AKKN ElectronicsA maintenance engineer reports a twenty-year-old servo drive that trips on undervoltage every time the line starts. The DC-link bus collapses shortly after the pre-charge contactor closes, and the drive refuses to run. The issue appears only at start-up, not during steady-state operation. This pattern points to a DC-link capacitor that can no longer hold charge under sudden load, a common failure mode in aging European-made drives still in service.
Field Symptom: Bus Voltage Sag at Power-On
When the drive powers up, the pre-charge circuit charges the DC-link through a current-limiting resistor. In a healthy unit, the bus reaches rated voltage within one or two seconds. A failed electrolytic capacitor shows a different behavior: the bus voltage rises slowly, then sags sharply when the inverter begins to switch. In some cases, the measured bus voltage never reaches 90% of nominal during the pre-charge window. The drive responds by triggering an undervoltage fault and locking out.
Do not assume the capacitor is the only suspect. The pre-charge resistor, contactor, and rectifier diodes can produce similar symptoms. The diagnostic sequence below separates capacitor degradation from other causes.
Layered Root-Cause Checks
Layer 1: Visual Inspection
Examine the capacitor vent, can seam, and solder joints. A swollen vent, cracked can, or dried electrolyte residue at the base is conclusive evidence of end-of-life. If the capacitor shows none of these signs, proceed to electrical measurement. Absence of visual damage does not clear the part.
Layer 2: Capacitance and ESR
Measure capacitance at 120 Hz and ESR at 100 kHz using a benchtop LCR meter. For large-can screw-terminal types, connect directly to the terminals with four-wire Kelvin leads. Clip-lead measurements on snap-in parts can add contact resistance that confuses the ESR reading. Record the ambient temperature; ESR values shift significantly with temperature, and a reading taken in a warm cabinet differs from one taken on a cold bench.
Layer 3: Leakage Current
Leakage current rises as electrolytic capacitors age. Apply rated DC voltage through a series resistor of 1 kΩ per 100 V of rating, and measure the current after five minutes of stabilization. Elevated leakage increases internal heating at start-up, which in turn accelerates further electrolyte loss.
Layer 4: Loaded Ripple Test
The most revealing check is the loaded ripple test. Run the drive at nominal load and measure the residual ripple across the DC-link with an oscilloscope through a differential probe. Ripple voltage above the manufacturer's stated limit indicates that the capacitor bank no longer absorbs switching current. Compare the measured ripple to the value recorded when the drive was new, if that data is available.
Measurement Method and Acceptance Values
The following table lists practical acceptance limits for a replacement-grade aluminum electrolytic capacitor at 20°C. These values apply to parts rated for general industrial use; check the original datasheet if the component has a tighter specification.
| Parameter | Test Condition | Acceptance Limit |
|---|---|---|
| Capacitance | 120 Hz, 20°C | ≥ 80% of rated value |
| ESR | 100 kHz, 20°C | ≤ 2× initial datasheet maximum |
| Dissipation factor (tan δ) | 120 Hz, 20°C | ≤ 1.3× datasheet limit |
| Leakage current | Rated DC voltage, 5 min | ≤ 0.01 C × V + 40 µA |
| Ripple current capability | Operating frequency, ambient temp | ≥ 1.1× measured load ripple |
For the leakage formula, C is rated capacitance in microfarads and V is rated DC voltage. A 4700 µF, 400 V part, for example, should draw no more than 0.01 × 4700 × 400 + 40 µA, or roughly 18.8 mA after five minutes. Values above this level indicate degraded dielectric behavior and warrant replacement.
Cross-Referencing Obsolete European Brands
Many European capacitor manufacturers have discontinued their large-can and snap-in electrolytic series over the past decade. When a drive needs a replacement, the cross-reference must match more than capacitance and voltage. Confirm the following parameters against the original specification:
- Ripple current rating at the switching frequency (100 Hz for rectified mains, 10–20 kHz for inverter applications).
- ESR at the operating temperature, not just the 20°C datasheet value.
- Case diameter, terminal spacing, and mounting hardware. A slightly larger can may fit, but check adjacent components for clearance.
- Lifetime rating at the actual ambient temperature and applied ripple, using the lifetime multiplier from the new part's datasheet.
- Surge voltage rating, which matters in applications with frequent line disturbances.
A common mistake is substituting a capacitor with the same capacitance and voltage but a lower ripple rating. In a DC-link application, the capacitor carries continuous ripple current; a part rated for lower ripple will overheat and fail prematurely, often within months. The reverse situation, a higher ripple rating, is always safe and extends service life.
Prevention Checklist for Procurement
For drives that still operate on legacy production lines, treat the DC-link capacitor as a scheduled maintenance item rather than a failure- driven replacement. Apply the following checklist when sourcing replacements for obsolete European brands:
- Record the original part's full specification, including ripple current and ESR temperature curves, before the drive is removed from service.
- Request a manufacturer cross-reference from the distributor, with the ripple and ESR data side by side, not just a capacitance match.
- Verify the replacement's shelf life. Electrolytic capacitors degrade during storage; a part sitting in inventory for five years may fail the leakage test before it is installed.
- Perform incoming inspection on every lot, measuring capacitance, ESR, and leakage on a sample per the table above.
- Keep spare units in stock for critical equipment, and rotate stock using a first-in, first-out policy.
- Document the measured bus ripple after installation as a baseline for future condition monitoring.
An aging electrolytic capacitor rarely fails without warning. Voltage sag, elevated ripple, and rising ESR all appear before catastrophic failure. A structured measurement routine at incoming inspection and at scheduled maintenance catches these signs early and keeps legacy European drives running.
Start-up undervoltage trips in legacy drives are often traced to a DC-link capacitor whose ESR has risen above the point where the pre-charge circuit can support the bus. Visual inspection, capacitance and ESR measurement, and a loaded ripple test provide a reliable diagnosis. Cross-referencing obsolete European brands requires matching ripple current and ESR behavior, not just capacitor values, because the replacement part must handle the same thermal and electrical stress as the original.
