# Forced Commutation Clarrification

## Main Question or Discussion Point

Hey all,

I am merely looking for clarrification as to what happens with the circuit that I have provided an image of. Please assume all elements are ideal (for simplicity). I come here for assurance as I cannot seem to find it anywhere else. Thanks in advance!

Thyristor T1 is fired at some angle and then it begins to conduct. The capacitor begins to charge until plate A in the schematic reaches the sources potential. If thyristor T3 is then turned on at this point, it provides a path for the current to discharge through T3. The current is a resonant oscillation through T3. Once the voltage across the capacitor has become fully reversed, D2 is forward biased. Once the forward current through T1 is less than the current through D2, T1 is such off. The capacitor is then allowed to discharge through D1, and then recharge from the load. Once the capacitor attains the potential of the source, it no longer conducts a current, and all of the current as an entirety remains in the free-wheeling diode.

Thanks again for any clarrifications.

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No one can verify if my perception of this circuit is skewed?

NascentOxygen
Staff Emeritus
Thyristor T1 is fired at some angle and then it begins to conduct. The capacitor begins to charge until plate A in the schematic reaches the sources potential. If thyristor T3 is then turned on at this point, it provides a path for the current to discharge through T3. The current is a resonant oscillation through T3. Once the voltage across the capacitor has become fully reversed, D2 is forward biased. Once the forward current through T1 is less than the current through D2, T1 is such off. The capacitor is then allowed to discharge through D1, and then recharge from the load. Once the capacitor attains the potential of the source, it no longer conducts a current, and all of the current as an entirety remains in the free-wheeling diode.

Thanks again for any clarrifications.
It sounds feasible. Component values and timing of the thyristor switching will be important.

Much appreciated! I was a little unsure about the resonant oscillation passing through T3 when it was turned on.

EDIT:

Perhaps I could have another questioned answered concerning the charging of a battery in rectifying circuits. If there is a resistor and inductor in series with a battery to he charged, why is the power transferred to the battery calculated with the average current through it, rather than the rms?

Last edited:
NascentOxygen
Staff Emeritus
Perhaps I could have another questioned answered concerning the charging of a battery in rectifying circuits. If there is a resistor and inductor in series with a battery to he charged, why is the power transferred to the battery calculated with the average current through it, rather than the rms?
The chemical activity going on in an electrochemical cell is proportional to the number of electrons entering the cell.

Nowhere in that statement can you divine a term (number of electrons)2.

RMS involves something squared, and there is nothing squared in the reactions that account for the recharging in the cell.

The squared term is evident in heating by a current, power = I2.R
so that's why RMS is involved in heating applications of electricity.

sophiecentaur
Gold Member
Much appreciated! I was a little unsure about the resonant oscillation passing through T3 when it was turned on.

EDIT:

Perhaps I could have another questioned answered concerning the charging of a battery in rectifying circuits. If there is a resistor and inductor in series with a battery to he charged, why is the power transferred to the battery calculated with the average current through it, rather than the rms?
The Power is the Current times the Volts. The Volts are constant (ideal battery) so the Power constant and is proportional to the Current.
I2R is another way of calculating Power dissipated in a resistor but that's when both current and resistance are varying (AC) - a different situation. The RMS value of current represents the equivalent (DC) current that would be flowing which would dissipate the same mean power.

The RMS value of an unvarying current is also the same as the DC value.