A freewheeling diode is connected across an inductive load is

A freewheeling diode is connected across an inductive load is

Right Answer is:

To restore conduction angle on phase


Many circuits, particularly those which are half or uncontrolled, include a diode across the load as shown in Fig. This diode is variously described as a commutating diode, flywheel diode or bypass 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. A freewheeling diode is connected across a load in the reverse direction. The energy stored in load inductance is feedback to load through the freewheeling diode.

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 maw rectifier, thereby allowing all of its thyristors to regain their blocking states.

Effect of inductive Load

In the case of an inductive load, as A.C voltage passes through zero after 180°, the SCR current may not reach zero and SCR may continue to conduct even when voltage wave is passing through the negative half cycle. This is due to the fact that for inductive loads current lags voltage and cannot be suddenly reduced to zero. During the negative half cycle current continues to now till the energy stored in the inductance is dissipated in the load resistance and a part of the energy is feedback to the source. As the current reaches zero, the SCR has turned off automatically and regains its blocking state. The figure shows the waveform of a half-wave rectifier with an inductive load.

Effect of Inductive load in the bridge rectifier

  1. Power factor reduces because of an inductive load.
  2. Output current ripple reduces due to inductive load.
  3. Output current tends to become continuous.

Effect of Free Wheeling Diode

The figure shows a diode D connected across a load. This diode is called the freewheeling diode. This diode prevents the conduction of SCR beyond 180°. At 180° ac voltage is zero and the voltage induced in the inductance change its polarity as di/dt changes its sign. As a result, the diode D starts conducting and stored energy in the inductance is discharged.

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 diode changes its sign and the freewheeling diode 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°, 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, the thyristor returns the energy stored in the load inductance to the supply line. With freewheeling diode the 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.

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