Variable frequency drives — DC bus, output stage and cooling
VFD Tripping — Reading Drive Faults and Finding the Real Cause
- Applies to
- Variable frequency and variable-speed drives on pumps, fans, compressors and conveyors, single- and three-phase input
- Difficulty
- advanced
- Competence required
- qualified electrician
- Diagnosis complexity
- Moderate. The drive names the electrical symptom precisely, which makes it easy to replace a healthy drive when the cause is the load, the ramp or the cooling.
- Electrical system
- Three-phase 415 V 50 Hz input nominal; DC bus and output per drive rating
- Safety classification
- stored energy
- Author
- EmersonEIMS Engineering
- Technical review
- Mr. Kararaho
- Last reviewed
- 2026-07-27
Scope — read this before relying on the guide
This guide sets out diagnostic method, ranked causes and safety requirements. It deliberately does not state equipment-specific numbers — test voltages, resistance and component values, torque figures, acceptance windows and pin assignments are referred to the manufacturer's documentation for your exact model rather than given here, because a plausible-looking figure for the wrong machine is more dangerous than no figure at all.
Direct technical answer
Read which fault the drive reports and when it occurs, because the combination usually names the cause outright. A trip during deceleration is almost always DC bus over-voltage: the motor is being driven by the load's inertia and returns energy to the bus faster than the drive can absorb it, so the answer is a longer deceleration ramp or a braking resistor, not a new drive. A trip during acceleration is usually over-current from a ramp too fast for the load inertia. A trip after running for a while, particularly on a hot afternoon, points at cooling — a blocked heatsink, a failed fan or a cabinet running above the drive rating. An earth fault or over-current at the instant of start points at the motor or its cable rather than the drive. Two rules matter regardless of the fault. Never fit power-factor correction capacitors on a drive output, and never switch a contactor on the output while the drive is running — both destroy output stages. And before opening anything, verify the DC bus has discharged with a meter; the stated waiting time is guidance, not proof.
01Symptom description
Controller / display
- Over-voltage, DC bus over-voltage or OV fault
- Over-current or OC fault, often qualified by the phase of operation
- Under-voltage or DC bus under-voltage
- Overload, motor overload or thermal fault
- Over-temperature — drive heatsink or ambient
- Earth fault or ground fault
- Input phase loss
Indicators
- Fault relay operated with the motor stopped
- Drive powering up but refusing to run
- Repeated trips with automatic restart cycling
- Display blank, which is a supply or drive fault rather than a trip
Sounds
- Cooling fans not running, or running at maximum continuously
- A bang at the moment of failure, indicating output-stage or capacitor failure
- Motor noise changing markedly at certain speeds, which suggests a resonance rather than a drive fault
- Contactor operating on the drive output — a practice that destroys drives
Smells
- Burnt-electronics smell — do not re-energise
- Hot or fishy smell associated with failed electrolytic capacitors
- Dust burning off the heatsink, indicating cooling neglect
Behaviour
- Trips on DECELERATION — the classic regenerative over-voltage signature
- Trips on ACCELERATION, suggesting the ramp is too fast for the load inertia
- Trips at the instant of start before the motor turns, pointing at motor or cable
- Trips after running for some time, and more readily on hot days — cooling
- Trips more often as the day warms, which is thermal even without an explicit temperature fault
- Ran correctly for years and began tripping recently, which suggests an ageing or environmental change rather than design
- Started tripping after the driven machine was serviced, indicating the load changed
Visible
- Heatsink and filter condition, and whether cooling fans turn
- Cabinet ventilation, filters and ambient temperature
- Bulged or vented DC bus capacitors
- Output cabling — length, routing, screen termination
- Any capacitors or contactor fitted on the drive output
- Motor and cable condition
- Braking resistor presence, condition and connection
02What the fault means
In plain language
A variable frequency drive controls a motor by converting the incoming supply to DC and then synthesising a variable-frequency output. When it trips it names the electrical symptom accurately, but the symptom is often produced by something outside the drive — the load, the ramp settings, the cooling or the motor cable. Replacing the drive without understanding which usually produces the same fault on the new one.
