The DC-bus capacitors are the most wear-prone components inside any variable frequency drive. They typically last 8–15 years depending on ambient temperature and load. This guide covers the five warning signs — and how to confirm a suspect capacitor before ordering a replacement.
| Symptom | Likely cause | Action |
|---|---|---|
| Bulging or leaking capacitor top | Electrolyte dry-out, over-temperature | Replace immediately — do not power on |
| Rattling or humming from the drive | Loose internal connections, dried electrolyte | Stop and inspect |
| Low uF reading (below -10% of nameplate) | Capacitance loss from aging | Replace the capacitor bank |
| Undervoltage / bus fault alarms at load | Capacitors can no longer hold the DC bus | Replace before the drive fails under load |
| Drive trips on startup but runs when warm | Reduced ripple-current capability | Plan replacement at next shutdown |
The safety vent on the top of an electrolytic capacitor opens when internal pressure rises. Any visible bulge, split, or dried residue is a definitive fail — powering the drive further risks rupture. Shut down, lock out, and replace.
Healthy capacitors are silent. A rattle usually means internal elements have loosened as the electrolyte dried; a loud hum can indicate the capacitors can no longer filter ripple current effectively.
Measure with an LCR meter set to 120 Hz (or 100 Hz for 50 Hz drives). Replace when the reading is more than 10% below the printed uF value. Always discharge the bus and verify 0 V before probing.
If the drive reports undervoltage at the DC bus only when the motor is loaded, the capacitor bank can no longer store enough energy — the classic late-stage failure signature.
Cold capacitors have higher ESR; if pre-charge alarms vanish after warm-up, capacitance has degraded. Schedule replacement — this failure mode escalates to a hard fault within weeks.
Typically 8–15 years. Every 10°C above rated ambient roughly halves their life — hot electrical rooms are the main killer.
No — capacitors age as a bank. Replacing a single unit with mismatched ESR causes uneven current sharing and repeat failures.
Yes — it can rupture or vent hot electrolyte. Do not power the drive; discharge and replace.