Power draw measurements


#1

I hooked up my multi-meter to measure the current draw of my ROV and here are my initial findings.

Test #1 - Powered on sitting idle

Light percentage, Power draw

0, .70A

10%, .76A

20%, .80A

30%, .83A

40%, .861A

50%, .895A

60%, .915A

70%, .96A

80%, .997A

90%, 1.02A

100%, 1.064

Test #2 - In the water

For this test I simply engaged the port and starboard motors in the forward motion. My multi-meter only goes up to 10A and it was very easy to draw that much current.

I found that at around 25% of full power the ROV was drawing about 9.8A, I left the motors running for 10 seconds and that is where it settled.

Conclusion

Although I didn't get a lot of data points about the current draw for the motors under various load conditions I got the general answer I was looking for....A LOT! I'm going to guestimate that the thrusters could easily draw over 20A to 30A depending on how much power you send to them. The ESCs can easily handle that, their specs say 18A continuous and 50A burst.

I'm able to have it in the water and use a multi-meter through the use of my external battery adapter thing, http://www.thingiverse.com/thing:94381


#2

This is great info!

Were you able to get all thrusters running at once to see the power draw?

Any data on surge current?


#3

No, I wasn't able to get surge or total power draw. I'm unable to test the full power draw due to the 10 Amp limitation of my multi-meter.

I saw this article that talks of using a "shunt" to measure over 10 amps but sounds too complicated for me. http://www.dutchforce.com/~eforum/index.php?s=b77d3a15f45a0839f31b8929be9b667d&showtopic=20058&st=0&pid=172340&#entry172340

Like I wrote initially my goal of this test was just to get a general idea of current draw and it's very apparent it is a lot!

I need to write up another forum post describing my tests at separating the motor power supply (two 12V 7AH sealed lead acid motor-scooter batteries) from the main ROV's power supply. Using this approach of separating the power sources I no longer get brownouts which was my entire goal, BUT I've ruined the port and starboard motors by MELTING the a motor lead(s) right where they enter the motor. What I believe is happening is that I'm able to push more power to the motors than they can handle and at a certain point one or more of the leads just physically melts. On one motor I had just one lead out of roughly 15 melt. Once one lead melts the motor is basically worthless because the full torque potential is lost and also when you give it max power the motor starts doing this jerky stop-start action.

I've got three more motors on order and I'm probably going to fry them, if they do fry then I know for sure that there is a "too much power" limit.

I'm good at breaking things :)


#4

Breaking things is one of the most undervalued skills I can think of. Before you break it, you don't really know what it was capable of.

When you say we are pushing more power to the motors than they can handle, are you doing that through the ESCs?

Does this mean the ESCs are oversized?

Please excuse my ignorance here, I have not really launched into this yet, as I am investigating the possibility of running power over my tether.

I looked at this http://openrov.com/profiles/blogs/preliminary-propeller-efficiency-testing?id=6365107%3ABlogPost%3A47251&page=2#comments

which is the efficiency analysis of different thruster propellers.

They also top out at 10 amps.

Do you find that the thrust you get at 10 amps per thruster is insufficient to propel the ROV?

I am eagerly awaiting your next test results!


#5

Hi,

if my ROV gets under construction, i use ROXXY BL Outrunner Motors. Max. 300KV with a reted current of 7Amps and maximum 10Amps. Hopefully i´m able to reduce the cable size to 0,75mm² (19 or 20 AWG).


#6

Yes, I am doing it all through the ESCs.

I'm trying to determine if the ESCs are oversized, which I didn't think would be possible. The ESC specs state 18A continuous and 50A burst. I thought the motors could only draw as much as they are rated for (9.5A). So even though the ESCs are beefier than the motors technically that shouldn't be a problem.

I think 10A per thruster is enough to propel the ROV, but personally I don't think you'll ever get that with the default battery pack, even with the Li-Ion 3.7V 5000mAh batteries. As I've found as soon as you start increasing the power to the motors brownouts occur and the only way to restart the system is to physically disconnect the batteries to do a 'hard-reboot'.

The brownouts are why I switched to using a separate dedicated battery pack for the motors. If you look at the specs for Li-Ion batteries it says the batteries can only discharge up to 10A peak and thus this is why brownouts occur (in my opinion)

Battery spec (details tab): http://www.meritline.com/trustfire-rechargeable-li-ion-battery-5000mah---p-104043.aspx?source=fghdac&gclid=CNLsyLG4-LgCFe1_QgodcB8AZQ