PLC Motor and Drive Troubleshooting: A Layer-by-Layer Method

Direct answer

Troubleshoot a PLC-controlled motor by finding the first point where commanded state and observed state disagree. Preserve the PLC status, drive or overload fault, safety state and process conditions before resetting anything. Then follow the energy and control chain in order: start request, PLC permissives, safety/STO status, PLC output or network command, relay/contactor or VFD input, motor power/current, motor and cable condition, mechanical load, and run/speed feedback. Test one boundary at a time and predict what each result will prove.

If the PLC output is on but the motor is stopped, the PLC is not automatically faulty. The break may be an open safety channel, missing interposing-relay voltage, failed contactor coil, tripped overload, VFD not-ready state, wrong command source, lost network control word, output-stage fault, motor/cable problem or seized load. Conversely, a running motor does not prove the sequence is healthy if the PLC never receives auxiliary, drive-running or speed feedback.

Motor and drive systems contain hazardous electrical energy, stored DC-bus charge and mechanical energy. A stop command, PLC output-off bit, HMI button or safety input is not an energy-isolating device. Use the site's documented energy-control procedure, qualified personnel, correctly rated instruments and manufacturer discharge/verification instructions. Never insulation-test through a connected VFD, and never bypass a safety function to “see if it runs.”

Define the motor-control boundaries

A motor “circuit” is several coupled systems. The PLC executes sequence logic. A safety system determines whether torque may be produced. A contactor starter or variable-frequency drive controls power. The motor converts electrical input to torque. A coupling, brake, gearbox, pump, fan or conveyor turns that torque into work. Feedback tells the PLC what actually happened.

The seven-boundary model

Boundary	Command or energy entering	Expected evidence leaving	Common break
operator/sequence request	start mode, recipe or automatic step	latched/active run request	wrong mode, stale request, sequence not at step
PLC permissive logic	request plus process interlocks	run command bit	permissive false, timer, fault latch, hidden inhibit
safety system	closed guards, reset, valid channels	safety output/STO enabled state	guard open, channel discrepancy, safety fault, reset condition
PLC-to-starter/drive interface	output voltage or network control word	relay/contactor pickup or drive command accepted	output fault, open circuit, wrong source, lost connection
power controller	line power plus accepted command	motor current/torque-producing output	overload trip, drive fault, contactor pole, input/DC-link/output fault
motor and cable	three-phase electrical output	shaft torque and plausible current	phase/ground fault, winding, connection, insulation, thermal state
mechanical process and feedback	shaft torque	movement, flow/pressure and run/speed feedback	seized load, brake, coupling, sensor or feedback mapping

This model prevents two common errors: treating a PLC bit as physical proof and treating a drive fault description as a root-cause verdict. Each indication is evidence for one boundary. Correlate it with evidence on both sides.

Preserve the fault before resetting

Resetting a drive or overload can erase the sequence of events, release a fault latch and allow an unexpected restart. First capture a consistent evidence packet. Use synchronized timestamps where possible; a PLC trend, drive clock and HMI alarm list that disagree by minutes can reverse the apparent cause-and-effect order.

Minimum evidence packet

Evidence	Capture	Diagnostic value
symptom and time	stopped, slow, reverse, noisy, hot, tripped; first occurrence and duration	selects workflow and correlation window
operating context	mode, product, speed, load, recent maintenance, ambient state	exposes condition-dependent faults
PLC state	request, permissives, command, feedback, fault and sequence step	separates logic decision from physical response
safety state	safe-input channels, reset, output/STO status, diagnostic code	identifies valid safety inhibition without bypassing it
drive/starter state	ready, running, direction, reference, actual, warning, fault history, overload state	locates interface or power-controller rejection
electrical evidence	input conditions, output current, imbalance, DC-link/output measurements when authorized	distinguishes supply, controller and load paths
mechanical evidence	shaft/load state, brake, coupling, jam, pressure/flow, vibration and temperature	separates electrical current demand from process resistance
identity/version	PLC program revision, drive model/firmware, motor nameplate, parameter backup	makes manuals, limits and prior baselines applicable

Save raw values rather than only colored HMI indicators. A green “motor available” object may combine several tags and delay states. The underlying ready, faulted, interlock, safety and communication bits are reproducible evidence.

