Many VFD repairs begin with “drive won’t start” or “bus overvoltage at idle.” Before ordering a new control board or IGBT module, measure the DC-link bus. In three-phase drives up to roughly 75 kW, the bus is smoothed by banks of aluminum electrolytic capacitors that age into higher ESR, lower capacitance, and rising ripple heating. When the vent opens or the case bulges, the decision is no longer whether to replace the capacitor, but which parameter set to specify. This guide covers the application boundary, the numbers to check, and the minimum data a procurement engineer must send to a distributor for a correctly matched replacement.
Where the DC-Link Sits in a VFD
The DC-link capacitor bank is wired between the input rectifier bridge and the IGBT inverter stage. It holds the bus voltage during load steps, absorbs the rectifier ripple, and provides a low-impedance source for the switching loop. For a 400 VAC line, the bus sits near 540 V DC (1.35 × line RMS); for a 230 VAC line, near 325 V DC. The capacitors therefore need voltage ratings well above the measured bus value.
Ripple current is the main life-limiting stress. A 7.5 kW drive’s DC-link typically handles an order of 6–10 A of combined 100/120 Hz and switching-frequency ripple. In a sealed cabinet, ambient around the bank can reach 55–65 °C, so hot-spot temperature rises far above the 85 °C or 105 °C datasheet reference. Before ordering any part, confirm which capacitor is actually the DC-link element: snubber capacitors across each IGBT module and output EMI filter capacitors are separate components with different voltage and ripple profiles.
Parameter Thresholds That Decide the Replacement
Measure capacitance with an LCR meter at 100 Hz. A healthy 4700 µF part reads within ±20 % of nameplate; a reading 15 % or more below nameplate at operating temperature justifies replacement. Never drop below the original voltage class. For 400 VAC input, select ≥ 450 V DC rated electrolytics; for 230 VAC input, ≥ 250 V DC. Operating below 80 % of rated voltage extends life, which is why many VFD designs select 450 V parts for a 540 V bus rather than 500 V parts.
Compare datasheet ripple current at 10 kHz and 105 °C with measured values. At 25 °C, a 4700 µF/450 V snap-in electrolytic typically shows 20–60 mΩ ESR. When ESR at 100 kHz exceeds roughly twice the expected value for the capacitor’s age, plan a replacement. As ripple capability falls below the drive’s continuous output requirement, hot-spot temperature climbs and the lifetime estimate collapses.
Apply the halving rule: for every 5–8 °C the core temperature exceeds the datasheet reference, service life halves; for every 10 °C below, it roughly doubles. If the calculated life at your actual ambient is under 60 % of the designed maintenance interval, schedule a bank replacement rather than a single part.
Choosing between Replacement Styles
| Style | Typical range | Ripple capability | Mounting / labor | Typical drive power |
|---|---|---|---|---|
| Snap-in electrolytic | 220–1000 µF, 250–500 V DC | Moderate; enough for most single-drive DC links | Snap-in terminals; quick retrofit | 0.75–22 kW |
| Screw-terminal electrolytic | 1000–10 000 µF, 400–500 V DC | Higher; suits multi-unit banks | Busbar or terminal lugs | 30–90 kW |
| Polymer hybrid electrolytic | 470–820 µF, 450 V DC class | Higher at elevated ambient; lower ESR | Snap-in; occupies same footprint | Compact drives, 70 °C ambient |
| Film DC-link (polypropylene) | 300–900 V DC/AC class | Very high ripple, low ESR | Larger physical size; busbar interconnects | Multi-inverter or ripple-dominated links |
The ranges above are typical generic numbers, not a promise of any specific brand. Confirm exact capacitance, ripple rating, and ESR from the manufacturer datasheet of the part your distributor proposes.
Procurement Procedure in Six Steps
- Record the drive’s rated power, input line voltage, and switching frequency from the nameplate or service manual.
- Using a clamp meter or current probe, measure DC-link ripple at full load; ripple above 20 % of nominal bus voltage indicates a degraded bank.
- Measure capacitance and ESR per module; note can diameter, height, terminal pitch, and busbar spacing.
- Measure ambient temperature around the bank and check cabinet ventilation.
- Send your distributor a complete set: nominal capacitance, rated DC voltage, ripple capability (A at 10 kHz), ESR tolerance, mounting dimensions, and terminal layout. State RoHS/REACH compliance needs and preferred lead time; distributors usually cross-match to the same series or an equivalent with equal or better ripple rating.
- On delivery, verify capacitance within ±20 % of nameplate, ESR within datasheet limits, and vent/terminal condition. Keep these records for the next maintenance cycle.
Replacing a whole bank rather than a single failed unit avoids current sharing between a new low-ESR part and an aged high-ESR neighbor. With the numbers above, procurement can specify a DC-link replacement without waiting for an OEM serial number.
