Brushless Propulsion 101

I originally wrote this article almost a year ago for the MWC's TF144 newsletter. By popular demand, I have updated the content based on the latest lessons learned over the past spring/summer. Please note that while I like Hobby King, the links in the article are not endorsed by RCNavalCombat.com.
Credit for brushless outrunner testing and data goes out to Chris Kessler.
By Leaps and Bounds
volume 3
By Mike Mangus

Brushless Motor Systems

In the first BLaB article I alluded to how brushless motor systems along with lithium battery chemistries have changed the face of electric powered aircraft. The lightweight high-capacity lithium batteries and powerful efficient brushless motors have allowed model aircraft to fly longer with more power than comparable combustion powered planes.
We have already touched on lithium batteries in previous articles. Let’s explore the other latest technology finding its way into our ships; the brushless motor.
A brushless motor offers many advantages over brushed motors. Brushless can be up to 25 - 30% more efficient. Because there are no brushes to wear out brushless motors can last virtually forever with proper care. Additionally, the electronically-controlled precise timing in a brushless system promotes more available torque at most rpms.
For us, the torque advantage is the main selling point. With more torque comes better acceleration as well as speed during tight turning.
Notice I didn’t mention lower maintenance. One of the brushless disadvantages is some extra maintenance to keep ball bearings from going bad. Additionally, brushless motor systems typically cost more than brushed motor systems. Lastly, choosing a motor out of the thousands of motors on the market can be daunting.
Luckily, that is where this article can help. J
I’ve come up with a few guidelines for choosing a motor based on ship size and gearbox or direct drive setups. Most of these observations are based on testing and retesting in various ships over the past couple years as well as input from other captains in the hobby.
Brushless motors are usually sized by diameter, length, and rpms per volt (aka kv). Motor size can be matched up to brushed motor sizes as a starting point for selecting a brushless motor:
380 = 28mm diameter motor
540 = 36mm diameter motor
775 = 42 - 47mm diameter motor
We will skip motor length since it is the least important factor for selecting a motor, though longer is usually better for torque.
After motor size, rpms per volt (kv) is the most important item for picking the correct motor. The goal is to match the motors rpm with the prop speed the ship needs. From recent testing, we are seeing prop rpm ranges from 1300 for slow 28 second twin-shaft ships to 3000 rpm for large and fast 24 second single-shaft ships. There are some exceptions as we will see below.
Several factors must be considered to figuring out what kv motor will be needed, such as how many volts will the motor run on and what gearbox ratio (if any) the ship has. Some examples are:
HMS Erin has 3 to 1 ratio gearboxes and runs on a 6.4 volt battery. Prop rpm is approximately 1400 due to twin shafts and slow 28 second speed. That math would look like: Prop RPM (1400) X gearbox ratio (3) divided by battery voltage (6.4) = 656 kv. Since I like to have a little extra rpm in case the ship needs more speed to make 28 seconds or to account for low pitch props, I will add around 10% more kv for an even 750 kv motor requirement.
So a good brushless motor for the Erin may be a 36mm diameter with 750kv.
Chris Kessler’s Bismarck amazed us all at the IRCWCC Nats with its incredible turning and power due to careful consideration and selection of a brushless power system. The Bismarck has a 2.5 to 1 gearbox and runs on a 12.8 volt battery. Prop RPM is around 2500’ish rpm due to a single shaft and a fast 24 second speed. That gives us: (2500 prop rpm x 2.5 gearbox ratio) / 12.8 volts = 488kv. I do not know if Chris is running the Bismarck at full throttle, but the 580kv brushless motor he has in the ship is capable of giving 3000 prop rpm.
Direct drive power systems are just as easy. The Verite PDN does not have a gearbox and runs on a 6.4 volt battery. As a small slow single-shaft ship with a tiny prop, the Verite’s prop rpm is around 5800 rpm. If we do the math: 5800 prop rpm / 6.4 volts = 910kv. Adding in a little extra for speed changes gives us a nice 1000kv figure.
So a good motor for the direct drive Verite may be a 28mm to 36mm (currently has a 36 mm) with 1000kv.
Before we start searching Hobby King for our new motors there is one more thing to consider. There are two types of brushless motors; inrunner and outrunner. The difference is how the motors are constructed. Inrunner motors are designed like the typical brushed motor with a fixed outside case and a spinning internal armature. Outrunner type motors have a fixed central core and a rotating outer case. Performance wise, an outrunner motor typically has less kv and more torque than an inrunner. Although higher torque is desirable, select the motor type by the kv needed for the ship. We also may consider how much space is available inside the hull … is there enough space to prevent internal components from rubbing against an outrunner’s spinning case?
The lowest cost brushless motors and ESCs are on HobbyKing.com. Although there are stories about bad orders, late shipping, damaged shipments, I can honestly say that in over 4 years and 20+ orders through Hobby King that I have not had a single issue with an order.
Turnigy is the best quality brand offered on Hobby King. They have a good reputation for affordable high-quality electronics.

