Drive-level protection

VFD DC-Bus Support for Voltage Sag Ride-Through

VFD DC-bus support supplies energy directly to the DC link of a Variable Frequency Drive (VFD) when grid voltage is too low to maintain the bus.

Protection pointThe VFD DC link, where undervoltage causes the drive to trip.
Normal current pathThe support source need not be in series with normal AC load current.
Project gateVerified DC-bus access and coordinated electrical protection.

WS300 application role

High-Power DC-Link Ride-Through for VFD-Dominated Industrial Loads

WS300 is designed to provide short-duration high-power support to critical VFD-driven production equipment, allowing customers to protect the loads that actually stop the process without necessarily installing a full-line energy solution.

Why use load-layered protection instead of full-line DVR?

How it works

Connect the disturbance to the protected boundary.

A typical VFD rectifies AC input into a DC link. During a voltage sag, available rectified voltage falls and the DC-link capacitor discharges. If the bus crosses the drive's undervoltage threshold, the inverter stops even if the grid recovers moments later.

A coordinated DC support source holds the link inside an approved operating window. The design must prevent unsafe backfeed, respect the VFD manufacturer's limits, and coordinate with precharge, braking, fault protection, and control logic.

Application fit

When it fits—and when it does not.

When it fits

  • The critical process is dominated by one or more VFD-driven motors.
  • The drives expose a suitable common or individual DC link for engineered connection.
  • Only selected production-critical loads need ride-through.
  • The disturbance is short enough for a high-power, short-duration source to cover the energy gap.

When it does not fit

  • The VFD manufacturer does not permit or document DC-link access.
  • Protected loads need galvanic behavior or fault performance that the proposed interface cannot provide.
  • Controls, contactors, pumps, or auxiliaries outside the drive still trip and stop the process.
  • The requirement is long-duration backup rather than short ride-through.

Engineering boundary

When a DVR May Be the Better Boundary

VFD DC-bus support is highly targeted, but not every production trip originates inside the VFD. If PLCs, contactors, servo controls or other AC-side devices also need ride-through protection, a feeder-level Dynamic Voltage Restorer may provide a more appropriate protection boundary.

Compare with WS-DVR

Engineering inputs required

Define the project with measured information.

  • VFD make, model, and approved DC-link access
  • DC voltage window, precharge, and braking arrangement
  • Time-aligned critical-drive power demand
  • Isolation, anti-backfeed, and fault-protection requirements

Next engineering step

Wilson Load-Layered Architecture →Compare whole-line protection →Why VFDs trip during sags →Describe the Event →
Sources / References
  1. IEC 61000-4-30: Power quality measurement methods. Measurement and interpretation methods for voltage dips, interruptions, harmonics, transients, and other power-quality parameters. Accessed 2026-08-24.
  2. IEEE 1159-2019: Monitoring Electric Power Quality. Recommended practice for describing, measuring, and interpreting power-quality phenomena. Accessed 2026-08-24.

Preliminary diagnosis

Tell Us What Happened

Not sure whether the VFD or another control device caused the trip? Tell us what happened.

No waveform or power-quality record is required. Just tell us what stopped and what you observed.

Describe the Event

Engineering review

Tell Us What Happened

Not sure whether the VFD or another control device caused the trip? Tell us what happened.

Describe the EventEmail an Engineer