Half Controlled vs Fully Controlled Bridge Rectifiers 1

[nfi]

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

 

[ScottyUK]

A full converter is capable of regenerative operation when the firing delay angle is greater than 90°.


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If we learn from our mistakes I'm getting a great education!

 

[nfi]

Hiya,

sorry, can you explain that to me, please?

Thanks,

NFI

 

[nfi]

Do you mean that, if the load is capable of generating power itslef (e.g. a motor) and the firing angle is 90° or more, then for the first 90°, power can be taken from the load and fed back into the supply?

wouldn't the effective supply from the load be blocked by the polarity of the SCRs?

NFI

 

[Skogsgurra]

The voltage of a fully controlled thyristor bridge is zero at delay angle 90 degrees. When angle is 90+ (up to near 180 degrees), the voltage goes negative and if the load (motor) 'wants' to drive current against the negative polarity, the net effect is that you get a negative voltage times a positive current, which is a negative power - or, in other words - regeneration.

It is important to understand that current always has the same direction. It is the voltage that changes polarity.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[nfi]

First, thanks very much for your replies and patience with me here

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

 

[Skogsgurra]

First, the DC current needs to be continuous so that one couple of thyristors is always conducting.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Some of the first google hits are not very correct. This one is a good introduction:

http://www.industrialcontroldesignline.com/howto/211601108

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[nfi]

Thanks very much for your help - I have to admit, I did try reading up on this on google before coming here, but I felt none-the-wiser for what I found

Thanks for the link and the explanation - it;s very much appreciated

NFI

 

[nfi]

Hello,

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

 

[Skogsgurra]

Yes. You need to reverse polarity. That can be done in several ways. First, you can reverse the armature winding with contactors. Second, you can also reverse the field winding (that also reverses armature polarity) and Third, you can use two rectifier bridges connected in anti-parallel. That makes the number of thyristors twice as many (twelve instead of six).

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[nfi]

Thanks very much

I now feel fully educated Many thanks for your patience with me.

All the best,

NFI