OTW Dive Module - operation weirdness

[JWLaRue]

Okay guys, I could use a little help!

Today Jim Butt and I were troubleshooting one of his OTW dive modules, getting it ready for the Carmel FunRun. The problem that Jim has been having is that the motor does not run smoothly. It runs with occasional, random stops-and-starts...it even changes speed, sometime radically, all by itself. Over the past couple of days and following normal problem solving procedure, just about everything in the dive module has either been swapped out and replaced with new/different parts or just simply disconnected from the receiver. I think the only electronics that were not replaced is the dive management electronics and the two servos.....though, again, we tried with all of them disconnected.

I should note that the dive module works fine on the workbench. The problem manifests itself only when the module is in the water. The problem occurs if just the motor, ESC and receiver are connected.

So here's the weird thing that we discovered. You ready for this one? The problem *only* appears to occur if the motor shaft is in the water. We even held the module out of the water at an angle and just touching the shaft in the water causes the problem. We can even get the problem to occur, though not as badly, if the entire module is out of the water and a jumper with one end in the water touches the motor shaft.

None of my several OTW modules exhibits this behavior. I don't recall anyone having a problem like this.

So....any suggestions?

-tnx,
Jeff

p.s. Jim will be along shortly to add his thoughts to the days troubleshooting activities. If I've neglected to mention something, he'll be sure to add it in!

[apa-228]

A couple of questions regarding your test procedure- 1- Are you submerging the entire module to test the motors? 2- Have you checked continuity between the motor casing and the ESC and any other conductive components? 3- Can you bypass the ESC easily to run the motors direct using the mission switch to turn them on and off, eliminating everything else in the module but the battery and motors?

[subdude]

I'll answer your questions first, then move further.

1. The problem does not change no matter how much of the module is submerged, from fully on the bottom to nothing but the motor shaft in the water. I'll add the water in my tank is fresh city water, no issues with any other cylinders, components, etc.
2. I have not checked continuity because there is nothing to connect the motor casing to any other conductive parts. The motor is mounted to a plastic mount. Nothing but wires connect it to anything else.
3. I could bypass the ESC and receiver, but see no need to really. The ESC has been swapped with a new one, the receiver has been swapped with another one (twice). The battery does not reside in the WTC, but outside and is connected thru posts on the end cap opposite end from the motor.

I've been heavily involved in R/C since the mid 70's, have dabbled in electronics for about as long or longer. I have never seen anything like this.

As Jeff said, I have changed (or disconnected / bypassed) the below with no effect on the issue:

Transmitter, receiver, battery, battery wiring, ESC, pitch controller, dive plane servo, rudder servo, ballast pump control, motor, motor caps, servo wire extension to the pump control, and probably some other things I'm forgetting. We tried the transmitters at different ranges from the cylinder, shop lights on/off, shielding the cylinder with aluminum foil, etc. I rearranged all the wiring to eliminate any possible RFI. Nothing, I mean NOTHING, made a difference. As soon as the prop shaft touches the water, things go stupid.

I'm baffled beyond belief.

Jim

[JWLaRue]

Both motors (original and replacement) work perfect when connected directly to a 12-volt DC power source.

-Jeff

[eckloss]

Is this the same cylinder that was in the Holland at the YMCA a few months ago? I had that boat in my view finder for it's initial sailing then. It seemed to work ok, if a bit slow. Very strange.... Hope it gets figured out!

[JWLaRue]

Erich,

Yes, the same cylinder, but Jim swapped out the 3:1 gearbox and motor for the standard OTW direct drive.

-Jeff

[subdude]

Erich,

The loose (slipping badly) universal joint combined with the slooooow speed masked the stuttering. You could see / hear it up close.

[bob the builder]

This is really weird, as I just went through testing two subs yesterday and they both exhibited exactly what you're talking about. Neither is an OTW unit. One is an older D&E 3.5 and the other is a newer 2" SubDriver.

Bench tested perfectly for both boats. Put them in the water, and the motor stutters. Servos seem to work properly. Both units are running MTronikis ESC's. One was running a Futaba Skysport and the other a VEX.

Something strange be going on out there!


Bob

[apa-228]

OK, the motor shaft contacting the water and the battery residing outside the cylinder could have a bearing on the problem. IF the commutator to armature shaft insulation is poor the motor shaft can become energized also. With the motor fully isolated by your plastic motor mounts it would normally be no big deal as long as nothing else electrical goes to ground at the same time. If however the insulation over the connections at the endcap terminals or battery terminals faults with the motor submerged that could allow current to flow causing all sorts of problems.

If you have a means of testing the voltage with the motor running out of water and then with the drive end submerged it would be interesting to see any derivation between the two readings and would eliminate grounded components. It sounds crazy, but since working around electricity and water since 1960 nothing surprises me anymore.

JUST HAD ANOTHER THOUGHT - test for voltage between the motor casing and the battery leads with the motor running and see if you get a reading, if not this will eliminate the commutator/armature being grounded.

[JWLaRue]

While the battery is outside the dive module, it is in it's own watertight cylinder. Either way, the problem occurs when the *only* thing touching water is the motor shaft. :0

We can (and will) certainly test for voltage leaking between the motor casing and the battery leads, but I'm not clear what this could show us as the motor casing is entirely insulated and/or air gapped from everything?

re: "testing the voltage with the motor running out of water and then with the drive end submerged" - could be promising, what would be the recommended test point(s) to do this?

-Jeff

[apa-228]

The motor will run normally with the battery voltage, whatever that is, when out of the water since the problem only occurs when it is in the water. When the motor is placed in the water a parallel circuit might be being made and there should be a fluctuation in the voltage reading. The voltage fluctuation would cause both the stuttering and stopping problems

[greg w]

I don't really have anything to add except for a question. Is the prop on the shaft when this occurs?

[JWLaRue]

Nope....we’re just holding the dive module.

-Jeff

[Ralph --- SSBN 598]

Besides the propeller shaft what other metal control rods bolts etc are coming from inside the cylinder to the outside of the cylinder via the end caps or other exits?

Try using your multi meter and see if you can get a voltage reading across any two of these.
Or disconnect the batteries and then you can use your resistance part of the meter.

[mike dory]

Guy's is this a brushless or brushed motor? I had a problem, not unlike what your are talking about, in an Engle 212. It turned out to be in the speed control. Best Wishes Mike