Field service content from Malloy Electric, with cross-references to the practitioner education resources at waywardleaders.com.

A VFD that has tripped is not telling you what is wrong. It is telling you what it noticed. The fault code reports the symptom that crossed the threshold and shut the drive down. The actual cause sits somewhere upstream of that symptom: a specification gap, an installation defect, a degraded component, a process change, an environmental shift, or a wiring problem that nobody has touched in five years. Effective troubleshooting starts with the recognition that the displayed fault is the end of the story, not the beginning.

This guide walks through how to triage and resolve VFD faults across the categories that account for the overwhelming majority of field calls our service teams respond to: overcurrent, overvoltage, undervoltage, overtemperature, ground fault, input phase loss, motor stall and overload, communication loss, encoder and feedback errors, and safety circuit interruptions. It also covers persistent and intermittent fault patterns, the diagnostic methodology our field engineers use to separate root cause from symptom, and the industry specific patterns that show up across mining, grain, water and wastewater, pulp and paper, food and beverage, oil and gas, HVAC, and material handling.

The scope is constrained to low voltage VFD applications (600 volts and below, occasionally up to 1000 volts), matching the motor and application scope covered in the companion pillars. Medium voltage drive troubleshooting has additional considerations and warrants separate treatment.

The content is drawn from more than seventy five years of motor and power transmission service experience at Malloy Electric, where our field service engineers and application engineers troubleshoot VFDs across eight Centers of Excellence in the northern plains and mountain west. Authorized VFD partner brands include ABB, Toshiba, and Fuji.

Why VFD Troubleshooting Methodology Matters

The expensive mistake in VFD troubleshooting is treating the fault code as the diagnosis. A drive that trips on overcurrent rarely has anything wrong with the drive itself. The drive is reporting that the current it measured exceeded a threshold. The cause might be an undersized drive, a control mode mismatch between variable torque and constant torque, an acceleration ramp that is too aggressive for the load inertia, a short circuit on the output, a ground fault, mechanical binding in the driven equipment, or a degraded motor winding. Replacing the drive because it tripped on overcurrent is the kind of decision that turns a one-day service call into a six-month recurring problem.

A correct diagnostic approach starts with three questions, in order. First: what does the fault history actually show? Most modern drives store the last 10 to 20 faults with timestamps, operating conditions at the time of trip, and the state of the drive when the fault occurred. That history is usually more diagnostic than the current fault display. Second: what changed? A drive that ran for three years without a fault and started tripping last week is telling a different story than a drive that has tripped every time someone tried to commission it. Third: is this a first-week fault, a steady-state fault, or a degradation fault?

First-week faults trace to specification errors, installation defects, or commissioning gaps. They appear during commissioning or in the first weeks of operation and usually do not resolve themselves. Steady-state faults appear after months or years of clean operation and usually trace to process changes, environmental changes, or a developing component issue elsewhere in the system. Degradation faults appear gradually, with rising frequency, and trace to component aging in the drive (capacitors, fans) or in the driven equipment (bearings, couplings, motor insulation).

Each pattern points to different root causes and different solutions. For a structured walkthrough of the fault categories and the diagnostic logic our field engineers apply, the practitioner reference is the VFD Troubleshooting Guide at waywardleaders.com/vfd-troubleshooting-guide.

Foundations: Reading the Drive and Building a Diagnostic Approach

Before working any specific fault, develop the inputs that make diagnosis possible.

Fault History and Operating Data

Every modern industrial VFD logs fault events with associated operating data. The minimum useful record includes the fault code, the timestamp, the running state at the time of trip (running, accelerating, decelerating, idle), the output frequency, the output current, the DC bus voltage, and the drive temperature. Higher-end drives capture significantly more, including the load reference, the speed feedback, the analog input values, and the digital input states.

Reading this log requires the drive keypad or the manufacturer software (ABB Drive Composer, equivalent tools for Toshiba and Fuji). Make this the first action on any troubleshooting call, before opening covers, before measuring anything, and before forming a hypothesis. The data the drive already captured is the cheapest information available.

