I didn’t do the math but better: I tried with 2 motors: same manufacturer, same length, kv 80 and 120.
Same prop.
Results: same exact thrust.
Same voltage.
Same BATTERY current
Different MOTOR current
Different DUTY.
Only the Duty cycle changes when you increase Kv.
Thus, you need less motor phase voltage to get the same RPM, and more MOTOR current.
Also, having a lower Duty will get your ESC warmer.
I am talking about keeping the motor Kv constant (ie same motor) and changing the size of the battery…
Tried it too, between 40 and 58V.
Same thrust. Only Duty and Current motor changes.
What motor and prop?
Have you got graphs for these motors and voltages?
My Bad, Current battery also changed, motor current stayed the same.
I can’t see anything. But I assume that when you upped the voltage the battery current reduced…
Edit* I managed to see the Metr graphs from the email I got.
Here are your Graphs:
This supports exactly what I said!
Here we see a 400w fly (at what speed?) , things might change a bit if the setup is push to the motor’s torque limit I think
I think a lot of misunderstandings happen if you want to transfer experience from airplanes to boats. In air the relationship between velocity and RPM is much weaker than in water. When driving a prop through water, the importance of limiting the phase current to protect the motor from saturation and overheating, is much higher than through air. Also experimentally finding a setup which works best under a certain condition will not protect the motor from being overheated in another situation, e.g. when you tow someone or the weight of the driver varies or you have strong wind against you. Also you want a headroom to reach the speed you want, so most of the time you will drive with a dutycycle below 100% while consuming medium power only. Here again the requirements between air and water differ, in air especially with a paraglider you want to have full power to start and climb steadily. With a foil you only need high power at the start (while you have to limit the phase current) and while foiling the phase current drops because of rising velocity and less torque demand.
So from my opinion to not overdimension the motor, you need a phase current limiting ESC. You would not need it in air with a balanced setup. But in water you need it. I burnt some ESCs and one motor because of these facts. Since i use the correct phase current limitation everything is stable and you also do not need to overdimension cables which costs a lot of wheight.
thank you for explaining everything to me. you should work as a developer for large companies!
I guess probably that why I still have my first swordfish boat esc working even after +250A pics and my heli version burned in 30sec …
for current control
So many ESCs burn because the engine set-up is not checked at full throttle.
The load on the ESC from the clock losses drives the temperature up. The lower the limit, the higher the losses and thus the temperature.
In my opinion the wrong way.
I sometimes fly with 150 € sets (motor and ESC) and nothing burns.
Greetings Frank
anyone interested in the relationship between speed, power consumption, propeller diameter, data logging, rpm limmits for compensation of the voltage drop of li ion packs. as well as creating performance diagrams can look forward to my facebook page (from april). the reason is why in future I will only post it there: in a forum you always automatically have people who always say the opposite, even though they have much less experience on the subject. therefore the treads become long and confusing. in principle, there are actually only controversies between 2-5 people. an interested person doesn’t have much of it. one can clearly say now. is what you write on your facebook page correct? I say so. What is written is documented in the form of live performances, videos including data and images and measurements. whoever thinks something is wrong can simply stop reading. I will definitely not waste the time and justify something and explain again several times that I can clearly prove in a posting. In my main hobby, this procedure has proven itself very well. Best regards
Many esc die not at full, but at half throttle because motor amps are way higher than battery amps at this stage
Had this exact situation with my buddy. He was runnin his submarine or whats the word on half throttle, system got pretty hot all the time. When he downsized prop and let motor spin full throttle, system was much cooler. And this is with basic low amp rc controllers.
What you mention is correct. However, the knowledge on a forum like this one is built by iteration. So it is normal that everybody who thinks he/she has understood something challenges other members to be proven wrong. It is also a question of “knowledge maturity”. When there is a minimum number of persons that agree, the group is ready to start a new level.
Now, externalizing the knowledge is always possible. But one would loose the search function power. As per the big picture, there’s also a wiki for the ones who want to fix their knowledge while sharing their understanding. The Wiki idea is nearly as old as this forum. As each post is editable, there is no (less ?) controversy. One doesn’t need to have build a nice board (80% of the forum members) to write down a nice Wiki. A good sense of synthesis is the key.
As you are the first one to show an interest to do it, honour is yours, would you create the
“Speed / power consumption / propeller diameter / rpm interaction - Wiki” here ? What this site needs is a shell to publish proven results.
As it is a Wiki, you wouldn’t be alone, to maintain it. I am sure it would create many vocations.
The efoil.builders wiki origins: Why not start a wiki? - Site Feedback - FOIL.zone (Nov 2017)
The new category: About the Wiki category - Wiki - FOIL.zone
The first and only topic: Propulsion Type Overview - Wiki - Wiki - FOIL.zone
That right!!!
They die at half throttle because the regulator’s clock losses put a higher load on it than at full throttle.
If the prop is chosen so that the current does not have to be limited, the ESC also survives.
