Why does the genset generate shaft current?

This blog explores the main causes of genset shaft current, its impact on performance and life, and effective solutions for detecting and preventing the problem

September 17, 2025
BY BISON

Gensets play a vital role in modern power generation. By converting mechanical energy into electrical energy, gensets ensure continuous operation in critical applications where power outages cannot be tolerated. However, an often-overlooked issue arises—shaft current. Shaft current refers to the unwanted flow of electrical current through the genset’s rotor shaft. While it may seem minor, it can lead to serious problems, such as bearing damage, excessive wear, overheating, and costly equipment failure.

Understanding shaft current is crucial because it directly affects safety, performance, reliability, and maintenance. If left unaddressed, it can shorten component lifespan, increase downtime, and raise operating costs. This blog explores the main causes of genset shaft current, its impact on performance and life, and effective solutions for detecting and preventing the problem.

What is shaft current?

Shaft current in gensets refers to unwanted electrical current flowing through the rotor shaft instead of its intended path. This occurs when voltage builds up on the shaft due to electromagnetic, electrostatic, or stray current effects. Rather than being safely grounded, the excess current travels through the shaft and bearings, causing damage over time.

Potential differences between the shaft and other moving parts, such as bearings, rotor, and housing, create conditions for this current to flow. Various factors can cause this imbalance, leading to electrical discharge inside the genset.

Causes of shaft current in gensets

1. Electromagnetic induction / magnetic asymmetry

When the magnetic field inside a genset is imbalanced, electrical current can be induced in the rotor shaft. This may result from flux leakage, uneven magnetic fields, rotor eccentricity, or improper winding during manufacturing. The imbalance forces stray currents to flow through unintended paths, including the shaft and bearings.

2. Capacitive coupling / electrostatic charge build-up

The rotating shaft and surrounding parts act like a capacitor, allowing static electricity to accumulate. This can occur due to friction between materials (triboelectric effect), static charge between the shaft and bearings, or high-frequency switching in the system. Once the charge reaches a critical level, it discharges through the shaft, creating harmful currents.

3. High-frequency common-mode voltage from Vfds

When gensets operate with variable frequency drives (VFDs), shaft currents often increase. Vfds use high-frequency switching through insulated gate bipolar transistors (IGBTs), creating common-mode voltages that travel through the shaft and bearings, causing electrical discharge and wear.

4. Grounding and insulation issues

Poor grounding or damaged insulation allows stray currents to travel through the shaft. Inadequate grounding can lead to voltage differences, ground loops, and higher electrical resistance, while insulation failures from aging, thermal stress, or mechanical damage remove the barriers protecting the shaft and bearings from electrical flow.

5. Imbalance and harmonics

Non-linear electrical loads can create imbalance and harmonic distortion. These distortions increase stray currents and voltage fluctuations, which contribute to shaft current problems.

6. External influences

Mechanical misalignment, dust, moisture, high temperatures, or excessive vibration can worsen shaft current issues. Contaminants and harsh operating conditions accelerate insulation degradation, increase electrical conductivity, and add mechanical stress to components, raising the risk of electrical discharge through the shaft.

Effects of shaft current

1. Bearing damage (pitting and fluting)

When shaft current discharges through bearings, it creates microscopic sparks that damage the bearing surfaces, leading to pitting (tiny craters) and fluting (groove-like patterns). This results in noisy operation, increased vibration, and eventual bearing seizure.

3. Performance loss

Bearing damage and electrical discharge increase friction, mechanical resistance, and power losses within the system. As a result, the genset’s power generation efficiency drops, reducing overall performance and reliability.

4. Accelerated wear and reduced lifespan

Stray electrical currents cause localized heating and stress on the rotor shaft, insulation, and bearings. This accelerates wear, significantly shortening component lifespan. Generators designed to last decades may require major repairs or replacement after only a few years if shaft current issues go unchecked.