Technical explanation
A VFD rectifies the incoming supply to a DC bus, smooths it with electrolytic capacitors, and inverts it to a variable-frequency output through a switching stage. Each protection maps to a specific physical condition. DC bus over-voltage arises when the motor is driven above synchronous speed by the load — a high-inertia fan or a descending load, so it acts as a generator and returns energy to the bus. The bus rises until protection trips, which is why over-voltage faults cluster on deceleration and why the remedy is extending the deceleration ramp or providing somewhere for the energy to go, typically a braking resistor. Over-current on acceleration is the mirror case: the ramp demands more torque than the drive can deliver within its current limit for the inertia present. Under-voltage points at the incoming supply, and input phase loss is particularly damaging because it raises DC bus ripple current, which stresses the bus capacitors and shortens their life well before it causes an obvious trip. Those electrolytic capacitors are the principal wear item in any drive; they degrade with time and temperature, and a drive stored unpowered for a long period may require its capacitors to be reformed before use rather than simply energised. Thermally, drive losses appear as heatsink heat, so blocked fins, failed fans and cabinets above the drive ambient rating cause trips with every electrical parameter normal. On the output side two practices reliably destroy drives: fitting power-factor correction capacitors, which present a near short to the drive's switched output, and opening or closing a contactor on the output while the drive is running, which forces the output stage to interrupt inductive current. Long motor cables also cause the drive's fast switching edges to arrive at the motor with overshoot, stressing the winding and, on unscreened installations, raising earth leakage that can present as an earth fault.
03Common causes, ranked
These are ordered by likelihood. Presenting every possible cause as equally probable is a failure of diagnosis, not thoroughness.
Most likely
start here- Deceleration ramp too fast for the load inertia, causing regenerative bus over-voltage
- Acceleration ramp too fast for the load, causing over-current
- Cooling obstructed — blocked heatsink, clogged filter or failed fan
- Cabinet ambient above the drive rating
Possible
check next- Braking resistor absent, undersized, disconnected or failed where the duty needs one
- Motor or cable earth fault
- Supply problem — under-voltage, imbalance or input phase loss
- Motor parameters not entered from the nameplate, or the wrong control mode selected
- Driven machine stiffer than before, so the load has genuinely increased
Less common
after the above- DC bus capacitors degraded with age and temperature
- Output-stage device failure
- Power-factor correction capacitors wrongly fitted on the drive output
- Contactor switching on the drive output while running
- Long unscreened motor cable causing earth leakage and reflected-wave stress
- Mechanical resonance at particular speeds, mistaken for an electrical fault
Model specific
verify per unit- Every fault code, its meaning and the parameter set are drive-specific — take them from the manual for that exact drive and firmware
- Ramp times, current limits and braking options are configurable and differ by model
- Ambient temperature rating and derating above it are model-specific
- Maximum motor cable length, and whether a screened cable or output filter is required, are stated by the manufacturer
- Capacitor reforming procedure after long storage is manufacturer-specific
Environmental
site conditions- High ambient temperature, which directly reduces drive capability
- Dust and fibres blocking heatsinks and filters — the dominant cause in industrial and agricultural settings
- Humidity and corrosive atmospheres
- Cabinet without ventilation sized for the drive losses
Installation related
built in- Drive mounted without the specified clearances for airflow
- No braking resistor on a high-inertia or overhauling load
- Unscreened motor cable, or screen not terminated correctly at both ends
- Motor cable longer than the drive permits without a filter
- Standard motor on a drive with a long cable run, rather than an inverter-rated machine
- Drive sized on motor rating alone without regard to the duty and starting requirement
Maintenance related
deferred work- Filters and heatsinks never cleaned
- Fan failure not noticed until the drive trips thermally
- Parameters never recorded, so unintended changes cannot be detected
- Fault history never reviewed, so a pattern goes unnoticed
- Drives held as spares for years without capacitor reforming
Component level
electronics- DC bus capacitors degraded
- Cooling fan failed
- Output-stage device failed
- Braking resistor or its circuit failed
- Input rectifier failed
04Safety requirements
Isolation
- Isolate the drive supply, lock off and prove dead before any work
- The DC bus holds a LETHAL charge after isolation — the manufacturer waiting time is guidance, and discharge must be verified with a meter
- A motor can be back-driven by its load and generate voltage at the drive output
- Confirm the driven machine cannot start or move during the work
Lockout and tagout
- Lock and tag the drive supply isolator
- Tag any process control or remote system that can command a start
- Where the driven machine can rotate under process conditions, physically restrain it
- Keep the only key with the person doing the work
PPE
- Arc-rated protection appropriate to the prospective fault energy
- Insulated tools rated for the DC bus voltage
- Eye protection — capacitors can vent and output devices can fail energetically
- ESD precautions for any board handling
Stored energy
- DC bus capacitors retain a lethal charge for a substantial period after isolation. This is the defining hazard of drive work.
