For a single-phase half-wave-controlled converter system, sketch waveforms for load voltage and load current for (i) RL load and (ii) RL load with freewheeling diode across RL. From a comparison of these waveforms, discuss the advantages of using a freewheeling diode?

Question

Answer

Waveforms for RL Load and RL Load with Freewheeling Diode

RL Load

During the positive half-cycle of the input voltage, the SCR conducts and current flows through the load.

During the negative half-cycle of the input voltage, the SCR is reverse biased and the load current becomes zero.

The load voltage is proportional to the load current and is therefore also zero during the negative half-cycle of the input voltage.

RL Load with Freewheeling Diode

During the positive half-cycle of the input voltage, the SCR conducts and current flows through the load.

During the negative half-cycle of the input voltage, the freewheeling diode conducts and current flows through the load in the opposite direction.

As a result, the load current becomes continuous and the load voltage is no longer zero during the negative half-cycle of the input voltage.

Advantages of Using a Freewheeling Diode

The freewheeling diode provides a path for the load current to flow when the SCR is reverse biased.

This results in a continuous load current and a non-zero load voltage during the negative half-cycle of the input voltage.

Without the freewheeling diode, the load voltage would be zero during the negative half-cycle, resulting in a pulsating DC output.

The use of a freewheeling diode results in a smoother DC output voltage and reduces the amount of ripple.

It also reduces the stress on the SCR by preventing it from experiencing a high reverse voltage during the negative half-cycle.

The freewheeling diode also provides a path for the load current to flow in the event of a sudden loss of input voltage, preventing damage to the load.

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