5. Electrical discharge and safety hazards

Excessive shaft current can cause sparks, arcs, or corona discharges inside the genset, leading to insulation failure, short circuits, and equipment malfunctions. These electrical hazards increase the risk of shocks, burns, fires, and other workplace accidents.

6. Vibration and noise

Bearing damage and mechanical irregularities caused by shaft current increase vibration and noise levels. Excessive vibration accelerates component wear, affects equipment stability, and creates additional risks for surrounding systems and operator safety.

7. Increased maintenance costs

Shaft current often remains undetected until visible damage occurs, leading to frequent inspections, premature bearing replacements, and unplanned servicing. These repeated expenses quickly add up, reducing return on investment for operators.

8. Unexpected downtime and financial loss

A failed genset in a hospital, data center, or factory can disrupt critical operations, halt production, and lead to significant financial losses. Emergency repairs, replacement parts, and lost productivity further increase costs while damaging operational reliability and reputation.

Detection and diagnosis

Diagnostic tools

Detecting shaft current in a genset often starts with a shaft voltage probe, which measures the electrical potential on the rotor shaft to confirm the presence of stray currents. Oscilloscopes are also widely used to capture and analyze voltage waveforms, making it easier to identify high-frequency discharges from VFDs or insulation failures. Other equipment also includes vibration analyzers, temperature sensors, insulation testers, electricity meters, and shaft grounding monitoring systems.

Monitoring techniques

In addition to instruments, here are some practical monitoring methods:

Audible bearing noise: electrical discharges often produce a whining, grinding, or crackling sound inside bearings.
Vibration analysis: increasing or unusual vibration levels may indicate bearing pitting or fluting caused by shaft currents.
Visual inspection: during maintenance, scorched lubricant, uneven wear, or physical damage on bearings can point to current-related issues.

Prevention and mitigation

Proper grounding and bonding: A solid grounding system gives stray electrical currents a safe path to discharge, preventing them from flowing through the shaft and bearings. Proper bonding between components also minimizes potential differences that cause unwanted currents.

Insulated bearings: Installing ceramic or coated insulated bearing blocks prevents electrical currents from passing through sensitive bearing surfaces, protecting them from pitting, fluting, and premature failure. Additional insulation options, such as insulated bearing housings or insulating coatings, further isolate the shaft and reduce current flow.

Shaft grounding brushes or rings: Shaft grounding devices provide a low-resistance path for stray currents to safely discharge, keeping harmful electrical energy away from bearings and significantly extending their lifespan.

Bearing housing shields (faraday shields): Faraday shields installed around bearing housings block induced voltages and electromagnetic interference, preventing currents from reaching the bearings. This is particularly effective in high-frequency environments.

Harmonic reduction with filters: For gensets connected to VFDs, harmonic filters reduce high-frequency voltages, electrical noise, and voltage spikes that contribute to shaft current problems. This improves system stability, protects components, and minimizes common-mode voltages.

Optimizing the operating environment: Controlling factors such as temperature, humidity, dust, and vibration helps reduce insulation stress and bearing wear. A cleaner, more stable environment minimizes risks of electrical discharge and extends equipment life.
Regular inspection and maintenance: Even with preventative measures in place, regular inspections of generator set shaft currents should not be neglected. Typically, inspections can be performed quarterly, semi-annually, or annually, with more frequent inspections required in high-risk environments or critical applications.

Summary

Shaft current in gensets is a hidden but serious problem caused by magnetic asymmetry, capacitive coupling, high-frequency voltages, grounding issues, and harmonics. If left unaddressed, it can lead to bearing damage, reduced efficiency, higher maintenance costs, and unexpected downtime—shortening equipment lifespan and creating financial risks for operators. Early detection and proactive prevention are essential.

As a professional genest manufacturer in China, BISON offers genests that improve power system efficiency, safety, and durability. Managing shaft current is more than just a technical requirement; it’s an investment in long-term reliability and reduced operating costs.

Business with BISON today to ensure your genset operates at peak performance, a reliable purchasing process, and a positive customer experience.