- Verify discharge by measurement at the bus terminals, every time, immediately before touching anything
- Capacitors can recover charge after being discharged — re-check before each work session
- Braking resistors remain hot long after the drive stops
Specific hazards
- NEVER assume the DC bus is discharged because the display is dark or the stated wait has elapsed. Measure it. This is what kills people on drive work.
- NEVER fit power-factor correction capacitors on a drive output. They present a near short to the switched output and destroy the drive.
- NEVER open or close a contactor on the drive output while the drive is running. Interrupting inductive current at the output destroys the output stage.
- Do not repeatedly reset and restart a tripping drive. Each trip is a real electrical event and repetition damages the output stage and capacitors.
- Braking resistors reach high temperatures and are a burn and fire risk if incorrectly mounted or enclosed
Stop and call a qualified professional if
- You cannot verify the DC bus is discharged
- There is a burnt smell or visible damage inside the drive
- Output-stage failure is established
- The installation has capacitors on the drive output, which needs correcting by design rather than adjustment
- The drive is part of a safety function whose integrity must be maintained
05Tools required
| Tool | Why it is needed |
|---|---|
| The manual for that exact drive and firmware | Fault codes and parameters are drive-specific; a code number means nothing without it |
| True-RMS multimeter rated for the DC bus voltage | Verifying bus discharge — the safety-critical measurement, and checking supply |
| True-RMS clamp meter | Input and output current, and phase balance |
| Thermal camera or infrared thermometer | Heatsink, cabinet and terminal temperatures; thermal causes are common and easy to confirm |
| Insulation resistance tester | Motor and cable insulation where an earth fault is reported — test the motor and cable, NOT through the drive |
| Drive fault history and parameter list | The pattern of trips and the parameter set usually name the cause |
| Anemometer or thermometer for cabinet ambient | Establishing whether the drive is operating within its rated ambient |
06Diagnostic decision tree
1. Has the DC bus been measured and proven discharged?
Yes → Continue
No → Stop. Do not touch anything until it is measured. The wait time is not proof.
2. Is there a burnt smell or visible damage inside the drive?
Yes → Do not re-energise. Escalate.
No → Continue
3. Are power-factor capacitors fitted on the drive OUTPUT?
Yes → Remove them — they destroy drives and must not be there
No → Continue
4. Does a contactor switch on the output while the drive runs?
Yes → Correct the control scheme — this destroys output stages
No → Continue
5. WHEN does it trip — decelerating, accelerating, at start, or after running?
Yes → The timing names the cause; match it below
No → Read the fault history to establish the pattern
6. Trips on DECELERATION?
Yes → Regenerative bus over-voltage — extend the decel ramp or provide a braking resistor
No → Continue
7. Trips on ACCELERATION?
Yes → Ramp too fast for the load inertia — extend the accel ramp and check the load
No → Continue
8. Trips after running, or worse on hot days?
Yes → Thermal — heatsink, fans, filters and cabinet ambient
No → Continue
9. Trips at the instant of start, or reports earth fault?
Yes → Test the motor and cable separately from the drive
No → Investigate supply, parameters and bus capacitors
07Step-by-step diagnosis
Step 1Isolate and PROVE the DC bus is discharged
- Inspect
- DC bus voltage at the bus terminals
- Where
- Across the DC bus
- Instrument
- True-RMS multimeter rated for the bus voltage
- Expected result
- Safe, near-zero potential that stays there
- If the result is abnormal
- A dark display does not mean a discharged bus. Measure, wait, measure again. This is the step that kills people when skipped.
- Next
- Step 2
Safety: The manufacturer waiting period is guidance, not evidence. Verify by measurement every time.
Step 2Read the fault history and establish WHEN it trips
- Inspect
- Fault codes, their order, and the operating phase at each trip
- Where
- Drive display or service software
- Instrument
- Drive interface and its manual
- Expected result
- A clear pattern tied to deceleration, acceleration, start or run time
- If the result is abnormal
- The combination of which fault and when it occurs usually names the cause outright, and it costs nothing to establish. Repeated trips over weeks change the diagnosis from a failure to a design or duty problem.
- Next
- Step 3
Verify for your unit: The fault code meaning for this exact drive and firmware — codes are not transferable between makes.
