This is the only version of this checklist. Soft starters are typically treated as non-critical drive components — if yours is protecting a critical process, cross-reference the VFD Preventive Maintenance Checklist for expanded electrical condition monitoring tasks.
⚠️ 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.
Soft starters fail quietly. The thyristors degrade. Connections loosen. Cooling paths clog. None of it announces itself until the motor won't start, the bypass contactor welds shut, or the drive throws a fault code nobody bothered to investigate last quarter. PM on a soft starter takes less than an hour. The failure it prevents can take your process offline for days.
This checklist covers both field-level inspection tasks and a full reference task library for maintenance managers building or auditing soft starter PM programs.
For the broader motor drive maintenance context, start with electric motor preventive maintenance.
How to Use This Checklist
Record findings with specificity. "Checked connections" is not a finding. "Terminal block T2 showing light discoloration, torqued to spec, no active arcing" is a finding. The difference matters when you're looking at that unit a year from now and trying to decide whether the pattern is trending toward failure.
Trend your results over time. A single IR scan means nothing in isolation. Insulation resistance values only tell you something when you're watching them move. A soft starter that tested fine at the last four PMs and reads low today is a story — the number alone is not.
A bad finding looks like this: "Cooling fan checked — OK." A good finding looks like this: "Cooling fan operational. Slight bearing noise noted. No vibration. Logged for trending."
Field Checklist — Critical Tasks
Visual Inspection
| Task | Freq | Type |
|---|---|---|
| Inspect the soft starter enclosure for physical damage, corrosion, moisture intrusion, or loose/missing hardware. | Every PM | ELE |
| Verify cooling vents and fan openings are clear of dust, debris, and obstructions. Clean with compressed air if needed. | Every PM | ELE |
| Inspect input and output power cables for insulation damage, abrasion, or heat discoloration. | Every PM | ELE |
Electrical Inspection
| Task | Freq | Type |
|---|---|---|
| Check all power and control terminal connections for tightness, discoloration, or signs of arcing or overheating. | Every PM | ELE |
| Confirm status and fault indicator LEDs are functioning and no active fault codes are displayed. | Every PM | ELE |
| Verify bypass contactor (if equipped) operates correctly and contact tips show no excessive wear or pitting. | Quarterly | ELE |
| Inspect thyristor/SCR heat sinks for dust accumulation, corrosion, or loose mounting. Clean if needed. | Semi-Annually | ELE |
| Verify all parameter settings match the approved setup record. Document any deviations. | Annually | ELE |
Operational Checks
| Task | Freq | Type |
|---|---|---|
| Test soft start and soft stop ramp function by cycling the starter through a normal start/stop sequence. Confirm smooth acceleration and deceleration without excessive current spikes. | Quarterly | ELE |
| Check internal cooling fan (if equipped) for operation, unusual noise, and bearing condition. | Quarterly | ELE |
Reference Checklist — Full Task Library
Visual Inspection
| Task | Freq | Type |
|---|---|---|
| Inspect enclosure exterior for physical damage, corrosion, moisture entry, and integrity of gaskets and door seals. | Every PM | ELE |
| Verify cooling vents, airflow baffles, and internal fan(s) are unobstructed. Clean with dry compressed air. Confirm fan rotation direction if accessible. | Every PM | ELE |
| Inspect power cables (input/output) for insulation damage, heat discoloration, or jacket abrasion. Check strain reliefs and cable entry points. | Quarterly | ELE |
Electrical Inspection
| Task | Freq | Type |
|---|---|---|
| Inspect and torque-verify all input and output power terminal connections per manufacturer spec. Check for discoloration, arcing marks, or corrosion. | Every PM | ELE |
| Inspect control wiring terminals (24V, analog, digital I/O) for tightness and condition. Check for wire chafing near terminal blocks. | Every PM | ELE |
| Confirm no active faults or warnings are stored in the fault log. Document and investigate any historical faults since last PM. | Every PM | ELE |
| Verify LED indicators, display (if equipped), and HMI control interface are functional. | Every PM | ELE |
| Verify bypass contactor (if equipped): inspect contact tips for pitting and wear, check coil voltage, and confirm proper operational sequencing with the starter. | Quarterly | ELE |
| Check thyristor/SCR assemblies and heat sinks for dust buildup, corrosion, and secure mounting. Clean as required. | Semi-Annually | ELE |
| Perform insulation resistance (megger) test on input and output power conductors. Record values; investigate any reading below 1 MΩ. | Semi-Annually | ELE |
| Verify current limit, ramp-up time, ramp-down time, and motor full load amp (FLA) settings match the approved configuration record. | Semi-Annually | ELE |
| Check ground fault protection settings and verify proper operation of protection relay functions (if equipped: overload, phase loss, phase imbalance). | Semi-Annually | ELE |
| Review and document all parameter settings against the approved baseline. Update configuration record if any parameters have been adjusted. | Annually | ELE |
| Verify firmware/software version. Consult manufacturer recommendations for any available updates relevant to known issues. | Annually | ELE |
| Inspect and exercise all auxiliary contacts, I/O terminals, and interlocks associated with the soft starter control circuit. | Annually | ELE |
| Confirm spare thyristor/SCR modules or soft starter unit are available in inventory if this is a critical application. Flag if spares are absent. | Annually | ELE |
Operational Checks
| Task | Freq | Type |
|---|---|---|
| Cycle the soft starter through a controlled start/stop sequence under normal load. Monitor current draw during ramp-up and compare to baseline. Flag deviations >10%. | Quarterly | ELE |
Thermal Imaging Reference Points
| Task | Freq | Type |
|---|---|---|
| Perform thermographic (infrared) scan of the soft starter and associated power connections under load if IR camera is available. Document and flag any hot spots exceeding ambient +40°C. | Annually | ELE |
Failure Modes This Checklist Targets
Thyristor/SCR Degradation Thyristors are the core switching components in a soft starter. Heat cycling, overcurrent events, and contamination degrade them over time — the result is uneven phase control, erratic starting behavior, or complete starter failure with no prior warning if nobody was watching the heat sink temperature or trending insulation values.
Loose or Corroded Power Connections Soft starters carry significant inrush current on every start cycle. Connections that are slightly loose work themselves looser, develop resistance, generate heat, and eventually arc. The discoloration is always there before the failure — if someone looked.
Bypass Contactor Wear Many soft starters transfer motor load to a bypass contactor after the ramp is complete. Those contact tips wear. They pit. They weld. If the contactor fails mid-run, the motor sees the full soft starter control circuit under load — which it was never designed for continuously. Inspect contact condition every quarter.
Cooling Path Blockage Soft starters generate heat through the thyristors, and they depend entirely on airflow to dissipate it. A clogged vent or failed cooling fan doesn't cause an immediate fault. It causes the unit to run progressively hotter until it doesn't anymore.
Parameter Drift or Unauthorized Changes Ramp time, current limit, motor FLA settings — these get changed in the field for convenience and the configuration record never gets updated. Then someone troubleshoots a nuisance trip by adjusting the overload setting. Then the motor starts pulling 130% FLA on every start and nobody knows why. Verify parameters annually at minimum.
Insulation Breakdown on Power Conductors The cables feeding a soft starter are subject to heat, vibration, and the occasional overcurrent. Insulation breaks down. A megger test twice a year catches the trend before it becomes a ground fault on a live circuit.