Motor does not start

Use a left-to-right trace. The objective is not to “get the output on”; it is to identify the first missing prerequisite or response without defeating protection.

Ordered no-start workflow

1. Confirm the requested mode and start condition are valid for the process.
2. Observe the PLC run request, every permissive and the final command in the same scan window.
3. Confirm the safety function reports its intended enabled state and no channel discrepancy.
4. At the interface, prove the physical output voltage or network command and the destination's accepted-command status.
5. For a starter, trace control voltage through relay, stop chain, overload contact and contactor coil; then check main-pole and auxiliary response.
6. For a VFD, check ready/not-ready, command source, speed source, STO state, active fault/warning and actual command/reference.
7. If power is delivered, compare motor currents and verify mechanical movement or load resistance using approved methods.
8. Confirm feedback changes within the designed start timeout.

Observation	Most useful interpretation	Next check
no PLC request	upstream mode, sequence or HMI issue	request owner, mode arbitration and sequence step
request on, final command off	permissive/interlock logic is intentionally blocking	first false condition and its physical source
PLC command on, physical output off	output module/channel, wiring or load-current problem	module status, terminal voltage and channel diagnostic
output on, contactor coil not energized	interposing relay, stop chain, overload contact or control wiring open	voltage across each boundary under command
VFD sees no command	source selection, network mapping or digital-input circuit	command source and accepted control word/input state
VFD commanded but not ready	safety/STO, active fault, enable or internal state	drive status/fault manual and safety diagnostics
controller produces output, motor has no current	open output path, contactor pole, cable or motor connection	isolated continuity/connection tests per procedure
motor current rises but shaft does not move	locked rotor, brake, seized motor or mechanical load	stop; isolate energy and inspect mechanical system

Motor starts and then trips

The time from command to trip is evidence. An immediate high-current or ground fault suggests a different branch from an overload after several minutes, an overvoltage during deceleration or an intermittent network loss. Record elapsed time, command/reference, actual speed, current and process load.

Trip-timing matrix

Timing or event	Candidate families	Discriminating evidence
immediately on start command	short/ground fault, wrong motor connection, seized load, very short ramp, excessive boost, contactor/output fault	exact trip code, current rise, isolated motor/cable test, shaft/load state
during acceleration	current limit, overload, insufficient torque, brake not releasing, load too high, acceleration too short	current/torque trend, ramp, brake feedback and mechanical load
at steady load	sustained overload, cooling loss, phase/current imbalance, process restriction	current by phase, load baseline, temperature, fan/airflow and process values
during deceleration	regenerative DC-bus overvoltage, braking configuration or overhauling load	DC-bus trend, deceleration ramp, brake/chopper/resistor status
only at low speed	inadequate cooling, torque/boost issue, mechanical resonance or minimum-speed process problem	motor thermal method, current/torque, speed and vibration trend
after a repeatable time	thermal model, timer/interlock, lubrication or process accumulation	fault history, PLC timer, motor/drive thermal state and load trend
random under vibration/heat	loose connection, encoder/feedback, fan, contamination, network or insulation deterioration	event trend plus environmental correlation

Rockwell's PowerFlex 525 product information demonstrates why the raw code and context matter: separate faults exist for power loss, over/undervoltage, stall, motor overload, heatsink temperature, hardware/software overcurrent, ground fault, phase loss, communication loss and encoder loss. Other manufacturers use different code numbers and thresholds; use the exact installed manual.

Direct-on-line starter path

A direct-on-line or reversing starter has a low-voltage control path and a three-phase power path. The PLC usually commands an interposing relay or contactor coil. An overload auxiliary contact and contactor auxiliary feedback return state. The main poles switch motor power. Proving only one path is insufficient.

