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Shaft voltage: How to prevent wear on your powertrain

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Danger to your assets!

How to prevent wear on your powertrain caused by shaft voltage using a sound grounding concept.

Are you familiar with this? Unusual noises in bearings or gears occur and an unspecific trouble shooting process begins because the root cause of the problem can be very diverse. Typical causes are wear and tear or imbalances which can both lead to vibrations. Another factor contributing to noise development and wear which is often neglected is the so-called shaft voltage. With our expertise as a maintenance provider, we would like to introduce you to this phenomenon, methods of evaluation and effective countermeasures.

What is shaft voltage and why does it occur?

In electrical processes, like in electric power trains, there is always a physical interaction between voltage, the resulting amperage and consequently, the resulting magnetic and static fields. There’s also a negative side-effect that occurs: the so-called bearing current.

Static charges occur on conveyor belts, plastic films, paper and foil webs – these lead to high-frequency voltages on the surface. Unlike the conventional ground currents, these will not be diverted through the grounding cable cross-section, but through the surface of the grounding cable.

To safeguard your system, you need fine grounding cable for a sound grounding concept. It is also important how careful the ends of the grounding cables are connected to your machine components. After changing the motor, the cable’s coloured insulation is often not properly removed, so that the shaft voltage cannot be correctly diverted even though the right cable was chosen.

In case there is a change in processing, e.g. when taking up the paper, the shaft voltage is not diverted through the paper, but discharged through the powertrain. Voltage peaks of up to 400 volt are not uncommon. Due to bearing tolerances and high voltage on the powertrain, sparks occur which in turn lead to material damage, causing premature wear of components and costly repair and maintenance measures. A sound grounding concept and choosing the right materials for grounding is a good method to prevent those costs. A shaft-voltage test is necessary to choose the right grounding concept and assess the existing shaft voltage in your systems.

Shaft current and alternating voltage: how are they related?

Shaft current is a special type of direct current. It results from rectification of AC power. The diods in use reverse the negative half of the sinusoid. This results in a sequence of peaking waves. These waves are what define shaft currents.

Formation of shaft voltages is an unwanted side effect on electrical machines, especially on synchronous motors. These shaft voltages are systemic and do not impair function at first – as long as constructive steps are taken to make sure they do not carry current. If the shaft current does start to carry current because the isolation was interrupted due to installation errors or contamination, there will be increased current density on the connections points. This could happen in the gear tooth profile, causing wear or even outages.

How do I identify and measure shaft voltages?

As your specialists for powertrains, we use a so-called digital oscilloscope with special measuring tips and a simulator for grounding rings to measure shaft voltages. Firstly, we measure the voltage curve on the drive shaft over a certain time. This is done in order to find out what kind of voltage builds up during operation and if there is any other electrical discharge in another part of the machine, e.g., in the bearings of a cross joint or in gear teeth. A second measurement is performed to assess the effectiveness of a shaft grounding by attaching the grounding simulator. The correct use of these costly grounding simulators requires a high amount of expertise, training and experience. That is the only way you can ensure the correct shaft voltage measurement in the right locations during the various processes of machines and systems. HINT: Read this helpful study to learn how a shaft voltage measurement is conducted in practice!

Countermeasures against high shaft voltage

If bearings currents are elevated, the following technical measures are recommended:

Measure 1: Shaft grounding rings divert harmful discharges

You can use a shaft grounding ring with conductive microfibres which creates a low impedance path from the motor shaft to the ground. Shaft grounding rings are useful in diverting harmful discharges and ensure that your drive trains, machines and system run reliably. It is recommended to consult with a competent power train specialist for commissioning.

Measure 2: Isolated bearings prevent conduction of shaft current

Isolated bearings ensure that shaft currents cannot travel through the bearings. Please note that by using isolated bearings, the shaft current will not be eliminated. Harmful discharges can still be transferred to other powertrain components, causing unwanted technical damage which require repair or maintenance on the powertrain.

Conclusion: let the experts deal with shaft voltage!

Shaft voltages on the powertrain of your machines or systems are a complex topic and acquiring and handling the necessary measurement technology requires a lot of experience. Our recommendation is to consult an expert for powertrain technology. Maintenance service partners like us support you in troubleshooting your powertrain and offer a comprehensive solution to safely keep your machines running.

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