Some more updates on brushless motors ... Before the MWCI Nats, the Erin's twin brushless system was swapped out for a single motor two shaft BC gearbox. A 1300kv outrunner motor from the Richelieu was pressed into service on the gearbox which was then installed into the Erin. The idea was to try to match up the motor kv to the power requirements to push the Erin 28 seconds while attempting to gain efficiency and torque over the original 2600kv motors. Oh, not to mention wanting to lighten the Erin's weight. Unfortunatly, it didn't work as intended. The Erin only managed 30 seconds on the new gearbox motor combination. I surmise that by doubling the load (two shafts on one motor vs a motor per shaft) and dropping the kv to 1300, the outrunner motor didn't have enough power to propel the ship to 28 seconds. I had two options to get the ship on speed for the next day's battles: replace the props with higher pitched props or change the motor. Since I liked the props already on the ship (Raboesh curved blade props), the Erin got a motor change back to a single 2600kv brushless. That worked well enough to get the Erin on speed at 54% throttle. So the lessons learned? If using a single motor powering two shafts, expect to need more motor kv than a ship with twin motors powering each shaft. I don't have exact figures yet, but will try to get some sort of idea of how much added kv may be needed. On a brushless related note, there was a handful of ships using brushless pumps at the Nats. Clark (aka Tugboat) had a 28mm brushless outrunner in his Sharnhost CA that performed very well. The motor has 2100kv running on 6.4 volts. Interestingly enough, the Musashi driver (Carl C) used the exact same motor in his ship's pump. Although I didn't get specific amp and voltage data, it was visually apparent by his pump stream that the motor was as strong as a 540 sized brushed motored pump. So it is looking like 2000kv might be the target for an effective brushless pump motor on 6 volts. In a way, this is gratifyingly close to the kv figure I put into the original brushless article written over a year ago. With guys like Carl, Chris K, and Clark taking the lead on brushless pumps, we could start seeing them more often in the future. I know I want to start expiriementing again.
The pump in Scharnie was tested at 1.4 gallons per minute on 7.4V using a half-unit restrictor*. I regretfully don't have data on the current draw, but Darth Kessler told me that he had read 16A maximum on his. I am planning to buy a datalogger for KGV prior to the Labor Day Leadfest in Statesboro, and KGV is running brushless pump and drive. I wish you'd told me you needed a 2100kv motor, I had several in my parts box! Anyhow, KGV will have two 380-size 2100kv motors direct-driving the props, and the same 2100kv motor running the pump. I'm building the pump this week, and will be testing it over the weekend. Two-blade brass combined-flow impeller, 1.5" diameter, 1/2" diameter outlet and hose up to the restrictor. * If the maid from the hotel is reading this, I am SO sorry for the watery apocolypse in the bathroom!!!
So with brushed you have to pay attention to the windings. Do you have to keep track of that also with brushless or just choose the KV you want? Are there multiple versions of a 2600kV motor where one is a high speed and another high torque?
Windings don't seem to be as important to keep track of. All motors with the same KV SHOULD turn the same RPM on the same Voltage with no load, however they wont all have the same torque. Highly simplified, the larger the diameter the more torque. Generally speaking faster motors are smaller while larger motors have more torque.
Kess, what was the name of the onboard datalogger you had at Nats? Monkey see, monkey do Monkey wants cool datalogger like Kess. Ook ook.
Also, keep in mind that if you are not running the motor at the design voltage, you will see a decrease in the available power out of the motor... I believe the power should trend with Voltage squared but I am not certain of that. the RPM needs to be correct and the power also. For those using pumps, if you want a quick and dirty estimation of the power you need, take your target flowrate, multiply by pressure rise acrosss the pump (flow in m^3/s pressure in Pa) which will give you watts to the fluid... make an estimate of efficiency (for our pumps I usually use 8% to 11%) and divide that power to fluid by the efficiency to get a guesstamate of watt's electrical. E.G. water at 1gpm (6.309e-5 m^3/s) and a pressure rise of 25 psi (172369 Pa) gives you about 11W into the fluid... use the efficiency number to get 98W-135W I've used this to help in motor choice in the past for pumps/guesstamating for props based on measured or used setups -Greg
the problem is that you don't know current, you do know design voltage for the motors spec sheet and the voltage you are running. reformat to P=V^2/R which was my assumption (DC, not sure how that works in the Motor world)
It does vary like that, but it gets complicated. The reason that the speed is at all proportional to the voltage is that these are 3-phase DC motors. Like 3-phase AC, but it never goes negative. (Thanks to Kess for educating me, I'd thought they were 3-ph AC) so the role that the voltage plays is not directly determining of speed, since the frequency applied to the motor by the ESC determines motor speed. The voltage helps determine the strength of the rotating field(more voltage applied means more current and therefore higher torque), which determines how strongly the rotating field can hold on to the rotor as it speeds up (or maintains a set speed as torque of the load increases). That's why when you have your 24 secod battleship running in a straight line with the motors at (just say) 12,000rpm and you throw the rudder hard over (which builds up pressure between the prop and the rudder, increasing the torque required to turn the prop at its set speed) that the brushless motor draws more current but maintains its set speed, and the brushed motor responds by lowering its speed to a point on its torque-speed curve where it can develop the torque needed (but at a lower speed). And that's why I like brushless motors Within the capacity (current-wise) of their ESCs, they WILL stay on speed in a turn. IIRC Kess has the Eagle Tree datalogger. I will soon be gathering current and voltage data on my drive and pump! Might not get it before the Labor Day battle, but in the coming months.
