Best Practices for Protecting Motors

Motor Maintenance Best Practices

For best results, a comprehensive, proactive strategy is needed to maintain your motors properly. The development of this strategy should be based on asset criticality, ambient conditions, size, supplied voltage, and local capabilities. The recommendations below apply to critical machines supplied with 600V or greater and will need to be modified (in terms of frequency and scope) for less critical assets.

Critical Motor PM/PdM Strategy

Visual Inspections (Required Daily)

• Inspect the integrity of the motor frame ground
• Verify the oil level in oil reservoirs (if applicable)
• Inspect the oil cooling/pump system for proper operation (if applicable)
  • Look for leaks
  • Ensure temperature is appropriate
  • Ensure pressure is appropriate
  • Listen for unusual noise
• Inspect motor vents/end bells to ensure air paths are clear
  • Look for excessive dirt build-up on the frame/end bell
  • Look for excessive dirt/oil build-up on vents/fan cover
  • Look for excessive dirt/oil build-up on air filters
• Inspect for lubrication/oil leaks at motor end bells, reservoirs
• Inspect motor base for unusual condition(s), such as:
  • Sound
  • Vibration
  • Loose or missing bolts/nuts
• Inspect motor base integrity, including:
  • Motor connection box(s)
  • Feeder cables
  • Temperature (RTDs)
  • Vibration probes/guards
  • Fan
  • Coupling
• Inspect motor frame/conduit integrity, checking for:
  • Chemical attack
  • Water leaks
  • Fatigue cracking

PdM Activities (Required Quarterly to Yearly, depending on history of motor)

• Vibration analysisThis Predictive Maintenance technique is widely used to evaluate mechanical rotating equipment to determine if any undesirable changes are present that might give an early indication of imminent failure. Uses transducers to translate a vibration amplitude and frequency into electronic signals to determine the equipment’s actual condition. This may lead to the recommendation of a logical course of maintenance actions to correct the problem before secondary damage or catastrophic failure can occur. Additionally Vibration Analysis can be used for the modeling, prediction, measurement and analysis of structural dynamic response in design and root cause failure analysis. In design, vibration modeling and prediction is used to anticipate and avoid undesirable dynamic response. In root cause failure analysis it is used both to understand undesirable response and as a factor in determining true root cause. (monthly to quarterly)
• ThermographyA predicitive maintenance method that detects infrared energy emitted from an object, converts it to temperature, and displays an image of the temperature distribution./infrared inspections (quarterly to bi-annual)
• Oil analysis for motors with oil reservoirs (annual)
• Off-line motor analysis (bi-annual to annual)
• Motor winding test (insulation resistance test)
  • Resistance to ground ─ indicates condition of insulation to ground for each phase(should trend this)
  • Resistance phase to phase ─ indicates condition of phase-to-phase insulation (should trend this)
  • Phase resistance comparison (winding imbalance) ─ indicates condition of turn-to-turn (winding) condition (should trend this)
• Cable test (starter to motor)
  • Resistance to ground for each feeder cable ─ indicates condition of feeder cable insulation to ground (should trend this)
  • Resistance Phase to Phase for each feeder cable ─ indicates condition of feeder cable insulation to the other phases (should trend this)
• PI polarization index test ─ a resistance-to-ground test that eliminates the need to do temperature correction
• Capacitance to ground ─ indicates the cleanliness of the motor

PM Activities (Required/As-needed)

• Lubrication: Greasing of rolling element bearings (as needed)
  • Use ultrasonic detection/monitoring to determine the frequency and amount of lubrication needed (refer to company lubrication policy).
  • The type of grease and operational environment may require more frequent lubrication.
  • It is suggested that the type of grease used on each motor be recorded in order to avoid bearing failure. In many cases, you may be able to standardize the type of grease used in the bulk of your motors.
  • Another good practice is to let your motor repair center know the type of grease in case the standard grease used by the repair center conflicts with your company’s standard motor grease.
  • Frequency:
    • Normal speed and horizontal mount grease every 6 months
    • High-speed and/or vertical mount grease quarterly
  • Bearings should be shielded on the winding side, to prevent grease from entering the motor windings.
• Grounding and bonding inspection (annual)
• Grounding and bonding continuity test (annual)
• Motor overhaul on equipment condition
  • Vibration analysis, on-line and off-line motor analysis ─ determines when a motor should be overhauled
• Connection/terminal inspection
  • Visual (annual)
  • Continuity test ─ performed when motor overhaul is performed or when indicated by on/offline testing
• Motor alignment
  • Laser or dial indictor alignments (annual)
• Special considerations
  • DC motor inspection/maintenance ─ must include commutator and brush inspection, brush spring adjustment, and brush replacement (as needed)
  • Any motor with individual surge protection (metal oxide varistor) ─ needs to be disconnected prior to on/offline testing, as it interferes with readings

Other Recommended Activities (As-needed)

• On-line motor analysis
  • In-rush/start-up test ─ can help identify rotor/stator faults and shows motor startup characteristics
• Motor current signature analysis, or motor current analysis ─ used to investigate loose or broken rotor bars, rotor eccentricity, and stator/rotor air gap issues discovered by vibration analysis
  • A good vibration analysis program and a certified technician will discover loose or broken rotor bars, rotor eccentricity, and stator/rotor air gap issues.
  • Use motor current signature analysis to pinpoint diagnosis.
  • If your company has online motor test equipment, it should be used regularly (bi-annual to annual). If not, then contract this testing procedure on an as-needed basis.

Summary

It may work to apply a run-to-fail strategy in certain cases, i.e., if there is a plan for when that failure occurs. However, most motors require a more proactive approach with regular PMs, PdMs, and inspections based on equipment condition and how critical that equipment is to achieve the desired safety, environmental, and performance goals.

Allied Reliability’s highly skilled consultants have the skills and knowledge across a wide range of industries to support your maintenance team in solving a specific problem or developing a complete motor maintenance strategy for your manufacturing facility.

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