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Short Answer
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.
It can protect selected critical drives without placing the normal AC load current through a series protection device.
It is not plug-and-play for every VFD: DC-bus access, isolation, anti-backfeed design, voltage window, precharge, fault protection, and control coordination must be verified.
When This Applies
- 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.
Engineering Explanation
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.
GRID → VFD Rectifier → DC LINK ● ← Engineered DC Support ↓ Inverter → MotorSupporting Multiple VFDs
Multiple drives may be supported from a coordinated DC architecture only after reviewing bus topology, independent protection, fault contribution, voltage compatibility, simultaneous power demand, regenerative conditions, and sequencing. A shared bus is an engineering project, not an assumed wiring shortcut.
| Review item | Why it matters | Evidence required |
|---|---|---|
| DC voltage window | Support must remain inside every connected drive's acceptable range | Manufacturer limits and measured bus behavior |
| Fault isolation | One drive fault must not propagate across the shared support path | Single-line, protection study, and device ratings |
| Simultaneous load | All protected drives may demand power during the same event | Time-aligned kW profile and process sequence |
| Control coordination | Support, drive logic, and process interlocks must recover together | I/O list, trip logs, and functional test plan |
Calculation Example
If three critical drives draw a combined 600 kW and the required support interval is 0.5 seconds, the ideal energy gap is 600 kW × 0.5 s = 300 kJ, or about 0.083 kWh. Conversion losses, overload margin, actual motor torque, and control reserve must be added during detailed design.
Alternative Solutions
Use whole-line sag protection when the DC link is inaccessible, mixed AC loads must remain energized, or modifying drive connections is not acceptable. Use an online UPS or hybrid backup system when required autonomy is much longer than the measured sag window.
Limitations / When Not to Use
- 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.
- Site data is insufficient to establish safe isolation and anti-backfeed behavior.
Recommended Architecture
Protect the drive where the disturbance actually causes the trip, but only after mapping the complete process. A DC-bus architecture is strongest when the critical boundary is clear, the interface is verified, and every auxiliary needed for production continuity is included.
Traceable sources
Sources / References
- 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.
- IEEE 1159-2019: Monitoring Electric Power Quality. Recommended practice for describing, measuring, and interpreting power-quality phenomena. Accessed 2026-08-24.