Rectification - converting AC to DC
A direct current (DC) generator, as is used in Eniquest’s PowerMaker models, incorporates an alternator to convert the mechanical energy from its engine into electricity. The alternator inherently generates alternating current (AC) electricity. To enable the AC electricity to be stored in a storage battery bank it must be converted to DC electricity.
AC electricity is converted to DC electricity using a rectifier.
A rectifier uses diodes. A diode allows electrons to flow through it in one direction only. This is the symbol for a diode:
The blue arrows show the direction in which electrons can flow; electrons can’t flow back in the other direction.
The normal waveform for AC electricity looks like this:
When AC electricity is passed through a diode only one half of the complete cycle can pass through; the circuit looks like this:
Passing AC electricity through a single diode produces a waveform like this:
This process is called half-wave rectification, because only half of the full cycle is used.
Because all of the electricity flows in one direction, this is a form of DC electricity, but it’s very uneven and won’t run DC-powered equipment, including inverters, very well. Half-wave rectification is usually only used for very-low-power applications, because at very-low power the waveform can easily be smoothed using a capacitor.
Four diodes can be arranged so that the half of the AC electricity cycle that isn’t used in half-wave rectification gets flipped over, and both halves of the cycle are used. The four-diode arrangement is called a bridge rectifier and it looks like this:
In each half cycle a different pair of diodes carries the electricity to the load, so that the load always received electricity from the same direction. The flow of electricity through the bridge rectifier for each half cycle looks like this:
When AC electricity is passed through a full-wave bridge rectifier a waveform like this is produced:
This process is called full-wave rectification, because both halves of the full cycle are used. The resulting DC waveform is much more even that half-wave rectification, but is still not very smooth, and is usually smoothed using a capacitor.
Full-wave rectification is used by most mains-powered battery chargers and DC power supplies.
Three-phase full-wave rectification
An alternator can be constructed so that it produces more than one separate AC electricity supply at the same time. Commonly, alternators produce three separate electricity supplies, which are each timed so that they start at intervals of one-third of the cycle apart. This gives a much smoother overall flow of power. The three waveforms look like this:
The red circle indicates the same point on each of the three waveforms, and you can see that they are timed one-third of a cycle apart.
Each supply is called a phase, and the combined supply is called a three-phase supply. A single supply, such as normal household electricity, is called a single-phase supply.
A three-phase electricity supply can be rectified using six diodes arranged as a bridge rectifier like this:
When a three-phase supply is rectified both halves of all of the phases are added together so that they overlap to produce a much smoother DC waveform, which look like this:
This wave form is smooth enough to run most DC-powered equipment, including inverters, very well; however, it is usually smoothed further using a capacitor.
The alternator that is used in Eniquest’s PowerMaker generators is a three-phase alternator.
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