Trace control and power separately

Test point	Commanded-off expectation	Commanded-on expectation	Fault indicated by mismatch
PLC logic output tag	false	true after permissives	sequence/logic owner if false unexpectedly
output terminal to control common	de-energized per design	rated control signal within documented tolerance	channel/module/supply/wiring issue
interposing relay contact	open	closed with correct contact voltage drop	relay coil/contact or socket issue
overload NC auxiliary	closed if healthy	remains closed during normal start	overload trip, wrong contact or wiring
contactor coil	de-energized	rated coil supply present	upstream control-path break or wrong coil rating
contactor main poles	open	closed with balanced supply to motor	worn/failed pole, mechanical contactor fault
auxiliary feedback	off	on within pickup time	auxiliary contact/mapping or contactor response
motor current	zero	plausible and balanced for state/load	power path, motor or load problem

Measure a coil across its terminals, not merely from one side to an assumed ground. The correct expected voltage is the coil nameplate/control-circuit value and manufacturer tolerance. A high-impedance meter can display coupled or leakage voltage that collapses under load; interpret it using the approved circuit diagram and suitable test method.

VFD-controlled motor path

A VFD introduces command-source selection, speed-reference selection, enable/STO inputs, rectifier and DC link, inverter output, motor-model parameters, acceleration/deceleration profiles, current/torque limits, network state and internal fault history. Divide the system into input, conversion/control and output/load zones.

Verify state before energized measurements

Start with the keypad/software diagnostics: ready, active command source, accepted run/direction, reference source and value, actual frequency/speed, output current, DC bus, active limit, warnings, fault queue and network status. These non-invasive values often identify the boundary before a qualified person performs energized measurements.

VFD zone	Non-invasive evidence first	Authorized measurement concept	Frequent error
input supply	line/phase loss history, input/DC-bus diagnostic, fault timestamp	phase-to-phase supply and power quality with appropriately rated equipment	blaming the motor for upstream dips or imbalance
DC link	displayed bus and over/undervoltage history	only at designated terminals using discharge and live-work procedures	assuming the display/off LED proves absence of voltage
control	ready, source, command, reference, limit and STO status	digital/analog/network signal at defined reference	command exists in PLC but drive listens to another source
inverter output	output frequency/current and fault diagnostics	PWM-capable meter or motor-drive analyzer as manufacturer requires	interpreting PWM with an unsuitable general-purpose meter
motor cable	ground/phase fault and reflected-wave context	isolated cable/motor tests after disconnecting drive	applying insulation-test voltage to drive electronics
motor/load	current, torque estimate, speed feedback and thermal model	current, vibration, temperature and mechanical checks	clearing overload without finding excess load/cooling cause

Fluke's output-measurement guidance explicitly separates DC-bus, output voltage/current/frequency, unbalance, volts-per-hertz and output reflections. It also calls for instruments suited to PWM drive output. An ordinary meter result that looks unlike line-frequency sine-wave voltage does not by itself prove a failed inverter.

PLC command, permissive and feedback logic

Online monitoring must distinguish desired state, issued command and confirmed state. A well-instrumented motor object exposes the reason it is inhibited, the owner of the command, transition timing and feedback disagreement. Forcing a bit can make the display appear correct while the physical chain remains unsafe or invalid.

Logic evidence checklist

PLC signal	Diagnostic question	Better implementation evidence
start request	which source requested run and in what mode	source enum plus timestamp, not one merged Boolean
permissive summary	which exact permissive is false	bitfield/array and first-out capture
run command	did logic issue forward/reverse/run	command state with sequence step and owner
output/network health	could the controller deliver that command	module channel diagnostic or connection status
run feedback	did contactor/drive acknowledge running	raw feedback plus debounced state and transition time
speed feedback	did motion reach the commanded band	reference, actual, tolerance and timeout
trip/fault	which device originated the stop	latched first-out source plus raw device code
restart state	can the motor restart automatically after recovery	documented restart mode and operator notification

If a command chatters, examine the rung and input transitions at a sampling rate that can see the event. HMI history polling may miss a one-scan permissive drop. Use a PLC trace, sequence-of-events function or high-speed trend appropriate to the controller, while keeping trace load and retention controlled.

Safety and STO boundary

Safe Torque Off normally prevents the drive from producing motor torque; it is not a general energy-isolation method, and it does not necessarily remove hazardous voltage from the drive or motor terminals. The installed safety design, risk assessment, drive safety manual and applicable standards define its role.

