I started with 120a car ESC’s. Vesc’s where just starting to hit the scene then. I had a program card and tried various settings. The problem is that no matter what setting I chose for the brakes, they where still rough and unpredictable. If I was coasting down a steep hill and tried to apply the brakes, I would push the trigger 1/2-3/4 out before the brakes would respond and then they would grab hard. The only way to make them work right was to first accelerate and then apply the brakes. I was already going too fast and the last thing I wanted to do was accelerate! Acceleration is also not very smooth but jerky and hard to control. When the Vesc came out, it changed everything. Smooth predictable acceleration and braking.
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I’ve been working on a 1S skateboard drivetrain. The intent is to get the heat generation outside of the motor where it is easier to cool things. Also would eliminate need for BMS other than temp sensing on the battery and simplify things to a single pouch or prismatic cell. There are some efficiency gains as well but it’s a bit tricky to actually make those gains feasible. It’s really hard to get a lot of copper in a motor of smaller diameter as the one I’m making. The continuous current is much higher around 30A RMS on 1S if doing around 125W per motor. If your wires aren’t pretty big, you’ve got some problems.
If anyone is interested in learning about my motor, I was thinking of doing a little reveal. I have been working on it a long time and need the support of a forum I think. Preorders would be nice if enough people become passionate, but it’s been hard for 1 person to handle so you’d have to respect my time trying to get stuff to you.
- We all want power
- Power = voltage * current
- Current is limited to a max of 75 amps per motor for 6374 200 Kv motors, the most powerful motors that are commonly used. Other motors have lower current capabilities, often significantly lower. Sure you can pick a higher current ESC than the 60A VESC that is popular, but it wont really matter because you’ll start melting your motor
- So the only way to get more power is to increase the voltage
QED
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I’m designing for about 200ARMS battery current per motor. or about 700W input power. Hopefully around 400W peak output power per motor.
200 amps sounds HUGE, but it sounds plausible given you are using unconventionally low number of turns with crazy thick magnet-wire. I’m really curious what you’re up to from an academic standpoint, but as of yet it doesnt sound too compelling vs conventional designs, given that you’re targetting 400W peak output which is frankly pretty low (except when compared to hub motors). Sure the BMS simplicity has some value, but I figure that’s pretty much irrelevant in the face of the fact that I’ve never seen a 50 A*h 1s battery for sale. I’m skeptical, but eager to be proven wrong I guess.
It’s about in line with boosted boards output power if I’m not mistaken… certainly a good benchmark to meet.
I have a sheet I made that compares 7S to my 1S I should share. I never made a 1S motor, but I have most of the materials. I only made a 7S motor before using conventional magnet wire.
If you do the math, you’ll find that it is possible to get higher efficiency in the motor iteself from a low voltage system simply because it becomes easier to fit more copper in the motor when going to a 1 turn design. However, to get overall better efficiency, the resistance in the phase leads also needs to be scaled up as well, which is the most annoying part.
Gearing is the main reason I use low kv motors.
If you use belts, and wheels 97mm or smaller you have a limit on the smallest motor pulley and largest wheel pulley you can use. On the motor side you cant go below 15t or they wear out too fast and skip more. With a low kv motor you can run a 20t motor pulley, because more teeth engagement they never slip, the pulley and belts last much longer. If the wheel pulley is too larger the belt is close too the ground and gets munched by rocks.
Im on 12s dual and my vesc battery amps only 15amps (30 total) and I still get good performance. Everything stays cool. I personally wouldn’t want to subject my batteries to more than 50 amps.
My motor amps are at 65a which gives good low speed acceleration / hill climbing. It is only lacking a little mid speed acceleration but its enough for my needs.