Operating Context

The operating context at the time of the fault is often more important than the fault code itself. An overcurrent during a start attempt points one direction. An overcurrent during steady-state running points a different direction. An overcurrent that happens only on cold mornings points a third direction. The fault code tells you the threshold that was crossed; the context tells you why.

Useful context questions: was the drive running or starting when it tripped, what was the production process doing, what was the ambient temperature, what other equipment was operating, was there a thunderstorm or utility event, when was the last successful run, and what changed since.

Visual Inspection

Visual inspection before any electrical work catches a meaningful fraction of problems. Items to check: fan operation (heatsink fans and any cabinet fans), heatsink condition (clean or coated in process debris), enclosure interior temperature (warm but reasonable, or uncomfortably hot), evidence of moisture or corrosion, any displaced wiring or loose connections visible without removing covers, indicator LEDs in expected states, and the keypad responsive.

Safe Access

VFDs store dangerous energy in the DC bus capacitors even after power is removed. Standard practice is power down through lockout-tagout, wait for the documented bus discharge time (typically 5 to 15 minutes depending on drive size), and verify zero volts on the DC bus terminals before touching anything inside the drive. Never assume the bleed resistors worked correctly. The pre-energization safety framework and the verification practices that apply to both initial commissioning and subsequent service are covered in the VFD Installation Guide at waywardleaders.com/vfd-installation-guide.

Investigating Power-Side Faults

Power-side faults concern the rectifier, the DC bus, and the input wiring up to the drive terminals. They are common, they often masquerade as drive problems when the actual issue is upstream, and they usually leave a trail of evidence in the fault history.

Overcurrent (OC)

Overcurrent is the single most common VFD fault category. The drive measures output current exceeding the rated value (the specific threshold and trip time depend on the duty rating and overload curve). Root causes fall into five categories.

The first category is sizing and configuration error. An undersized drive on a constant torque load will trip on overcurrent during normal starting events. A drive set to variable torque (normal duty) mode on a constant torque application has reduced overload capability and will trip under conditions that a heavy duty configured drive would handle. Acceleration ramp too short for the load inertia produces overcurrent during the start. Verify drive sizing against motor full load amps and verify the duty mode matches the application. The conceptual framework for selecting between variable torque and constant torque duty ratings is covered in the VFD Selection and Sizing Guide at waywardleaders.com/vfd-sizing-guide.

The second category is output-side short circuit or ground fault. Insulation breakdown in the motor cable, the motor winding, or a connection in the motor terminal box can produce sustained or intermittent overcurrent. Megger the motor cable and motor winding (drive disconnected) to verify insulation resistance against the motor manufacturer specification. New installations should read in the hundreds of megohms; aged motors should still read above 10 megohms.

The third category is mechanical overload. Bearing degradation, coupling misalignment, gearbox binding, process plugging (in grain, water, or pulp applications), or a process change that increased actual load can drive output current up. The fault history will show whether the overcurrent is happening at start or after running, and the current level relative to the rated current at the time of trip is diagnostic.

The fourth category is motor problem. A motor with a developing winding fault (turn-to-turn shorts, phase imbalance) draws elevated current without showing other symptoms initially. A motor that has been rewound to incorrect specifications can present as overcurrent. Verify motor nameplate data against drive parameter settings.

The fifth category is parameter or control mode issue. A drive in vector control mode without a valid motor ID run can compute motor current incorrectly. A motor parameter mismatch (wrong nameplate frequency, wrong rated current, wrong rated speed) corrupts the drive's internal model and produces unexpected current behavior. Re-run the motor ID procedure with the motor uncoupled, then with the motor coupled if the application requires it.

Overvoltage (OV)

Overvoltage faults indicate the DC bus voltage exceeded the upper threshold. The DC bus voltage rises when the motor regenerates energy back to the drive faster than the drive can dissipate it.

Common causes include deceleration ramp too short for the load inertia (the motor regenerates during decel, raising the DC bus), absent or undersized braking resistor on applications with significant regenerative duty, supply voltage above nominal (a 480V drive on a 528V supply will trip at idle), supply voltage spikes from utility switching or upstream equipment, and a brake chopper failure in drives equipped with dynamic braking.

