Why Three-Phase Drives?

Advantages Over Single-Phase Drives:

1. Higher Power Ratings: Suitable for \(100\;\text{kW}\) to \(1500\;\text{kW}\) and beyond

2. Higher Ripple Frequency:

   • Single-phase full-bridge: ripple at \(2f\) (e.g. \(120\;\text{Hz}\) for \(60\;\text{Hz}\) supply)

   • Three-phase full-bridge (6-pulse): ripple at \(6f\) (e.g. \(360\;\text{Hz}\) for \(60\;\text{Hz}\) supply)

3. Lower Filtering Requirements: Smaller smoothing inductor required

4. Better Motor Performance: More continuous armature current

5. Lower Torque Pulsation: Smoother shaft torque

6. Higher Efficiency: Better utilisation of transformer and supply capacity

Three-Phase Drives – Power Ranges

Classification of Three-Phase Drives

Based on Converter Configuration:

1. Three-Phase Half-Wave (3-pulse) Converter

   • Requires a neutral connection; rarely used industrially

2. Three-Phase Semiconverter (half-controlled bridge)

   • One-quadrant operation (forward motoring only)

   • Up to \(115\;\text{kW}\)

3. Three-Phase Full-Converter (fully-controlled bridge)

   • Two-quadrant operation (motoring + regenerative braking)

   • Up to \(1500\;\text{kW}\)

4. Three-Phase Dual-Converter

   • Four-quadrant operation

   • Up to \(1500\;\text{kW}\)

Three-Phase Supply – Notation

Standard Three-Phase System:

• Phase sequence: \(a\)-\(b\)-\(c\)

• Phase voltages (Y-connected supply):

  \[\begin{aligned} v_a &= V_m \sin\omega t \\ v_b &= V_m \sin\!\bigl(\omega t - 120^{\circ}\bigr) \\ v_c &= V_m \sin\!\bigl(\omega t - 240^{\circ}\bigr) \end{aligned}\]

• \(V_m = \sqrt{2}\,V_p\) — peak phase voltage

• \(V_{Lm} = \sqrt{2}\,V_L = \sqrt{3}\,V_m\) — peak line-to-line voltage

• Relation: \(V_L = \sqrt{3}\,V_p\)

Ripple Frequency Comparison

Key Observation:

• Single-phase bridge (2-pulse): ripple period \(= T/2\), ripple frequency \(= 2f\)

• Three-phase bridge (6-pulse): ripple period \(= T/6\), ripple frequency \(= 6f\)

Consequence

A six times higher ripple frequency means the peak-to-peak voltage ripple is much smaller, so a smaller smoothing inductor \(L_a\) is needed for the same current-ripple specification.

Three-Phase Semiconverter – Introduction

Key Features:

• One-quadrant drive: forward motoring only

• Power range: up to \(115\;\text{kW}\)

• Half-controlled bridge: 3 thyristors (positive group) \(+\) 3 diodes (negative group) \(+\) freewheeling diode \(D_F\)

• Output voltage always \(\geq 0\): cannot regenerate

• Firing angle range: \(0^{\circ} \leq \alpha \leq 180^{\circ}\)

Circuit Topology

Components:

• Thyristors: \(T_1\), \(T_3\), \(T_5\) (positive group)

• Diodes: \(D_2\), \(D_4\), \(D_6\) (negative group)

• Freewheeling diode: \(D_F\)

• DC motor: armature resistance \(R_a\), inductance \(L_a\), back-EMF \(E_a\)

Operating Principle

Conduction Sequence

The semiconverter operates with the following sequence:

1. Positive group (thyristors): \(T_1\), \(T_3\), \(T_5\) are fired at \(60^{\circ}\) intervals with delay angle \(\alpha\)

2. Negative group (diodes): \(D_2\), \(D_4\), \(D_6\) conduct naturally when forward-biased

3. Freewheeling mode: When all thyristors are off, current flows through \(D_F\), shorting the motor terminals

Output Voltage Analysis

Average Output Voltage

For continuous conduction mode, the average armature voltage is:

where:

• \(V_m = \sqrt{2}\,V_p\) is the peak phase voltage
• \(\alpha\) is the firing delay angle (\(0^{\circ} \leq \alpha \leq 180^{\circ}\))

In terms of RMS line voltage \(V_L\):

Maximum and Minimum Values

• At \(\alpha = 0^{\circ}\): \(V_{a,\max} = \dfrac{3\sqrt{3}\,V_m}{\pi} = \dfrac{3\,V_{Lm}}{\pi}\)

• At \(\alpha = 180^{\circ}\): \(V_{a,min} = 0\)

Waveforms

Observations:

• Output voltage has 6 pulses per cycle (6-pulse converter)

• Ripple frequency: \(6f\) (e.g., \(300\;\text{Hz}\) for \(50\;\text{Hz}\) supply)

• Freewheeling intervals appear when \(v_a = 0\)

• Armature current is relatively smooth due to motor inductance

Performance Characteristics

Input Power Factor

For continuous current operation:

Advantages

• Lower cost (only 3 thyristors vs. 6 for full-converter)

• Simpler control circuitry

• Lower harmonic distortion than full-converter

• Inherent freewheeling action

Limitations

• One-quadrant operation only (no regeneration)

• Cannot reverse motor without mechanical contactors or field reversal

• Limited to lower power applications (\(\leq 115\;\text{kW}\))

Three-Phase Full-Converter – Introduction

Key Features:

• Two-quadrant operation: motoring in one direction + regenerative braking

• Power range: up to \(1500\;\text{kW}\)

• Fully-controlled bridge: 6 thyristors

• Output voltage can be positive or negative

• Firing angle range: \(0^{\circ} \leq \alpha \leq 180^{\circ}\)

• Most common industrial drive topology

Circuit Topology

Components:

• Positive group: \(T_1\), \(T_3\), \(T_5\)

• Negative group: \(T_2\), \(T_4\), \(T_6\)

• All devices are controlled thyristors

• No freewheeling diode required

Operating Principle

Conduction Sequence

Each thyristor conducts for \(120^{\circ}\):

1. Positive group: \(T_1\), \(T_3\), \(T_5\) fired at \(60^{\circ}\) intervals

2. Negative group: \(T_2\), \(T_4\), \(T_6\) fired at \(60^{\circ}\) intervals, displaced by \(180^{\circ}\) from positive group

3. At any instant, one thyristor from the positive group and one from the negative group conduct

Operating Modes

Output Voltage Analysis

Average Output Voltage

For continuous conduction mode:

In terms of RMS line voltage \(V_L\):

Voltage Range

• At \(\alpha = 0^{\circ}\): \(V_{a,\max} = \dfrac{3\,V_{Lm}}{\pi} \approx 1.35\,V_L\)

• At \(\alpha = 90^{\circ}\): \(V_a = 0\)

• At \(\alpha = 180^{\circ}\): \(V_{a,\min} = -\dfrac{3\,V_{Lm}}{\pi} \approx -1.35\,V_L\)

Waveforms

Speed Control Strategies

Below Base Speed (Constant Torque Region)

Armature Voltage Control:

• Field current \(I_f\) kept constant at rated value

• Speed controlled by varying \(\alpha\) (and hence \(V_a\))

• Motor equation: \(V_a = E_a + I_a R_a \approx K_\phi \omega_m\)

• Maximum torque available (constant torque operation)

Above Base Speed (Constant Power Region)

Field Weakening Control:

• Armature voltage at maximum (\(\alpha \approx 0^{\circ}\))

• Speed increased by reducing field current \(I_f\)

• Torque capability decreases: \(T \propto I_a \phi\)

• Constant power operation: \(P = T\omega_m = \text{constant}\)

Four-Quadrant Operation with Field Reversal

Full-converter provides two-quadrant operation. For four-quadrant capability:

• Forward direction: Quadrants I (motoring) and II (regeneration)