Admittedly, I haven't been paying much attention to a motor's designed voltage range. I did at first but it became apparent that there are very few brushless motors that run at the kv we need and the low voltages we use. The fix is to boost the available voltage like Chris K did. He went to 12.8v in his Bismarck which put the voltage within the range his motor operates best at. He did his homework to get the best power out of the motor ... and it shows with the outstanding performance of his Bismarck's propulsion system. Maybe this is the start of a new paradigm in our hobby. With the growing use of high performance, high capacity, lightweight LiFE batteries, there is little holding us back from finally moving away from 6v power.
Correct, I have an eagle tree data logger. However I think the whole "turning drags down the motor" bit is mostly bunk, I'm sure there is some truth to it but I think it is in the noise. Atleast I haven't seen any clear indication which shows it to be happening on any appreciable scale.
In response to Mike's comment: One of the things the efficiency that brushless provides does is makes staying at a lower voltage possible, there are plenty of "2S" brushless motors out there which would work in a NiMH setup or even probably a 6v gel cell setup. Brushless actually makes the huge LiFE packs pretty pointless. Across both IRCWCC and MWCI NATS my Bismarck never drew more than 5 Ah for an entire battle. I'm running a 20 Ah pack. I have two (one for the morning one for the afternoon). Now I'm not going to go out and buy something smaller as that would be pointless but at the same time theoretically I could run an entire NATS with only charging my battery packs at the start of the week......
Heh. Coming from the airplane world, dead weight was a bad thing. If we needed weight, we tried to put something useful into the plane i.e. bigger battery pack. Naturally, the thinking carried over to ships when I got into the hobby, thus my ships tend to have LOTS of AH in them. I wonder ... will the Eagle Tree measure motor rpms for brushed and brushless? It would be interesting to see if we could actually measure an rpm drop when a ship is turning tightly. I know that I can hear an audible drop in rpm while turning with a brushed motor setup and see the speed drop off. Depending on the brushless motor setup that was in the Erin (one motor, twin motors, brushed, brushless, etc), I could visually see differences in acceleration, decelleration, and turning speed based on availble power going to the props (either via rpm, torque, or whatever is making the difference sans voltage). The issue with brushless motors designed for 2S (6.4 - 6.6 volts) is they tend to have moderately high kv, i.e. 2K and higher. When we start getting into the lower kvs like 1K and lower, motors tend to be designed to run on 3S or higher voltage. Even so, I have largely ignored the recommended voltage ranges for matching up kv to our propulsion applications and taking the small hit in efficiency. I betcha 2S motors will be perfect for 6v pumps.
Eagle tree had a setup to measure brushed RPM but it is optical or a hall effect if I remember correctly where the brushless is electrical off the motor leads. Maybe it is more apparent in a brushed setup but I've looked at my data multiple times and haven't been able to pick it out so if it is happening it is a very small effect. As for 2S motors the higher Kv's are fine as you're running half the voltage of a 4S setup. so a 1000kv 4s motor would turn at roughly the same rpm as a 2000kv 2s motor, of course there is a torque difference but realistically these motors have way more power then we need anyway.
Kess, I love you, but you get the finger Bunk, indeed I SINK YOU! MANY TIMES!!!! Oh, wait, waking up. Nevermind. But I think it's a slight but noticeable effect. Noticeable enough for me to waste 10 minutes of taxpayer time to theorize on it Take THAT taxpayers!! Now I have to get the motor speed sensors for both kinds of motors Can't abide not having data to go by. I blame Kess for getting me turned on (oh my!) by his datalogger. How is this man unmarried? There are hot nerd chicks out there who'd swoon dead away for a man with a datalogger
LOL!!!!!!!!!!!!!!!!!!!! My head hurts from reading all of this! The unmarried comment was funny. My next project will be replacing 550's in both the Arizona and Alabama with twin brushless. Probably the pump motor size (2100). I'm still working on the details....
Hey I could still be wrong, it's happened before, it will happen again. I just have yet to see data which clearly supports that argument, I even sat and spun for a bit before battle monday morning to make sure I had some on the logger. Still I could have missed it or something. I'm just happy my girlfriend accepts the hobby as well as she does. The datalogger is useful, not a must have but worth it to people like myself.
Well, I made a claim, and however well-supported it is by theorizing, there is only one way to prove or disprove it 'SHHH! I'm doing science!!'
I recently performed some tests on brushless motors to support motoring my Henri IV. I need to to do more tests with other motors and props, but it's a start. All tests were done with a 2 cell 6.6 volt LiFe battery. Where the the current draw exceeds the motor limits, the data should be considered suspect, because the motor is way down the efficiency curve at those amperages. Motor Prop Amps Watts RPM Notes Astro 01, 14 turn, geared 687 Kv 1.75" 25p 4 blade 4 25 3600 1.5" 25p 4 blade 3 20 4000 1.25" 40p 3 blade 2.8 18 4100 0.75" 25p 3 blade 0.8 5 4900 1.75" 25p 3 blade 2.5 16 - Himax HC2208 870 Kv 1.75" 25p 4 blade 6.9 40 3800 Exceeded max motor amps 1.5" 25p 4 blade 6.5 38 4500 Exceeded max motor amps 1.25" 40p 3 blade 5.6 35 4900 Exceeded max motor amps 0.75" 25p 3 blade 1 7 6800 Axi 2204 54 1400 Kv 1.5" 25p 4 blade 6.8 40 4400 Exceeded max motor amps Neo 400 750 Kv 1.5" 25p 4 blade 4 25 4200 1.5" 25p 3 blade 1.4 9 5000
