Troubleshooting “Little Jake”: Page 4 of 5

Next, with the generator brushes reseated, we used the Sun Chaser battery bank to run Little Jake’s generator like a motor. With controller power still disabled and the tail furled, we back-fed 48 volts to the wind turbine from the junction box at the tower base. Indeed, the machine began to spin and “motor” with ease. This was both promising and unsettling—if it worked as a motor, why wasn’t it delivering power as a generator?

The final test was a simple voltage test, but had to be done with an analog voltmeter instead of a digital multimeter. I recruited friend and fellow windsmith Dave Kiedrowski to come to the MREA the following week to do this test with me. The idea was to watch the needle of the voltmeter (connected to positive and negative terminals on the generator) as we spun the rotor slowly at first, and then gradually up to a “significant” rpm, but not so out of control that we were in danger of being hit by a spinning rotor while strapped to the top of the machine in our climbing gear. According to Mick, we should have seen the voltage gradually rise, and the needle should have steadily climbed, with no spikes, jumps, or drops. But we never saw more than 2 volts register on the meter. The armature was simply not delivering voltage and, for the first time, we were pretty certain that the problem was within the armature windings.

It was too windy to unfurl the tail and work safely while we were on the tower, so we repeated this voltage test at the base of the tower, with the leads of the analog voltmeter connected to the conductors in the junction box. We manually unfurled the tail. At high wind speeds with a swiftly moving rotor, we were still only seeing a couple of volts. I relayed our results to Mick, and he confirmed that it was time to call a crane operator and take down Little Jake for repair. He agreed that the problem was in the armature.

Just to be sure the maintenance class hadn’t missed anything, Dave and I repeated the continuity checks and high-resistance megger tests from the BOS to the generator. This time, we used a digital megger with batteries instead of a hand-crank, but the results were identical.

We also checked for shorts from the conductors to the metal tower using a high potential (hipot) tester I borrowed from fellow Jacobs enthusiast Jeff Nichols. Hipot or dielectric-withstanding voltage tests look for insulation breakdown and short circuits, and are often used to check electrical appliances for faults before they leave the assembly plant. Our hipot tests showed no ground faults or shorts to the tower. This was very similar to our tests with the meggers, but was just another tool to use and verify our original findings.

MREA regional training officer Clay Sterling and I worked with a crew and a crane operator to take down Little Jake in September 2011.

Bench Tests & Repairs

Once Little Jake was safely on the ground, MREA site manager Mike White disassembled the machine and packed up the generator for a trip to Mick’s wind turbine repair shop, just a few hours away. On a frigid December day, the three of us cracked open Little Jake and hoisted the armature onto the test bench, a factory original 1940s Jacobs wind generator testing system. A variable-speed DC motor arced and sparked as it drove the shaft of the armature from a slow crawl up to full output, and the needles on the hand-drawn calligraphic meter scales showed us voltage, current, and rpm.

With the generator on the test bench, the problem became quickly obvious. The voltage needle on the analog voltmeter was volatile—but only at an extremely low rpm; it spiked and collapsed with each slow rotation, indicating that there was indeed a short somewhere in the armature windings, likely due to insulation breakdown. Over time, wire insulation can break down with heat, movement, or extreme environmental conditions—three things that are unavoidable in any generator. The Jake is a 1946 machine, and the insulation was 64 years old, having seen some 50 years of service.