Background: Power electronics is one of the most important elements of today’s industry. Many of converters applications are being considered as critical for plant production process. Offering enhanced functionality in modern ac drives is a way to gain a competitive advantage in power converters market. One of the values to the customers comes from low cost condition monitoring of critical equipment and from opti-mized maintenance costs. Material and methods: Although the designers pay a lot of attention when designing DC-link circuit, breakdowns and failures of the DC circuit components can still happen. Unfortunately this means that the whole converter must be shut down in emergency mode to be repaired. Nowadays, monitoring of the bus capacitor is becoming one of the desired features from customer side, as well as from the converter manufacturer. Results: Proposed approach presents a method which assures the online evaluation of the equivalent series resistance of DC-link capacitor. The ESR increase trend is an important indicator of accelerated aging pro-cess or incoming failure of the capacitor. Conclusions: The important feature of the proposed condition monitoring method is that it does not require any additional sensors. For estimation purposes already installed sensors and other internal converter’s signals are used.