If the prop is selected too large and part load is constantly in control mode, the controllers boil.
Regardless of that the engine should reach 80% of its idling speed at full throttle in order to achieve a good ETA.
Just as an info.
I have produced about 2500 brushless motors.
Among other things also worked with Torqueedo (Torcman), Kontronik and others.
Greetings Frank
So lets look at some real world efoil data and see whats going on. The following data was logged in January while I was on a trip in South Africa. The setup I was using was as follows (if anyone wants pics I can add them in too):
Me: 90kg
Board: 5ft x 60cm x 8cm
ESC: Freefly Arc200 with naturally aspirated water cooling (through nose cone)
Battery: 12s12p (30Ah), Samsung 25r cells
Motor: Maytech 65161 120Kv
Prop: FR 150mm with 6" pitch
Foil: Unifoil Hyper 170
Now for the data:
Firstly it’s important to understand that this motor and pretty much every motor used for efoils hits its torque threshold when propped and in water and therefore can not get the motor up to the same no load RPM it can on land.
Now lets investigate this data…
As you can see there are a series of high “phase current” peaks (you can also see the battery current increases at the same time). These coincide with when I was riding at full throttle and putting the motor into it’s torque threshold. If you then investigate them more closely, as time goes on you can see that for the full throttle runs, the phase current starts to increase and hits higher peaks as the battery voltage goes down. The reason for this is pretty simple: Torque is directly proportional to power. Since P = V x I, as the voltage drops, the motor is forced to take more phase current to hit the torque threshold of the motor.
If I were to move this to a 14S battery all of a sudden, you would see a few things happen:
Now onto another very interesting point that has been raised about running at 50% throttle and the temperature effects on speed controllers…
While the speed controller is less efficient at 50% throttle, I don’t believe that is why guys are blowing them up. In my opinion the reason guys blow them up is because they are not adequately cooling them at all speeds.
If you look at point C on the graph you can see that I was running at about 40% throttle for a prolonged period of time and that the ESC was running cooler than it was when I was doing full throttle runs before hand.
Can you elaborate on that?
I understand that if the power keeps the same and prop also keeps the same but just battery voltage rises, the duty will be lower and the battery amps will be lower (U1 x I1= P1; P2=P1=U2 x I2; so if U2>U1 => I2 <I1). So in my opinion you will still need same torque (phase amps) and rpm(phase voltage). Same amps means same phase amps, not same battery amps. Same rpm means same phase voltage, same phase voltage at lower inputvoltage means lower dutycycle.
Other opinions?
It is not the efficiency, it’s the phase amps. At low rpm the motor can draw over 1000A phase amps. This kills every ESC without phase current limit or over current protections.
Torque for a brushless motor is calculated as a constant multiplied by the phase current.
It can also be calculated by the power divided by the rpm of the motor.
Now because power is equal to voltage multiplied by current. Therefore if you increase the voltage but keep the power and torque at the same, you will see a reduction in phase current as battery current.
Now the reason I say that the ESC becomes less efficient is because there seems to be a belief that the more energy is converted to heat at 50% throttle than at 100% throttle.
Yes, T=Km x Iphase
sure, P=pi x RPM x T x 1/30 => T~P/RPM
So, sure P = Irms x Urms, you increase input voltage 12 =>14S. So Pbat = Ibat x Ubat => Ibat gets lower.
Torque keeps same. So from “T=Km x Iphase” Iphase has to be same.
I guess you mix up phase voltage and input voltage.
Phase voltage correlates with backEMF and therefor RPM. So if you keep the prop and motor the same, for same power you will get the same equilibrium (it gets a differential equation as soon as you take the inductance into account, still inductance will keep the same as it is a motor constant, so we can neglect it here) of RPM and torque=> same phase voltage, same Iphase.
Depends on the commutation algorithm, for FOC you got a fixed PWM frequency at the ESC, so the ESC doesnt differentiate between 50 and 100% duty.
But current goes in square to heating. So at low rpm 50% duty will kill the ESC same as 100% duty.
Do the math: Iphase=(Uphase-UBEMF- (L/ (di/dt))/Rmotor
The L/ di/dt makes it a differential equation. With a solution of something like 1-e^(t/tau), so you simply have a ramping effect by the inductance of the motor. Since we are in low RPM and having low inductance motors we can neglect this term and directly calculate the maximum (tau is around 2ms, at 1200RPM with 3polpairs one electric FOC commutation takes ~8.5ms, so after 3x tau you can neglect it with less 5% error or after 5tau with less than 1% error).
So R motor is like 10-30mOhm.
UBEMF directly correlates with KV, so for 120KV motor you get 10V BEMF at 1200RPM
Uphase = Duty * Uinput, so at 14S and 50% duty => ~25V
All put into the formular: Iphase will peak out between 500A (30mOhm) and 1500A (10mOhm). This kills any ESC.
If you instantly go on full throttle on even lower RPM you can imagine what will happen.
