Single-Phase Dual Converter

Introduction

A single full-controlled bridge can already do two things: it can rectify (deliver power from the AC line to a DC load) and, at large firing angles, it can invert (push power the other way). What it cannot do on its own is reverse the direction of the load current — its thyristors only conduct one way. Many drives, however, need to control a motor in both directions of rotation and brake it back into the supply. That calls for control over both the polarity of the voltage and the direction of the current.

A dual converter solves this by using two converters working as a team across the same load. One is arranged to supply positive current, the other negative current; between them they cover all four combinations of voltage and current sign. This page looks at the single-phase dual converter — its circuit, the firing-angle relationship that ties the two converters together, its two current modes, the output waveforms, and how it delivers true four-quadrant operation.

What is a Single-Phase Dual Converter?

A single-phase dual converter is a converter built from two single-phase full converters connected in anti-parallel (back-to-back) across a common load. At any instant one bridge behaves as a rectifier (converting AC to DC and feeding the load), while the other is held ready to run as an inverter (returning energy from the load to the AC source). Because the two bridges point in opposite directions, the load can receive current of either polarity and voltage of either polarity.

Each bridge is phase-controlled by its own firing angle. If the two bridges are meant to present the same average voltage to the load, their firing angles are locked to one simple rule:

With this rule the two average output voltages are equal in magnitude, so the converters "agree" on the DC level seen by the load. That common voltage is set by whichever angle you choose, which is how a single control knob steers the whole four-quadrant drive.

Positive Group & Negative Group

The two bridges of a dual converter are given names according to the direction of load current each can supply:

• Positive group (Converter 1, "P"): the bridge that carries positive load current. When it is the active converter it usually runs as a rectifier (firing angle below 90°), sending power from the AC line into the load.
• Negative group (Converter 2, "N"): the bridge that carries negative load current. When it becomes active it typically runs as an inverter (firing angle above 90°), returning stored energy from the load back to the AC source.

Whether a group acts as a rectifier or an inverter is decided purely by its firing angle: below 90° the average output is positive (rectifying), above 90° it is negative (inverting). Swapping the roles of the two groups is exactly how the dual converter reverses motor torque or direction.

Circuit Diagram & Construction

The single-phase dual converter is assembled from a small, well-defined set of parts:

• Converter 1 — a single-phase full bridge of four thyristors (the positive group).
• Converter 2 — a second single-phase full bridge of four thyristors (the negative group), connected in the opposite sense.
• A common load — most often a separately excited DC motor armature, or any inductive DC load.
• A current-limiting reactor (L_r) — an inductor placed between the two bridges. It is essential in the circulating-current mode, where it absorbs the instantaneous voltage difference between the bridges and keeps the circulating current within safe limits.

Both bridges are fed from the same single-phase AC supply and their DC terminals face the load from opposite directions. A control circuit generates the two sets of gate pulses and enforces the α₁ + α₂ = 180° relationship so that the two average voltages match.

Principle of Operation (α₁ + α₂ = 180°)

The average DC output of a single-phase full converter fired at an angle α is E_dc = (2V_m/π) cosα, where V_m is the peak of the AC supply. Writing E_max = 2V_m/π for the no-delay maximum, the two bridges of the dual converter produce:

For the two bridges to present the same voltage to the load, their averages must be equal and opposite when measured around the loop, i.e. E_dc1 = −E_dc2. Substituting the formulas above gives cosα₁ = −cosα₂, whose solution is exactly the control rule:

So as one bridge rectifies at a small angle, its partner sits at the supplementary angle and is ready to invert. Move the control angle and both bridges track together, holding the same average load voltage while allowing current to be handed smoothly from one group to the other.

Modes of Operation

Although α₁ + α₂ = 180° makes the two average voltages equal, the two bridges do not produce identical instantaneous waveforms — their ripple differs from moment to moment. How the circuit deals with that difference gives two distinct operating modes.

1. Non-Circulating Current Mode

In this scheme only one bridge is allowed to conduct at a time; the gate pulses to the idle bridge are completely removed. Because the two bridges are never on together, no current can flow around the loop between them, so no reactor is needed. When the drive must change the direction of current, the control first blocks the active bridge, waits a short dead time (typically 10–20 ms) to be sure its thyristors have fully turned off, and only then starts the other bridge. This mode gives better efficiency and input power factor, but the changeover delay makes the load current briefly discontinuous and the control logic more demanding.

