Best Practices for Protecting Motors


Motors are the “prime movers” of all manufacturing plant processes, without which manufacturing processes would come to a screeching halt. Therefore, they demand our utmost attention to ensure top performance and ongoing reliability. Whether you are dealing with electric motors, air motors, or hydraulic motors, there are separate and specific classes/subclasses that require different control, monitoring, and maintenance programs to run at top efficiency. These differences become more important when gathering data for a computerized maintenance management system (CMMS).

To properly maintain motors via CMMS, you must not only enter accurate nameplate information but also be sure to assign the correct subclass and include complete motor specifications. This data helps to fine-tune how predictive maintenance (PdM) and preventive maintenance (PM) procedures are developed and allotted to each type of motor. For this blog article, the focus is on general motor maintenance recommendations regardless of type.

For more detailed information about various types of motors, refer to Allied Reliability’s white paper, Prime Movers, the Driving Force Behind Manufacturing Processes.

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 analysis (monthly to quarterly)
  • Thermography/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.


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.

About the Author

Mark Thomas

Mark Thomas

In his current role with Allied Reliability, Mark is a reliability consultant responsible for working closely with clients to design, train, and implement preventive maintenance (PM) and reliability continuous improvement programs. Prior to his role with Allied Reliability, Mark held several roles with Emerson Automation Solutions.

Mark holds the CMRT, ARP-A, and Lean Six Sigma Yellow Belt Certifications. He is a seasoned field and plant service technician with a demonstrated history of working in the mechanical or industrial engineering industry. He is skilled in petroleum, technical support, inspection, databases, and plant maintenance. Mark is a strong support professional with an Associate of Science (AS) focused on Industrial Electronics Technology/Technician from Missouri State Fair Community College.

Connect with Mark on LinkedIn.


Allied Reliability provides asset management consulting and predictive maintenance solutions across the lifecycle of your production assets to deliver required throughput at lowest operating cost while managing asset risk. We do this by partnering with our clients, applying our proven asset management methodology, and leveraging decades of practitioner experience across more verticals than any other provider. Our asset management solutions include Consulting & Training, Condition-based Maintenance, Industrial Staffing, Electrical Services, and Machine Reliability.

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