Class A.pptx

it is about class a commutation

1. Class A: Self Commutated by a Resonating Load Presented by : Muhammad Talha(364518)

2. Class A: Self Commutated by a Resonating Load Class A commutation of  thyristor  is a type of forced commutation and is also referred to as Load Commutation . Class A commutation is a sub-classification of forced commutation sometimes called self or resonant commutation. To commutate the thyristor , two necessary actions must be considered: I A  > I H  i.e., anode current must be less than holding current. The potential at anode must be lower than the cathode . In the circuit given below it is clearly shown that load i.e ., R is connected serially with the commutating components i.e ., L and C. Generally, when values of R, L, and C are low then in that case , the elements are arranged serially along with SCR as shown above.

3. Class A: Self Commutated by a Resonating Load On the contrary when the load resistor possesses a high value then it is connected in parallel across the capacitor while this parallel combination is connected in series with the inductor.

4. Working of Class A Commutation On applying an external dc input signal, the current starts flowing through the circuit. In order to turn on the SCR in the circuit, a gate trigger pulse is required. So, simultaneously gate signal is applied that will put the thyristor in forward conduction mode. We have already discussed the same that gate pulse is necessarily required to turn on the SCR. Hence, after the SCR gets on, the forward current that flows through the SCR begins to charge the capacitor. At the same time, the inductor connected in the circuit stores energy . We know that it is the property of the inductor, that it opposes the change in current. So, once the capacitor gets charged up to the peak of the supply input, the polarity of the inductor connected in the circuit will get reversed and now the inductor will oppose any further flow of current through it. As the inductor does not further allow the flow of current, the output current starts to decrease and reaches zero.

5. Necessary Condition We know that the current flowing through the circuit is given as : Further , writing the above equation in Laplace transform , R + Ls + 1/ sC = Z that corresponds to the overall impedance of the circuit due to serially connected elements . On rearranging,

6. On simplifying, The characteristics equation for the above equation is given as : However, the standard characteristics equation is given as : : ξ corresponds to the damping ratio and ω n   denotes the natural angular frequency On comparing the above two equations, we will get ,

7. Here , For underdamped system, ξ < 1 This is the condition for an underdamped system . The resonant frequency is given as: The maximum conduction time of SCR is given as: For  α  as firing angle,