2. Circulating Current Mode

Here both bridges conduct at the same time, held at α₁ + α₂ = 180°. Their averages cancel around the loop, but their instantaneous ripple does not, so a small circulating current flows between the two bridges. A current-limiting reactor L_r is inserted in that loop to absorb the ripple voltage and keep the circulating current small and safe. Because both bridges are always active, current can be transferred from one group to the other with essentially no delay (0–15 ms), the load current stays smooth and continuous, and the control is simpler. The price is the extra reactor, higher device ratings and slightly lower efficiency.

Output Voltage Waveforms

The traces below are computed directly from the formulas above for the circulating-current case: converter 1 fired at α₁ = 60° (rectifying) and converter 2 at α₂ = 120° (inverting), both feeding a highly inductive load so each full bridge conducts continuously. The three panels share one time axis (ωt): the AC supply, then the output of each bridge with its average level marked.

Reading the Waveform

• Supply voltage v_s: the ordinary AC sine, V_m sinωt, over one full cycle. The pink dashed lines mark the firing instants α₁ and α₂.
• Converter 1 output v_o1: a two-pulse full-converter waveform fired at α₁ = 60°. Each thyristor pair conducts for a full half-cycle, so the trace dips briefly negative before the next pair takes over. Its average E_dc1 is positive (orange dashed line) — converter 1 is rectifying.
• Converter 2 output v_o2: the same style of waveform but fired at α₂ = 120°. Because α₂ > 90°, its average E_dc2 is negative — converter 2 is inverting.
• Equal magnitudes: since α₁ + α₂ = 180°, the two averages are the same size (|E_dc1| = |E_dc2|). That is what lets the two bridges share one load voltage.
• Circulating current: the instantaneous traces are not identical, so their difference appears across the reactor L_r and drives the small circulating current — the reason a reactor is needed in the circulating-current mode.

Four-Quadrant Operation

Plotting output voltage V_o against output current I_o gives four regions. A dual converter can work in all four, which is why it suits reversible drives. When V_o and I_o have the same sign the load absorbs power (motoring); when they have opposite signs power flows back to the supply (braking / regeneration).

Ideal vs Practical Dual Converter

It helps to separate the textbook "ideal" picture from what really happens in hardware:

In other words, the α₁ + α₂ = 180° rule guarantees equal average voltages, which is all an ideal converter needs. A real converter still has ripple, and it is that leftover ripple — not the averages — that creates the circulating current the reactor is there to tame.

Key Parameters & Formulas

For a single-phase dual converter with peak supply voltage V_m and firing angles α₁, α₂:

Choosing α₁ sets the load voltage through E_dc1 = E_maxcosα₁, and α₂ = 180° − α₁ follows automatically, keeping the idle bridge poised to take over the current whenever the drive needs to brake or reverse.

Advantages & Disadvantages

Advantages

• Full four-quadrant control — both voltage polarity and current direction can be reversed, ideal for reversible DC drives.
• Smooth transition between motoring and braking, especially in the circulating-current mode.
• Regenerative braking — energy from a decelerating load is returned to the AC supply instead of being wasted as heat.
• Fast dynamic response for speed and torque control of DC motors.

Disadvantages

• More devices and cost — two complete full bridges instead of one.
• Reactor required in the circulating-current mode, adding size, weight and loss.
• Complex control — precise firing so that α₁ + α₂ = 180° and safe changeover in the non-circulating mode.
• Lower efficiency in the circulating mode because of the continuous circulating current.

Applications

• Reversible DC motor drives that need forward and reverse running with braking — rolling mills, hoists and cranes.
• Traction and elevator systems where regenerative braking recovers energy on the down-run or during deceleration.
• Machine-tool and paper/textile drives that demand fast, four-quadrant speed and torque control.
• Test rigs and dynamometers that must both drive and absorb power from a machine under test.

Frequently Asked Questions – FAQs

Related Topics

• Single-Phase Full-Wave Controlled Rectifier (Full Converter)
• Single-Phase Semi-Converter (Half-Controlled Bridge)
• Single-Phase Half-Wave Controlled Rectifier
• Cycloconverter (Direct AC-AC Conversion)
• What is a Rectifier?