Rectifying Three Phase Power

Our wind turbine produces three phase alternating current (AC), which we'd like to rectify to direct current (DC). Recall the bridge rectifier circuit of Chapter 4, which used 4 diodes. In the bridge circuit, two diodes were active during the positive part of the waveform, and the other two diodes during the negative part.

By adding two more diodes, this circuit can be expanded to rectify all three phases from our wind turbine. With 6 diodes, the three-phase rectifier can rectify all three phases. Each phase uses 3 of the 6 diodes at any given time. These 3 diodes work just like the diodes in the bridge rectifier, one for the positive part of the waveform, the others for the negative part of the waveform. At different times, different diodes are used by different coils.

The diagrams below trace the current flow through the three-phase rectifier. Each frame freezes a moment during an interesting event (phase max positive/negative values) and analyzes how current is steered and rectified within the three-phase rectifier. Figure 6-7 shows the moment of maximum voltage generated at each phase, and Figure 6-8 shows the moment of minimum voltage generated at each phase.

Diode (conducting) Diode (not conducting)

. A ■ / \ • ' \

✓ \

Phase 1 at its maximum positive value.

V1 V2

V1 V2

Phase 2 at its maximum positive value.

Phase 3 at its maximum positive value.

Figure 6-7

Wye Coil Arrangement Schematic voltages are percentages of maximum values.

V1 = -70% V2 =+100% • V3 = -70%

1

\

(

*

1

Phase 3 at its maximum positive value.

1

1

Wye Coil Arrangement Schematic Diode (conducting) 1 Diode (not conducting) voltages are percentages of maximum values.

V2 V3

Figure 6-8

Wye Coil Arrangement Schematic Diode (conducting) 1 Diode (not conducting) voltages are percentages of

V2 V3

Phase 1 at its maximum negative value. V1 V2 V3
Phase 2 at its maximum negative value. V1 V2 V3

Phase 3 at its maximum negative value.

Phase 3 at its maximum negative value.

1

1 "

1

As you can see from the images, regardless of the polarity and intensity of any phase's voltage, the steering abilities of the three-phase bridge rectifier allow current to flow in only one direction through the load.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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