Brushless motors and combat

All, since I have been using brushless motors for a while in rc and successfully in my karlsruhe last season for combat I figured that I would post my experiences with them as it pertains to combat and try to answer any questions you all might have.

First off, the disadvantages. Cost Cost and more Cost. While the ESC will run you about the same as the new viper marine brushed ESC's, the motors are not cheap. Expect to spend between 40-100$ per motor (however a ship has not been made which can not be run well off of one brushless motor, appropriately chosen, geared or belted to the rest of the shafts). If you (for some reason) end up buying the 100A continuous ESC that I will talk about later, you will end up out about 150$

Disadvantage number two: You WILL have to waterproof or encase the ESC yourself. There are no waterproof, brushless, reversing ESC's that I am aware of in the reasonable price range.

Disadvantage number three: Should your pump be governed by motor can diameter (per your ruleset, it would appear that the IRCWCC dispensed with that rule in the latest version available online), be forewarned... Brushless motors come in all shapes and sizes.


Now we will go onto the advantages.

First, no brushes to corrode and the bearings are replaceable. The efficiency is better and the torque (if the correct motor is chosen) is far higher than brushed motors. These are motors that you can (if chosen correctly) can be run direct drive. you may even have to gear them up if you get a particularly torqy motor.

The motor (if properly chosen) will drain better and take up far less space than a can motor. this allows for more compact gearboxes and drive trains.

The ESC's can be plugged into your computer. You can set throttle curves, sensitivities, start impulse, voltage cutoff (e/g. it kills motor power until brought back to idle if the supply voltage drops below a certain level while still keeping the servos powered), reverse timing (variable delay from zero, break before reverse, all programmable) which you can use to set the ESC to stop the prop before slamming in reverse to save wear and tear on the drivetrain. The ESC's also are definitely very conservatively rated and have very good current/temp (I suspect it is temp based for the limit at least partly) overload kill (e.g. it prevents too much current from being pulled through the ESC to preclude the release of magic smoke.


Now for the types that work well. Any outrunner brushless motor should work well for you, however it is IMPERATIVE that you find one that has large drainage holes. And be aware that in this type of motor, the outer housing. DO NOT BUY AN INRUNNER. Inrunning motors (E.g. the inner spindle turns, the outer housing stays stationary) generally turn at very very very high speed (one of my hacker speed ~200 equivalents runs at ~40000 rpm) and are sealed (not well enough for full immersion.

see this picture and you can see the drainage holes. The motor in question should power my Tirpitz nicely (although if it is too small, I can always go larger). It is a 275W motor which I will probably run off of 12VDC.


The ESC's that I have found that work are the Castle Creations Mamba and Mamba Maxx ESC's. Combined with the Phoenix Link (or whatever castle creations is calling it this week) you can update the firmware, change all the previously mentioned settings, etc. from your computer.

Now onto waterproofing the motor. Yes you can do it. I like Aeroplate. Simply dunk the motor in it. and then brush some on periodically. That will keep water out of the nooks and help your bearings last longer.


I ran a small motor (the E-flite Park400) in my karlsruhe and I was able to run for about an hour on 3.2 Amp Hours at 8.4V full charge. which means that even though I was, until I turned the ESC down, running at ~20 seconds per 100 feet, I was pulling on the order of 3 amps through that motor (compared to some 20A that I could have pulled). All in all it seemed to me that I was running far more efficiently than in any of my previous ships with brushed motors. That and the engine space was about 50% what it would have been with a brushed motor and gearbox.


Well, I have rambled on enough I suspect. I think that my motor choice will work for the tirpitz but due to the lack of experience as far as what motors work best in what ship there is still a great deal of playing to be done. Most helpful thing would be some knowledge of the speeds at which the propellers need to turn for the good running ships as well as the current draw to the brushed motors on said ships. with those two bits of information, one can make a good estimate at what motor will be required.

any questions/comments, that is what this thread is for.

-Greg

 

Brushless motors require an ESC. A brushless DC motor is really more of an AC motor at the inputs as the ESC puts out a different voltage on the three leads, each one 120 degrees out of phase with the other, however from the word I hear it is not a sine wave, more of a ramp to constant and a ramp down with each phase or something like that.

Now I am speaking of the Outrunners ONLY. DO NOT USE A NORMAL INRUNNING BRUSHLESS as they do not drain and run at a very high speed. what you are looking for is the outrunner type which the outer can spins and the wound core does not.

there are no non-permanent electrical connections in the motor (E.G. Brushes) When wet the get more drag on them and slow down a bit but still run (notice how high mine is in the tirpitz so that when it gets wet the big T is already on the way down)

I dip the entire motor in Aeroplate before running it in water simply to protect the iron stator assembly from rust. (also helps protect the bearings).

Now if a bearing goes bad, the motor can be pulled apart, the bearing can be pressed out and replaced. Not to bad.

Another advantage is the wide array of motors available. the kv rating (eg 1000kv) approximately means that the output speed is xxxx rpm per maximum voltage appled to motor (e.g. 1000 rpm per volt or 6000 rpm for a 6V pack). We can get motors down in the 600-700kv range all the way up to 4000-10000kv (naturally we generally would prefer the former). This means that there are many options out there.

As an aside, the good folks at castle creations have told me that their Mamba ESC is waterproof (not sure if I quite believe them or if they understood that I really mean waterproof when I say it) but they are good folks and stand by their products. The mamba is a 20A continuous ESC based on an airplane ESC that my plane buddies have that is also 20A rated (but they pull more than 45 through consistently). THe Mamba MAXX esc is NOT waterproof however, but we really don't need it (I suspect) as it is a 100+ amp continuous ESC.

-Greg

 

There are two types of brushless motors... the outrunner type and traditional (as we call them). In traditional the windings are on the outer can and the center shaft has the magnets on it, which rotate.

In an outrunner, the windings are in the middle of the motor and the outer can rotates. this usually is connected to a shaft that runs through the center of the motors and out the other side. So if you look in the picture, the part of the motor that says Eflite on it rotates (the base where the wires goes in is separate from it). Outrunners will drain better and tend to be lower speed, higher torque motors.

 

Well... we have ample power here, power loss is of little concern to me. Belt gives me a few advantages. One is less shock on the drivetrain when running over rocks or jolting the motor forwards due to the belt stretch. Another is that it is different than most, which is as good a reason as anything to try it. It seems to run nice and quiet too (Far quieter than any gear drive that I have used).

 

A little side note... these outrunners have very high torque. USE GROUND FLATS ON ALL SHAFT AND LOCKTITE ON ALL SETSCREWS.


-Greg

 

thanks for reminding me. I need to revisit this and add more examples. I just wish the brushless manufacturers would publish the performance curves....

as an aside, there are now a whole boatload of very inexpensive brushless motors out there....

 

I ran one motor to both shafts. check out the karlsruhe refit thread in the ship builds section. I found there that three bladed props can be set so that one blade intersects the path of another but does not hit due to timing.... worked well, now the new gearbox I built from plastic... I ran a single speed 400...

 

the new K-class cruiser gearbox (plastic) is good. the old plastic flat plate tirpitz one didn't work so well. I ran 2S lithium polymer. I am going to plastic machined as it is perfectly suitable if done right.

 

Nope. the cost of the esc is still high, even though the cost of the motors has come down... I started looking at them for a couple of reasons, efficiency being one of them as I was trying to get a small light cruiser running with enough batteries and co2 to last more than one sortee... since I had the equipment, using it in the tirpitz was a no-brainer...