Two common practices damage variable frequency drives (VFDs) far more often than electrical faults do: disconnecting the drive’s input power whenever the machine stops, and installing electromagnetic contactors or switches on the drive’s output. Both feel reasonable — one looks like saving energy, the other like adding a safety break — but both shorten drive life and cause unexplained trips. This article, updated from our original maintenance note for an international audience, explains the failure mechanisms and the correct alternatives.
Mistake 1: Cutting Input Power at Every Normal Stop
Many operators switch off the VFD’s AC input whenever the driven equipment stops, believing it is safer and saves energy. In practice, a drive that spends long periods de-energized in a humid plant environment suffers slow condensation and oxidation on its internal circuit boards. Moisture-driven leakage paths develop gradually, and the result is familiar to maintenance teams everywhere: after an extended shutdown, the drive throws repeated soft faults at the next power-up, with no obvious cause.
What to do instead
- Keep the drive energized during normal production pauses; only isolate it for maintenance work and lockout.
- Maintain airflow in the drive cabinet — keep the cabinet’s intake and exhaust fans operational so air circulates and moisture cannot settle.
- Control cabinet humidity — place desiccant inside the cabinet, or better, fit an automatic temperature and humidity controlled anti-condensation heater. This is standard practice in our own VFD cabinet builds.
- After long de-energized storage, allow the drive to dry and, where the manufacturer recommends it, re-form the DC bus capacitors before applying full load.
Mistake 2: Contactors on the VFD Output
Wiring an electromagnetic contactor between the VFD output and the motor — to stop the motor “positively” or to switch between motors — creates two serious hazards.
Opening under load
If the contactor opens while the drive is still running, the load is interrupted suddenly. The resulting surge triggers the drive’s overcurrent protection at best; at worst, the transient stresses the inverter’s IGBT output modules to destruction. Drives are designed to modulate current into a continuously connected motor, not to survive having that connection snatched away at full output.
Inductive voltage spikes
A motor is a large inductance. When the circuit is broken abruptly, the stored magnetic energy has nowhere to go and appears as a high-voltage spike across the opening contacts. That spike degrades the insulation of both the motor windings and the connecting cable, accumulating damage with every switching event.
What to do instead
- Connect the VFD output directly to the motor cable with no switching device in between, wherever possible.
- Start and stop the motor through the drive’s control terminals (run/stop commands), which gives controlled soft-start and soft-stop ramps — the behavior a VFD exists to provide.
- If an output contactor is genuinely unavoidable — for example, in bypass schemes or multi-motor switching — interlock it with the drive so the contactor only ever opens or closes when the drive output is disabled. The interlock must be enforced by the control logic, not by operator discipline.
Why This Matters for Plant Reliability
VFDs drive the pumps, fans and compressors that plants depend on daily, and an inverter module failure typically means days of downtime plus a repair bill approaching the cost of a new drive. Both failure modes described here are entirely preventable through installation and operating practice — no extra hardware budget required beyond, at most, a cabinet heater and an interlock relay. WELK Meters builds VFD control cabinets with ventilation, anti-condensation measures and correctly interlocked output circuits as part of its automation panel scope.
Frequently Asked Questions
Should a VFD ever be switched off?
Yes — for maintenance, lockout/tagout, and genuinely long shutdowns. The point is not to cycle input power as a routine stop/start method, and to manage humidity when the drive is de-energized for extended periods.
Can I use a contactor on the VFD input side?
An input-side contactor for isolation is acceptable and common, but it should not be used to start and stop the motor in normal operation. Frequent input power cycling stresses the drive’s inrush circuit and DC bus capacitors.
How do I switch one VFD between two motors safely?
Use output contactors with an electrical interlock that guarantees switching only occurs while the drive output is disabled, and ensure the drive’s parameters (motor data, protection settings) match whichever motor is selected.