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Smoothing DC electricity

A direct current (DC) generator set, as is used in Eniquest’s PowerMaker models, incorporates an alternator to convert the mechanical energy from its engine into electrical energy.  The alternating current (AC) electricity from the alternator is rectified  to convert it into direct current  (DC) electricity.  The DC electricity that is created this way still bears some of the characteristics of the AC electricity from which it is made and so is not very smooth.  The waveform for rectified DC electricity looks like this:

Half-wave rectified, full-wave rectified, and three-phase full-wave rectified DC electricity can all be further smoothed; the most common way of doing this is to use a capacitor

A capacitor can directly store electricity; that is, rather than covert electrical energy to some other form such as chemical energy as a rechargeable storage battery does, a capacitor directly stores electrons on its internal plates.

When a capacitor is connected to a rectified DC electricity supply it charges with electricity from the supply.  As the voltage coming from the rectified DC electricity falls after each peak the voltage is maintained with electricity from the capacitor.  

The circuit of a full-wave rectified single-phase electricity supply that is smoothed with a capacitor looks like this:

smoothing circuit 

Unlike a battery, for which the voltage remains relatively constant as it discharges, the voltage of a capacitor falls steadily and continuously as it discharges, so the voltage between the peaks always falls.  The greater the load the faster the capacitor is discharged, and the more steeply the voltage falls.

The waveform of a full-wave rectified single-phase electricity supply which is smoothed with a capacitor looks like this:

smoothed waveform 

In this illustration the time is divided into green and brown bands.  During the brown bands the capacitor is supplying the energy to the load, and during the green bands the rectified AC supply is both supplying the energy to the load and charging the capacitor.

Like the three-phase full-wave rectification, this wave form is smooth enough to run most DC-powered equipment, including inverters, very well.

Frequency

During the part of the waveform that the capacitor is supplying the electricity to the load the voltage falls steadily and continuously as the capacitor discharges.  The sooner the electricity to the load is being supplied by the rectified AC supply the less time the voltage from the capacitor has to fall, so the smoother the electricity supply is.

This timing depends on the frequency of the original AC electricity supply.  The higher the frequency of the original AC electricity supply the sooner the capacitor will be recharged and the better it will be able to smooth the rectified DC electricity supply.  The difference looks like this:

The red lines show the lowest voltage.  The low frequency voltage dips lower than the high frequency voltage does.  

Normal mains electricity has a frequency of 50 Hz.  The alternator that is used in Eniquest’s PowerMaker generator has a frequency of around 350 Hz, and so gives a much smoother DC supply than rectified mains frequency dose.

Full-wave rectification is used by most main-powered battery chargers and DC power supplies.


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