Diagnose without bypassing protection

Read both safety input channels, reset/acknowledge conditions, discrepancy time, safety-controller diagnostics, safety output state and drive STO status. Compare the raw channels with the safety function's validated truth table. A guard can be physically closed while one contact or channel remains open. Do not bridge STO terminals, force safety outputs or change a safety signature as a troubleshooting shortcut.

Safety observation	Candidate cause	Approved evidence path
both channels open	open guard/E-stop, missing supply or series-circuit break	device state and channel-by-channel wiring diagram
channels disagree	contact timing, broken conductor, misalignment or channel fault	safety diagnostic code and physical inspection under procedure
inputs valid but reset denied	reset edge, monitored reset, other zone or latched fault	function block status and validated reset sequence
safety output valid, drive STO active	downstream safety wiring, drive terminals/configuration or status mapping	safety output terminals and drive safety diagnostics
motor stopped but hazardous power present	expected STO behavior may be misunderstood	do not touch; isolate energy using formal procedure

OSHA identifies push buttons, selector switches and control-circuit devices as not being energy-isolating devices for hazardous-energy control. That principle is crucial when a PLC screen shows “off.”

Supply, DC bus and drive-output faults

Begin drive electrical diagnosis at the input, then move through the DC link to the output only when evidence and authorization require it. Fluke's input-side guidance recommends checking supply voltage, current and frequency because upstream capacity, imbalance, dips or distortion can affect drive operation and life.

Symptom-to-zone map

Symptom/fault family	Probable zone	Evidence before replacement
undervoltage/power loss	upstream supply, fuse/contactor, connection or severe load dip	timestamped input and bus trend, input protection and other-load correlation
overvoltage at deceleration	regenerated load energy, decel ramp or braking system	bus trend, load direction/inertia, brake chopper/resistor state and configured ramp
input phase loss/imbalance	feeder, fuse, contactor pole or connection	phase-to-phase values and current under comparable load
output phase loss	output connection, cable, motor winding or drive output	drive code, current pattern and isolated motor/cable evidence
ground fault	motor cable, motor insulation, junction or output stage	disconnected cable/motor tests per vendor method; never megger through drive
overcurrent/stall	jam/brake, acceleration, motor data, cable/motor fault or tuning	current/torque trend, mechanical state, ramp and motor parameter verification
drive/heatsink overtemperature	ambient, airflow, contamination, fan or load	temperature history, fan and enclosure/filter condition, output current
communication loss	network path, adapter, PLC connection or timeout/action configuration	connection diagnostics, switch/cable counters and drive comm-loss history

A fault-clear that temporarily restores operation is not corrective evidence. If the same condition returns, preserve its timestamp and trend, compare to operating context and correct the cause before authorizing normal service.

Motor, cable and insulation checks

Only test after applying the equipment's isolation procedure, proving absence of hazardous energy and disconnecting sensitive electronics as required. Record motor nameplate, connection, cable length/type, drive carrier/output context, ambient temperature and prior readings. A resistance value without test voltage, temperature, duration and connection is poor evidence.

De-energized motor evidence

Check	What it can reveal	Important limitation
visual/connection inspection	loose lugs, heat damage, contamination, wrong star/delta link, damaged cable	torque and configuration follow manufacturer procedure
phase-to-phase winding resistance	gross imbalance, open joint or connection problem	very low values require appropriate instrument and lead compensation
insulation resistance to frame/PE	insulation contamination or deterioration	disconnect drive/sensors; test voltage, duration, temperature and limits are motor-specific
phase-to-phase insulation test where specified	winding/cable insulation issues	do not invent procedure; coordinate with motor manufacturer for sensitive designs
cable-only insulation/continuity	separates feeder cable from motor winding	disconnect both ends and protect other connected devices
rotation/mechanical feel under isolation	seized bearing/load, brake or coupling issue	stored mechanical energy and load movement still require control
temperature-corrected trend	deterioration relative to baseline	one generic pass number cannot replace product/site criteria

Siemens motor instructions warn that high-voltage insulation tests can damage insulation when the procedure is not appropriate, require compliant test equipment and define motor-specific limits and connections. Apply those instructions only to the named motor family. They demonstrate why a value copied from a different motor manual is not a universal acceptance criterion.

