Asymmetrical thyristor firing causing DC component

[mull982]

Can someone explain to me how two single phase thyristors on one leg of a single phase circuit (each thyristor handling the positive and negative half) misfiring or firing aysmmetrically can cause a DC component or DC offset to be present in the AC waveform?

I have even heard that this aysmmetrical firing can cause DC current to flow in the circuit.

Can anyone explain the theory as to why this happens?

 

[Electric-Light]

Can someone explain to me how two single phase thyristors on one leg of a single phase circuit (each thyristor handling the positive and negative half) misfiring or firing aysmmetrically can cause a DC component or DC offset to be present in the AC waveform?

I have even heard that this aysmmetrical firing can cause DC current to flow in the circuit.

Can anyone explain the theory as to why this happens?

It causes a 2nd harmonic. When thyristors are not firing on both sides, meaning that its only firing on half a cycle, it creates an asymmetry.

When only one polarity is loaded, it suffers a slight voltage drop only on the loaded side. The voltage asymmetry between the loaded half and unloaded half creates a DC component.

It's kind of like an unbalanced tire. If you already have a perfectly balanced tire and you put two matching weights directly opposite from each other balance is not disturbed, but if one of them comes off, the tire will become off balance.

 

[Besoeker]

Can someone explain to me how two single phase thyristors on one leg of a single phase circuit (each thyristor handling the positive and negative half) misfiring or firing aysmmetrically can cause a DC component or DC offset to be present in the AC waveform?

Each thyristor carries the current in one direction only.
If one fails to fire, you then have current in one half cycle only so current in one direction only i.e. DC.

 

[mull982]

Each thyristor carries the current in one direction only.
If one fails to fire, you then have current in one half cycle only so current in one direction only i.e. DC.

I understand that this is the extreme case. But what about when the two thyristors dont fire at exactly the same angle and are firing at different angles? How far off would the firing angles have to be to create a signifigant DC offset?

 

[Besoeker]

I understand that this is the extreme case. But what about when the two thyristors dont fire at exactly the same angle and are firing at different angles? How far off would the firing angles have to be to create a signifigant DC offset?

Off even by a small margin would result in a DC component. Significance would depend on application.

 

[mull982]

So even if the firing angles are off by a small amount there would be a DC component? Could this DC component be seen by looking at the waveform and seeing the resulting waveform shifted either up or down from zero axis?

Although the thyristors may not be conducting during each full half cycle and may only conduct for a small portion of each half cycle will the resulting waveform still be shifted up or down from zero axis?

Would this DC off set be the same as having a DC current flowing in the circuit? In other words would you essentially have an AC current and a DC current flowing in the circuit at the same time? Would this DC current have the same effect on load components as just a DC only current of the same magnitude flowing in the ciruict alone have?

 

[dbuckley]

You need to add a time element to understand where the DC component is coming from.

Take one complete cycle, two half-cycles. The same amount of power flows on each half cycle. Dim that sine wave through a SCR dimmer working properly, and you still get the same amount of power flowing each way, even thoughy the waves are chopped.

Now make it asymetical, and more power flows one way than does the other. So the two half-cycles do not deliver the same amount of power. So imagine that 100W flowed in one direction, and 95W flowed in the other, you have a difference of 5W, and its always in the same direction, so its a DC effect when integrated over the time of a full cycle.

This doesn't matter a hoot on an incandescent lamp, but put a transformer out there, and you get trouble. The transformer magnetises in one direction, and then the other, on each sucessive half wave. If the half-waves are not symetrical, then the current flow has a net magnetising effect. Transformers dont like that.