1. Class A: Self Commutated by a
Resonating Load
Presented by : Muhammad Talha(364518)
2. ClassA: 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:
IA > IH 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. ClassA: 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
• ωn 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,
