Many circuits, particularly those which are half or uncontrolled, include a diode across the load as shown in Fig.1. This diode is variously described as a commutating diode, flywheel diode or by-pass diode. This diode is commonly described as a commutating diode as its function is to commutate or transfer load current away from the rectifier whenever the load-voltage goes into a reverse-state.

This diode serves two main functions:

(i) It prevents reversal of load voltage except for small diode voltage-drop.

(ii) It transfers the load current away from the main rectifier, thereby allowing all of its thyristors to regain their blocking states.

Figure 2 shows a half-wave controlled rectifier with a freewheeling diode $D_{f}$ connected across $R-L$ load. The load-voltage and current waveforms are also shown in Fig.3. With diode $D_{f}$ thyristor will not be able to conduct beyond $180^{\circ}$ .

We know that during the positive half-cycle, voltage is induced in the inductance. Now, this induced voltage in inductance will change its polarity as the $d_i/d_t$ changes its sign and diode $D_{f}$ will start conducting as soon as the induced voltage is of sufficient magnitude, thereby enabling the inductance to discharge its stored energy into the resistance.

Hence, after $180^{\circ}$ , the load current will freewheel through the diode and a reverse-voltage will appear across the thyristor. The power flow from the input takes place only when the thyristor is conducting. If there is no freewheeling diode, during the negative portion of the supply voltage, thyristor returns the energy stored in the load inductance to the supply line. With diode $D_{f}$ the free- wheeling action takes place and no power will be returned to the source. Hence, the ratio of the reactive power flow from the input to the total power consumed in the load is less for the phase-control circuit with a freewheeling diode. In other words, the freewheeling diode improves the input power-factor.

Mathematically:

$$\frac{E I \sin \phi}{E I}=\operatorname{less} \quad \therefore \sin \phi=l e s s$$

since, $$\phi=l e s s \quad \therefore Power-factor \cos \phi=\operatorname{more}$$

Hence it is clear that the freewheeling diode helps in improvement of power factor of the system.