000kv and ran it on 11.1V, 3S, your still turning 22200 rpm, BUT, your pulling more amps? Am I doing this correctly.

Kinda. The Kv is not at unloaded rpm....from some makers anyway. It is at a specific load, maybe 20 amps, maybe 40 amps. But it still gives you an idea of the capabilities of the motor. You also need to know the size of the motor to help determine torque - a 380 motor has much less torque than a 540 motor even if both have the same Kv.

In reality, there are so many variables - motor, cell quality, ESC quality, wiring lenghts, boat setup - that trying to calculate too much will just give faulty results. The best way to not make a mistake is to profit from the experience of others. The problem with that is that some posters know nothing about what they post yet still try to influence others. To help reduce that problem, choose your experts or at least listen to several before you buy. Just because a setup worked for one guy does not mean it will work for you. You may have cheaper components or you may not think that six runs on a motor is good motor life....

A very rough guide to 2-pole motor choices:

Sport running - Kv times volts = 20,000 - 25,000 rpm
Oval racing - Kv times volts = 25,000 - 30,000 rpm
SAW racing - Kv times volts = 35,000 - 45,000 rpm


.

[Boatman]

Listen to Jay SING! I think his post should be stuck like chuck on top of the info forum on setups. IF ONLY someone would have put it in such perfect words while I learned. Instead of running a 8XL 2000KV (roughly) on 6S! Needless to say it did not last much longer before it became a paper-weight.

[domwilson]

The problem with that is that some posters know nothing about what they post yet still try to influence others. To help reduce that problem, choose your experts or at least listen to several before you buy. Just because a setup worked for one guy does not mean it will work for you. You may have cheaper components or you may not think that six runs on a motor is good motor life....

A very rough guide to 2-pole motor choices:

Sport running - Kv times volts = 20,000 - 25,000 rpm
Oval racing - Kv times volts = 25,000 - 30,000 rpm
SAW racing - Kv times volts = 35,000 - 45,000 rpm


.

Agreed. Ask questions. Evaluate the info. that others give you. See if there is a consensus from other posters. Read and experiment. Admittedly I don't know nearly as much as Jay. I have about 8 or so boats I tinker with and four more to build. I've trashed alot and just plain thru some in the garbage. Some of my experience comes from some of my mistakes. Money lost makes an interesting motivator for learning more.

[tiqueman]

Fluid, it makes perfect sense that a 380 would have less torque than a 540. And that so many variables take place into calculating exactly what a motor will do in a specific set up. And it makes sense that for example, Boatmans set up w/ an approximate 2000kv motor on 6S is theoretically going to spin over 44000 rpm, so this is why that was a bad thing.? But Im still not quite grassping, or maybe IM just making it a lot more difficult than it is, why isnt a 2045 well suited for a 32" cat, but a 1500 is? And arent the large scale boats, like the HPRs and others alike running like 600 KV or something. So thats an extrememly efficient motor, but takes A LOT of voltage to get it there??? And... LOL, sorry, I know... You guys are like radio shack, Ive got questions, youve got answers.... So is it more of a "you can only be as efficient as the size of your boat can only handle so many cells??? In other words, your not going to get 12S in a 16" mono hull w/ a 600KV motor just so you can sport boat w/ 26000 rpm. Is that the logic? But a 2400 w/ 3S will fit in that boat and keep you in the same rpm range.

[tiqueman]

One more question, Fluid mentions three different scenarios for rpm, sport oval and SAW. Are these different because of run times. ie, If your sport boating, as I do, you are going to run longer than what a oval 5 lap event would be so you would obviously be running your set up longer (sport) creating more heat etc. Where as if you are running a SAW event, 2 passes and your done so it can take the abuse becasue its not drawn out over a longer period of running?

[domwilson]

Me myself, I just sport run. SAWS from what I understand are like running down the drag strip. It's just hard on everything. Ovals you are looking for a balance of speed, handling and runtime. Sport is usually not as hard on equipment as you can bring it in anytime, check things and not be penalized. This is just what I've gathered. Not sure on this tho'. Somebody who's a member of a club or participates in these events might have a better answer.

[Brushless55]

I

A very rough guide to 2-pole motor choices:

Sport running - Kv times volts = 20,000 - 25,000 rpm
Oval racing - Kv times volts = 25,000 - 30,000 rpm
SAW racing - Kv times volts = 35,000 - 45,000 rpm

.

Awesome!

[G Doggett]

The only thing I would add to Fluid's excellent summary is -

RUN TIME

Sports Running - 5 minutes +
Oval Racing - 2 - 3 minutes.
SAW - measured in seconds !

Graham.

[Brushless55]

The only thing I would add to Fluid's excellent summary is -

RUN TIME

Sports Running - 5 minutes +
Oval Racing - 2 - 3 minutes.
SAW - measured in seconds !

Graham.

Once again!

[Brushless55]

To be real, some of the most simple yet best info for FE boats yet.
That will help save motors, escs, and batteries
I put 5s to my 8XL in my MM and it was crazy fast, till the motor gave up running it to long..
If only this was posted 4 days ago..

[bwells]

Fluid mentioned a 380 verses 540 motor. These number seem to correspond to brushed, are they can size as the numbers are usually 4 digits kv on brushless. I'm confused as most brushless motors are rated with 2 4 digit number, i.e 2400-3650 (just made up). I'll keep an eye on this one as I'm still trying to figure out the terminology on brushless, it seems rather confusing!

[Brushless55]

Fluid mentioned a 380 verses 540 motor. These number seem to correspond to brushed, are they can size as the numbers are usually 4 digits kv on brushless. I'm confused as most brushless motors are rated with 2 4 digit number, i.e 2400-3650 (just made up). I'll keep an eye on this one as I'm still trying to figure out the terminology on brushless, it seems rather confusing!

380 and 540 are the cans sizes..
Like a 2845 (380) and 3660 (540) also 3674 (540 xlarge can)

[bwells]

I thought so, thanks for the clarification. Now on to step two!

[domwilson]

The 380 and 540 sizes are referenced back to the brushed counterparts as far as size. On the 540 sized motors some manufacturers (mainly chinese) use the 36 for the can diameter in mm and the 50,56,74 etc. as the length in mm. But here is a better article than my typing... http://www.ezonemag.com/pages/faq/a414.shtml