A quick check of Hobby King gives a few motors that may be suitable. Note that the motor size is typically listed a 4-digit number such as 2830 (diameter and width) and kv:
28mm diameter:
http://www.hobbyking.com/hobbyking/store/__8481__Turnigy_2830_Brushless_Motor_800kv.html
http://www.hobbyking.com/hobbyking/store/__8139__Turnigy2836_Brushless_Outrunner_1000kv.html

35mm diameter:
http://www.hobbyking.com/hobbyking/store/__18228__Turnigy_D3536_8_1000KV_Brushless_Outrunner_Motor.html
http://www.hobbyking.com/hobbyking/store/__18232__Turnigy_D3542_6_1000KV_Brushless_Outrunner_Motor.html
http://www.hobbyking.com/hobbyking/store/__18231__Turnigy_D3542_5_1250KV_Brushless_Outrunner_Motor.html
Note that these motors are slightly smaller (by 1mm) diameter than 540 size motors and may run afoul of the "pump motor no larger than the drive motor" rule in some rulesets.
36mm diameter:
http://www.hobbyking.com/hobbyking/store/__17720__XK3674_B_1400KV_Brushless_Inrunner.html.
http://www.hobbyking.com/hobbyking/store/__17718__XK3665_B_1200KV_Brushless_Inrunner.html. It is the newest in 4 pole inrunner motors (gives stump pulling torque compared to the standard 2 pole inrunner motors).
http://www.hobbyking.com/hobbyking/store/__6679__KD36_74_15XL_1100kv_Brushless_Inrunner.html
http://www.hobbyking.com/hobbyking/store/__8486__Turnigy_3648_Brushless_Motor_850kv.html
http://www.hobbyking.com/hobbyking/store/__8482__Turnigy_3639_Brushless_Motor_750kv.html
http://www.hobbyking.com/hobbyking/store/__8484__Turnigy_3648_Brushless_Motor_600kv.html

38mm diameter:
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=11182.

40mm diameter: http://www.hobbyking.com/hobbyking/store/__17710__XK4060_1500KV_Brushless_Inrunner.html. http://www.hobbyking.com/hobbyking/store/__17711__Turnigy_XK4074_B_1400KV_Brushless_Inrunner.html.
42mm diameter outrunner motors:
http://www.hobbyking.com/hobbyking/store/__8488__Turnigy_4240_Brushless_Motor_1300kv.html
http://www.hobbyking.com/hobbyking/store/__8489__Turnigy_4250_Brushless_Motor_1000kv.html
http://www.hobbyking.com/hobbyking/store/__8487__Turnigy_4240_Brushless_Motor_800kv.html
http://www.hobbyking.com/hobbyking/store/__8506__Turnigy_4250_Brushless_Motor_580kv.html
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=8492 (400kv)

49mm diameter outrunner motors:
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=11102 (1100kv)
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=11143 (880kv)