ABB guidance for motors on PWM drives adds another system concern: the converter output can impose insulation stress, while cable arrangement, grounding, filtering and motor design affect reflected voltage and bearing-current risk. A replacement motor of the same power rating is not automatically equivalent for inverter duty, cable length and switching conditions.

Separate the motor from the mechanical load

Electrical current reflects the torque demanded by the mechanical system. A healthy motor can trip because a conveyor is jammed, a pump valve/process condition raises load, a brake remains applied, a bearing fails or a gearbox binds. Conversely, a damaged motor can produce abnormal current with an unloaded shaft.

Electrical-versus-mechanical evidence

Evidence pattern	More consistent with	Confirmation approach
balanced high current with high process load	genuine mechanical/process overload	compare pressure/flow/tension and mechanical inspection to baseline
high current with zero speed	locked rotor, brake or severe jam	stop immediately; isolate and inspect before another start
one phase current abnormal	supply/output connection, winding or measurement issue	phase comparison at defined points and isolated tests
normal current but low process output	coupling slip/break, pump/process issue or feedback error	shaft/load speed and process evidence
vibration at particular speed	resonance, alignment, bearing or mechanical looseness	controlled speed/vibration analysis by qualified personnel
rising bearing temperature over time	lubrication, alignment, load or bearing damage	temperature/vibration trend and mechanical inspection
drive current estimate disagrees with external suitable instrument	scaling, sensor, PWM measurement or drive issue	verify instrument method and drive diagnostics

Decoupling a load can be useful only if the manufacturer and site authorize it and the motor, key, coupling, brake and machine are secured for an uncoupled test. Removing a guard and running exposed rotating parts is not a troubleshooting shortcut.

Speed, direction and feedback errors

A motor that runs at the wrong speed may have the correct run command but the wrong reference owner, scale, limit or feedback interpretation. Direction errors can come from command logic, phase sequence in a starter system, drive direction configuration or feedback polarity. Establish the intended direction and safe test condition before changing leads or parameters.

Command-versus-actual matrix

Command evidence	Actual evidence	Likely branch
zero reference, motor turns	minimum speed, jog, local mode, stale network command or mechanical back-drive	source/mode, minimum limits and local controls
nonzero reference, actual remains zero	not enabled, STO, brake, current limit, load or feedback failure	ready/status/limit plus shaft observation
reference correct, output frequency capped	max-frequency limit, current/torque limit or process override	active limit and parameter ownership
output frequency plausible, shaft speed wrong	pole/nameplate data, slip/load, belt/gear ratio or encoder scaling	motor data and independent speed measurement
shaft direction wrong	reverse command, phase order or configured polarity	command bits and approved rotation test
motor runs, PLC reports stopped	auxiliary/drive status mapping, input wiring or debounce/timing	raw feedback at device, input and PLC tag
oscillating speed	unstable control loop, noisy reference, load variation or encoder issue	reference/actual/torque trend at sufficient sample rate

When changing two motor leads for direction on a VFD installation, follow the drive/motor procedure with energy isolated; do not switch output contactors while the drive is producing output unless the approved design explicitly supports that operation.

Intermittent motor and drive faults

Intermittent events need synchronized history, not repeated manual snapshots. Trend command source, run request, critical permissives, safety status, drive ready/run/fault, raw fault code, reference, actual speed, output current/torque, DC bus if exposed diagnostically, device temperature, network status and process load.

Triggered capture plan

Trigger	Pre-event window	Post-event window	Supporting environmental signal
first drive fault bit	long enough to see command/load change	through reset decision	cabinet temperature, line event, adjacent large load
feedback lost while command remains	several start/steady-state cycles	through timeout and stop	vibration, connector movement, network counters
current exceeds baseline	ramp and prior process state	through trip or recovery	pressure, material load, brake state
safety channel discrepancy	guard/stop transitions	through reset attempt	door position, vibration, channel raw inputs
network connection loss	connection health before event	reconnection and command restoration	switch port errors, topology change, power dip

Use the broader intermittent PLC fault troubleshooting guide for vibration, moisture, temperature, EMI and evidence-window methods. The motor/drive page remains focused on the command-to-torque chain.