Step 3Inspect the output side for the two fatal practices
- Inspect
- Any capacitors on the drive output, and any contactor that switches while running
- Where
- Between the drive and the motor
- Instrument
- Visual inspection and the control scheme
- Expected result
- Neither present
- If the result is abnormal
- Power-factor capacitors on a drive output present a near short to the switched output. A contactor opening on a running output forces the stage to interrupt inductive current. Both destroy drives, and both are found in the field more often than they should be.
- Next
- Step 4
Step 4Check cooling before anything electrical
- Inspect
- Heatsink fins, filters, cooling fans and cabinet ambient temperature
- Where
- At the drive and in the enclosure
- Instrument
- Thermal camera, thermometer
- Expected result
- Clear fins, all fans turning, ambient within the drive rating
- If the result is abnormal
- A drive that cannot reject its losses trips with every electrical parameter normal. A single failed fan is easy to miss because the drive keeps working until it overheats. This is among the commonest causes and the cheapest to fix.
- Next
- Step 5
Step 5For over-voltage trips: examine the deceleration duty
- Inspect
- Deceleration ramp time, load inertia, and whether a braking resistor exists and works
- Where
- Drive parameters and the braking circuit
- Instrument
- Drive interface, visual inspection of the resistor
- Expected result
- Ramp appropriate to the inertia, with braking provision where the duty demands it
- If the result is abnormal
- A high-inertia load drives the motor above synchronous speed on decel, returning energy to the bus. Extending the ramp or providing a braking resistor is the fix. Replacing the drive is not.
- Next
- Step 6
Verify for your unit: Braking resistor rating and duty cycle for this drive and load — take it from the manufacturer sizing data.
Step 6For over-current trips: examine acceleration and the load
- Inspect
- Acceleration ramp, current limit, and whether the driven machine turns freely
- Where
- Drive parameters and the driven machine
- Instrument
- Drive interface; turn the machine by hand with the drive isolated and locked off
- Expected result
- Ramp achievable for the inertia; driven machine free
- If the result is abnormal
- A machine that has become stiff — bearings, a blockage, a seized component — raises the torque demand and trips the drive. The drive is reporting the load accurately.
- Next
- Step 7
Safety: Lock off and restrain the machine before turning it by hand.
Step 7For earth faults and start-instant trips: test the motor and cable, not through the drive
- Inspect
- Insulation resistance of the motor and its cable, disconnected from the drive
- Where
- At the drive output terminals, with the drive disconnected
- Instrument
- Insulation resistance tester
- Expected result
- Healthy insulation on motor and cable
- If the result is abnormal
- Never insulation-test through a drive — it will damage the output stage. Disconnect and test the motor and cable separately. Long unscreened cables also raise earth leakage that can present as an earth fault without an actual insulation failure.
- Next
- Step 8
Safety: Disconnect the drive before applying an insulation tester to the output cabling.
Step 8Check the supply, including phase loss
- Inspect
- Input voltage on all three phases under load, and balance
- Where
- Drive input terminals
- Instrument
- True-RMS multimeter, clamp meter
- Expected result
- Three phases present and balanced
- If the result is abnormal
- Input phase loss raises DC bus ripple current and shortens capacitor life long before it produces an obvious trip. Under-voltage and imbalance stress the drive continuously.
- Next
- Step 9
Step 9Assess parameters and bus capacitor condition last
- Inspect
- Motor parameters against nameplate, control mode, and physical condition of the bus capacitors
- Where
- Drive configuration and, with the bus proven discharged, the capacitors
- Instrument
- Drive interface, visual inspection
- Expected result
- Parameters matching the motor nameplate; capacitors not bulged or vented
- If the result is abnormal
- Capacitors are the principal wear item and degrade with time and temperature. A drive held as a spare for years may need its capacitors reformed by the manufacturer procedure rather than simply energised.
- Next
- Refer output-stage and capacitor faults for workshop repair or replacement
08Repair procedure
Ramps, limits and parameters — usually the actual fix
configuration- Extend the deceleration ramp where regenerative over-voltage is confirmed
- Extend the acceleration ramp where over-current on accel is confirmed
- Enter motor parameters from the nameplate and select the correct control mode
- Configure skip frequencies where a mechanical resonance is causing trips
- Record the full parameter set after any change
Where the fault is a ramp or a duty mismatch, no hardware change will help and a replacement drive fails identically.
