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How Does an Alternating Current 3-Phase Generator Work?

In an AC single-phase generator, mechanical energy is used to create electrical energy. According to Faraday’s laws of electromagnetic induction, a generator produces voltage when an armature comprising wound copper coils around a metal core rotates through a magnetic field produced by two magnets. Single-phase units produce voltage in one single sine wave. An AC 3-phase generator is simply three AC single-phase unit combined to produce three sine waves sequentially with an offset of 120° between them.

A 3-phase generator produces three waves of AC voltage in one revolution, delivering a consistent supply of constant voltage. It is particularly useful when the power requirement is high and demand is constant. A 3-phase unit will typically have an output voltage of 480 volts.

A 3-phase is superior to single-phase, as it will weigh less, be physically smaller, and cost less for the same amount of power generated than an equivalent single-phase unit. A 3-phase unit is inherently more efficient than a single-phase unit.

Selecting a 3-Phase Generator

It is imperative when deciding what type of unit best suits your environment that you understand your electrical articulation and that you purchase a machine correctly configured for your electrical environment. One needs to know the phase usage, voltage, and frequency of the power required on site and also make an accurate assessment of the total power needed in order to correctly specify a suitable generator. It is often referred to as a genset because it consists of two primary components, namely an internal combustion engine (this can be diesel, natural gas, or propane powered) and a generator end. The engine produces power in kilowatts at a certain amount of revolutions per minute (RPM), while the end converts this mechanical rotational energy into electrical energy.

A Single- or 3-Phase Generator?

Single-phase units are used for smaller single-phase loads, loads typically associated with households and offices. Single-phase gensets are not usually made in capacities larger than 50 kVa. They are commonly used in environments where no 3-phase equipment is used. 3-Phase machines are found mainly in larger commercial and industrial environments and can be made in sizes from 5 kVa to utility-sized units. These generators are generally more flexible than single-phase machines are. They can provide both single- and 3-phase power and can run industrial scale motors with high kilowatt ratings. A 3-phase machine can power multiple lines and has a power factor of 0,8.

The Flexibility of a 3-Phase Generator

A 3-phase unit may be connected in two configurations: the Delta connection or the ”Y” connection (the ”Y” connection is also known as the Star connection). In a Delta connection, the ends of the three coils join to form a closed loop, which looks like the Greek letter Delta. In a “Y” or Star connection, one end of each coil joins together, leaving the other ends for connecting externally. This configuration has the appearance of the letter Y or a star.

It is possible for the machine to be derated by converting it from 3-phase to single-phase if required. Typically, power output would be reduced by approximately 30%. So, a 125 kVa 3-phase generator would become an 87 kVa unit when converted to single-phase. The generator end in question will have to be of the reconnectable type. Should it not be reconnectable, it can still run single-phase loads as long as the rated amps on each individual phase is not exceeded.

Conversely, an increase in the power output rating of a single-phase machine can be achieved by converting it to 3-phase output. A gain of up to 30% is possible. The generator end needs to be of the reconnectable type and there are factors, such as load balance that need to be considered. A specialist will be able to advise in this regard.

It is important to check upfront whether a machine is equipped with a generator end that is reconnectable. The ends are designed to be reconnectable or non-reconnectable. A 3-phase machine that is non-reconnectable will not be capable of having its output voltage changed to single-phase voltage. 12 lead reconnectable generator ends can be switched between three- and single-phase output voltages. 10 lead reconnectable ends can be converted to various three-phase voltages but not to single phase.

Generator sets may be connected in parallel for redundancy or increased capacity. Paralleling generators most often occurs where increased capacity is needed, usually where additional load is added to the system after the initial generator has been installed. Two or more generators are electrically joined to combine their power output. It is more desirable, and easier, to parallel identical generators than it is to join generators of different capacities and manufacturers. More thought and engineering expertise needs to go into overall system design based on the specific intended purpose of the system.

In general, a commercial spec 3-phase unit will be able to output 50 or 60 hertz frequency. This is usually achieved by simply running the engine at 1800 RPM for 60 Hz output and 1500 RPM for 50 Hz output. In some cases, modifications may be required. Higher capacity units may already be set to run at low rpm and the capability to change frequency should be canvassed with a specialist before purchase.

Sizing a 3-Phase Generator

How does one determine what size generator is required? The best approach is to engage with a supplier with a proven track record and a large install base of successful projects. A machine that can handle all the power generation needs of a specific site is one of the most critical aspects of the purchasing decision. Many of the uninitiated believe that purchasing a generator that has far more capacity than required will “more than cover” them.

Sizing is far more nuanced than simply purchasing the highest capacity machine the budget allows. Too small a machine puts undue stress on the unit and may even cause damage to the equipment connected to it. Too large a generator leads to inefficiencies and possible damage to the diesel engine when it runs for extended periods on very light loads. Determining exactly which size machine to install can be complex involving a number of factors and considerations.

Our qualified technicians offer support and advice in the selection of the right power solution for your needs by calculating your power requirements.

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