AC to DC - Rectifiers

What is the main function of a rectifier?

To convert AC into DC.

Name three rectifier classifications.

Uncontrolled (diodes), Controlled (SCRs), Semi-controlled (diodes + SCRs).

What devices are used in uncontrolled rectifiers?

Diodes.

What devices are used in controlled rectifiers?

Thyristors (SCRs).

What devices are used in semi-controlled rectifiers?

Mix of SCRs and diodes.

What is a single-phase half-wave rectifier?

One-diode circuit passing only positive half-cycles of AC.

What is a single-phase full-wave rectifier?

Circuit converting both halves of AC to pulsating DC using a center-tap or bridge.

What is a three-phase full-wave rectifier?

Six-diode or six-SCR bridge providing smoother DC with less ripple.

What is the function of a freewheeling diode?

Maintain current in inductive loads when the main device turns off, reducing ripple. It protects rectifier components from reverse voltage that could damage diodes or SCRS. This all minimizes losses and torque ripple in DC motor drives

Define CCM.

Continuous Conduction Mode: inductor current never reaches zero.

Define DCM.

Discontinuous Conduction Mode: inductor current falls to zero each cycle.

How is DC output of a controlled rectifier adjusted?

By changing SCR firing angle alpha.

Effect of increasing firing angle alpha?

Average DC output voltage decreases.

What happens if alpha > 90 degrees?

Average output can go negative (inversion region).

Common rectifier applications?

DC supplies, chargers, DC motor drives.

What is a dual rectifier?

A four-quadrant converter enabling bidirectional power flow (rectification and inversion).

Advantages of three-phase over single-phase?

Lower ripple, higher efficiency, better utilization.

How long is 1 degree at 60 Hz?

1/360 * 1/60 s which is approximately 46.3 us.

Describe half-wave rectifier output.

Positive half-sine pulses separated by zero; negative halves blocked.

Describe full-wave rectifier output.

Both halves become positive pulses, forming pulsating DC.

Describe RL load with freewheeling diode.

When main device turns off, current continues through diode, keeping current nearly continuous and load voltage positive.

Ripple Factor (definition)

Ripple factor measures the amount of AC content remaining in the rectified DC output. It is defined as the ratio of the RMS value of the AC component to the DC component of the output voltage:
r = \frac{V_{AC(rms)}}{V_{DC}} .
A lower ripple factor indicates smoother DC output.

Ripple Factor – Single-Phase Rectifiers

Approximate ripple factors for single-phase rectifiers:

• Half-wave rectifier: 121 %

• Full-wave rectifier: 48.2 %.

Ripple is significantly reduced in full-wave rectifiers since current flows during both halves of the AC cycle.

Ripple Factor – Three-Phase Rectifiers

Approximate ripple factors for three-phase rectifiers:

• 3-phase half-wave rectifier: 18.3 %

• 3-phase full-wave rectifier: 4.2 %.

Increasing the number of phases reduces ripple and improves DC smoothness.

Effect of Ripple

Ripple causes fluctuations in the DC output, producing voltage variations that can heat loads, reduce efficiency, and interfere with control circuits. Filtering or increasing the number of pulses helps reduce ripple.

Methods to Reduce Ripple

Ripple can be reduced by:

• Using filters (capacitors, inductors, LC or RC filters).

• Increasing the number of phases.

• Adding smoothing capacitors across the load to store charge and fill voltage gaps.

Conversion Efficiency (rectifier)

Rectifier efficiency is the ratio of DC output power to total AC input power: \eta = \frac{P_{DC}}{P_{AC(input)}} \times 100\% .

It indicates how effectively AC power is converted into usable DC.

Typical Efficiency Values

Approximate efficiencies for common rectifier types:

• Half-wave rectifier: ≈ 40 %

• Full-wave rectifier: ≈ 81 %.

Efficiency improves as both the conduction period and the number of pulses increase.

Effect of Load Type on Ripple

With a resistive load, current is discontinuous, giving higher ripple.
With an inductive load, current tends to remain continuous, lowering ripple.
Adding a freewheeling diode across an RL load also reduces ripple.

Relation between Ripple and Filtering Capacitor

A larger capacitor reduces ripple by storing more charge. Ripple voltage (approximate): V_{r(pp)} = \frac{I_{load}}{f \, C} . Here f is the ripple frequency and C is the filter capacitance.