Fourteen months after commissioning, an inverter-driven pump station begins tripping on DC-link undervoltage whenever the driven motor passes through its heaviest load point. The trip is intermittent and clears after a reset, but it worsens over two weeks. Inside the drive, the aluminum electrolytic capacitor mounted closest to the bus bar shows a visibly bulged pressure-relief vent and dried electrolyte around the case. The capacitor is still warm minutes after the load is disconnected. The rest of the DC-link board is intact.
This is a common failure pattern in industrial inverters. The swollen capacitor is the symptom, not the cause. The investigation below follows three layers: ripple and thermal stress, voltage and ESR behavior, and mechanical and sourcing checks. In most cases, at least two of these layers are involved.
Layer One — Ripple Current and Hot-Spot Verification
In a rectified DC-link, the bus capacitor supports the difference between the rectified voltage and the inverter load demand. The capacitor carries two ripple components: a low-frequency component at twice the line frequency, and a high-frequency component related to the inverter switching. Both components pass through the capacitor’s ESR and contribute to the same heating mechanism. The measured ripple current should not be treated as merely charging current.
Measure the ripple current directly on the capacitor’s positive lead with a current probe. Record the waveform over a load cycle that includes the heaviest steady load the machine is expected to produce. Compute the RMS value from the complete cycle, not from an instantaneous reading. For the failed capacitor in this example — a 450 V, 470 µF snap-in electrolytic — the vendor-rated ripple was 1.2 A RMS at 85°C case temperature, while the measured waveform contained a repeatable RMS value of 1.8 A.
| Parameter | Measure with | Acceptance value / rejection limit |
|---|---|---|
| DC-link ripple current, RMS | Current probe on capacitor lead, full load cycle | ≤ 80% of vendor rating at expected case temperature; reject if measured RMS exceeds 100% |
| Case temperature at rated load | Type-K thermocouple on the can side, or IR gun | ≤ vendor maximum case temperature; reject if the reading exceeds the rating by more than 5°C |
| Hot-spot temperature rise | Calculated from I_rms² × ESR and vendor thermal resistance | Typical rise 10–15°C above case temperature in still air; above the vendor maximum is a rejection |
The 0.8 ratio is a conservative engineering margin. Many aluminum electrolytic datasheets allow 100% ripple at the rated case temperature, but the safe ratio drops at high ambient temperatures. A common working rule is to reduce the permitted ripple current by roughly 10% for each 10°C by which the expected case temperature exceeds the vendor’s rated case temperature.
Layer Two — Voltage Margin, ESR, and Leakage Checks
Measure capacitance at 120 Hz and ESR at 100 kHz with an LCR meter after the capacitor is removed from the circuit. In-circuit ESR readings are misleading because the surrounding diodes and bus bars affect them. For typical inverter-grade electrolytic parts, capacitance within −20%/+30% of the nominal value and an ESR no higher than 1.5× the vendor’s initial limit are acceptable. If the ESR has doubled from its documented initial value, the part is near end of life and should be replaced even if the inverter still runs.
The second layer also covers operating voltage. In a 230 V AC rectified bus with 310 V nominal DC, a 450 V-rated capacitor operates at roughly 69% of rated voltage, which is a reasonable continuous margin. The same part operating from a 400 V AC mains (≈ 560 V DC bus) would be continuously overvoltage — a condition that generates gas, opens the vent, and produces the same field symptom. A practical delivery check: a capacitor intended for continuous DC-link service should be rated at least 1.2× the maximum DC bus voltage considered in the design, including AC mains tolerance.
Two additional measurements belong in this layer:
- Leakage current: apply the capacitor’s rated DC voltage through a 1 kΩ limiting resistor for two minutes at 25°C, then read the leakage. The acceptable limit, in milliamps, is the lower of 0.01 × C(µF) × V(V) or 2 mA.
- Surge margin: compare the application’s maximum transient DC bus overshoot with the capacitor’s surge rating. A capacitor voltage rating of at least 1.2× the maximum observed steady-state bus voltage leaves a practical margin for short-duration grid transients.
If the failed part is in a drive operating from a 230 V supply, the more likely contributors are the ripple current overload from Layer One, a substandard replacement part, or a combination of both.
Prevention and Incoming Inspection Checklist
The field failure in this example was eventually traced to a replacement capacitor installed during a previous repair. The part carried the correct capacitance and voltage markings, but its ESR at 100 kHz was 2.7× the vendor’s nominal limit, and the vent profile was visibly different from the reference part on the adjacent position. This raises a sourcing point: a DC-link capacitor for an inverter should be purchased against a defined specification, not only against a part number printed on a label.
- Define the required ripple current rating at the actual switching frequency. The 100 kHz ripple rating on many electrolytic datasheets does not always translate directly to the 4–12 kHz switching frequencies used in general-purpose inverters.
- Confirm that the DC-link capacitance and ESR together, not only the capacitance value, meet the inverter’s ripple-voltage calculation at the heaviest load step.
- Apply a minimum voltage rating of 1.2× the maximum DC bus voltage, including AC mains tolerance, for continuous duty.
- Measure capacitance and ESR on 100% of incoming lots of DC-link capacitors from new suppliers; for established suppliers, perform a periodic sample check of three units from each lot.
- Discard any part with a bulged vent, a displaced vent plug, or dried electrolyte around the terminal seal. These features are not cosmetic.
- Use the correct tightening torque for capacitor clamps; uneven compression across the can can simulate a failed vent.
Set a maintenance trigger: replace the capacitor at the next maintenance window if the half-yearly ripple-current reading has risen more than 15% against the previous one. Also compare capacitor case temperature against the value recorded at commissioning; a rise of more than 5°C at equal load is an early warning.