Obviously there are far more motors to select from. These are just a handful. Yet knowing the diameter and kv of the motor we want helps when searching for the right motor.
One last note on motors before we go into electronic speed controls (ESCs); most of these motors have a 5mm shaft which will require a 5mm bore pinion if using a gearbox. Tower Hobbies and various RC Rock Crawler web sites will have a selection of 32 and 48 pitch pinions. Metal gears are highly recommended.
Brushless waterproof ESCs with instant reverse are difficult to find. Most have none, limited, or delayed reverse unsuitable to the fast paced throttle changes we do during battle. There are a few non-waterproof ESCs that we can use though. Each motor will require its own ESC. Unlike brushed motors, multiple brushless motors cannot be run from one ESC.
There are also two types of ESCs: sensored and sensorless. Do not get a sensored ESC (or motor either). We want a sensorless ESC.
The most common ESCs we are using are:
http://www.hobbyking.com/hobbyking/store/__8993__Hobby_King_Brushless_Car_ESC_100A_w_Reverse_Upgrade_version_.html
http://www.hobbyking.com/hobbyking/store/__11743__HobbyKing_Brushless_Car_ESC_60A_w_Reverse.html
http://www.hobbyking.com/hobbyking/store/__13445__HobbyKing_Brushless_Car_ESC_45A_w_Reverse.html
http://www.hobbyking.com/hobbyking/store/__11742__HobbyKing_Brushless_Car_ESC_30A_w_Reverse.html
This is a low cost R/C car ESC with instant reverse. It will handle up to “4S” 14.8 volt batteries.
Although the ESC can be programmed by counting beeps in programming mode, a programming card is far faster, easier, and low cost: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9550. Only one programming card is needed even with multiple ESCs of the same model/type.
The Hobby King ESC will require waterproofing, either with resin epoxy, Scotchkote, or your favorite waterproofing method.
Newer yet untested ESCs that should work (NOTE these will only handle up to “3S” or 11.1v batteries):
http://www.hobbyking.com/hobbyking/store/__7188__Turnigy_Brushless_ESC_60A_w_Reverse_prog_v2_2_.html
http://www.hobbyking.com/hobbyking/store/__7186__Turnigy_Brushless_ESC_35A_CAR_ESC.html
Programming card: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7187
Really new ESCs that are totally untested … we do not know if they have an “instant” reverse:
http://www.hobbyking.com/hobbyking/store/__14634__Turnigy_TrackStar_1_10th_Scale_60A_Car_ESC_Version_2.html with programming card http://www.hobbyking.com/hobbyking/store/__16615__Turnigy_Track_Star_60A_ESC_Programing_Card.html
http://www.hobbyking.com/hobbyking/store/__14630__Turnigy_TrackStar_18A_1_18th_Scale_Brushless_Car_ESC_.html with programming card http://www.hobbyking.com/hobbyking/store/__14631__Turnigy_TrackStar_ESC_Programing_Card.html

There are many new ESCs joining the market almost daily. The key things to look for is an “instant” no delayed reverse and a 100% reverse power (may need programming to get it).
Before drawing this article to a close I have to say something about maintenance. Brushless motors have ball bearings. Ball bearings do not like corrosion. Corrosion WILL happen if the bearings are not oiled every evening after a day’s battling. Within a couple days after a weekend battle, pull the motors out of the ship and spray soak the bearings with WD-40 or LS-2. If this is not done, the bearings WILL go bad! You have been warned!
So if the idea of having better acceleration, deceleration, and turning speed sounds like something your ship needs and you do not mind the higher cost or maintenance, look into installing a brushless motor system. The advantages outweigh the disadvantages.
If anyone needs help selecting a motor, please feel free to contact me via the email lists or at pbpow@hotmail.com.
Mike

 

The issue I ran into was connecting two ECS's with two motors. I just could not get them to work well with the system. I can see a single motor and esc, has anyone else had this issue with brushless?

 

I have a Radetzky setup with 2- 28mm outrunners powered by 2- Mtroniks Hydra 15A Brushless. Zero glitching/issues. Running on 9.6-9.9V (3 LiFe cells).

 

I need to double check when I get home but I believe I disconnected the BEC portion on all but one ESC (I have 3 in there). From what I've read it seemed prudent to do.

