Machine Design

Microfiber Ring Protects Bearings from Shaft Currents

The ceramic coated, insulated generator bearings at the top of one wind-turbine tower failed only 11 months after the tower was brought online. The company that owns and operates the wind farm replaced the bearings and slip rings, but the new ones failed only five months later. Once again, that caused costly downtime and repair.

In fact, a failed 1.5-MW generator can account for over $48,000 of lost revenue if down for a month, and repair costs can add up to as much as $50,000.

Parasitic capacitive coupling between the rotor shaft and stator windings induces high-frequency currents in wind-turbine-generator shafts. These currents can reach levels of 60 A and 1,200 V or greater. If not diverted, the currents discharge through the generator’s bearings, causing pitting and fluting, premature bearing failure and, potentially, catastrophic turbine failure.

After a third bearing failure 11 months later, the owner added an Aegis Wind Turbine Grounding (WTG) conductive- microfiber bearing-protection ring and shaft collar from Electro Static Technology, Mechanic Falls, Maine, to the drive end of the turbine in addition to replacing bearings and slip rings on both ends of the generator. Technicians also replaced the generator’s two standard carbon-block, spring-loaded brushes, which rub on the slip ring at the nondrive end.

Technicians from Electro Static Technology came out three months after the last bearing swap to measure shaft voltage on the generator with and without the bearing-protection ring and collar engaged. During the tests, wind speed ranged from 10.2 to 13.4 mph, and average voltage on the 5.824-in. shaft was 6.41-V peak-to-peak with the conductive-microfiber bearing-protection ring and collar versus 41.35-V peak-to-peak with carbon-block brushes only, a reduction of 84.5%.

The voltage waveform with the protective ring and collar was smooth as compared to the waveform without the protection, in which voltage peaks an average of 6.5 times higher indicated bearing-current discharge.

Electro Static’s conductive-microfiber bearing-protection ring is said to channel induced currents in the shaft safely to ground. The ring surrounds the generator shaft with millions of strong microfibers less than 10 μm in diameter. The fibers provide high-density, parallel paths of least resistance from the motor shaft to ground.

Each fiber can conduct up to 60 A and discharge several thousand volts with frequencies in the megahertz range. This capability lets the fibers significantly reduce voltage buildup on the generator shaft. The fibers also compensate for variations in shaft-surface roughness and microscopic misalignments.

If microfibers lose mechanical contact with the rotating shaft, local field emissions quickly reestablish electric contact somewhere along the ring. A gap >5 μm between the shaft and the fibers causes gaseous or electric breakdown, the flow of secondary electrons freed by impact ionization of gas ions in the gap. A 5-nm to 5-μm gap prompts Fowler- Nordheim tunneling in which electrons “tunnel” through a barrier in the presence of an electric field.

These types of field emissions let the ring perform much like conventional spring-loaded carbon brushes. But because there is less mechanical contact between the shaft and microfibers, there is less direct frictional and thermal wear.

Multiple microfibers also dissipate heat better than single-conductor devices, so the ring can tolerate higher current densities. Electro Static’s wind-turbine bearing-protection ring is rated for up to 120 A of continuous current at frequencies as high as 13.5 MHz and discharges of up to 3,000 V. The microfibers function equally well when contaminated by oil, grease, dust, moisture, or other substances.

TAGS: Energy
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