Propeller RPM


#1

As it's said at the topic, would like to know a few datas from OpenROV builders.

Could you please tell me what the underwater working RPM of your actual axial propellers is ?

By the way .... May I know what your expected thrust is with those same propellers ?

Thanks a lot ......

- Ion -


#2

have you seen these data? maybe they are helpfull for you:

http://openrov.com/profiles/blogs/preliminary-propeller-efficiency-testing

maybe the guys have the plotted data in editable form, then you could try to calculate the rpm with the current consumption, kv rating(800 for the motor in the test) and the applied voltage(11.1)

the uS number from 1550 to 1900 can be related to 6-100% voltage applied to the motor, minus the drop in the H-bridge.

the motor controller has a deadband between 0-6%, so the motors lowest applied voltage(and then speed) is 6% of the applied voltage to the controller.

if i try out then with ideal numbers(without knowing the drop in the H-bridge)

6% of 11.1V = 0.666V * 800kV = 532.8Rpm at ~1550uS

Most efficient area of prop according to data: 1610uS on the Graupner 2308.65

1900uS-1550uS=350uS span for 6-100%(94%)

350uS/94%=3.72uS/%

Target efficiency uS=1610

1610uS-1550uS=60uS /3.72= 16.12%

then i guess this should be added to the starting 6% + 16.12%=22.12% *0.111V *800kV= 1964rpm

most likely the motor runs slower than this because of the drop of rpm vs torque. so again, these are ideal numbers.


#3

Thanks a lot Thomas.

I'll check with my results

Regards


#4

Hi Ion:

The motor testing we did in the spring did not include gathering RPM data. There's been a task on our "to-do" list since then to upgrade the motor test stand, both to gather RPM data and to automate the test process. But that task got put on the back burner this summer to get the Version 2.5 ROV out the door.

In the near term, we've got a lot of work on the 2.5 vehicle to do- updating the documentation, working on the software, and tweaking the design of the endcaps to improve the sealing. Once those are locked down a bit, we'll probably turn back to the test stand and optimizing the motor-propeller combination.

Building and running a test stand at home would be a great way for other people to help out in advancing the design of OpenROV.

-Walt


#5

Hi:

Real tests are best way, but most times results slow and expensive. In order to "fence in" the tests samples a mathematical model could be applied.

This would reduce the propellers to be tested.

The calculation chain would be:

1-] ROV total resistance in a preset speeds interval (Rt(v) ), paying special attention on projected cruise speed.

2-] Set propeller restrictions (mainly diameter).

3-] Set blades number

4-] Set desired RPM

5-] Calculate "ideal" propeller pitch for desired RPM and Thrust (Thrust ≈ Rt(v) )

6-] Calculate "ideal" propeller Torque (Prop Torque ≈ Motor torque at desired RPM and Load(Amps) )

7-] Change ideal prop until fitting Torque and Thrust.

8-] Select closest to "ideal" comercial propeller.

9-] Calculate real propeller+motor working conditions.

10-] Repeat from point -7- until approaching the best selection.

11-] Start real tests with closer propellers to the calculated one.

This way, may be only three/four propellers would have to be tested, reducing time and money.

Regards


#6

Here is a link to some work that was done using different props that may be of some help:

http://openrov.com/forum/topics/interesting-link-to-home-built-thrusters\

Regards

Dave


#7

Thanks a lot again David.

For the closest prop (Graupner 2308.50), and from that link results, I found a small interval (+/- 50 RPM) around 1530 RPM as the propeller working turnrate.

J=0.3879

Regards