DC link, braking resistors and regeneration
What happens in the drive's DC link when a motor brakes, why overvoltage trips occur, and the options: longer ramps, Vdc-max controller, braking chopper and resistor, regenerative supply, common DC bus.
The DC link
A standard VFD rectifies the mains into a DC link (≈ 1.35 × line voltage, e.g. ≈ 540–560 V DC at 400 V AC) smoothed by capacitors. The inverter takes energy from there to drive the motor. A diode rectifier cannot feed energy back to the mains.
When the motor becomes a generator
During deceleration, lowering a load or with overhauling loads the motor returns energy. It charges the DC link capacitors; voltage rises until the drive trips with DC link overvoltage (e.g. F30002 on SINAMICS, see the Diagnostic Database).
Options
| Option | How it works | When |
|---|---|---|
| Longer deceleration ramp | Less power returned per second | Simple; if stopping time allows |
| Vdc-max controller | Drive automatically extends the ramp when voltage rises | Default on many drives; stopping time becomes variable |
| Coast stop | Motor stops freely, no energy returned | When stopping time does not matter |
| Brake chopper + resistor | Transistor switches surplus energy into a resistor as heat | Frequent stops, hoists, fast deceleration |
| Regenerative (active) front end | Feeds energy back to the mains | Large or continuous braking power (cranes, test benches) |
| Common DC bus | Braking drives supply motoring drives | Multi-axis machines |
With a braking resistor, size peak power (resistance limits the current) and average power (duty cycle → resistor heat), and protect it thermally (thermostat contact to the drive or PLC) — overheated resistors are a fire risk. When a braking resistor is installed, disable the Vdc-max controller, otherwise the two functions interfere.
DC braking
"DC braking" injects DC current into the motor to hold or stop it at low speed. The energy is dissipated in the motor — use it briefly; it heats the motor.
Connections
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Sources
Updated 2026-10-02