Rectification is a process whereby alternating current (AC) is converted into direct current (DC.)
Rectification is commonly performed by semiconductor diodes. Before the development of solid state rectifiers vacuum tub e diodes were used.
'Half-wave' rectification
In half wave rectification, either the positive or negative half of the AC wave is passed easily, the other half is blocked. Half wave rectification eliminates one half of the wave, and so is very inefficient.
'Full-wave' rectification
Full-wave rectification converts both polarities of the input waveform to DC, and is more efficient. However,depending on the transformer configuration, it can require four times as many rectifiers as half-wave rectification. This is due to each output polarity requiring 2 rectifiers each, for example, one for when AC - 'X' is positive and one for when AC - terminal 'Y' is positive. The other DC output requires exactly the same, resulting in four individual junctions. Four rectifiers arranged this way are called a bridge rectifier.
Automotive Application
Automotive alternators use diodes in the rectifier assembly.
A diode allows current to flow in the forward direction, but blocks the flow of current in the reverse direction.
A 3-phase bridge rectifier has 6 diodes to rectify the total alternator output -
One of each is used to rectify current in each of the 3 phase windings. The positive diodes let current flow out to the battery terminal B -positive. The negative diodes complete the return circuit from the battery terminal B - negative.
In each revolution of the magnet, the polarity of each phase winding changes, and as a result, the current changes direction. To provide a uni-directional, or DC output, a complete circuit is needed for current to flow when each change in polarity occurs. As the rotor turns, it induces a voltage in the winding, which generates current flow.
In this position, and with this polarity, the current path is as follows: output of winding A, positive diode A, alternator terminal B-positive, battery positive terminal, battery ground [B-negative], alternator ground, negative diode B, output of winding B, neutral or star point.
When the magnet rotates further to this position the polarity of winding A changes. The current path then is: winding A, at star point, winding C, positive diode C, alternator terminal B+, battery positive terminal, battery ground, alternator ground, negative diode A and output of winding A.
As the rotor moves through its various positions, individual phase currents change in magnitude and polarity, but the output current to the battery and the electrical circuits remains unidirectional.
Source: CDX Global & Wikipedia - en.wikipedia.org
