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Author Topic:   Chronic Stator Failures
HANK56 posted 05-03-2010 12:03 PM ET (US)   Profile for HANK56   Send Email to HANK56  
I have a 2006 175 Sportjet that is going through stators like crazy. I have installed three [stators] since new and only averaging 100 hours on each one. I did a voltage test yesterday and [measured] 14.2-volts at top speed. (The tachometer is reading about 6,200-RPM, but I think that it is out as well as the top rpm should be about 5,700-RPM.) I do not troll with the boat as I have a 9.9-HP. I have no extra electrical gadgets added, either. Battery is new and fully functioning. Any suggestions?
jimh posted 05-03-2010 08:14 PM ET (US)     Profile for jimh  Send Email to jimh     
In a permanent magnet alternator (which is what I assume is being discussed), the stator is a passive component. It is really just some copper wire wound on forms. The wire is insulated with an enamel-like insulation, sometimes called FORMVAR. The insulation is very thin compared with the usual polyvinyl-chloride insulation on hook-up wire. The stator is stationary.

The most likely modes of failure of a stator are from either short circuit or open circuit. If the stator temperature rises to the point where the wire insulation becomes soft or melts, it is possible for coil turns in the stator to short out to each other or to the chassis of the engine. Really high temperature could lead to heating of the copper wire to the point where it might melt and cause an open circuit. Heat is the most likely cause of failure, although abrasion of the wire from mechanical interference could also occur. Mechanical damage to the wire could occur as a secondary effect from heat. The wiring could sag too much if heated.

We now look to sources of heat. The engine is a very good source of heat. If the engine is not getting proper cooling, the extra heat from the engine may cause the stator temperature to rise. Another source of heat is the stator itself. The flow of electrical current through the stator windings will generate some heat, and the more current flow the more heat generated. If the boat charging system is constantly required to produce a high current, this can lead to heating of the stator.

A stator could be damaged by excessive voltage. If running at very high speeds the stator will generate higher than normal voltages. Any weakness in the wire insulation might be overcome by an arcing, leading to a short circuit.

If you give us a description of how the stators have failed, we could use that information to form an opinion of the circumstances that lead to the failures.

jimh posted 05-03-2010 10:18 PM ET (US)     Profile for jimh  Send Email to jimh     
In a stator assembly there are usually some multi-strand PVC-insulated wires connected to the stator coil winding. There is some sort of butt-connection between the coil winding and the wiring, and the wiring usually has some type of connector on the far end. These connections and connectors are also sources for problems which might cause a stator to fail.
HANK56 posted 05-04-2010 11:40 AM ET (US)     Profile for HANK56  Send Email to HANK56     
Thanks Jim
What happens when the stator starts to malfunction is when I run at high speeds for more then a half an hour or more and I go to let off the throttle the engine will start running rough and will quit at the low speeds.If I keep the throttle at high speeds it seems to still work.I was told that the low speed stator is the one that burns out.
I have inspected the malfunctioning stator and could not visually see anything wrong with it.I was also told that I may have a grounding problem.I have had this problem since new and am getting no help from Mercury or the dealer.If there is any more information needed please let me know.

Thank you

jimh posted 05-04-2010 12:30 PM ET (US)     Profile for jimh  Send Email to jimh     
Mark--You've made the mistake of calling every coil winding involved with your outboard motor "the stator." When you gave the voltage output from "the stator" as 14.2-volts at top speed, I made the natural inference that you were talking about the battery charging circuit.

Please identify the coil winding that is causing you problems. It sounds like it has nothing to do with the battery charging circuit. Perhaps the coil winding that keeps failing has something to do with the ignition circuit. You can't just talk about "the stator" and make any sense.

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