 

I have two ESC's, one for each motor. Never could get them to synch, one would just not work or they both would work for a short time. The ECS's were linked together by a Y harness for the receiver. This is what caused "Project Overkill" to be placed inactive.
I still want to do this, I am sure after NATS seeing Mike's and Chris's boat it will be a priority for my Iron Duke.

 

The older HK car ESCs had a desyncing issue. They did not like to be run in pairs or with a high amp draw pump motor. I fought that issue for 3 months last year before finally seperating the pump power from the main power by giving the pump it's own battery. The updated versions linked in the article have been fixed and no longer has the desync problem.

All is not lost for the older versions though. A person on another forum seems to have traced the desyncing to faulty programming in the low-voltage cutoff part of the ESC. The reputed fix (I still have yet to try it) is to manually program the ESC's low voltage cutoff from "0.00" volts to something like "2.00" volts. Seems the older ESCs, when set to 0.00, automatically set the low voltage cutoff artificially high which would cause the ESC to shut down prematurely, aka desync.

 

Just came back in from the garage where I finally had some time to play with the voltage cutoff limits on the first version HK ESC. In a nutshell: success!

The Verite PDN was the test ship since it only has a single battery powering the entire ship. It has had problems constantly with the pump shutting down the ESC whenever the pump is turned on and the ESC was in neutral. The temporary fix was to make sure to turn the pump on when the ESC was moving the ship forward or backwards. So the ship is the perfect test subject to play with the ESC's cutoff voltages. The test consisted of plugging in the ship, ops checking everything to make sure it works, then flicking the pump on and off while watching the ESC indicator lights. The usual behavior is the ESC "desyncing" or shutting down after the pump was turned on. It wouldn't happen every time the pump was turned on, but eventually it would happen and leave the ship with no propulsion.

Using the programming card, I adjusted the cutoff to 2.0v and ran the test. Although the ESC did not "desync", it would reinitialize and keep working. This was a better result in a way. At least the ESC would simply reset instead of shutting down completely, allowing the ship to continue moving after a brief few seconds pause.
Adjusting the cutoff to 1.0v didn't change the result. The ESC would still reset but not shut down.

Success came after setting the cutoff to 0.5v. The ESC stayed synced, did not reset, and continued working no matter how quick or often the pump was toggled on/off.

I have yet to try it on the water where the pump would have a higher amp draw when turned on with a load (water in hull). But the initial testing gives hope that anyone who bought the first version HK ESC can use them without fear of shutting down.

Please note that this only applies to the first versions of the HK ESCs. The ESCs linked in the article above are for the updated version which has a software change to prevent the desync/shutting down issue.

Now to head back into the garage and take the second battery pack for the pump out of the HMS Erin. That ship desperately needed to loose a couple pounds. Heh

 

Awesome Mike, I will have to try this. Do we still need 1 ECS per Motor? I think you have your Erin setup with a dual drive gearbox, would you do that again or go to two motors?

 

Yup. A brushless motor requires its own brushless esc ... cannot put multiple brushless motors on one esc and expect them both to work.

This past spring after the Brouhaha, the Erin was switched to a twin motor/gearboxes setup to get more torque to the props. It worked to a certain extent though was limited due to trying to use high kv motors at low throttle (25%) settings to stay under speed.
Based on the testing we did at the IRCWCC Nats last month, the Erin is getting a pair of lower kv motors for the MWC Nats. That should make the ship a lttle more efficient power wise and help get the most out of the brushless motor setup.

I think with the correct kv outrunner motor, the single motor/twin shaft gearbox from BC would work very well. I have a motor for it, but do not want to change the Erin over before the Nats. Big changes before Nats are not good. Heh.

 

I ebay'd some Not terribly difficult to find. I think mine came from the RC car market. They also came with sleeves to take them to 1/8" if I remember right.

 

RC Rock Crawler web sites have 5mm bore 48p and 32p gears. Tower Hobbies also carries them. Just google "5mm 48p gears" and a few sites will pop up.

 

Speaking of brushless pumps ... I will be bringing an updated brushless pump for testing on Wednesday.