SCRs Usage - Fundamental Question

Discussion in 'General Electronics Chat' started by Healthycola, Apr 3, 2010.

  1. Healthycola

    Thread Starter New Member

    Mar 8, 2010
    Hey guys, I've been researching about SCRs in 3 phase rectifiers. Yes, I have read the Ebook, and I'm pretty sure I understand how they work.
    I'm just wondering, why using an SRC is a better idea. If you trigger it at lets say, 30 degrees, won't you get a more choppy output as compared to 0 degrees (i.e. it functions like a diode). [Scroll down for quick demo] I know I have some sort of misconception in my fundamentals which is why I don't see why, so any sort of guidance is appreciated.
  2. kkazem

    Active Member

    Jul 23, 2009
    I guess me answering this shows my age--I've actually used SCR's in power conversion circuits, albeit many, many years ago. There really are not many benefits to using SCR's compared to today's modern power switches like Power MOSFETs and Power IGBTs. But, the SCR has one or two big assets. First, they can handle simultaneous very high voltage and very high current and are used often for hundreds of kilowatt inverters and motor controllers like for big and fast electric trains. Also, they are very tough. They can be subject to abuse and still survive without damage whereas other switches would likely have failed. They are easy to drive as well. The bad part it that in order to turn them off, you must lower the SCR or Triac current to near zero (below their "holding" current). If used in an AC to DC power supply, no problem as the SCR will rectify the AC and shutoff at near the zero crossings. But it used as an Inverter and DC-DC Converters, you need a 2nd SCR for every SCR you use as a switch. This 2nd SCR will be wired in the opposite polarity and needs a properly-timed drive signal. It will shutoff the first SCR by discharging a large capacitor into the first SCR such that it momentarily lowers the current in the first SCR to below the holding current. So, you need at least 2 SCRs for every power switch location in your circuit in most cases. They are used these days mainly where their awesome reliability and large power capacity is worth the hassle of using them, which is in large, electric trains, magnetically-levitated trains, other large motor drives, and large DC-AC Inverters, DC-DC Converters, and AC-AC Cycloconverters (changes one AC frequency to another). Moreover, they do not switch fast and must be used below 20KHz, which makes the circuits audible and annoying to people. There low-frequency switching requires much larger magnetics (transformers and inductors) compared to today's very fast switching parts that can run at hundreds of kilohertz or even in the megahertz region in some cases, allowing very small magnetics.
    I hope this helps.
    Kamran Kazem
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    In the 3-phase SCR rectifier application the intention is to obtain a controllable DC output - rather than a fixed DC output as would be case for a simple diode rectifier.

    Sure there is increasing ripple in the output as you delay the firing angle from 0°. With suitable post rectification filtering one can reduce the ripple to an acceptable level over the full operating range [phase angle control range] of the rectifier.

    Another consequence of simple rectifier circuits - phase controlled or otherwise - is that they create undesirable harmonics - particularly in the line side supply, which can be costly / difficult to eradicate, especially at very high power levels.

    My thanks to kkazem for his insights.