[PSUBS-MAILIST] Motor Issue

Mon Jul 19 08:27:02 EDT 2021

 James, a couple of thoughts. After diving my boat with the same MK-101 motors, it is not uncommon to have some decolorization of the compensation oil. This is just small particles of the carbon brushes in suspension in the oil. This fluid is a dispersion and not a solution. The particles are in suspension in the low dielectric oil and not solubilized into the oil.  As such, the electrical conductivity of suspension should be that of the virgin oil.  If the electrically conductive carbon brush dust was solubilized into the oil, then it could slightly affect the conductivity.  To verify this, put some of the dark oil on a microscope slide and view under a microscope.  You should observe particles in suspension.  
 On reassembly of the MD-101, I found the best way to deal with the brushes is to jam a thin piece of wood between the armature and permanent magnets so that the commutator is axially extended by about 1 cm.  Then pull the springs on the brushes off to the side so they are not engaged.  Slide the end of the housing that has the brushes into place then with a small screw drivers slide the springs back into place.  Then pull the wood wedge out and slide the armatures into its correct location.  Then, install the remaining end cap and insert the long retaining bolts and tighten being carful to centralize the fined section.

As to noise and overheating issues.  I found these MD-101's are sensitive to radial alignment of the brushes, that is position of the housing that has the fin and brushes to the permanent magnet section of the thruster.  To experience this, loosen the two long bolts enough so that if you grab the fin on the body of the thruster, you can rotate the brushed end relative to the body. Put some painters tape on both the fined side and the body and with a pen draw a mark across both.  When you rotate the fin, these marks will show the misalignment.   While the motor is running, rotate the fin.  When the fin is perfectly centralized the motor has maximum speed but when you move off center it slows down slightly and makes a different sound.  By rotating you can hear when the motor in set to the sweet spot.  It may have been that the brushed end might have been slightly out of alignment during the dive.
Best Cliff
Sorry you will not be able to attend PSubs 2021 convention.  i would have liked to meet you in person.

    On Monday, July 19, 2021, 06:22:56 AM CDT, James Frankland via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Hi All
Does anyone have any ideas on this?
Following the dive i did last week, i had a motor issue.  The motor did not actually stop or fail, but during the dives it just didnt feel right.
The positive wire going into the rotating penetrator would get hot after about 10 seconds of motor power.  I could also hear a kind of electrical noise from within somewhere.
So, the next day i took the motor apart.  The oil (Silicone oil) was VERY black.
I was expecting to see something had been rubbing, but i couldnt see anything.  It all looked fine.  
I put it all back together and it runs fine in air.  However, there does seem to be a louder motor running noise than the others.
I am unsure what the issue is.  My feeling is that its carbon in the oil, making it conducting and hence slightly shorting out, causing the heat on the cable.
The other motor cable did not get hot, nor did the oil go black.
Again, another video here.  https://www.youtube.com/watch?v=v_VxVfhM9t8&t=2s
Would appreciate any input.  Im not sure what to do.  Buy a new motor?  Air compensate it?
Many thanks
James

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