Resolution depends on cause. If decel ramp is the issue, extend the decel time or add a properly sized braking resistor. If supply voltage is high, verify the supply with a recording power quality meter; some installations require a buck transformer or a higher voltage rating drive. If supply spikes are the issue, address upstream (line reactor, isolation transformer, snubber). If the brake chopper or resistor is failed, repair or replace.

Undervoltage (UV)

Undervoltage faults indicate the DC bus voltage dropped below the lower threshold. The drive cannot continue operation without adequate DC bus voltage. Common causes include input phase loss (a blown input fuse or loose terminal disconnects one phase, dropping DC bus by approximately 13 percent and triggering UV under load), low supply voltage during heavy load periods, voltage sag from upstream events (other equipment starting, utility switching), and a degraded DC bus capacitor bank that cannot hold voltage under load.

Verify supply voltage with a recording instrument across at least a 24 hour period covering the operating cycle that produced the fault. Verify input fuses and input terminal tightness. If the supply is clean and the fuses are good, suspect the DC bus capacitors, particularly on drives more than 7 years old.

Ground Fault (GF)

Ground fault indicates current flowing through the equipment ground path exceeding a threshold. The drive monitors phase current imbalance, and persistent imbalance reads as current to ground. Common causes include actual insulation breakdown in the motor winding (most common in aged motors, in motors that have seen moisture, or in motors operating beyond their insulation class temperature rating), insulation breakdown in the motor cable (water ingress in conduit, damaged jacket, abrasion against metal edges), moisture inside the motor terminal box or the drive enclosure, and EMI being interpreted as ground fault on some drive types in installations with poor shield termination.

Megger the motor and motor cable separately to localize the fault. Verify the drive enclosure interior is dry. Verify VFD cable shield is terminated correctly at both ends.

Input Phase Loss

Input phase loss indicates one or more incoming phases are missing or significantly imbalanced. The drive cannot operate on two phases. Causes include blown input fuse, loose input terminal, failed input contactor or breaker, and utility phase loss on the supply.

Verify all three input phase voltages at the drive input terminals with a calibrated meter. Verify input fuses and breakers. If the supply is bad, the utility or upstream service needs attention.

Output Phase Loss

Output phase loss indicates the drive cannot detect current in one or more output phases. Causes include loose output terminal, broken motor lead, failed contactor in the motor circuit, and motor winding open circuit. Verify output wiring continuity (drive locked out) and verify motor winding resistance against nameplate.

Investigating Thermal Faults

Thermal faults concern the drive's ability to dissipate the heat it generates. They are sometimes diagnosed as drive failure when the actual problem is the installation environment.

Drive Overtemperature (OT)

Overtemperature indicates the drive heatsink or internal ambient temperature exceeded its limit. Common causes include cooling fan failure (heatsink fan or cabinet fan stopped or stalled), heatsink dust accumulation (particularly in grain, mining, aggregate, woodworking, and similar applications where airborne particulate accumulates on heatsink fins and reduces thermal transfer), enclosure ventilation blocked (drives in NEMA 12 enclosures depend on the enclosure designer's thermal calculation; obstructed vents or failed enclosure fans cause buildup), ambient temperature exceeding the rated value (drives derated for high ambient applications, drives installed in unconditioned enclosures in hot summer conditions), carrier frequency setting too high for the application (higher carrier produces smoother current and lower acoustic noise but increases drive heat losses; many overtemperature trips resolve by reducing carrier frequency from 8 or 12 kHz back to 2 or 4 kHz), and adjacent equipment generating heat in a shared enclosure.

The fault history usually shows whether the trip occurred during heavy load (real thermal load problem) or during light load (cooling system problem). Clean the heatsink as the first remedial action; this resolves a meaningful fraction of overtemperature complaints with no parts cost.

DC bus capacitor aging and cooling fan wear are both temperature driven and represent the two leading life-limiting mechanisms in industrial VFDs. Repeated thermal cycling accelerates both. For the complete maintenance framework including thermal baseline trending, capacitor health monitoring, and scheduled fan replacement, see the VFD Maintenance and Reliability Guide at waywardleaders.com/vfd-maintenance-guide.

