Unbalanced 3 phase current from 3 leg thyristor control

[ArchieMedes]

Hi all,

Is it possible to create an unbalanced circuit with a 3 leg thyristor control circuit? Like probably, one or more leg of the circuit controlled by thyristor is misfiring or not synchronized with the other 2 legs? Please explain as I can not understand how would such scenarios would create an unbalanced circuits with 3 different current values on the three phase circuits.
The load is fully resistive, i.e. heaters. Thanks for the help.

 

[gar]

101115-2156 EST

An SCR has the characteristic that until the gate is triggered the anode-cathode path is close to an open circuit. Once triggered it continues to conduct current in one direction until that current drops below what is called the holding current. If the anode to cathode voltage was negative on the anode relative to the cathode when the gate was triggered the SCR would not conduct. It only conducts in one direction. A Triac can be triggered in both anode-cathode polarities.

To adjust load current to a resistive load with an SCR in an AC circuit the timing of the gate trigger is adjusted. The more the trigger pulse is delayed from the positive zero crossing of the applied anode voltage the shorter is the time the SCR will be on and thus less average current to the load. In the resistive load circuit the SCR turns off at the negative anode zero crossing because the anode current goes below the holding current at this negative zero crossing.

If there is a difference in the trigger time of an SCR relative to its anode positive zero crossing for for a particular phase relative to the other phases, then there will be unbalanced currents.

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[steve66]

Are you sure there are equal heater wattages on all three legs?

For example, a 8KW heater may have a 2KW element on the A-B phase, a 2KW element on the B-C phase, and (2) 2KW elements on the C-A phase.

That would obviously be unbalanced, but would not normally be a problem.

 

[ArchieMedes]

Thanks for the replies.

The heater legs are balanced. I checked the resistance per stage all are within the range. As for the triggering of the thyristor. The triggering method used is zero crossing detection single cycle firing control. Is this prone to unbalanced circuit when there is a misfire of triggering?

Thanks

 

[gar]

101117-2125 EST

Archie:

Look at the circuit diagram of the trigger circuits.

If one timer is in some fashion used for the three different phases, it could be done, then unlikely that timing is a problem, and failure to trigger randomly could cause unbalance.

If there are three different timing circuits, one for each phase, then the three timer periods might not be the same.

If full cycle triggering is used, in other words an on-off system or bang-bang servo, then failure to trigger uniformly would cause an unbalanced current.

Any loss of any conduction periods changes the average current.

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