Controlled Rectifiers

Controlled rectifiers convert AC to adjustable DC by firing SCRs at an angle α after the natural commutation point. Increasing α reduces the average output voltage. With a highly inductive load the current is continuous and the standard cosα formulas apply.

A full converter (four SCRs in single phase) can operate in inverter mode (α > 90°) if the DC side has an active EMF, returning power to the AC line — the basis of regenerative DC drives.

Relation 1 —

Write this relation with symbols exactly as in Power Electronics — PS Bimbhra before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

Write this relation with symbols exactly as in Power Electronics — PS Bimbhra before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

Write this relation with symbols exactly as in Power Electronics — PS Bimbhra before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

For continuous conduction with an RL load, the single-phase full-converter output is V_dc = (2V_m/π)cosα, where V_m is the peak of the AC supply. At α = 0 it equals the uncontrolled diode-bridge value; at α = 90° the average is zero.

Input power factor degrades as α increases because the current lags the voltage by α (displacement) and is non-sinusoidal (distortion). This is why controlled rectifiers draw reactive power and harmonics.

A freewheeling diode across the load in a semiconverter improves power factor by preventing the output from going negative and allowing load current to circulate.

State assumptions explicitly before using any relation for controlled rectifiers — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Power Electronics viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Power Electronics papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to controlled rectifiers.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Controlled Rectifiers appears in drives, UPS, and grid interfaces. In Indian electrical curricula this topic is tested because it connects theory to controlled power conversion.

GATE and semester exams often combine controlled rectifiers with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use controlled rectifiers?" — answer with a lab, mini-project, or plant visit example if possible.

• Using peak line-to-line voltage where phase peak is required (or vice versa) in three-phase converters
• Applying the continuous-conduction formula when conduction is actually discontinuous
• Forgetting the full converter gives 2V_m/π (twice the half-controlled average)
• Ignoring the input displacement factor cosα when computing power factor

Before attempting controlled rectifiers problems, confirm you can:

1. Firing angle α controls average output voltage
2. Discontinuous conduction with RL load at large α
3. Inversion when α > 90° with active DC source

Revise the solved examples in Power Electronics — PS Bimbhra and one previous-year GATE or university paper for this unit.