Motor Overtemperature

If a motor thermal sensor (PTC, RTD, klixon) is wired to the drive, the drive can trip on motor overtemperature. Causes include actual motor overload, reduced cooling at low speed (TEFC motors lose cooling as the square of speed), drive carrier frequency too high (motor heating from harmonics), wrong motor parameters in the drive (the drive's thermal model runs incorrectly), and sensor or wiring fault on the motor thermal input.

For a deeper treatment of motor compatibility, bearing protection, and inverter duty insulation requirements that prevent the secondary thermal issues common to mismatched motor-drive pairings, see Before the First Fault: A Field Guide to VFD Installation and Reliability by Dr. Carl Lee Tolbert, PhD, CMRP.

Investigating Motor-Side Faults

Motor-side faults concern the cable between the drive and the motor, the motor itself, and the load.

Motor Stall

Motor stall indicates the motor is not following the speed command. The drive expects the motor at a commanded speed (sensorless vector estimation or encoder feedback) and reads zero or below threshold. Causes include mechanical jam in the driven equipment, brake not released (mechanical brake not energized when drive commands run), load exceeding motor capability (process change, plugging condition), failed motor ID run leading to wrong parameters in the drive's vector model, and wiring error on the encoder for closed loop vector control.

Verify the load can rotate freely (manually, with the drive locked out). Verify any mechanical brake is releasing. Re-run motor ID with the motor uncoupled.

Excessive Vibration or Audible Issues

A motor that vibrates excessively or produces unusual audible noise on a VFD usually has one of three issues. Resonance: a frequency in the operating range matches a mechanical natural frequency of the motor, the driven equipment, or the foundation. Solution: configure skip frequencies in the drive to avoid the resonance band. Carrier frequency related noise: a carrier frequency too low (1 to 2 kHz) produces an audible whine at the carrier; increasing carrier above the audible range (or modifying carrier modulation pattern, where supported) resolves the whine. Output filter or cable issue: a missing or wrong dV/dt filter on a long cable run can produce reflected wave noise audible at the motor.

Motor Insulation Failure

A motor that runs cleanly for a period and then trips on ground fault or overcurrent, with megger readings showing degraded insulation, has an insulation failure. On VFD applications, several factors accelerate insulation degradation beyond what a line-fed motor would see.

Reflected wave overvoltage produces voltage peaks at the motor terminals that exceed the supply voltage, particularly on long cable runs. Standard motor insulation rated for sinusoidal supply is not designed for these stresses; inverter duty rated motors per NEMA MG1 Part 31 are. A standard motor on a VFD with a 200 foot cable will see insulation degradation that the same motor on a line supply would not.

PWM common mode voltage produces capacitively coupled currents through bearings. The discharge erodes bearing race surfaces in an electrical discharge machining action, producing characteristic fluting. Insulated bearings or shaft grounding rings (or both) interrupt the current path.

Common mode voltage also stresses motor winding insulation at the neutral point, contributing to neutral-end failures that line-fed motors rarely see.

When a motor fails after a period of clean VFD operation, the root cause analysis should determine whether the failure was a normal end-of-life event or a VFD-driven failure that better motor specification (inverter duty, bearing protection, output filtering) would prevent.

Cable Issues

Cable issues between drive and motor produce a spectrum of fault patterns. Long cable runs with standard building wire instead of VFD-rated cable produce reflected wave issues at the motor and EMI on adjacent circuits. Damaged cable jackets allow moisture ingress, leading to gradual insulation degradation and eventually ground fault. Improper shield termination (shield grounded at one end only, or not grounded at all) defeats the EMI protection the shielded cable was supposed to provide.

The complete installation framework, including cable specification, shield termination at both ends, panel layout, and the verification practices that prevent these issues from showing up in the first place, is covered in Before the First Fault by Dr. Carl Lee Tolbert.

Investigating Control-Side Faults

Control-side faults concern the wiring and signals between the drive and the plant control system, the operator interface, and the safety system.

Communication Loss

Communication loss between the drive and the plant control system manifests as the drive going to a defined fail-safe state (depending on configuration, this may be coast, hold last speed, or trip on communication fault). Causes include cable damage or loose connectors on the communication wiring, missing or wrong network terminator (particularly on RS-485 networks like Modbus RTU and DeviceNet), address conflict on the network, EMI on the communication cable from nearby VFD output cables, and controller or master device fault.

