Question for other using VESC regarding motor RPM

[Mattchatman]

Question for others using VESC regarding motor RPM.

I’m running at 50 volts, My ERPM is maxing out at 18000 which indicates 3000 rpm unloaded (spinning in air) 18000ERPM/6 poles is 3000rpm, I’ve verified that is correct using an android app that uses a strobe light function to check rpm (really easy effective way to check rpm). With a 130kv motor I should get 6500rpm at 50v right? I measured ~30v AC between motor wires at full throttle, has anyone else measured the ac voltage output of a VESC when not connected to a motor? Am I missing something? Is the KV rating wrong on this motor? What should the voltage between two of the motor wires be from the ESC at 100% duty cycle? I’m in FOC mode, max ERPM is set to 50k, and I’ve checked other minimums to see if something is limiting my rpm, and I can’t find anything.

ESC - Flipsky 75100
Motor: flipsky 65121 130kv
Hydrofoil: Gong Allvator M70 1500cm²
Battery: 12s1P li-ion
2kwh battery from battery hookup.com
Prop: 7 1/2 x 7 BA 3-blade cut down to 110mm

[sat_be]

Are you sure about your measurement? Isn’t it rpm=erpm/polepairs? At least it was like this in metr before it was changed to actual rpm when poles is configured in settings. At the time I tested it with a 63100 (14 poles/7pole pairs, 130KV) I got the following without load: 12Sx4Vx130x0.95=5928
0.95 because of vesc max duty cycle of 95%.
This could be measured with a strobe tool connected to my phone. Under load at full speed, voltage will be lower than without load. You will not reach the same rpm as without load. Rule of thumb is 0.7 x no Load RPM if the prop is not too big. In my setup I get to around 4000RPM@40km/h.
If no load rpm is ok but you don’t get enough rpm in water, your motor lacks torque, you need a bigger one (or a smaller prop but you’ve reduced it alreay). Normal efoil props are between 135 and 150mm in diameter, 6-7” pitch.

[Mattchatman]

I agree, my motor, the 65121, has low torque, almost half that of the larger 65161. Your calculations look correct for rpm based on my KV rating. I’m getting around 2000 rpm under load, but with no load ( spinning in air) I am topping out at 3000rpm which is low for 130kv at 50 volts… But maybe the KV rating isn’t correct. That indicates a KV rating of about 60.

[FoilEngineer]

You might have done this already but just checking:

• You ran the motor detection with no load, no prop, so only the bare shaft
• Once you ran the detection did you measure the rpm with the same bare shaft?
• What amps did it detect on the motor?
• What is the motor amps vs the battery amps when you do the KV test?
• What mode are you running the control on, Current, Duty, Speed?

There might something in the answers to these questions

If not the only other way is to run it under load, measure the rpm and then double check the KV which should still result in the same KV (60KV in your case) under load. In other words, your motor amps under load at that voltage (for the motors rated power) should give you the same KV as the no load case

[Mattchatman]

-I ran detection with the propeller, I should probably read up on recommended procedures

-I ran the strobe test without the propeller to measure rpm, just a piece of tape to judge where the shaft is with the strobe tachometer

-Pulls around 2-3amps unloaded, around 150w (battery is 50v)

-I will rerun the test tomorrow and record more data

-running FOC mode, max 95% duty cycle, I’ve selected the duty cycle control. At full throttle I am reading 95% on duty cycle.

[Toto44]

You know that the strobe will give you also a good reading at 6000 if you get a good result at 3000.
So maybe ist just half of the real speed you measured? That would fit the kv rating.

[sat_be]

I built a “laser tachometer” according to the instructions below. It uses a laser diode and photo transisor and can be connected to a mobile phone. Put a piece of aluminium tape on the shaft to reflect the laser. Might be a bit of an overkill but this way you can measure the actual rpm.

https://usefulmobileapps.com/en/strobe-light-tachometer.php?s=app-slt-1

[Mattchatman]

You could be right Toto, that crossed my mind, I thought I had somehow ruled that out that it was spinning twice as fast, but Now I’m doubting my measurement…All good suggestions. I’ve switched to a larger 8’ paddle board and 3d printed a mast clamp ( same one fits the larger 65161) which has much less drag than my original wooden one. I’m now close to getting on foil just from the motor, but I still need an assist from a boat to get on foil. I’m thinking this motor may just not have the torque. It’s possible with the perfect propeller it will work, But I think I’m going to buy a 6384 or 63100 outrunner.

tldr - It’s looking like the flipsky 65121 ( smaller version of 65161 with about half the torque ) is not a good hydrofoil motor unless you weigh much less than I do (80kg)

[FoilEngineer]

You have to run detection without any load so it must be bare shaft no load.

I think thats where your first problem is

[Mattchatman]

First successful ride of my electric hydrofoil!

I take it back! I was able to get on the foil today! See the VIDEO ABOVE. A different propeller did the trick! I was originally using a very cut down 7 1/2 x 7 BA that shipped with my motor - flipsky 65121 ( smaller version of the popular 65161). Now I’m using a 2-blade 7 x 4 3/4, it’s an old propeller I found on eBay, new old stock. Fits the 9-spline shaft, I did cut the diameter from 7" to 6" . See my build thread for more info if you are interested. I suspect I was counting half the RPM before. I’m requiring 75amps to get on foil, and then around 40 I think. I had some better rides, but that’s the only one I filmed.

[Larsb]

Erpm = rpm*polepairs, nothing else, as one electric revolution consists of both a north pole and a south pole (=pair)

18000/3=6000 rpm