DC-bus residual voltage — capacitor discharge time on VFDs and PV inverters

The Safe Torque Off (STO) guide covers why STO safely stops the motor without interrupting the supply to the drive itself. This article covers a related, often underestimated hazard that remains after the supply has indeed been switched off: the residual voltage on the internal DC link (DC bus) of a variable frequency drive or PV inverter.

Why the DC bus retains its voltage

A variable frequency drive rectifies the incoming AC supply and buffers that DC voltage in large electrolytic capacitors on the DC link, before an output stage converts it back into a variable AC supply for the motor. On a 400 V AC mains supply, this DC bus voltage typically sits around 560-680 V DC in operation. After the supply is switched off, that voltage remains on the capacitors until it has sufficiently discharged through an internal bleeder resistor (or, in its absence, through leakage currents) — a process that, depending on the drive's power rating, the load, and the presence of an active discharge circuit, can take anywhere from a few seconds to several minutes. Manufacturers of larger drives typically specify an explicit minimum waiting time (for example, 5 minutes) in the manual that must be observed before opening the enclosure.

Why STO does not solve this problem

Safe Torque Off interrupts the drive signal to the output stage so the motor can no longer produce torque, but STO does not interrupt the drive's own supply voltage and therefore does not discharge the capacitors on the DC bus either. For the safety function during operation (for example, safely stopping a machine shaft) that is sufficient, but for maintenance work on the drive itself (opening the enclosure, replacing a board) STO is not sufficient: that requires physical isolation of the supply and observing the specified discharge time, regardless of whether STO is active.

Establishing the actual discharge time

Physically switch off the supply to the drive and lock out according to the LOTO 5 steps.
Wait at least the discharge time specified by the manufacturer in the manual.
Then always measure the residual voltage yourself across the DC bus terminals with a suitable voltage tester (see the two-pole voltage tester guide) before making contact with any internal components — the specified waiting time is a guideline, not a guarantee, since a faulty internal bleeder resistor can significantly extend the actual discharge time.

PV inverters: an additional complication

With a PV inverter, an extra complication arises: as long as daylight falls on the connected PV strings, those strings continuously supply voltage on the DC input side of the inverter, regardless of whether the inverter itself has been switched off. See the PV DC-side insulation resistance testing guide for the broader background of this "cannot be de-energised in daylight" problem. The ordinary capacitor residual-voltage issue on the inverter's internal DC bus applies in addition to that.

Note: never rely solely on the fact that the supply voltage is switched off, or that an STO signal is active, as proof that the DC bus is safe. Only your own measurement of the residual voltage, after the specified waiting time, provides that certainty.

Practical relevance

Before opening the enclosure for maintenance, repair or replacement of a variable frequency drive or PV inverter, the specified discharge time must always be observed and the residual voltage measured yourself — STO, a switched-off main switch, or a locked-out LOTO padlock, none of these replace that measurement.

Common mistakes

Assuming STO discharges the DC bus — STO only interrupts the drive signal to the output stage, not the supply voltage or the charge on the capacitors.
Not observing the specified waiting time and opening the enclosure immediately after switch-off, while the capacitors may still hold hundreds of volts.
Not performing your own residual-voltage measurement after the waiting time — a faulty internal bleeder resistor can significantly extend the actual discharge time compared with the specified value.
Treating only the AC side of a PV inverter as de-energised, without accounting for the fact that the DC side keeps supplying voltage in daylight regardless, independent of the inverter's own DC-bus residual voltage.