Verify physical cable continuity. Verify network terminators present and correct. Verify drive node address against the control system configuration. Check for VFD output cables running parallel to communication cables without separation; communication cables should be physically separated by at least 12 inches from VFD output cables, or routed in separate grounded conduit.

Encoder or Feedback Faults

Drives in closed loop vector control depend on encoder feedback. An encoder fault manifests as either an encoder loss alarm or as erratic speed control. Causes include encoder cable damage, encoder mechanical coupling failure (encoder no longer turning with the shaft), encoder failure, and EMI on encoder cable from VFD output.

Verify encoder cable continuity and shield termination. Verify encoder coupling. Check encoder output with a scope or with the drive's diagnostic display while manually rotating the motor.

External Reference Signal Issues

Drives controlled by an analog input (4-20 mA or 0-10 V from a process controller) can present apparent control issues that trace to the reference signal. A noisy or drifting reference produces erratic speed. A signal that reads zero when the process expects it to be non-zero produces unexpected stops. Verify the analog signal at the drive terminal with a calibrated meter, not at the source.

Safe Torque Off (STO) and Safety Circuit Faults

Modern drives integrate Safe Torque Off as a safety function. Loss of STO inputs produces a defined safe state and a fault. Causes include actual e-stop or safety circuit activation (legitimate), STO wiring fault (broken wire, loose connection), STO interlock from a safety PLC that is not receiving the expected reset, and safety relay failure.

STO is a safety function. Troubleshoot in conjunction with the plant safety system documentation and do not bypass.

Persistent and Intermittent Faults

A drive that trips reliably under a specific condition is easier to troubleshoot than a drive that trips occasionally with no obvious pattern. Intermittent faults require different diagnostic approaches.

Pattern Hunting

Capture multiple fault events and look for the common factor. Time of day (suggesting thermal, supply voltage, or other equipment correlation). Day of week (suggesting production schedule correlation). Weather (suggesting moisture, ambient temperature, or utility correlation). Production state (suggesting process change correlation). Most intermittent faults have a pattern, and the pattern points to the cause.

Recording Power Quality

Many intermittent faults that show up as overvoltage, undervoltage, or overcurrent trace to supply quality issues that are invisible without a recording instrument. A power quality monitor logging supply voltage, current, and harmonic content across a representative operating period catches issues that a snapshot measurement misses.

Recording Drive Data

Most modern drives support logging output (analog out, communication, or internal logging) of operating parameters. Capturing output current, DC bus voltage, output frequency, and drive temperature across the operating cycle that produces the fault often reveals the precursor pattern that leads to the trip.

Environmental Correlation

Intermittent overtemperature trips that correlate with summer afternoons usually indicate a marginal cooling situation that crosses the threshold under specific ambient conditions. Intermittent ground faults that correlate with rain or high humidity indicate moisture ingress somewhere in the cable or motor system. Intermittent overvoltage trips that correlate with utility transients indicate a need for input protection (line reactor, isolation transformer, surge protection).

Industry-Specific Troubleshooting Considerations

The diagnostic principles above apply universally. Specific patterns vary by industry.

Mining and Aggregate

Heavy duty drives in mining and aggregate environments commonly trip on overcurrent from shock loading (crusher entry, conveyor restart under load) and on overtemperature from dust accumulation on heatsinks. Routine heatsink cleaning, properly sized overload margin, and active front end on the largest drives for regenerative duty address most chronic complaints.

Grain Handling and Agriculture

Seasonal grain handling applications often present trips after months of idle time. The pattern is usually moisture ingress during idle months leading to ground fault or insulation issues on first startup, dust accumulation on heatsinks producing overtemperature, and rodent damage to wiring producing phase loss or ground fault. Pre-season inspection and megger testing prevent most of these.

Water and Wastewater

Water and wastewater drives running continuous duty 24/7 on variable torque pump applications usually run cleanly for years and then begin showing capacitor aging symptoms (occasional undervoltage during heavy load periods, slow drift in measured DC bus voltage). Predictive replacement at 7 to 10 years prevents unexpected failures.