Controlled restoration and acceptance

After a cause is corrected, restoration is a test with predicted results—not a reset followed by immediate production. Review the change, confirm guards and connections, remove test equipment and temporary configurations, restore parameters from controlled records, notify affected personnel and follow the site's restart procedure.

Acceptance evidence

Acceptance item	Pass evidence	Failure response
command/permissive behavior	correct owner and every interlock proven in intended state	return to logic/sensor diagnosis
safety function	validated response and reset behavior per safety procedure	stop; safety-qualified review
start transition	ready, command, pickup/run feedback within designed time	preserve trace and locate delayed boundary
current/load	plausible, balanced and within equipment/application limits	investigate motor/load/supply before release
speed/direction	correct across approved operating points	verify source, scaling, parameters and mechanics
stop/deceleration	intended stop category with no bus/overload fault	review load energy, ramp and braking design
fault handling	correct first-out, alarm, reset authorization and restart policy	fix diagnostic/control design before release
sustained run	temperature, vibration, process and network remain stable for defined window	extend monitoring or reopen root-cause analysis

Record what changed, why it corrected the predicted boundary, the before/after evidence and any remaining risk. “Replaced VFD and it runs” is not a strong root-cause record if a loose input connection or seized load can damage the replacement.

Simulator lab for diagnostic practice

A simulator can inject command, permissive, feedback, overload, network and drive-state faults without bypassing a real safety function or experimenting with production outputs. The learning objective is to choose a discriminating observation, predict its result and stop when evidence changes the fault boundary.

Six fault cases to rehearse

Case	Injected condition	Correct first evidence	Common wrong conclusion
PLC output never energizes	process permissive false	individual permissive and sequence step	replace output module
output energizes, starter stays open	overload auxiliary open	control-path voltage across documented contacts	PLC logic is wrong
VFD ready but ignores run	local/keypad command source selected	active command source and accepted run bit	network cable failed
motor accelerates then trips	mechanical load plus short acceleration	current/torque versus ramp and load	drive is undersized
motor runs, PLC times out	auxiliary feedback input open	physical running state versus raw PLC input	motor failed to start
intermittent comm loss	switch/connection event	timestamped network and drive fault history	increase every timeout

Practise the VFD command, reference, feedback and fault chain in the VFD simulator. The lab continues the diagnostic task; it does not reproduce every manufacturer's protection model, output waveform or safety certification.

Diagnostic answer map for search and AI-assisted troubleshooting

User or AI query	Concise answer	Essential condition
PLC output is on but the motor is not running	Trace the physical interface, safety/STO, starter or drive ready state, power output, motor and load.	An online PLC bit is not proof of terminal voltage or motor power.
Motor starts then trips after a few seconds	Save the exact trip and trend current, ramp, brake and mechanical load from before the start.	Do not reset until fault history and operating context are captured.
VFD says run but motor does not turn	Check ready/STO, active source, accepted reference, output frequency/current, motor path and mechanical brake/load.	Use the exact drive status model and safe measurement procedure.
Contactor coil has voltage but does not pull in	Verify voltage across the coil matches its rating under load, then inspect coil/mechanism under isolation.	A high-impedance ghost reading can mislead; do not hold the contactor manually.
VFD overvoltage fault during stopping	Regenerated load energy may raise the DC bus; examine decel ramp and braking design.	Input overvoltage and device-specific faults must also be excluded.
VFD overcurrent fault during acceleration	Check jam/brake/load, acceleration, motor data, boost/tuning and cable/motor fault evidence.	A code identifies protection action, not one guaranteed cause.
Motor current high on all phases	The motor may be genuinely overloaded or under-volted, incorrectly connected or driven with bad parameters.	Compare process/mechanical load and supply to nameplate/application limits.
Motor runs but PLC reports stopped	Trace auxiliary/drive run feedback from device contact/status through wiring/network and PLC mapping.	Confirm the motor is physically running before altering feedback logic.
Can I megger a motor connected to a VFD?	Normally disconnect the motor cable from drive electronics and follow both manuals before insulation testing.	Never apply insulation-test voltage through a connected VFD.
Can STO be used as lockout	STO prevents torque when correctly functioning but is not automatically an energy-isolating device.	Follow the site's energy-control procedure and drive safety manual.

