A 22 kW variable speed drive began tripping on undervoltage after roughly ten years in service. The HMI showed no fault other than the DC bus voltage sagging from 540 V to below 430 V under load. When the cover was removed, one of the four screw-terminal electrolytic capacitors in the bus bank had a swollen vent and slight electrolyte residue. The other three measured close to the rated 470 µF, but all four showed clearly elevated ESR at 100 kHz.
This failure pattern is typical of European-built machines whose original capacitor brands have been discontinued. The repair decision is not about finding a better capacitor; it is about confirming the failure, measuring the correct parameters, and sourcing a cross-referenced part that fits both the busbar and the procurement workflow.
Layer 1: Isolate the Capacitor Bank from the Rest of the Drive
Before ordering parts, confirm that the capacitor bank — not the rectifier or pre-charge circuit — is causing the DC bus sag. Measure the rail voltage immediately after the pre-charge contactor closes. If it tracks the rectified line voltage within a few volts, the diode bridge and the contactor are working.
Use an oscilloscope with a 10x probe and set the bandwidth limit to 20 MHz to reject switching noise. Measure the ripple voltage directly across the positive and negative bus terminals at full load. A healthy DC link typically holds ripple between 1% and 3% of the nominal bus voltage. Ripple above 3% points to the capacitor bank, even if the capacitance readings still look normal, because ESR — not capacitance — dominates ripple at the switching frequency. Also check case temperature. A capacitor that runs more than about 20 °C above cabinet ambient at full load, in a normally ventilated enclosure, is being over-stressed by ripple current.
Layer 2: Measure Capacitance and ESR Before Cross-Referencing
Remove each capacitor from the circuit for accurate readings. Discharge the bus through a resistor until the voltage drops below 10 V; do not short the terminals directly. Set the LCR meter to series equivalent mode and record capacitance at 120 Hz and ESR at 100 kHz. For aluminum electrolytic capacitors, ESR at 100 kHz is the more sensitive aging indicator. Capacitance at 120 Hz catches open or shorted cells but often stays near the rated value long after ESR has climbed.
Measure at room temperature. ESR decreases as the capacitor warms, so a part that has just been running can look healthier than it is. Leakage current behaves in the opposite direction and should also be tested only after the part has cooled to 20–25 °C.
| Parameter | Condition | Acceptance limit |
|---|---|---|
| Capacitance | 120 Hz, 20–25 °C | Within ±20% of the rated value |
| ESR | 100 kHz, 20–25 °C | No more than 150% of the original datasheet maximum |
| Dissipation factor | 120 Hz | Per original specification; typical limit around 0.2 for the 400 V class |
| Leakage current | Rated voltage, 2 minutes | 0.01 × C × V (µA) or 3 µA, whichever is greater |
| Ripple current | At operating temperature | Measured AC current no higher than the rated ripple at that temperature |
As a field reference, a 400 V, 470 µF screw-terminal capacitor typically measures around 0.2–0.3 Ω at 100 kHz when new. A reading above 0.5 Ω, with capacitance still within tolerance, indicates a part that should be replaced. In a series-connected bank, the capacitor with the highest ESR absorbs a disproportionate share of the ripple voltage and will fail first — replace bank capacitors in matched sets rather than swapping a single unit.
Layer 3: Source the Replacement and Verify After Installation
Cross-referencing an obsolete European part number is not a label-matching exercise. The engineering parameters that matter are capacitance, DC voltage rating, ripple current at the operating temperature, ESR class, terminal type, can diameter, length, and mounting pitch. If the original was a long-life grade, the replacement should come from the same class rather than a standard-grade part of equal capacitance.
- Voltage rating: select at least the original class. For a 400 V DC bus, a 450 V rated capacitor is the standard choice.
- Capacitance: match the original value. Increasing capacitance raises inrush current and can stress the pre-charge circuit.
- Ripple current: compare the rated ripple at the actual operating temperature, not only the 105 °C figure printed on the label.
- Mechanical fit: confirm can diameter, length, screw terminal size, and busbar pitch before ordering.
- Compliance documentation: require current RoHS and REACH declarations with every shipment. A supplier that cannot provide them for a cross-referenced part should be treated with caution.
- Storage history: aluminum electrolytics stored for more than two years, especially at temperatures above 40 °C, typically need re-forming. Check date codes and storage conditions on incoming goods.
- Procurement terms: confirm export lead time, MOQ, and payment terms before the repair is planned around a single shipment. Replacement orders for obsolete parts are often small, so a distributor that tolerates low MOQ makes the logistics simpler.
After installation, measure the DC bus ripple again at full load. The new bank should bring it down to the healthy 1–3% range, and the case temperature rise should settle in the single digits to low teens above ambient for a correctly rated capacitor. Record the readings and the cross-reference used, so the next maintenance interval has a baseline to compare against. Distributors such as GOPI International maintain cross-reference data for obsolete European part numbers and can supply electrically matched equivalents with compliance files and global shipping.
