Tips from PdMA

PDMA TOTW Tips from PdMA

A collection of really good tips from PdMA Corporation, an industry leader in the field of predictive maintenance (PdM) and condition monitoring in the field of Electric Motor Testing.

Resistive Imbalance vs. Current Imbalance vs. Thermal Imaging

It is well-known that a high resistive imbalance on a three-phase motor due to a hot spot, or a high resistance connection, may not immediately show up as a current imbalance. Well… it should be well-known for those who read the PdMA Tip of the Week. As the high resistance connection worsens, eventually it does show up as a current imbalance. However, waiting for a current imbalance may result in advanced localized damage around the high resistance connection. Another technology well-known for its early indication of developing high resistance connections is thermal imaging. If the motor is running and the high resistance connection is occurring in an exposed and accessible environment like the starter cabinet, thermal imaging can provide visual evidence of the high resistance identified through a de-energized resistive imbalance. Both precision resistive imbalance and thermal imaging will identify early indications of developing high resistance connections long before AC current is affected. Using them together to identify these defects early may prevent a current imbalance and prevent advanced levels of heat damage to localized components. To read a case study on a high resistance connection and the data associated with it Click Here.

Mission Critical

On any given day there are hundreds, or maybe thousands, of assets driving a plant environment. Each application within the plant has various tasks hence establishing the need for assets of different sizes, cost, and criticality. All assets need to be cared for cost effectively; however, there are usually several assets per plant that are labeled critical assets which receive a little extra care. Review your plant’s assets, identify and verify the condition of the critical assets regularly, and use the PM tools and methods that are available to optimize the integrity of those assets before other less critical assets. Based on the risk to business, on average less than 20% of assets within the plant should be labeled critical.

Source: Maintenance and Reliability Best Practices by Ramesh Gulati

Field Resistance Condition Assessment

Field resistance is temperature corrected and compared to nameplate information for condition assessment. Accurate temperature of the field winding is required for proper comparison to the nameplate field resistance calculations. Incorrect nameplate information may also create an invalid reference for resistance and a potential alarm condition. If the corrected field resistance is significantly different from the value of resistance calculated from the nameplate voltage and current, verify the nameplate information is valid, refer to the operating manual for information on the expected field resistance, ensure the dissipation resistors are disconnected, and test directly on the slip rings, in an effort to isolate the cause. Remember, the difference may actually be a field anomaly and not a problem with the nameplate or temperature entry.

Nameplate Speed vs. Actual Speed

Identifying the speed of the motor is important for proper data analysis in many predictive technologies like electric motor testing and vibration analysis. It is not uncommon to identify a slight difference between measured speed and nameplate rated speed at the listed nameplate full load amps (FLA). NEMA MG1 allows +/- 20% of the difference from synchronous speed to rated speed under rated conditions. Although technologies, like the PdMA MCEGold software, may provide a field calibration of the speed search algorithm to allow for these differences, we should not automatically dismiss them as inaccurate nameplates.

Instead we should evaluate the possible anomalies that could result in speed changes to verify a problem does not exist. Examples of anomalies affecting the measured speed to FLA relationship include a rotor cage anomaly reducing the operating speed, reduced operating voltage increasing the current amplitude for a given load, and a frequency change, which directly affects the synchronous speed. Always assume the nameplate is correct, until you prove it is not.

Teamwork Leads to Home Run

Having teamwork or a team mentality is highly beneficial in creating a successful reliability program. “What do I need to provide to others, and from others, to ensure meeting my responsibility?” (Moore, p379) Working with other departments and communicating clear goals and objectives as you all strive to maximize equipment and process reliability for the good of the company should be a minimum standard. Having quality leadership to set the process expectations and support the team through execution is valuable to a team mentality. When this occurs, you’ve hit a home run, costs are minimized and production uptime is maximized.

Source: Making Common Sense Common Practice, Ron Moore.

Phase-to-Phase Inductance Measurements

Routine offline and shop stage gate tests should include Phase-to-Phase Inductance measurements. Phase-to-Phase measurements can be useful for diagnosing a variety of situations:

  • Poor or incorrect rework such as reversed coil winding leads
  • Faults in power cables or main contacts in the power circuit
  • Air gap eccentricity problems
  • Shorted turns such as stator phase-to-phase and coil-to-coil
  • Rotor porosity and lamination damage
  • Broken/cracked rotor bars or end rings

For more information read Influence of Residual Flux on the Measurement of Inductance.

You are invited to submit an Electric Motor Testing Tip of your own and receive a free PdMA mug or hat if they publish it! Contact Lou at 813-621-6463 ext. 126 or


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