Answer first
Short Answer
A Variable Frequency Drive (VFD) converts AC input to a DC link before creating controlled motor output.
During a voltage sag, rectified input may be too low to sustain the DC link, so stored capacitor energy falls and the drive reaches its undervoltage threshold.
The exact ride-through time depends on load torque, DC capacitance, control power, manufacturer settings, residual voltage, and event duration.
When This Applies
This explanation applies when a drive reports undervoltage, DC-link low, or a related input-power trip during a short voltage reduction. It does not prove that every process stop is caused by the VFD; PLC power, contactors, instruments, and auxiliaries may fail first.
Engineering Explanation
The VFD input rectifier charges a DC-link capacitor. The inverter draws energy from that link to maintain motor operation. During a sag, the input cannot replenish the link at the normal rate. The capacitor discharges until the drive recovers input power or crosses its protection threshold.
Load matters. A lightly loaded motor may coast or ride through longer than a motor producing high torque. Control power and process logic may also impose a shorter limit than the DC link itself.
Calculation Example
For an idealized 300 kW load that must be supported for 0.3 seconds, the energy gap is 300 kW × 0.3 s = 90 kJ, or 0.025 kWh. This does not size a product: conversion losses, motor dynamics, overload, reserve, DC voltage window, and protection coordination still apply.
Alternative Solutions
- Support an accessible VFD DC bus when selected drives define the critical boundary.
- Use whole-line voltage sag protection when mixed AC loads must remain energized.
- Harden control power separately when controls or contactors trip first.
- Use an online UPS, Battery Energy Storage System (BESS), generator, or hybrid system for longer backup requirements.
Limitations / When Not to Use
Do not connect an external source to a VFD DC bus without manufacturer review and an engineered design for isolation, anti-backfeed, precharge, braking behavior, fault protection, and voltage coordination. Do not assume every VFD exposes a safe, supported interface.
Recommended Architecture
Record the voltage waveform and align it with VFD trip logs. Confirm the first failure, accessible electrical boundary, required ride-through interval, and every auxiliary needed for the process. Then compare DC-bus support with feeder-level and UPS alternatives.
Sources / References
- IEC 61000-4-30: Power quality measurement methods — voltage dips, interruptions, and other power-quality measurement methods. Accessed 2026-08-24.
- IEEE 1159-2019: Monitoring Electric Power Quality — descriptions and monitoring practice for conducted power-quality phenomena. Accessed 2026-08-24.