Pulp and Paper

Coordinated multi-drive systems on paper machines with DC bus sharing present unique troubleshooting patterns. A fault on one drive can affect the entire bus. Diagnostic approach requires understanding the bus topology, the load sharing scheme, and the inter-drive communication.

Food and Beverage

Washdown environments produce moisture-related faults on drives located in process areas, even with NEMA 4X enclosures, particularly around door seals and conduit entries that degrade over time. Drives located in protected control rooms with cable running to washdown motor locations are more reliable long-term.

Oil and Gas

Remote drive locations with environmental conditioning units present trips when the conditioning system itself fails. Verify the drive shelter HVAC before assuming a drive problem. Long cable runs to remote motors require dV/dt filtering; missing or undersized output filters produce motor insulation faults.

HVAC

HVAC drives in mechanical rooms operate cleanly for years and then begin showing fan failures and capacitor aging in parallel with the broader 7 to 15 year drive lifecycle. Sleep/wake function configuration errors can produce apparent control faults that resolve with parameter adjustment.

Material Handling and Conveying

Conveyor restart under load is the dominant fault driver. Verify drive duty mode is heavy duty (constant torque), verify acceleration ramp is appropriate for loaded restart, and verify any required brake release timing. Regenerative duty on downhill conveyors requires properly sized braking provision; trips on overvoltage usually trace to undersized or absent braking resistors.

How Malloy Field Services Supports VFD Troubleshooting

Malloy Electric field service engineers respond to VFD troubleshooting calls across the customer base. The typical engagement starts with a phone consultation to capture the fault history, the operating context, and the recent change history, then proceeds to onsite work when the issue cannot be resolved remotely.

Onsite work covers fault history extraction using manufacturer software (ABB Drive Composer and equivalent tools), full system diagnostic including motor megger testing, cable verification, supply quality measurement where indicated, drive parameter review against application requirements, and corrective action including parameter adjustment, component replacement, motor work, or upstream service work as the diagnosis warrants. The commissioning practices that establish the baseline against which subsequent fault investigations are measured are covered in the VFD Commissioning Guide at waywardleaders.com/vfd-commissioning-guide.

For drives that require component-level repair, our service shops handle DC bus capacitor replacement, cooling fan replacement, gate driver and IGBT module replacement, control board replacement, and full drive rebuilds across our authorized partner brands ABB, Toshiba, and Fuji. For installations where root cause analysis points to specification or installation issues, our application engineers work with the customer to develop the corrective specification and execute the corrective work, including motor compatibility upgrades, cable replacement, filter addition, and panel modifications through our UL508A and UL698A panel shop.

The objective in every case is to fix the actual cause, not just clear the displayed fault. Building and maintaining the in-house diagnostic competency that allows your team to handle the routine issues internally and escalate the right problems to us at the right time is covered in the VFD Training.

About Malloy Electric

Malloy Electric has provided motor and power transmission services to industrial customers since 1945. Our VFD service line spans application engineering, specification support, new drive sourcing, panel integration through our custom UL508A and UL698A control panel shop, field installation and commissioning, field troubleshooting and repair, predictive maintenance, and engineered upgrades including the comprehensive modernization of legacy multidrive systems. We serve customers across the northern plains and mountain west from eight Centers of Excellence in Sioux Falls, Dakota Dunes, Fargo, Mandan, Omaha, Cedar Rapids, Gillette, and Billings. Authorized VFD partner brands include ABB, Toshiba, and Fuji.

For practitioners who want to go deeper, Before the First Fault: A Field Guide to VFD Installation and Reliability by Dr. Carl Lee Tolbert, PhD, CMRP covers the full VFD lifecycle from specification through long term reliability and is available at waywardleaders.com/book.

We Service What We Sell. We Solve Problems.

Frequently Asked Questions About VFD Troubleshooting

What is the most common cause of a VFD overcurrent fault?

The most common single cause across all applications is a control mode mismatch between the duty rating of the drive configuration and the actual application. A drive set to variable torque (normal duty) mode on a constant torque application has reduced overload capability and will trip on overcurrent during normal starting events. Verify the drive is configured for the correct duty rating, then verify drive sizing against motor full load amps, then verify acceleration ramp is appropriate for the load inertia. After those three checks, investigate output-side faults, mechanical overload, motor condition, and motor parameter accuracy.

