Saturday, 21 September 2013

Thruster Oil Temperature (and Volume) Increase

After a small tweak to the pressure tolerant (hopefully, yet to be tested) brushless DC motor drive PCB I'm getting very reliable use out of the thruster now.

So in view of this I thought I'd now put the PCB back into the thruster and fill it with oil.

I was testing the PCB outside the thruster with the motor control lines, three phases, power to the hall effect sensors and feedback from the hall effect sensors running to the motor in the thruster through the flexible tube and placing the PCB in a small oil filled container outside of it.

Now with the PCB in there, there's just power (24VDC) and communication (RS485) from my laptop travelling down the flexible tube.

So with my load cell (fish scale :)) and temperature sensor in situ it was time to do some testing!

The other thing I noticed is, as I was using a PVC pipe originally for the thruster housing from the local plumbers merchants, but this didn't seal very well and it wasn't quite the right size, so I now have an Acetal tube which I made on my lathe (which is a lot nicer than PVC to machine, as PVC is a bit "stringy"), is that I can see any air bubbles trapped in the thruster through the housing.

I'm not sure if this is good or bad thing as I haven't pressure tested the housing yet (probably the next thing i'll do), but we'll see.

Anyway I could purge the thruster of air by jiggling it around, see how many bubbles were in there and letting the air bubbles run up the flexible tube :)

So here's a picture of the setup I used today.

I let it run for three hours at full load and the oil temperature in the flexible hose/tube rose

The load cell is measuring about 1.82kg of force and the temperature sensor (the white thing with a probe sticking into the flexible tube) initally started off about 17 degrees C.

After three hours the temperature rose to about 19 degrees C and the thruster housing was definitely a lot warmer to the touch, but not hot. The Acetal is transferring some of the heat from the brushless DC motor control PCB and then the oil into the water.

The thing that was very noticeable though, which I didn't expect and thought was initially a leak was the oil moving up the tube as the temperature rose! I had to empty some out as the test went on initially. The reason I know it wasn't a leak is when the oil cooled down the level dropped back down in the flexible tube and the oil remains clear not cloudy.

I intend doing a few more detailed measurements on the oil movement in the flexible tube when I test the thruster next time, but the main thing is I now know that the ESC PCB is happy to carry on turning the motor and propeller, under full load (about 3 amps) with oil in the housing (as it will be in the final ROV) for over three hours, as compared to the my last test with the BLDC control PCB being external to the thruster and in the small oil filled container (for initial testing purposes), so it's a good result all in all :>