Half Controlled vs Fully Controlled Bridge Rectifiers
Half Controlled vs Fully Controlled Bridge Rectifiers
(OP)
Hello,
can anybody explain to me what the benefit of fully controlled bridge rectifiers are over half-controlled bridge rectifiers, please?
It seems to me that their output is the same and that the triggering arrangement of a fully-controlled bridge is more complex than for a half-controlled bridge, so why bother with them?
Thanks for any help,
NFI
can anybody explain to me what the benefit of fully controlled bridge rectifiers are over half-controlled bridge rectifiers, please?
It seems to me that their output is the same and that the triggering arrangement of a fully-controlled bridge is more complex than for a half-controlled bridge, so why bother with them?
Thanks for any help,
NFI






RE: Half Controlled vs Fully Controlled Bridge Rectifiers
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If we learn from our mistakes I'm getting a great education!
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
sorry, can you explain that to me, please?
Thanks,
NFI
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
wouldn't the effective supply from the load be blocked by the polarity of the SCRs?
NFI
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
It is important to understand that current always has the same direction. It is the voltage that changes polarity.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
Current maintaining the same direction and voltage changing polarity seems contrary to how I understand A.C. Conventional current must surely flow from +ve to -ve at all times?
At 90° phase, the source is at V-peak, isn't it? Not until 180° phase would the source voltage go -ve. I can see that i don't really understand this in the way I need to - if you could possibly explain it to me, I'd be very grateful.
Thanks again,
NFI
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
Second, the voltage across the DC motor is not measured as instantaneous voltage, but mean voltage over one period.
Third, the firing angle is NOT counted from the voltages zero crossing, but from the natural commutation point, which is at 30 degrees.
When all these conditions and prerequisites are in place, it can (quite easily) be shown that the DC voltage follows cos(alpha) where alpha is the firing angle. As you know, cos(90)=0 and cos(90+) is negative.
Once this has been understood, you must start all over again and look at commutation and overlap, which changes the rules somewhat. But basic principles remain the same.
Google thyristor control for a deeper treatment.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
http://w
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
Thanks for the link and the explanation - it;s very much appreciated :)
NFI
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
again, many thanks for all the help here and the very useful links.
If somebody could answer a few more questions for me, that would be very helpful and much appreciated;
Does regenerative braking only take place if the armature windings of the motor in question are reversed? That may sound like a stupid question, but I want to make sure I've got this right...
Is it the inductance of the motor windings that makes Vdc (as experienced by the motor) follow cos(alpha)?
Does the bridge rectifier have to be fully controlled so that the back EMF (the generated Vdc) can be synchronised with the A.C. supply (i.e. it's fed back into the appropriate side as the polarity switches)?
I think I actually understand this now :)
Thanks,
NFI
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
Second question. Yes, the inductance keeps current flowing (continuous current) and that keeps the thyristors conducting. Resistive load (no inductivity) makes the cos(alpha) relation invalid. Low current will eventually lead to discontinuous current. The cos(alpha) relation doesn't hold then either.
Third question. Yes. That is the reason. If you have a half controlled bridge (three thyristors and three diodes), The diode part will produce full positive voltage all the time. The controlled part will then not be able to produce more than the same negative voltage - and not even that, due to firing angle restrictions - so the sum of the full positive voltage and the nearly, but not quite, full negative voltage is still positive. That means that the bridge cannot commutate and you get a bridge short as soon as you reverse armature polarity.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Half Controlled vs Fully Controlled Bridge Rectifiers
I now feel fully educated :) Many thanks for your patience with me.
All the best,
NFI