Cooling and terminations
cleaning and connections- Clean heatsinks and replace filters
- Replace failed cooling fans promptly — a failed fan is an urgent fault
- Restore cabinet ventilation and the drive mounting clearances
- Re-torque power terminations and thermally survey under load
Braking, capacitors and output
component replacement- Fit or replace a braking resistor correctly sized for the duty where regeneration demands one
- Replace degraded DC bus capacitors, or the drive where that is more economical
- Refer output-stage failure for workshop repair or replacement
Installation faults that must be corrected
wiring- REMOVE any power-factor correction capacitors fitted on the drive output
- Correct any control scheme that switches a contactor on a running output
- Fit screened motor cable with the screen terminated correctly, or an output filter where cable length requires it
- Correct input phase loss at its source
These are design defects rather than adjustments, and they will keep destroying drives until corrected.
Beyond field repair
manufacturer level- Refer output-stage, rectifier and control-board faults
- Follow the manufacturer capacitor reforming procedure for drives held in long storage
- Provide the fault history, parameter set and measured supply values
09Post-repair validation
- Confirm the drive completes a full start, run and stop cycle under real load without tripping
- Confirm it decelerates from full speed under the worst-case load without an over-voltage trip
- Measure input and output current on all phases and record them
- Thermal-survey the heatsink and terminations under sustained load
- Confirm cabinet ambient stays within the drive rating through the hottest part of the day
- Confirm every cooling fan runs
- Record the complete parameter set after commissioning the change
- Review the fault history after a settling period to confirm trips have stopped rather than become less frequent
10When not to repair
- Where the fault is a duty or inertia mismatch — that is a sizing and settings question, not a repair
- Obsolete drives where boards and capacitors are unobtainable
- Drives repeatedly damaged by an uncorrected installation defect, such as output capacitors
- Small drives where replacement costs less than a competent board repair
- Where the cabinet cannot be brought within the drive ambient rating
11Prevention
- Clean heatsinks and filters on a defined schedule; thermal causes dominate in industrial settings
- Treat a failed cooling fan as an urgent fault, not a cosmetic one
- Record the complete parameter set at commissioning so drift and unintended changes are detectable
- Review drive fault history at every service visit — repeated trips precede failures
- Monitor cabinet ambient temperature; drive losses are a heat source the enclosure must handle
- Never allow power-factor capacitors on a drive output, and audit for them on inherited installations
- Specify screened motor cable and inverter-rated motors, particularly on long runs
- Reform capacitors on drives held in long storage before putting them into service
12Questions engineers actually ask
The drive trips on over-voltage every time it stops. Is the drive faulty?
Almost certainly not. On deceleration a high-inertia load drives the motor above synchronous speed, so it acts as a generator and pushes energy back into the DC bus faster than the drive can absorb it. The bus rises until protection trips. The fix is a longer deceleration ramp, or a braking resistor to give that energy somewhere to go. A replacement drive will do exactly the same thing.
Can I fit power-factor capacitors on the motor to improve the installation?
Not on the drive output — never. A drive produces a rapidly switched output, and capacitors present a near short to it, which destroys the output stage. If power-factor correction is required it belongs on the supply side of the drive, and a drive already presents a different load characteristic to the supply than a direct-on-line motor. Audit inherited installations for this; it is found more often than it should be.
Is it safe to megger the motor through the drive?
No. An insulation tester applies a high voltage that will damage the drive output stage. Disconnect the motor cable from the drive and test the motor and cable on their own. And before you touch anything at all, verify the DC bus has discharged by measuring it — a dark display and an elapsed waiting time are not evidence.
It only trips on hot afternoons. Nothing electrical looks wrong.
That is the signature of a thermal cause, and every electrical parameter can be perfectly normal. Check heatsink fins and filters for blockage, confirm every cooling fan is actually turning, and measure the cabinet ambient rather than the room. Drives derate above their rated ambient, and an enclosure sized without accounting for drive losses will trip predictably in the afternoon.
Standards and references
- IEC 61800-5-1 — adjustable speed electrical power drive systems: safety requirements
- IEC 61800-3 — adjustable speed electrical power drive systems: EMC requirements, including motor cable and screening
- IEC 60034-1 — rotating electrical machines: rating and performance
- IEC 60204-1 — safety of machinery: electrical equipment of machines
- The drive manufacturer's manual for the exact model and firmware, which is the only valid source for fault codes, parameter meanings, DC bus discharge time, ambient rating, permitted motor cable length and braking resistor sizing referred to throughout
This guidance is written from engineering principle and is not a substitute for the manufacturer's model-specific documentation. Where a figure is model-specific, confirm it against the service data for your unit before acting on it.
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