The chopper circuit employing the load commutation technique is known as the load commutation chopper. The load commutation technique is, falling of the load current flowing through the thyristor to zero due to the nature of the load itself.

So that the conducting thyristor gets commutated once the current flowing through it falls below the holding current value. Let us see the circuit diagram, operation, and waveforms of the load commutated chopper.

Circuit Diagram of Load Commutated Chopper :

The circuit diagram of the load commutated chopper is shown below. The circuit consists of four thyristors (T₁, T₂, T₃, and T₄) and a commutating capacitor. The four thyristors operate in two pairs, thyristors T₁, T₂ form one pair, and thyristors T₃, T₄ form another pair. The two thyristor pairs are triggered alternatively and conduct load current.

In load commutated chopper, at an instant, there are two main thyristors that conduct load current along with the commutating circuit. Whenever thyristors pair T₁, T₂ are acting as main thyristors, the thyristors pair T₃, T₄ along with the capacitor form the commutating circuit.

Whereas, when thyristors pair T₃, T₄ are acting as main thyristors, the thyristors pair T₁, T₂ along with the capacitor form the commutating circuit. A free-wheeling diode is connected across the load to free-wheel the inductive load current back to it.

Working of Load Commutated Chopper :

Now let us see the operation of the above load commutated chopper. In the above circuit, it should be assumed that the losses in diode and thyristors are zero and the load current is assumed to be constant. The operation of the circuit can be explained in the following modes.

Before starting the circuit, the capacitor C is assumed to be charged to the negative source voltage (-V_dc) with the upper plate as negative, and the lower plate as positive as shown below.

• Mode-I : At instant t = t₀, the thyristors pair T₁, T₂ is triggered. Hence, the load current flows through the path, V_dc⁺ → T₁ → C → T₂ → Load → V_dc^– as shown below. The load voltage will be equal to V_L = (V_dc + V_C) = 2V_dc.
  

  During this, the capacitor is charged linearly by the constant load current from negative source voltage (-V_dc) to positive source voltage (V_dc). When it is fully charged to V_dc, the thyristor pair T₁, T₂ are reverse biased. Once the load current falls below the holding current value of T₁ and T₂, they will be turned OFF which will take place at instant t = t₁.

• Mode-II : When T₁ and T₂ get turned OFF at t₁, the freewheeling diode (FD) is forward-biased. Hence the load current is transferred from T₁ and T₂ to the freewheeling diode. Therefore, the load current flows through the freewheeling diode from t₁ to t₂ and the load voltage becomes zero as shown below.
  
• Mode-III : The thyristors pair T₃, T₄ are triggered at instant t₂. Hence, the load current flows the path V_dc⁺ → T₄ → C → T₃ → Load → V_dc^– as shown below.
  

  The load voltage will be equal to 2V_dc i.e., V_L = V_dc + V_C = 2V_dc. At instant t₃, the capacitor is charged to -V_dc from V_dc, due to which thyristors T₃ and T₄ are reverse biased.

The output voltage again becomes zero and the load current is transferred to the freewheeling diode from the conducting SCR’s T₃ and T₄. The capacitor voltage V_C will be equal to -V_dc.

The thyristor pair T₁ and T₂ are triggered again and the whole modes of operation will repeat again. The waveforms of load voltage, load current, capacitor voltage, and gate signals of thyristors are shown below.

Advantages of Load Commutated Chopper :

• The load-commutated choppers are capable of operating at high frequencies which in turn reduces filtering requirements for smoothening the load current.
• In load commutated choppers inductor is not used for commutation purpose which is generally costly and bulky.
• The chopper can commutate any amount of current.

Disadvantages of Load Commutated Chopper :

• When the circuit is used for high power applications, the switching losses will be high, and also due to the series connection of thyristors with load, there will be considerable losses in it. Hence the efficiency will decrease for high power applications.
• The freewheeling diode is subjected to double the input voltage (2V_dc), thus diodes with short reverse recovery time must be used.
• In load commutated chopper, the maximum load voltage will be double the supply voltage. However, by using filters we can reduce the maximum value of load voltage.
• The commutating capacitor has to carry the full load current at a frequency of half the chopping frequency.
• In this circuit, in order to turn ON a thyristor pair, it should be ensured that the other thyristor pair is in a blocking state (turned-OFF state).