Energy Unlimited by Victron

7. Generators

7.1. AC Generators

7.1.1. A diesel engine will last longer if it has to work

In order to generate a stable 50 Hz or 60 Hz output, the diesel engine powering the generator must rotate at a fixed and stable frequency. For 50 Hz output this is 3000 rpm or 1500 rpm, depending on the number of poles of the generator (3000 rpm / 60 seconds = 50 rotations per second = 50 Hz). When a diesel engine runs at relatively high rpm and with nearly no load the internal temperature will be low and service life will be reduced. It is therefore not recommended to run a genset 24 hrs per day, with nearly no load. And the noise, fumes and odours are not to look forward to either.

7.1.2. A hybrid or battery assisted AC system

A first improvement is to run the generator during periods of high power demand only, and install a battery and inverters to generate AC when the generator is off.

An even better system is obtained by operating one or more Phoenix Multi’s or MultiPlus units in parallel with the genset (see for example par. 10.6). The advantages are: - uninterrupted AC supply - relatively more load on the generator, less space needed, less noise and less weight because a smaller genset can be used: the MultiPlus will absorb peak loads taking energy from the battery, and recharge whenever “surplus” power is available (see for ex. par. 10.6.5. or “Achieving the impossible” and many other examples on our website).

7.1.3. Don’t forget the problem of limited shore power

A washing machine, dishwasher, electric cooker, air-conditioning: it is all feasible with a big enough generator. But in Europe power from the shore side is often limited to 16 A or even less (16 A x 230 V = 3,68 kW). Here also the MultiPlus can help to increase available power to the required level.

7.1.4. 3000 rpm or 1500 rpm (in a 60 Hz environment: 3600 rpm or 1800 rpm)

A, more expensive, 1500 rpm genset is the right choice if intensive use is to be expected. A 3000 rpm genset is in general designed for a limited number of operating hours, and is not made to operate at full load for long periods of time. Some generator suppliers are wildly optimistic about the maximum output of their product. A way to find out is to look for gensets from different suppliers but with the same engine and then compare the rated output.

7.2. DC Generators

Next to conventional 50/60 Hz AC generators, some generator suppliers are also offering DC generators. Outputs of up to 10 kW, which means a battery charging current of up to some 300 A at 28 V, are attainable. DC generators are smaller and lighter, and have a higher efficiency than AC generators. Moreover, engine rpm can be harmonised with power demand, so that efficiency remains high even under partial load. The idea is to use the DC generator to charge the batteries, and use inverters to supply the AC load. Sizing of the DC generator is a question of acceptable running hours per day. Please keep in mind however that the battery should be sized for the huge charge current. For a charge current of 300 A for example, battery capacity should be 300 A / 5 = 1500 Ah (see par. 2.5.6.) It should be noted here that some manufacturers of AGM batteries claim much higher charge currents without appreciable reduction of service life.