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.
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.
“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.)