The role of electrolytic capacitor in the inverter

Electrolytic capacitor as rectifier filter capacitor of converter/inverter, it is generally believed that the main parameters of electrolytic capacitor is rated voltage, capacitance, often used as a rectifier filter capacitor, electrolytic capacitor which is affected by the conventional single-phase rectifier circuit of the electronic technology. In three-phase rectifier circuit, a total of six wave head, each power cycle was used as capacitor filter, is only a third of the time of each wave head is the guide to the output rectifier power supply, the remaining two-thirds of the time, the output power can only be provided by the capacitor, this time is about 1/9 of the power cycle, namely, 2. 22mS。 With the output power of 30 kw inverter, filter capacitor is usually used 3300 & mu; F / 400 v two strings and electrolytic capacitor. In the load conditions of average current of the rectifier output of about 50 a. 2 in rectifier does not work. 2 time of ms, filter capacitor caused by discharge voltage drop is 33 ~ 35 v, average rectifier output voltage is 600 v 0. 055, such as considering the equivalent series resistance of the electrolytic capacitor is about 68 m & Omega; ESR shed, 50 a ripple electric voltage drops to 3. 5 v, then the ripple voltage amplitude will be more than 6%, about half of no capacitor filter, shows that rectifier output filter capacitor is not actually used for filtering, but used to absorb from the ripple current of rectifier and inverter. Of frequency converter in the mainstream bus ripple current mainly has two aspects: the ripple current of rectifier power frequency filtering, for example for 3 phase 380 v direct rectification, each about filter capacitor through the kw output more than 6 a ripple current, for a 30 kw inverter, filter capacitor need to filter out 90 a even higher ripple current, of course, the ripple current to indirectly through rectifier and filter capacitor a reactor to reduce greatly. But produce ripple current another source ( A ripple current produced by inverter But absolutely can't use string into the reactor to solve; Another cause of the ripple current is generated when the inverter work under the output frequency of ripple current and switch frequency ripple current, ripple current of inverter output frequency inverter driven induction motor, for example, to generate high amplitude of the switching frequency ripple current, the second kind of ripple current is all converter/inverter cannot eliminate itself, can only use filter capacitor to absorb, such as 30 kw inverter driven induction motor, inverter dc bus to produce at least 60 a ripple current. This filter will be in the filter capacitor current in the ESR of the power loss. Due to the limitation of cost, until now, none of the frequency converter manufacturers will filter with aluminum electrolytic capacitor ripple current restrictions in electrolytic capacitor under rated ripple current, so the application for long life under the application field of converter/inverter using electrolytic capacitor as a filter capacitor will have to be replaced periodically filter capacitor, on occasions cannot be replaced periodically filter capacitor, can only be regularly scrap converter/inverter, this is not conducive to ensure reliability is unfavorable to use low cost. So the inverter can not filter capacitor on the dc bus? Conclusion is not possible. In the process of inverter switch tube switch, can produce 400 a / & mu; S even higher current changes in 1 & mu; H inductor will produce 400 v induced potential, therefore, need to use filter capacitor to further reduce the parasitic inductance of mainstream bus. In order to reduce the inverter dc bus impedance caused by the parasitic inductance, need to reduce the impedance of the inverter dc bus, the simplest way is on the dc bus and low ESR and low ESL capacitors, known as the electronic market surge absorbing capacitor often said. Can see from the above analysis, the role of rectifier filter capacitor prefers to absorb the ripple current of inverter, inhibition of inverter switch process of the switch tube voltage overshoot. Reduce the dc bus impedance.
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