• Reverse direction: Reverse field polarity using contactors, then operate in Quadrants III (motoring) and IV (regeneration)

Input Power Factor

For continuous current operation:

Observations:

• Power factor decreases as \(\alpha\) increases

• At \(\alpha = 0^{\circ}\): \(\text{PF} \approx 0.955\) (excellent)

• At \(\alpha = 60^{\circ}\): \(\text{PF} \approx 0.477\) (poor)

• Low power factor increases supply current and losses

Advantages and Limitations

Advantages

• Two-quadrant operation (motoring + regenerative braking)

• High power capability (up to \(1500\;\text{kW}\))

• Good speed regulation

• Energy recovery during braking

• Industry standard for medium/high-power drives

Limitations

• Cannot reverse without field reversal (slow process)

• Power factor decreases with firing angle

• Harmonics injected into AC supply

• More complex than semiconverter

Dual-Converter – Introduction

Key Features:

• Four-quadrant operation without mechanical contactors

• Fast electronic reversal

• Two 6-pulse full-converters connected in anti-parallel

• Power range: up to \(1500\;\text{kW}\)

• Total of 12 thyristors

Circuit Configuration

Components:

• Converter P (Positive): Thyristors \(T_1\)–\(T_6\), firing angle \(\alpha_P\)

• Converter N (Negative): Thyristors \(T_7\)–\(T_{12}\), firing angle \(\alpha_N\)

• Both converters share the same DC motor

Operating Modes

Non-Circulating Current Mode (Preferred)

Principle:

• Only one converter operates at a time

• The other converter is blocked (no gating pulses)

• No circulating current between converters

• More efficient, simpler control

Control Strategy:

Circulating Current Mode

Principle:

• Both converters operate simultaneously

• Firing angles constrained: \(\alpha_P + \alpha_N = 180^{\circ}\)

• Inter-group reactor \(L_r\) required to limit circulating current

• Faster response but lower efficiency

Four-Quadrant Operation

Reversal Sequence

Transition from Forward to Reverse:

1. Deceleration: Increase \(\alpha_P\) beyond \(90^{\circ}\) to regenerate and slow the motor

2. Zero speed: Motor stops momentarily

3. Transfer: Block Converter P, activate Converter N

4. Acceleration: Decrease \(\alpha_N\) from \(90^{\circ}\) to accelerate in reverse direction

Applications

Typical applications requiring four-quadrant operation:

• Rolling mills (frequent reversals)

• Hoists and cranes (four-quadrant operation essential)

• Machine tools (precise positioning with fast reversal)

• Traction drives (metro, trams)

• Elevators (smooth acceleration/deceleration)

• Paper mills and textile machinery

Advantages and Limitations

Advantages

• True four-quadrant operation

• Fast electronic reversal (no mechanical contactors)

• Smooth transition between quadrants

• Excellent dynamic performance

• Regenerative braking in both directions

Limitations

• High cost (12 thyristors + complex control)

• Increased complexity

• Requires careful coordination between converters

• Potential for circulating current if not properly controlled

Single-Phase vs. Three-Phase Drives

Comparison: Three-Phase Converter Types

Drive Selection Guide

Key Equations at a Glance

Summary

1. Three-phase converters offer higher power capacity, lower ripple (\(6f\)), and better efficiency than single-phase counterparts.

2. Semiconverter: one-quadrant, \(V_a \geq 0\) always; uses freewheeling diode; suitable for simple unidirectional loads.

3. Full-converter: two-quadrant (motoring + regenerative braking); the workhorse of industrial DC drives; \(V_a\) can be positive or negative by varying \(\alpha\) through \(0^{\circ}\)–\(180^{\circ}\).

4. Dual-converter: four-quadrant by anti-parallel connection of two full-converters; enables fast electronic reversal; non-circulating current mode is preferred.

5. Speed control below base speed uses armature voltage control (constant torque); above base speed uses field weakening (constant power).

6. Field converter should be a full-converter (or better) for any drive that requires fast field reversal or field weakening.