VFD Undervoltage Trip: Causes and Troubleshooting Solutions

Why does my VFD trip on Undervoltage

A VFD Undervoltage trip means the drive has detected that its internal DC bus (DC link) voltage fell below the allowed operating threshold for long enough that the drive can no longer control the motor reliably. Many manufacturers log these events as trips and faults (for example, Control Techniques uses “trip” language in its diagnostics), but the underlying concept is broadly the same across modern drives.

The trip describes what the drive measured on the DC bus; it does not automatically prove that the utility supply is continuously low, or that the motor is defective.

Why does a VFD trip on undervoltage?

A typical VFD rectifies the incoming AC supply to create a DC bus, then the inverter section uses that DC energy to generate the motor output. If the incoming AC sags, one phase is lost, a connection opens, or the drive’s internal power components cannot maintain the bus, the DC bus can drop below the undervoltage limit and trigger a trip.

The exact threshold, detection timing, and what the keypad shows (text, code, and/or a trip log entry) are model-specific. Use the manual for your exact drive and rely on the drive’s event history/diagnostics rather than limits copied from another product line.

Make the investigation safe before opening the enclosure

VFDs contain capacitors that can retain hazardous energy after the incoming supply is disconnected. Installation and maintenance should be performed by qualified personnel using the manufacturer’s isolation procedure for the specific model. Isolate all energy sources, apply lockout/tagout, observe the specified discharge time, and verify absence of voltage before touching conductors or internal components.

Do not bypass undervoltage protection to “keep it running.” If the drive is tripping, it is indicating a condition that can cause loss of control, nuisance downtime, or equipment damage.

Common VFD undervoltage fault causes

1) Incoming voltage dips, brownouts, or brief interruptions

Short supply disturbances can reduce the energy available to the DC bus. Common contributors include utility sags, facility distribution events, or large loads starting on the same feeder. If the undervoltage trip correlates with other equipment starting or with known site power events, treat that as a lead to verify with measurement.

2) Phase loss, blown fuses, or phase imbalance upstream of the drive

A missing input phase (or severe imbalance) can prevent the rectifier from sustaining the DC bus. Typical root causes include a blown fuse, a failed breaker pole, contactor issues, or a high-resistance connection at a lug/terminal.

Practical takeaway: inspect and test the entire incoming power path—disconnect, protective devices, contactors (if used), terminals, and cabling—rather than focusing only on the drive.

3) Excessive voltage drop due to supply impedance

Even if “open-circuit” voltage looks acceptable, the voltage can collapse under load when the supply is weak. Common contributors include undersized transformer capacity, long cable runs, undersized conductors, and poor terminations creating high resistance. The drive may only trip at specific times (for example, acceleration or peak load) because that is when current demand and voltage drop are highest.

4) High load demand or aggressive acceleration

If the trip happens consistently on start/acceleration, the drive may be demanding current faster than the supply can support without sagging. Mechanical issues (binding, process jams), incorrect motor data, or an overly short acceleration time can all increase current demand and make undervoltage events more likely.

5) Drive internal causes (charging/pre-charge circuit, rectifier, capacitors)

If the incoming supply is verified healthy at the drive terminals during the event, and upstream devices and connections are confirmed sound, the undervoltage trip may be related to internal power components such as the rectifier, DC bus capacitors, or the pre-charge/charging path. These checks are typically service-level and should be performed using the procedures for the specific drive series.

VFD undervoltage fault troubleshooting: a practical sequence

The goal is to identify whether the evidence points to (1) the supply, (2) the distribution path, (3) configuration/operation, or (4) an internal drive issue.

1) Preserve the fault/trip evidence before resetting

Record the exact trip message/code shown on the keypad and pull the drive’s trip history/diagnostic data (as available on your model). Capture:

• Drive identity: model/series and input voltage rating

• Trip details: the undervoltage identifier plus any additional trips/warnings present

• When it happens: power-up, run command, acceleration, steady load, deceleration, or stop

• Site conditions: other equipment starts, generator transfer, known power events

• Recent changes: parameter restore, wiring work, process changes, motor replacement

2) Verify the supply at the drive input (all phases, under load if possible)

Have a qualified technician measure all three line-to-line voltages at the drive input terminals and compare against the drive’s rated input range. If the drive trips intermittently, measurements taken after the fact may look normal—so prioritize capturing the voltage during the event (or as close as possible).

Also check for imbalance between phases and for any evidence of single-phasing upstream.

3) Check upstream protection and connections

With the equipment safely isolated, inspect/test:

• Input fuses/breaker poles for an open phase

• Contactor (if used) for worn contacts or coil drop-out

• Terminal torque/condition at the disconnect, protective device, and drive input

• Cable condition (overheating, damage, loose strands)

Correct any identified issues, then re-check input voltage balance under operating conditions.

4) Capture intermittent disturbances with the right instrument

If the undervoltage trip is occasional and not repeatable on demand, use a power-quality logger or recorder to capture sags, interruptions, and phase events on the drive’s supply. A simple handheld measurement taken later cannot prove what happened at the instant of the trip.

5) Review the operating profile and key parameters

If the trip correlates with starting or rapid load change:

• Increase acceleration time (and/or reduce commanded torque/current limits, if appropriate for the application)

• Verify motor nameplate data is entered correctly

• Check for mechanical binding or process overload

• Confirm the drive is correctly applied for the motor and load (duty, overload capacity, supply)

Make one change at a time and record results so the troubleshooting remains traceable.

6) If supply is healthy, proceed with the drive manufacturer’s internal diagnostics

When the supply is confirmed within limits at the drive input during the event, and upstream power path issues have been ruled out, follow the manufacturer’s service guidance for checking the charging/pre-charge function, DC bus components, and associated internal hardware. Do not bypass charging components or protective functions.

Undervoltage fault solutions (matched to root cause)

• Utility/facility sags: address upstream power quality, feeder loading, transformer capacity, or generator performance; consider application-appropriate ride-through strategies if permitted by your risk assessment.

• Phase loss/connection issues: repair/replace failed fuses, breakers, contactors, and correct terminations; verify voltage balance after repair.

• Supply impedance/voltage drop: upsize conductors/transformer, shorten runs where possible, correct high-resistance joints; consider input reactors where recommended for the installation.

• Acceleration/load-related trips: adjust ramps and confirm the driven machine is not overloading or binding; verify motor data and control mode suitability.

• Internal drive issue: service or replace the drive per manufacturer guidance after confirming external causes have been eliminated.

Confirm the repair before closing the job

Define a verification plan that recreates the operating condition associated with the original trip (start, peak load, specific process step). Document what was changed, what evidence confirmed the root cause, and what results verified the fix. If the cause cannot be confirmed, state that explicitly and document what has been ruled out so the next investigation starts stronger than “reset and running.”

Key takeaway: an undervoltage trip is best diagnosed from the incoming supply toward the DC bus—preserving trip evidence, measuring under the right conditions, and using the exact manual for your drive series to guide safe, model-specific checks.