This is the standard checklist for non-critical AC motors. For production-critical motors where unplanned downtime carries significant cost or long lead times, see the AC Motor PM Checklist — Critical.
⚠️ Disclaimer: These tasks are guidelines only. They do not include lockout/tagout (LOTO), energy isolation, or other safety requirements. Review and verify suitability for your specific equipment and application. Add all required safety procedures per your company's policies and regulatory requirements before use. You are responsible for the safe and appropriate execution of all maintenance activities.
AC motors fail the same way they always have. Overheating, insulation breakdown, bearing wear, contamination. None of it is mysterious. Most of it announces itself long before anything actually breaks.
The problem is PM programs that treat a motor inspection as a visual walk-by. You look at it. It's still there. You check the box.
This checklist is for standard AC motors — non-critical equipment where downtime is manageable and replacement moves faster than extended troubleshooting. It covers the checks that find developing failures before they become downtime events.
For a full discussion of what drives AC motor failure and how PM fits into the picture, start with Electric Motor Preventive Maintenance: Why Most Programs Miss Early Failure Signs.
How to Use This Checklist
Record what you find, not just whether something passed. "Bearing temp 68°C, up from 54°C last PM" is a finding. "OK" is not. That difference is the entire point of trending.
Every reading is only valuable in context. A single insulation resistance measurement tells you almost nothing. Six measurements over three years tell you whether you have a motor that's holding or one that's degrading. Write down the numbers. Date them. Keep them with the equipment record.
A bad finding looks like this: Phase C current 18.4A, nameplate FLA 15A, phase A 14.8A, phase B 15.1A — imbalance and overload, flagged for electrical follow-up. A checkbox answer looks like this: Current — OK. One of these is a maintenance record. The other is a liability.
Visual Inspection
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect motor exterior for signs of overheating, discoloration, cracked housing, or physical damage. Note any abnormalities. | Every PM | MEC |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Inspect motor exterior for overheating, discoloration, cracked or damaged housing, and physical damage. Compare to prior PM notes and flag any new findings. | Every PM | MEC |
Operational Checks
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Verify motor is running at or near nameplate RPM and without unusual noise or vibration. Note any changes from baseline. | Every PM | MEC |
| Measure and record motor operating current on all phases using a clamp meter. Compare to nameplate FLA. Investigate any phase exceeding nameplate rating. | Every PM | ELE |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Measure and record motor operating current on all phases using a clamp meter. Compare to nameplate FLA and prior readings. Investigate any upward trend or imbalance exceeding 5% between phases. | Every PM | ELE |
| Measure and record supply voltage at the motor terminal box. Verify within ±10% of nameplate voltage. Check phase balance — voltage imbalance exceeding 1% can cause significant current imbalance. | Every PM | ELE |
| Observe motor operation for unusual noise, vibration, or rough running. Compare to baseline. Unexplained changes should be flagged for vibration analysis. | Every PM | MEC |
Mechanical Inspection
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect motor ventilation openings and cooling fins for blockage from dust, lint, or debris. Clean to ensure adequate airflow. | Monthly | MEC |
| Check motor mounting bolts and base hardware for looseness or vibration-induced movement. Retorque as needed. | Monthly | MEC |
| Inspect motor coupling or drive connection for wear, cracking, or misalignment. Check set screws and keyways. | Monthly | MEC |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Check motor coupling or drive connection — inspect for looseness, wear, cracking, or misalignment. Verify set screws, keyways, and flexible elements. Flag any degradation. | Every PM | MEC |
| Inspect motor ventilation openings, screens, and cooling fins for blockage from dust, lint, or debris. Clean to ensure unrestricted airflow. Confirm integral cooling fan (if present) rotates freely. | Monthly | MEC |
| Inspect motor mounting hardware — base bolts, foot pads, and anti-vibration mounts. Check for looseness, cracking, or deterioration. Retorque fasteners to manufacturer spec. | Monthly | MEC |
