Inquiry Regarding Cracked Stator Core in T200 Thruster Used in ROV Application

[EliotBR (Eliot)]

Hi @AnkitPandia, sorry to hear you’ve had an issue with one of our thrusters.

@AnkitPandia

1. Possible Cause:
   What could cause the stator core to crack under normal ROV operation? Could this be due to overcurrent, vibration, or mechanical stress?

@tony-white

It’s typically related to operating in very hot or cold water, at high power output, which can cause rapid expansion/contraction of the stator leading to cracking and subsequent failure.

@AnkitPandia

2. Repair Guidance:
   Is there any recommended method or guideline to repair or replace the stator? Or is complete thruster replacement necessary in such cases?

Our Thruster Usage Guide includes guidance on disassembly and reassembly, and our T200 spare components product allows buying a replacement core, which includes the stator.

Cracked stators generally need to be replaced (rather than repairing with a marine epoxy), as the stator windings will corrode if exposed to salty water (which is not reversible), and any potential patch-ups would need to be smooth with the existing stator surface to avoid rubbing against the rotor during operation.

If a thruster fails while you are operating within the rated conditions, please let us know, so we can help resolve the issue (potentially sending a relevant replacement if the failure was within the rated operating conditions), and so we can track how and where our products fail, to identify issues in our manufacturing lines, and/or improve the product design in future iterations.

@AnkitPandia

3. Electrical Parameters:
   We are currently running the thruster at 16.5V DC using a 30A ESC. Is this within the safe operating range? If not, what are the ideal current and voltage specifications for long-term reliable operation?

Per the Technical Details section on the T200 product page, 16.5V should be fine, and at that voltage the thruster will not draw more than 30A of current. There may be some concern with your ESC overheating if used continuously without sufficient heat dissipation, but that kind of effect is not captured in power specifications.

@AnkitPandia

4. Preventive Suggestions
   Are there any measures we can take to prevent this issue in our other thrusters (e.g., current limiting, better mounting, dampening)?

Stress fractures and crack propagation are likely the main relevant considerations here, and we provide guidance on care and maintenance, especially with respect to abrasive and corrosive operating conditions.

There is some extra information on longevity and usage in the FAQs on the product page

@T200 FAQ - What is the lifespan or mean time between failures of the T200? How long can they run continuously?

It depends! Because our customers operate in varying conditions across a wide range of use cases, there is no one expected total lifespan that would apply to all situations. Factors such as water quality, sediment density, particle size and composition, throttle level, operating voltage, etc., all play a large role in affecting thruster longevity.

Likewise, these same factors affect how long a thruster can operate in non-stop, continuous use. Anywhere from a few days to several months of continuous usable life are realistic to expect depending on the exact operating conditions.

Thrusters do require routine maintenance, and this also affects how long parts will last. If components are well taken care of, they can last for many years. If they are abused and used in harsh environments, parts may wear down faster and will need to be replaced more frequently. For example, iron particles may be pulled from the water and collect inside the rotor on the magnets. This iron should be cleared out regularly to prevent corrosion and allow for proper operation.

Beyond that, as Tony mentioned, operating at high current in very hot or cold water can increase the material stresses, especially if there are frequent and rapid cycles between temperatures. If you happen to be in extreme conditions it may be worth having some kind of staging, with the vehicle resting in water before and/or after a dive to slow down the temperature transitions (as has been previously described by a forum user).

@AnkitPandia

5. Material & Structural Inquiry
   1. What material is used in the T200 stator core, and is there any known history of fatigue or cracking in similar cases?
   2. Are there any batch-specific manufacturing issues that may apply to the unit we received?

Per the revision history on the T200 product page (in the Technical Details section), the only relevant stator change was in 2019:

@T200 - Technical Details - Revision History

08 November 2019

• Changed stator from being epoxy coated to fully potted

We do not have any technical bulletins related to T200 stator cracking, so outside of the design change I don’t believe there are any batch-specific issues to be aware of.

@AnkitPandia

6. Environmental Considerations:
   1. Could pressure at depth, temperature changes, or corrosion be contributing to the stator damage?
   2. Is the T200 rated for long-term vertical thrust usage under semi-static load (such as maintaining hover)?

See the response to 4.

@AnkitPandia

7. Vibration & Mounting Feedback
   We mount the thruster directly to the ROV frame. Could vibration feedback or frame resonance be contributing to mechanical stress on the stator?

Vibrations are always a possible contributor to stress, although a plastic frame in water tends to absorb a fair amount of vibration energy, so this is not something we’ve identified as a point of concern. If your custom frame is metal and you’ve strongly screwed the thruster to it then vibrations may be more significant, but even then if it’s significant enough to be a problem it seems more likely that would cause cracks to the stator base rather than the stator itself.

@AnkitPandia

8. Warranty or RMA Possibility:
   1. Is the stator cracking covered under warranty or a return/replacement policy?
   2. If not, can we purchase replacement stator or internal parts for in-house repair?

See the response to 2.