Identification & Types of Damage
- Shaft current damage can be identified by close examination of the damaged component.
- Some indications are clearly visible.
- Often a 30x magnification is required.
Types of Electrical Damage Caused by Shaft Currents
Damages observed can be classified as:
- Not visible unless magnified to 30X
- Caused by very low current ( < 0.5 amp)
- Long term damage (3 to 5 years)
- Reduces thrusting capabilities leading to eventual wipe
- Contributes to many thrust bearing wipes
- Often misdiagnosed as thermal or abrasive damage
- More severe than frosting
- Caused by higher currents (~ 5 amps)
- Short term damage (~ 6 months)
- Random and Pits are > 1 mm in diameter
- Found often on rollers, balls, thrust pads, gear teeth and journal bearings
- Discharge is sustained by weak oil
- Current path is random, sometimes straight.
- Often regarded as scratches caused
- During handling or
- Due to debris in oil
- High energy - proof of residual magnetism within the machine
- Cause for concern – further severe damage can be expected.
- Damage is very severe
- Very high current discharge - sometimes over 1000 amps.
- Machine fails within minutes of the discharge
- Tremendous amount of potential energy.
- Energy can be dormant for many years.
- Energy often released during a rub or machine upset.
- Damage is very severe.
- Very high current discharge over a short period of time.
- Machine fails within days or weeks.
- Machine could self-excite and lead to self-destruction.
- Damage can be prevented if detected by monitors.
Monitoring and Fault Conformation
- The brushes can be connected to an online monitor that provides continuous monitoring of shaft voltage & current
data. The online monitoring system serves as a fault confirmation tool and is an invaluable tool for plant engineers
to determine the health of machines and confirm developing machine faults at an early stage.
- Protection of turbo generators usually involves the installation of 2 Shaft voltage sensing brushes & current
grounding brushes and the installation of 1 Online Monitoring System.
- Problems detected are determined by reducing shaft current and shaft voltage signals into minimums,
maximums, frequency, ratios and rate of changes. These may indicate several possible causes requiring
positive identification and confirmation from other factors to pinpoint a specific fault.
- By narrowing possible problems, effort can then be focused on effective troubleshooting, leading to orderly
planning and/or execution of maintenance or shutdowns, as appropriate.
- When used in conjunction with other monitors, abnormal trends of shaft voltages & current signals have been used in
the past to confirm the following faults:
- Confirmation of Shaft Rub
- Wet Steam in Turbine
- Loss of Bearing or Seal Insulation
- Developing Stator winding faults
- Stator Lamination Shorting & Melting
- Stator Lamination shorting and melting
- Shift in air gap
- Rotor winding faulting
- The need for a shaft voltage harmonic filter.
- Trending of each brush grounding current, with maximum current alarm on each brush. Each brush will alarm
on low brush grounding current indicating brush or grounding maintenance required.
Altair specializes in the detection, confirmation & resolution of faultscaused by Shaft currents. We provide the
- Online shaft voltage audits to confirm presence of shaft voltages.
- Supply of Brushes
- Supply of Online monitoring systems
Altair Industrial has a track record of over 12 years in trouble shooting & solving problems at leading power
plants (Reliance, Adani, NHPC, etc.), petrochemical (IOCL, BPCL, RIL) and fertilizer complexes (GNFC, IFFCO,
Tata Chemicals, etc.) on a proprietary basis.