Author Topic: Inductors for line voltage filtering  (Read 910 times)

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Offline XaviPachecoTopic starter

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Inductors for line voltage filtering
« on: July 21, 2019, 09:42:06 pm »
I would like to use these inductors at the input of a three phase rectifier like shown below for a low voltage power supply. Is it a good practice to do this?
 

Offline Stray Electron

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Re: Inductors for line voltage filtering
« Reply #1 on: July 21, 2019, 09:53:30 pm »
  What's on the output of the rectifiers?  Inductors alone are usually not a good idea for filtering because of the dv/dt factor. In other words, they generate LARGE voltage spikes if the load is quickly switched on or off. The voltage that they generate is proportional to the inverse of the switching time and is usually substantial.  Inductors need to be used in conjunction with with capacitor on their input or their output (or both).  Also inductors tend to be very large, very heavy and very expensive for anything that draws much actual current or power.
 

Offline T3sl4co1l

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Re: Inductors for line voltage filtering
« Reply #2 on: July 22, 2019, 01:54:56 am »
  What's on the output of the rectifiers?  Inductors alone are usually not a good idea for filtering because of the dv/dt factor. In other words, they generate LARGE voltage spikes if the load is quickly switched on or off. The voltage that they generate is proportional to the inverse of the switching time and is usually substantial.

If the load is another inductor (choke input filter), at worst the peak voltage is an inductor divider between mains and DC bus.  So, nothing important.

Even if the load is hard switched, the peak voltage is limited by the rectifier diodes, which for a three-phase rectifier I'm guessing would be chosen at 1kV or higher, and should be chosen for avalanche capability if this applies (if it's a bog standard cap input filter, who cares).

As long as the current ratings are adequate for the application.  Mind that, if peak current draw exceeds Isat, you will momentarily lose the benefit of that inductance.

Tim
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Offline XaviPachecoTopic starter

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Re: Inductors for line voltage filtering
« Reply #3 on: July 22, 2019, 02:19:55 am »
This is from a design a posted some weeks ago that I'm trying to improve. See image attached. The three phase input voltage can go up to 480 VAC. The bridge rectifier is built upon these diodes. The three line inductors L1, L2, and L3 are these ones and I would like to replace them with these ones. The original fuse is rated at 125 mA/250V, but I think it's underrated so I would change it to 500mA/ 1A/600V.

 

Online SiliconWizard

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Re: Inductors for line voltage filtering
« Reply #4 on: July 22, 2019, 12:18:19 pm »
You can take a look at this kind of off-the-shelf filters: https://www.te.com/usa-en/product-CAT-C8114-AY91.html?tcpns=CAT-C8114-AY11%7CCAT-C8114-AY91%7CCAT-C8114-AY61%7CCAT-C8114-B349%7CCAT-C8114-F299C%7CCAT-C8114-AY18%7CCAT-C8114-AY68

Excessively expensive, but the drawings include a schematic so that would give you an idea of how it's done.
Basically Pi LC filters.
 


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