Frequently asked questions

What should I check first when a PLC-controlled motor will not start?

Capture the fault state, then compare the start request, individual permissives, safety/STO state, final PLC command and physical starter/drive response. The first mismatch defines the next test. Do not begin by resetting the drive or forcing the output.

Why is the PLC output on but the contactor is not energized?

The output tag may not equal terminal voltage, or the downstream control chain may be open. Check module/channel diagnostics, voltage at the output, interposing relay, stop circuit, overload auxiliary contact, coil supply and return using the approved schematic and test method.

Why does a motor start and then trip on overload?

Possible causes include excessive mechanical/process load, brake not releasing, acceleration too short, incorrect motor parameters, low-speed cooling limits, phase/current imbalance or motor/cable damage. Save current, timing, load and the exact protection state before reset.

How do I tell whether the VFD, motor or load is faulty?

Separate input supply, drive state/output, motor/cable and mechanical load. Compare command, ready, output frequency/current, shaft response, current balance, process load, vibration and temperature. Use isolated tests only under the manufacturer and site procedure.

Can a normal multimeter measure VFD output voltage accurately?

Not necessarily. PWM output contains fast switching components and requires a meter or motor-drive analyzer with the appropriate VFD/PWM method, rating and bandwidth. Follow the drive and instrument manufacturers' connection and interpretation guidance.

Should I reset a VFD as soon as it faults?

No. First preserve the current fault, history, timestamp, command/reference, current, DC-bus diagnostic, temperature, network state and process load. A reset can erase first-out evidence and may permit an unexpected restart.

How do I test motor insulation on a VFD system?

Apply the formal isolation procedure, verify absence of hazardous energy, disconnect the motor/cable from sensitive drive electronics as required, and use the motor and drive manufacturers' test voltage, connection, time, temperature correction and acceptance criteria.

Why does the motor run locally from the VFD but not from the PLC?

Local operation proves much of the drive, motor and load path, but not remote command. Check remote/local mode, active command and reference sources, network ownership, digital inputs, control word, safety state and PLC-to-drive mapping.

Is Safe Torque Off the same as electrical isolation?

No. STO is a safety function intended to prevent torque when correctly applied, but hazardous voltage and stored DC-bus energy may remain. Use the equipment's documented energy-isolation and verification procedure for servicing.

What proves a motor troubleshooting repair is complete?

A controlled restart should prove command ownership, permissives, safety response, start timing, current/load, speed/direction, feedback, stop behavior and fault handling over a defined operating window. Record the corrected boundary and before/after evidence.

Sources, review scope, and limitations

This guide was reviewed on August 28, 2026. It provides a cross-vendor diagnostic method, not live-work authorization, a safety validation, device sizing or a substitute for the installed manuals and site procedures.

• PowerFlex 525 Adjustable Frequency AC Drive product information and fault descriptions — Rockwell Automation
• Installation Considerations for IGBT AC Drives — Rockwell Automation
• How to troubleshoot motors and drives starting at the inputs — Fluke
• How to measure output voltage from a VFD to a motor — Fluke
• Technical guide book for variable-speed drives — ABB
• Low-voltage motors in variable-speed operation — ABB
• 1FW68 motor operating instructions, including insulation-resistance precautions — Siemens
• 29 CFR 1910.147, control of hazardous energy — OSHA
• Control-circuitry prohibition for energy isolation — OSHA

Fault codes, thresholds, motor thermal models, discharge times, STO behavior, terminal functions, measurement categories, insulation-test limits, cable requirements and restart policies differ by product and application. Illustrations are conceptual and not wiring diagrams or measurement instructions. Do not infer safe access from a stopped shaft, dark display, open control output or absent run command. Energized diagnostics and safety-system work require the qualifications, risk controls and procedures applicable at the site.