Troubleshooting “Little Jake”: Page 3 of 5

Intermediate

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Troubleshooting “Little Jake”
Stepping through the troubleshooting and repair process on a vintage Jacobs wind turbine.
Little Jake needed to come down for repair.
Based on troubleshooting tests, Little Jake needed to come down for repair.
Little Jake’s tail was manually furled out of the wind
Little Jake’s tail was manually furled out of the wind so tower wiring could be checked.
The controller box
The controller box was tested and diagnosed.
Diversion controller, dump loads, and DC panel.
Diversion controller, dump loads, and DC panel.
RE systems’ balance of system (BOS) components
The RE systems’ balance of system (BOS) components, including the charge controller, battery bank, inverters, and DC and AC load power centers.
A megaohmeter (megger)
A megaohmeter (megger) was used to test the THWN conductor insulation.
Yaw slip-ring brushes.
Yaw slip-ring brushes.
Yaw slip rings.
Yaw slip rings.
A high-potential (hipot) tester
Testing for shorts from the conductors to the metal tower was accomplished with a high-potential (hipot) tester.
Testing in the shop
Testing in the shop revealed an internal short—a short from wire to wire—in the armature windings.
Testing in the shop
Testing in the shop revealed an internal short—a short from wire to wire—in the armature windings.
The front end of the generator
The front end of the generator. The blades would be attached to the long shaft.
The rewound armature.
The rewound armature.
Troubleshooting “Little Jake”
Little Jake needed to come down for repair.
Little Jake’s tail was manually furled out of the wind
The controller box
Diversion controller, dump loads, and DC panel.
RE systems’ balance of system (BOS) components
A megaohmeter (megger)
Yaw slip-ring brushes.
Yaw slip rings.
A high-potential (hipot) tester
Testing in the shop
Testing in the shop
The front end of the generator
The rewound armature.

Testing the Generator

Little Jake is a 48-volt generator. Generators have two windings—the field and the armature. Direct current (DC) is produced by spinning the armature within a stationary DC field. The induced current travels from the copper commutator, at the end of the rotating armature, through conductive graphite brushes, and then down to the slip-ring assembly. A second set of brushes contacting the slip-ring assembly allows the machine to yaw (pivot) freely with the wind, without twisting the wires that run down the tower and into the junction box at ground level.

We inspected all four brushes that ride on the generator commutator for excessive or irregular wear patterns, looked for evidence of overheating and pitting on the commutator, and examined the inside of the cover for signs of spattering solder—which would have shown us that the armature windings had been overheated. Everything appeared in good working order; the brushes were evenly worn, had plenty of remaining life, and were set with a good amount of tension against the commutator. (Each of the four brushes slides into a spring-loaded retaining clip with five notches for increasing tension. All four brushes should ride around the commutator with equal amounts of tension.)

We also inspected the conductors between the slip-ring assembly and the generator terminals for the field and armature windings. We used a multimeter for continuity checks and the megger for evidence of insulation breakdown, just like we did when checking wires from the junction box to the top of the tower. We also isolated and performed resistance checks on the lightning arrestor by using a multimeter to test each lead to ground and from lead to lead. They all showed no continuity, and infinite resistance, which is appropriate.

We measured the resistance of the field, and it was an acceptable 16 Ω. Field windings are essentially giant coils of thin wire, so resistance should be low—the same as if the coils were unwound and just very long conductors. We checked for shorts within the generator by using the megger to measure resistance from the commutator to the metal input shaft (23 MΩ), and from the field to the metal case (3.5 MΩ). We didn’t know exactly what kind of numbers we should see, but we knew that anything in the MΩ range was acceptable, since it shows high resistance with respect to ground.

None of these tests yielded suspicious results, and we were getting frustrated. We had tested every component and wire within the system, from the BOS to the top of the tower, and we still hadn’t found anything wrong—not even a tiny clue as to why Little Jake wasn’t delivering power.

Additional Tests & Final Diagnosis

We had no written documentation on the machine, so we called Mick. He validated what we had measured so far, and gave us advice about what to do next.

  • “Flash” the field;
  • “Motor” the generator;
  • Check output voltage with an analog meter.

“Flashing the field” means re-teaching the machine which field poles are positive and which are negative. Without a residual magnetic field in the field windings, no current can be induced when the turbine begins spinning. To verify that our field windings had a polarized magnetic field, we lifted the brushes off the commutator and connected MREA’s Sun Chaser (a portable solar power trailer) to the wires in the junction box going up the tower. After a full 15 seconds of delivering 48 volts of battery power up to the field windings, we were certain that the field poles contained magnetism with the correct polarity, but the Jake still yielded no output. (Machines with field windings using residual magnetism need to be “flashed” upon installation and after a high-transient event like a lightning strike.)

Comments (1)

Lindsey Roke's picture

I was interested in the trouble-shooting approach.
It appears to have followed what we might call “the aardvark method”.
(“I am thinking of a word in the dictionary, what is it?”
“Is it aardvark?”
“No”
“Is it aardwolf?”
“No”
Etc.)
This method will eventually find a fault but it but it is not the fastest – and without due care has the potential to introduce more problems if (for example) connections are not done up again correctly.
Here in New Zealand I work for an appliance manufacturer (Fisher & Paykel Appliances Ltd.). In managing production engineering I have always been intrigued by various approaches to trouble shooting. We learned a great deal from the late Dorian Shainin when we had him here running some training sessions for us. (Look him up on the web is you are not familiar with him.) The technique he would propose for the dictionary would find any word with about 17 questions requiring just yes/no answers. (Find the middle page and ask if the word is before that page. Then go to the quarter or three-quarter point as appropriate. 11 questions will get you to the right page in a dictionary with up to 2048 pages. The next question gets you to the right column. Assuming there are no more than 32 words in a column, 5 more questions will get you to the right word.)
I recall one factory break-down where the photocell sensor was supposed to detect a cabinet on the assembly line, send a signal to a microprocessor controller, which in turn was supposed to drive a solid state relay which switched a hydraulic valve that operated a piston that did something useful. The problem was that when the cabinet went in front of the photocell the useful action didn’t happen. While I could have started by checking that the photocell was working, it was much quicker to repeatedly “cut the problem in half”. Because we had indicator lights across the electrical connections on hydraulic solenoids, (put there when the equipment was installed for just such trouble-shooting) the first check was to see if there was power getting to the solenoid when the photocell was triggered, etc. etc. (Without the indicator lights, the same thing could have been done with a meter – but when an assembly line is stopped, time is of the essence.).
You can bend the rules a bit if there is a very easy point to check that is not mid-way through the problem – or if there are other clues as to where to start looking.
I have learned that, if a piece of equipment has been operating OK and it stops, unless somebody has already had a go at trouble-shooting it by swapping components in and out before you arrive, it will very very rarely have more than one fault.
My objective, when called to an intractable malfunction, is to determine which component is faulty, get the tradesmen to replace it and walk away with confidence that the machine would work when they have done so.
While the mid-point of the Jacobs system in the article might be the input to the slip-rings, to save climbing the tower, checking first whether or not there was a voltage at the bottom would have isolated the problem to either the Balance of system and wiring to it from the tower or – to the generator, slip-rings and tower wiring. The next check at the generator terminals would have put the fault in the generator or in the slip rings and tower wiring etc.
(I’m not decrying checking such things as the brush and slip-ring condition while up the tower – but as maintenance tasks not as trouble-shooting in this case.)

Shainin, in my opinion was better (at least in this area) than either Joe Juran or Edward Deming – but not so well known, probably because he was better at solving problems than writing books.
I trust this might be useful.

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