Circuit Operation

The presence of source inductance changes the way the circuit operates during commutation. The word commutation refers to switching conduction from one pair of SCRs to the other. Let v_s = E sin wt, with 0 < wt < 360^o. Let the load inductance be large enough to maintain a steady current through the load. Let firing angle a be 30^o. Let SCRs S₂ and S₄ be in conduction before wt < 30^o. When S₁ and S₃ are triggered at wt = 30^o, there is current through the source inductance, flowing in the direction opposite to that marked in the circuit diagram and hence commutation of current from S₂ and S₄ to S₁ and S₃ would not occur instantaneously. The source current changes from - i_L to i_L due to the whole of the source voltage being applied across the source inductance. When S₁ is triggered with S₄ in conduction, the current through S₁ would rise from zero to i_L and the current through S₄ would fall from i_L to zero. Similar process occurs with the SCRs S₂ and S₃. During this period, the current through S₃ would rise from zero to i_L and , the current through S₂ would fall from i_L to zero. The duration of the process of commutation is usually referred to as commutation overlap period.

The operation of the circuit is illustrated with the help of an animation. There is no scope for setting any variable for animation, since the purpose is only to show how the circuit operates. It is assumed that the load inductance is large to keep the load current at a steady value. You can observe how the currents through the devices and the line current change during commutation overlap. The running of the program can be halted by clicking on the Stop button. It can be resumed by clicking on the Run button. By clicking on the One Step button, you can step through the process. To start or run the process for one more cycle, click on the Start or Repeat button.

click here to open the applet

 
TO THE TOP