VFD electrolytic DC-bus capacitors typically last 5–8 years and cooling fans 3–5 years under normal duty; high ambient temperature and ripple current shorten both. The practical rule: check capacitor ESR and capacitance every year after year three, and replace when capacitance drops below 80% of rating or when bulging, leaking or high ripple is found. This article explains why capacitors age, how to spot and measure the warning signs, and how to manage fans and spares.
The DC bus uses electrolytic capacitors to smooth the rectified voltage and support the inverter. Their electrolyte slowly evaporates with heat and ripple current – the two dominant aging factors. For every 10 °C rise in core temperature, electrolytic capacitor life roughly halves. That is why a drive in a hot, dusty enclosure ages much faster than the same drive in a clean climate-controlled room. The same applies to the cooling fan: it runs continuously while the drive is powered, and bearing wear plus dust load decide its life.
A basic ESR meter or a multimeter with capacitance mode is enough for a health check. Discharge the bus completely (wait after power-off and verify with a meter), then measure each capacitor's capacitance and ESR and compare with the nameplate. Plan replacement when capacitance is below 80% of rating or ESR is clearly elevated. For safety, the bus must be discharged and locked out before any measurement – see the safety steps in our VFD preventive maintenance checklist.
The cooling fan is the most common VFD field failure after capacitors. Inspect fans every 6–12 months: check for noise, vibration, dust build-up and free rotation. Blow out filters and heatsink fins on the same schedule. A blocked heatsink silently raises internal temperature and shortens capacitor life, so fan and filter care is really capacitor protection. Replace fans at the first sign of bearing noise; on larger drives, plan fan replacement in the same maintenance window as the capacitors.
| Component | Typical life | Failure signs | Check frequency |
| DC-bus electrolytic capacitors | 5–8 years | Bulging, leaking, high ripple, low capacitance | Yearly after year 3 |
| Cooling fans | 3–5 years | Noise, vibration, not spinning freely | Every 6–12 months |
| Air filters / heatsink | Variable | Visible dust, airflow drop | Monthly inspection, clean as needed |
| Braking resistor | 5–10 years | Discolouration, open circuit | Yearly |
Store spare capacitors and fans according to the manufacturer's guidelines (capacitors are shipped at reduced or zero voltage to slow ageing and should not be stored indefinitely). If you are not sure about the correct spare part for your drive, contact the drive OEM – our OEM support page covers drive servicing options.
Can I run my VFD past 8 years without changing capacitors? You can, but failure risk rises steadily and a bus failure can damage the inverter module – far more expensive than the capacitor set. Schedule replacement rather than waiting for failure. Related DC-bus symptoms are covered in our overvoltage troubleshooting article.
Do capacitors age when the VFD is switched off? Yes, more slowly – electrolytic capacitors age through both operation (heat, ripple) and storage (electrolyte evaporation). Long-stored drives may need reforming (controlled re-forming) before use. See our VFD FAQs for more.
Planning capacitor or fan replacement for your drives? Our service team can supply matched spares and guide the exchange. For drive selection and sizing support, see our low-voltage VFD range.
Contact Us
Be the first to know about our new product launches, latest blog posts and more. Founded in 1984, Wolong is a global leader in motor and drive technology, providing safe, efficient,... Any question or request?
Click below, we’ll be happy to assist. Contact Us
