Prevention

The solution to the shaft current problem is of crucial importance for all turbomachinery applications!

Shaft currents are caused by the build-up of electrical charges on the machine shaft through 3 mechanisms: static discharge resulting from the flow of particles in liquids, motor and generator faults and the movement of magnetized components. The charge build-up creates a potential difference (voltage) between components, which is discharged by sparks. Sparking occurs when the voltage between the components exceeds the breakdown voltage of the liquid separating them, usually oil. The voltage at which sparking begins depends on a variety of factors, such as the distance between the components, the type of oil and the oil impurities, and ranges from 0.5 volts for rolling bearings to over 10 volts for large liquid bearings. Typically, sparks occur at the points with the smallest distance between stationary and rotating components.

Spark discharges generate intense heat (over 2500 °C), which causes pitting at both ends of the spark. Depending on the energy level of the sparks, this can lead to a slow and gradual erosion of the bearing material, sometimes taking years before the damage becomes noticeable in rising bearing temperatures. Or the sparking can be so severe that the machine cannot operate for more than a few hours before it has to be shut down. Problems with the shaft current can lead to considerable operating costs.

What can be done to protect machines from wave currents?

In some cases, shaft current is inherent to the process, so compressors, turbines and pumps are all subject to significant static discharge loads. Fortunately, static discharges are usually low energy events that can be dissipated to ground with careful design. On the other hand, magnetized components and motor/generator faults can cause high energy discharges and the only effective long-term solution is to correct the underlying fault. Regardless of the source of the shaft current energy, Sohre brushes can protect sensitive machine components and keep your machine running.

Sohre Turbomachinery® shaft grounding brushes use a proprietary silver and gold fiber compound that makes reliable electrical contact with the shaft, ensuring low electrical contact resistance and low contact pressure. As a result, there is little mechanical wear on the brush or shaft surface. Compared to solid brushes made of graphite, carbon or other materials, Sohre brushes have a significantly lower contact resistance. Shaft grounding brushes from Sohre Turbomachinery® eliminate deterioration of the shaft surface and the resulting loss of contact during prolonged operation in normal through-current operation. Solid brushes from other manufacturers, on the other hand, which are commonly used on electrical machines, have a much higher resistance after wear and therefore start to spark, leading to a deterioration of the shaft surface. If the resistance of the installed brushes is then higher than the resistance inside the machine, the currents no longer flow through the brush, but through the machine, which can lead to damage.

Brushes from Sohre

Sohre's brushes are characterized by excellent electrical conductivity, so that the brushes ensure very low electrical noise, even at high surface speeds and in aggressive environments. This, a low wear rate and easy replaceability enable long-term monitoring and dissipation of electrical discharges during system operation.

Sohre brushes are preferred in the industry over other grounding solutions such as copper braids, carbon blocks and other conventional tools due to their superior performance and low maintenance requirements.

There are now more than 10,000 brushes in use worldwide - in chemical plants, refineries, paper mills, drilling platforms, oil fields, ships, power generation plants, cryogenic processes, wind turbines and many other facilities. Several original equipment manufacturers (OEMs) use our brushes as standard equipment for their customers.