| Check motor coupling alignment using a straightedge or dial indicator as applicable. Misalignment causes premature bearing and coupling wear. Correct and document if outside tolerance. | Quarterly | MEC |
Lubrication
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Lubricate motor bearings per manufacturer type, quantity, and interval. Do not over-grease. | Semi-Annually | MEC |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Lubricate motor bearings per manufacturer specifications — correct grease type, quantity, and method. Do not over-grease. For purge-type fittings, run motor briefly after greasing to expel excess. Record lubricant used and quantity. | Semi-Annually | MEC |
| Check bearing housing temperature using a contact thermometer or IR gun while motor is running under normal load. Bearing housing temp should not exceed ambient + 40°C or manufacturer limit. Record and compare to prior readings. | Semi-Annually | MEC |
| Listen and feel for bearing noise or roughness while motor is running. Investigate any grinding, screeching, or rumbling sounds. Elevated noise combined with elevated temperature is a strong indicator of bearing failure. | Semi-Annually | MEC |
Electrical Inspection
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect all electrical connections at the motor terminal box for looseness, corrosion, or heat discoloration. Clean and torque to spec as needed. | Annually | ELE |
| Check motor winding insulation resistance using a megger (500V DC for 480V motors). Record reading in MΩ. Values below 1 MΩ require follow-up. | Annually | ELE |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Perform insulation resistance (megger) test on all motor windings to ground. Use 500V DC for 480V motors; 1000V DC for 600V+. Record results in MΩ. Values below 1 MΩ require immediate follow-up — trending is more meaningful than any single reading. | Quarterly | ELE |
| Inspect all electrical connections at the motor terminal box — check for looseness, corrosion, heat discoloration, or damaged insulation. Re-torque to spec and clean as needed. Apply anti-oxidant compound on aluminum connections. | Semi-Annually | ELE |
| Perform a polarization index (PI) test on motor windings: take megger readings at 1 minute and 10 minutes. PI = 10-min value ÷ 1-min value. PI below 2.0 on older motors warrants further evaluation; below 1.0 indicates degraded insulation. | Annually | ELE |
| Perform a full internal inspection with motor de-energized and LOTO applied. Inspect stator windings for discoloration, cracking, moisture, or contamination. Blow out interior with dry compressed air. | Annually | ELE |
| Verify motor nameplate data is legible and matches plant records. Confirm frame size, HP, RPM, voltage, FLA, insulation class, and service factor are documented in the CMMS. | Annually | MEC |
| Review and trend all recorded data from this PM period — current draw, voltage, insulation resistance, bearing temperatures, and defect history. Identify deteriorating trends and adjust PM frequency or scope as warranted. | Annually | ELE |
Failure Modes This Checklist Targets
Bearing wear and overlubrication The two most common ways bearings fail — not enough lubrication and too much of it. Both show up as elevated temperature and noise before they show up as seized motors.
Winding insulation degradation Heat, moisture, and vibration break insulation down gradually over years. Megger testing catches the trend. Missing one reading doesn't end a motor. Missing the trend does.
Phase current imbalance Voltage imbalance as small as 1% causes current imbalance several times larger. It runs the motor hotter, shortens winding life, and shows up clearly on a clamp meter if anyone bothers to check all three phases.
Thermal failure from blocked ventilation A motor that can't shed heat runs hot. A motor that runs hot consistently has a shortened insulation life. Blocked cooling fins and vents are a fixable problem that most PM walkthroughs never look for.
Coupling and drive connection failures Misalignment, loose set screws, and degraded flexible elements show up mechanically before they destroy bearings. They're also among the easiest things to find and fix during a PM.
Loose electrical connections A connection that's worked loose corrodes. A connection that corrodes heats up. A connection that heats up fails — and usually takes surrounding components with it. Terminal box checks catch this before it becomes a rewire or a fire.
Related Checklists
AC Motor PM Checklist — Critical — for production-critical AC motors where the standard task set isn't enough.
DC Motor PM Checklist — Standard — same structure, different failure modes.
Motor Control Center PM Checklist — if you're PMing the motors, the MCC feeding them deserves the same attention.
