The performance of inverter electrolytic capacitors?
Release time:
2023-02-14
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The emergence of any product or item is always driven by its unique advantages that set it apart from others—and inverter electrolytic capacitors are no exception. So let’s take a closer look at the performance characteristics of inverter electrolytic capacitors. Join us as we explore this topic together—hopefully, this will be helpful to all of you.
Every product or item that emerges does so because it possesses advantages that no other can match—and inverter electrolytic capacitors are no exception. Here, we’ll take a detailed look at the performance of inverter electrolytic capacitors. Let’s explore this together with the editor—hopefully, this will be helpful to everyone.
Another cause of ripple current in the electrolytic capacitors of inverters is the ripple current at the output frequency as well as the ripple current at the switching frequency generated during inverter operation. Taking the induction motor driven by the inverter as an example, the ripple current at the output frequency results in a very high-amplitude ripple current at the switching frequency. This secondary ripple current cannot be completely eliminated by all variable-frequency drives; it can only be absorbed by the filtering capacitors. For instance, in a 30-kW induction-motor drive equipped with a variable-frequency drive, the DC bus of the converter generates a ripple current of at least 60 A! This ripple current flowing through the filtering capacitor causes significant power losses due to the capacitor’s ESR. Due to cost constraints, currently no inverter manufacturer has managed to limit the ripple current in the filtering aluminum electrolytic capacitors below their rated ripple current specifications. Consequently, converters or inverters that use electrolytic capacitors as filtering elements will need to undergo regular replacement. If regular replacement of filtering capacitors is not feasible, the electrolytic capacitors in the inverter will frequently fail prematurely, which undermines both reliability and the goal of cost-effective operation.
So, can we skip adding a filtering capacitor on the DC bus of the inverter’s electrolytic capacitors? The answer is no—it’s simply not feasible. During switching operations, the inverter’s switching transistors generate current changes as high as 400 A/μs or even higher, inducing an electromotive force of 400 V across a 1 μH inductor. Therefore, it’s essential to use filtering capacitors to further reduce the parasitic inductance of the main bus. To lower the impedance caused by the parasitic inductance of the inverter’s DC bus, we need to reduce the impedance of the DC bus itself. A very straightforward way to achieve this is by using capacitors with low ESR and low ESL on the DC bus—these are often referred to as current-absorbing capacitors.
In the electrolytic capacitors of an inverter, the ripple current flows entirely through the electrolytic capacitors. This ripple current generates power dissipation within the ESR of the electrolytic capacitors, which is converted into Joule heat. Taking a 30-kW inverter as an example, the total ESR of the electrolytic capacitors connected in parallel across the DC bus is approximately 60–90 mΩ, and the ripple current ranges from 80 to 90 A. The power dissipation in the electrolytic capacitors connected in parallel and in two series-connected units amounts to 40–70 W. Of this, the power loss generated by the electrolytic capacitors themselves accounts for about 10–20 W. For electrolytic capacitors with poor thermal performance, this can lead to a relatively high temperature rise, further shortening their service life.
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Inverter electrolytic capacitor
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