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DC-optimised alternator

An 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 direct current (DC) electricity

An alternator designed for producing DC electricity, as PowerMaker alternators are, has very different requirements to an alternator that produces AC electricity to directly operate mains-powered appliances. An alternator must be designed from the very start for generating DC electricity if it is to be properly optimized to work well with the process of converting AC electricity to DC electricity.

Converting AC electricity to DC electricity

The flow of AC electricity rises and falls smoothly as it flows back and forth in a circuit.  AC electricity flow is represented in a waveform that looks like this:

AC waveform

AC electricity is inherently uneven because its voltage rises and falls with its cycle.  To convert AC electricity to good-quality DC electricity the AC electricity must be made to all flow in one direction, and the unevenness of its rise and fall must be smoothed out.

Rectification

The process of making AC electricity all flow in one direction is called rectification.

In the process of rectification one half of the AC electricity cycle gets flipped over, and added to the other half of the cycle to produce DC electricity.  The DC electricity that this process creates looks like this:

rectified AC waveform

While the electricity is now all flowing on one direction the flow is very uneven, and is very poor-quality DC electricity.

Three phases

An alternator can be constructed with more than one set of coils so that it produces more than one separate AC electricity supply at the same time. Commonly, alternators produce three separate AC 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:

3-phase waveform

As each of the three supplies is timed at a different phase of the same cycle they are called phases, and the combined supply is called a three-phase supply. A single supply, such as normal household electricity, is called a single-phase supply.

When one of these three supplies is at a peak voltage the other two are at a reduced voltage, and when any of them is at zero voltage the other two are still providing some voltage, so the three of them added together provide a very even supply of electricity.

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:

rectified 3-phase

The alternator that is used in Eniquest’s PowerMaker generator sets is a three-phase alternator.

Frequency

Whether the AC electricity supply that is being rectified is single phase or three phase, it can be further evened out by the process of smoothing. The most common process of smoothing uses a capacitor to store some of the electricity from the voltage peaks and uses it to fill in the voltage troughs. 

The higher the frequency of the AC electricity supply that is being converted to a DC supply the more effective the smoothing process is.

Electricity from an alternator is produced by a magnet rotating near coils of wire.  The magnet produces a magnetic field between its two ends, which are called poles; the poles are named the north pole and the south pole. As each alternate pole sweeps past a coil of wire electricity is induced into the coil in opposite directions, and the electricity flows back and forth from the coil through the circuit. A simple alternator looks like this.

2-pole alternator

 

One cycle of AC electricity is generated  for every rotation of the two poles of the magnet, so the rate at which the magnet rotates sets the frequency of the AC electricity that is generated.  

If two more magnetic poles are added then four magnetic poles will sweep past the coils each time they rotate, and two cycles of AC electricity will be generated, giving twice the frequency for the same rotation speed of the magnets.

 



More magnetic poles can be added to increase the frequency further. In practice, small magnets are used around the perimeter of the alternator’s rotor. This picture shows the arrangement of magnets in a 12-pole PowerMaker alternator:

12-pole rotor

Normal mains electricity has a frequency of 50 Hertz. The alternator that is used in Eniquest’s PowerMaker generators generates electricity with a frequency of around 350 Hertz.

With three-phases, high frequency from its twelve poles, and constant voltage due to its variable–speed operation, Eniquest’s PowerMaker generators are optimised to provide a smooth and stable DC electricity supply.


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