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Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195
Hi
Needing a variac capable of handling 10-15A but being discouraged by it price/bulkiness i decided to make one with thyristors. Everything is sort of finished and tested all that remains is to assemble all together.
So my question here is about the inductor in series with the thyristors:
Since I'll use this only before rectifiers (mostly in TC operation) how crucial this inductor is and if so how big and what specs to use? Can i make it on this core TX42/26/13-3E25?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The inductor improves power factor (so you can get more DC current out of an outlet) and helps reduce the rate of rise of current below the critical di/dt of slow SCRs.
With my controller circuits, it also helps stability when controlling a rectifier that charges a large capacitor bank. Under some conditions, the large current pulses can drag the mains down to almost zero, and the control circuit can mistake this for the zero crossing, causing a runaway that ends in popped breakers.
A value of about 1mH on a laminated iron core with air gap works well. I tried smaller ones on iron powder toroids, but these weren't so good.
This is mostly determined by PFC and stability. I don't think modern SCRs need any help with the critical di/dt.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Also the short big current pulses (if no inductor is used) cause an increased power loss in the filter capacitor(s) after the rectifier, distort the mains waveform and of course the loss in the semiconductors is also larger. The power factor in this mode of operation would be very poor (probably below 0.3) so the inductor is really a must.
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Phase angle controller question
