An electrical generator is a device that produces electrical energy from a mechanical energy source. The process is known as electricity generation. The electrical generator here refers mainly to Alternating current generators used for AC power generation driven by prime movers such as internal combustion engines, turbines etc.

First electrical generator[]

The dynamo was the first electrical generator capable of delivering power for industry, and is still the most important generator in use in the 21st century. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current. It is the most common way to generate electrical energy for bicycle lighting.

How it works[]

The generator rotor is turned by a device termed a prime mover, often a Diesel engine, steam turbine, water turbine or gas turbine coupled to the rotor shaft.

Equivalent circuit[]

Equivalent circuit of generator and load.
G = generator
VG=generator open-circuit voltage
RG=generator internal resistance
VL=generator on-load voltage
RL=load resistance

The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's and parameters, follow this procedure: -

  • Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance .
  • Start the generator. Before connecting the load , measure the voltage across the generator's terminals. This is the open-circuit voltage .
  • Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the on-load voltage .
  • Measure the load resistance , if you don't already know it.
  • Calculate the generator's AC internal resistance from the following formula:

Note 1: The AC internal resistance of the generator when running is generally slightly higher than its DC resistance when idle. The above procedure allows you to measure both values. For rough calculations, you can omit the measurement of and assume that and are equal.

Note 2: If the generator is an AC type (distinctly not a dynamo), use an AC voltmeter for the voltage measurements.

Maximum power theory[]

The maximum power theorem applies to generators as it does to any source of electrical energy. This theorem states that the maximum power can be obtained from the generator by making the resistance of the load equal to that of the generator. However, under this condition the power transfer efficiency is only 50%, which means that half the power generated is wasted as heat inside the generator. For this reason, practical generators are not usually designed to operate at maximum power output, but at a lower power output where efficiency is greater.

Power output[]

Maximum power[]

In power generating stations or thermal power stations where electrical generators are used the capacity ranges from 5 MW to 500 MW, driven by steam turbines. The details of these generators are available there.


Early motor vehicles tended to use DC generators with regulators. These were not particularly reliable or efficient and have now been replaced by alternators with inbuilt rectifier circuits. These power the electrical systems on the vehicle and recharge the battery after starting. Rated output will typically be in the range 50-100 A at 12 V, depending on the forecast electrical load within the vehicle - some cars now have Electrically powered steering assistance and air conditioning, which places a high load on the electrical system. Commercial vehicles are more likely to use 24 V to give sufficient torque at the starter motor to turn over a large diesel engine. Vehicle alternators do not use permanent magnets; they can achieve efficiencies of up to 90% over a wide speed range by control of the field voltage.

Lowest power[]

Some of the smallest generators commonly found are used to power bicycle lights. These tend to be 0.5 A permanent-magnet alternators, supplying 3-6 W at 6 V or 12 V. Being powered by the rider, efficiency is at a premium, so these may incorporate rare-earth magnets and be designed and manufactured with great precision. Nevertheless, the maximum efficiency is only around 60% for the best generators - 40% is more typical - due to the use of permanent magnets. A battery would be required in order to use a controllable electromagnetic field instead, and this is unacceptable due to its weight and bulk.

Aircraft have also switched from DC generators to alternators; these are typically powered by a takeoff from an engine.

Sailing yachts may use water or wind powered generator to trickle-charge the batteries. A small propellor, wind turbine or impeller is connected to a low-power alternator and rectifier to supply currents of up to 10 A at typical cruising speeds.

Engine-generator unit[]

An engine-generator is the combination of an electrical generator and an engine mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a genset. In many contexts, the engine is taken for granted and the combined unit is simply called a generator.

In addition to the engine and generator, engine-generators generally include a fuel tank, an engine speed regulator and a generator voltage regulator. Many units are equipped with a battery and electric starter. Standby power generating units often include an automatic starting system and a transfer switch to disconnect the load from the utility power source and connect it to the generator.

Engine-generators produce alternating current power that is used as a substitute for the power that might otherwise be purchased from a utility power station. The generator voltage (volts), frequency (Hz) and power (watts) ratings are selected to suit the load that will be connected. Both single-phase and three-phase models are available.

Engine-generators are available in a wide range of power ratings. These include small, hand-portable units that can supply several hundred watts of power, hand-cart mounted units, that can supply several thousand watts and stationary or trailer-mounted units that can supply over a million watts. The smaller units tend to use gasoline (petrol) as a fuel, and the larger ones have various fuel types, including diesel, natural gas and propane (liquid or gas).


Engine-generators are often used to supply electrical power in places where utility power is not available and in situations where power is needed only temporarily. Small generators are sometimes used to supply power tools at construction sites. Trailer-mounted generators supply power for lighting, amusement rides etc. for traveling carnivals.

Standby power generators are permanently installed and kept ready to supply power to critical loads during temporary interruptions of the utility power supply. Hospitals, communications service installations, sewerage pumping stations and many other important facilities are equipped with standby power generators.

Small and medium generators are especially popular in third world countries to supplement grid power, which is often unreliable. Trailer-mounted generators can be towed to disaster areas where grid power has been temporarily disrupted.

The mid-size stationary engine-generator can be a 100 kVA set which produces 415 V at around 100 A per phase. It's powered by a 6.7 litre turbocharged engine, and consumes approximately 27 litres of fuel an hour, on a 400 litre tank. Stationary generators used in big buildings are generally in size up to 2800 kW. These diesel engines rotate at 1500 rpm to give power at 50 Hz. In areas where the power frequency is 60 Hz, the generators rotate at 1800 rpm or another even multiple of 60.


See also[]

  • Alternator
  • MHD generator|Magnetohydrodynamic generator [1]
  • Motor-generator

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