Why does my VFD keep tripping on overtemperature even after I cleaned the heatsink?

If a clean heatsink is still producing overtemperature trips, the next investigations are cooling fan operation (a fan that spins slowly or stalls intermittently produces this pattern), ambient temperature at the drive location (drives in unconditioned enclosures in hot conditions trip even when clean), carrier frequency setting (higher carrier produces more drive heat), and enclosure ventilation (sealed enclosures need either adequate thermal calculation or active cooling). If all of those check out, suspect a degraded thermal interface between drive and heatsink, particularly on drives more than 10 years old.

What causes a VFD ground fault when nothing has changed?

Ground faults that appear without a known change usually trace to gradual moisture ingress somewhere in the motor or cable system, gradual insulation degradation in an aging motor, or rodent or environmental damage to cable that has progressed to the failure point. Megger the motor and the motor cable separately to localize the fault. Inspect the cable run for damage. If the motor is older than 15 years and the cable is sound, plan for motor rewind or replacement.

My drive shows undervoltage but my incoming voltage is fine. What else could it be?

After verifying supply voltage with a recording instrument across a full operating cycle, the remaining causes are input phase loss (a blown fuse or loose terminal on one phase drops DC bus by roughly 13 percent and triggers undervoltage under load even though all three input voltages measure fine when nothing is running), degraded DC bus capacitors (drives more than 7 to 10 years old can show this pattern), and voltage sag from upstream events that a steady-state measurement misses. A recording power quality monitor across the operating cycle that produces the fault usually identifies the cause.

How do I troubleshoot an intermittent VFD fault?

Intermittent faults require pattern hunting. Capture multiple fault events from the drive's fault history with timestamps and operating data. Look for correlation with time of day, day of week, weather, production state, or other equipment operation. Most intermittent faults have a pattern, and the pattern points to the cause. If the pattern is not obvious from the fault history, install recording instruments (power quality monitor on the supply, drive data logging on output current and DC bus) across a representative operating period.

When should I call for field service instead of troubleshooting in-house?

Call for field service when the issue persists after the basic checks, when component-level repair is required, when the diagnosis points to motor work or cable work beyond your team's capability, when safety-related systems are involved (STO faults, safety circuit issues), or when the fault is affecting a production-critical asset and the time cost of internal diagnosis exceeds the cost of bringing in a specialist. The fastest resolution is usually a phone consultation first to scope the issue, then onsite work if needed.

Can I just swap the drive to see if that fixes it?

Swapping a drive is the most expensive way to diagnose a non-drive problem. Most VFD faults trace to causes outside the drive itself: undersized specification, installation defects, motor degradation, cable issues, supply problems, environmental conditions, or process changes. A drive swap that does not address the actual root cause produces a new drive that trips on the same fault inside hours, days, or weeks. Diagnose first, then act on the diagnosis.

How long should troubleshooting take before I escalate?

A correctly trained technician with the drive manufacturer software should be able to extract the fault history, identify the fault category, and complete the first round of diagnostic checks (visual inspection, supply voltage verification, motor megger) within 60 to 90 minutes. If the fault is not understood after that initial investigation, escalation to a field service engineer or to the drive manufacturer technical support is usually faster than continued internal investigation. Time spent troubleshooting the wrong cause is more expensive than the field service call.

What information should I have ready when I call for field service?

The drive manufacturer, model, and serial number; the motor manufacturer, frame size, and nameplate data; the application (what the drive is running); the fault code displayed and the fault history (at least the last 5 events with timestamps if accessible); a description of when the fault started, what changed recently, and any pattern in when it occurs; the supply voltage and any known power quality issues at the facility; and the criticality of the asset to current production. Having this information ready reduces the time to resolution significantly.

This guide was prepared by the field service and application engineering teams at Malloy Electric in cooperation with waywardleaders.com. For specific VFD troubleshooting support, field service dispatch, or component-level repair, contact your local Malloy Center of Excellence. Visit malloyelectric.com for service line information across motor repair, gearbox and power transmission, VFDs, custom control panels, field services, and predictive maintenance.