Unveiling the Hidden Culprits: Navigating Vibration Issues and Power Quality in Motor Maintenance

Unveiling the Hidden Culprits: Navigating Vibration Issues and Power Quality in Motor Maintenance

By Blake Parker
Director of Industrial Field Services, Integrated Power Services

When it comes to motor repairs, excessive vibration frequently drives customers to seek solutions. The immediate focus is typically on mechanical problems like imbalance, misalignment, bearing issues, or failing couplings. However, this approach can overshadow other critical factors.  In this blog, we’ll explore the often-overlooked electrical causes of motor vibration and why they warrant equal consideration.

Addressing mechanical aspects is a common strategy when tackling vibration issues. The 2X line frequency or 120Hz vibration component, often the primary suspect, points to a naturally occurring phenomenon that can be exacerbated by changes in the air gap—an indicator of mechanical trouble. While alterations in air gaps usually suggest mechanical problems, it’s equally important to consider power quality as a potential culprit. Poor power quality can introduce a variety of unusual issues that are challenging to diagnose.

Test the winding resistance

One crucial aspect of motor health is the balance of winding resistance, a factor outlined in most three-phase AC induction motor standards. This test, which should be performed during repairs or general testing, is non-destructive and critical in determining motor winding health.  AC induction machines, once appropriately wound, respond only to load changes or the power provided to the machine. A machine with a healthy winding won’t create a current or voltage imbalance. We need to examine the incoming power to see if there is a current/voltage imbalance and if the winding resistance is appropriately balanced. This practical approach to testing winding resistance is critical to effective motor maintenance.

The problems with voltage imbalance

NEMA MG 1 does not recommend operating an AC induction motor above a five percent voltage imbalance condition, and depending on the level of imbalance, the motor must be de-rated. The imbalanced voltage between phases introduces negative sequence voltage, causing the motor to fight itself, generating additional current in the stator and rotor and increasing heat in the machine. According to NEMA MG 1, the currents at normal operating speed will often be greatly imbalanced, approximately six to ten times the voltage imbalance. In addition to increasing heat in the machine, this imbalance can lead to unpredictable vibrations and is challenging to replicate consistently.

The importance of a thorough power quality survey

Diagnosing issues related to voltage imbalances in complex machine instrumentation can be challenging, particularly when Potential Transformers (PTs) are located on a shared common buss with multiple machines.  Depending on the machines on the buss, many can cause imbalance without being affected. For example, heaters can also cause imbalanced voltage past the switchgear and PTs/CTs (Current Transformers). Additionally, faulty contactors and other electrical components can introduce further complications.

Conducting a power quality survey is recommended to diagnose these issues effectively. A thorough power quality survey examines the entire system and drills down to the individual machines, making it possible to identify unusual or complex problems. In addition, current signature analysis on individual equipment improves diagnostic accuracy. Commissioning a power quality survey before removing the machine should confirm or eliminate the possibility of a power quality issue. The motor may not require removal if you find a power quality issue.

Trusted advisors for hard-to-diagnose issues

The longevity of your equipment depends on the system as a whole and its performance according to industry standards. Degradation of even one component in the system can lead to challenging issues that are hard to diagnose. When selecting an advisor you can trust for machinery repairs, look to an organization with experience troubleshooting the complete electrical and electromechanical system. IPS field service includes troubleshooting and repair from the switchyard through the electromechanical drivers and, in some cases, extends to driven equipment.

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