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hah! That’s awesome and hilarious! Are you saying if you do a single, rectangular wire, you can basically fill 100% of the available volume with copper, but with conventional designs you are subject to hexagonal close-packing issues? I get that, but I dont think you can actually pull off a rectangular wire… Also yes, the phase wire is going to look RIDICULOUS. I just dont see how this will work lol
I agree with the idea of using the Boosted as a benchmark. Their batteries are capable of 50 amps according to the A123 specsheets, so the max power is 50 amps * 12 * 3.3V = 2 kW. Unless you go 4WD with your system, its just not there.
wouldn’t the esc be huge?
the motor amps would need to be huge for equivalent performance against something more typical with a lower kv. 1000 amps …I think you should do it direct drvie not a hub motor so you can make the motor leads stationary. and also might as well make it a heatsink as if you hadn’t thought of such an idea. that would be pretty awesome if the trucks were both completely mechanical and giant functional wires. with a bit of insulation between them. how many teeth and magnets?
It’s given that there is a set amount of resistance, so naturally more amps = more heat produced. But Of course resistance is the cause of the heat.
Any kv any volt as long as you have the watt capability to handle the load-and an adequate delivery system with proper mechanical setup. Low pole motors are more efficient due to copper/iron ratio and make more watts per cc, so naturally more power smaller package. Bargain esc’s are a mistake and you would be better off burning your hard earned cash. Get a high end esc with the expectation to run it at 60% of it’s capability. High end esc’s have powerful fully tunable brakes you can customize progressive rates, bomb down hills at 40+mph with confidence and not worry about hitting E-rpm limits. Figure out what you want your skate to do, overbuild it and de-tune it. Overloaded conductors and mechanical loss builds heat. Heat is the enemy of all electronics.
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I guess I missed the part where there is a set amount of resistance, please explain.
There’s x amount of resistance in every build no? Of course it’s different with every board but I’m just trying to explain it simply to him.
Amp flow is variable in an esk8 system while er is normally not(maybe when temps get too high). So it’s not untrue that more amps create more heat with the same resistance. Also not untrue that more resistance creates more heat with same amp flow.
Not trying to start a discussion here, just trying to make it as simple as possible for him to understand.
I suggest reading these for everyone interested in the discussion
http://vedder.se/2014/10/chosing-the-right-bldc-motor-and-battery-setup-for-an-electric-skateboard/
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Yeah, this is a good idea to try to use mechanical components as current carriers, but the issue is keeping them isolated. So the next best thing is to run copper along the aluminum truck hangar, which is what I was intending on trying.
The only time you should be worried about heat is if your build dosent have a system properly capable of delivering the demand. Resistance is not a “given” it’s a choice you make when you buy and build.
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here is a comparison of a set of dual hummie V4 hubs w/ 12S (46v) & 100a/60a motor/battery limits vs a dual 1500kv 4S (16v) system that obtains similar top speed, acceleration & heating…
findings:
in order for the 1500kv 4S system to obtain similar performance to a set of hummie v4 hubs, the system requires:
- 0.007 ohm windings (1/10th as much electrical resistance)
- 6.4:1 gear ratio (64 wheel teeth & 10 motor teeth)
- 320a motor current limit
- 172.5a battery current limit per motor (345a total system peak battery current)
- 84mm tires (same size)
- 24000 peak motor rpm (vs 3358rpm hummie v4)
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I’m quite impressed with the amount of discussion my little question has started, thank y’all for being significantly less full of jerks than most of the fora I frequent.
My take-away so far: High voltage and low kv seems to be a sweet spot for the standard size wheels(83->90mm), motors(<=65MM dia) and gearing used, allowing for a wide range of speeds without hitting a frequency limit for switching fields. My focus is larger wheels(120->200MM and a max speed of 25mph. I’d like it to be able to lift a 280# fellow up a wall if needed. Seems like the the 1/8th scale rc motors would satisfy what I’m thinking about
The reasoning for the VESC vs RC ESCs is laggy response to input and inadquate brake tuning (too strong or too weak) Has anyone considered that the laggy response could have been upsteam in the system (transmitter/receiver) rather than ESC?
can you punch the numbers for 120mm wheels on those charts? It would be much appreciated!
120mm / 84mm = 1.42
1.42 * 6.4 ratio = 9.08:1 ratio