Electrical power generators, also known as alternators, transform mechanical energy into electrical energy. They can be used as backup for emergency power or as an alternator on board a vehicle. Generators can produce either AC or DC power and are typically powered by a fuel engine.

Characteristics

Generators produce electricity from mechanical inputs through the physical process of electromagnetic induction. In their simplest form, generators harness mechanical energy by rotating a coil through a magnetic field or electromagnets around a fixed coil in order to induce an electrical current.

Electromagnetic induction was discovered by Michael Faraday in 1830 and the physical process that produces an electromotive force (EMF) can best be explained by Faraday's Law of Electromagnetic Induction:

 Where:

         Electromotive Force (EMF)

      Number of Identical Loops of Tightly Wound Wire

        Rate of Change in Magnetic Flux

This formula concludes that the amount of electricity generated is dependent on the rate of change in magnetic flux as well as the number of turns of wire in the coil. Furthermore, the rate of change in magnetic flux is dependent on the rotational speed between the coil and the magnet, and strength of the magnetic field.

A generator is inversely analogous to an electric motor

Features

The term electrical power generators is primarily used to describe gensets with an integral combustion engine. The broader interpretation includes what can be referred to as alternative power generators that utilize renewable resources such as solar, thermal, and wind power. More information about alternative power generators can be found in the published selection guide for alternative power generators.

Types

Generators use conventional fuel sources such as gasoline, diesel, propane, and natural gas. Since each fuel source has unique thermal properties, each genset is primarily designed to consume a specific fuel source.

Diesel Gensets

Diesel is a high octane fuel that is easy to refine while its shortcoming stems from the amount of pollutants it contains. Diesel generators boast high efficiency producing more energy on a normalized basis when compared to gasoline. Diesel fuel burns hotter, leaving fewer residues on internal combustion chambers, allowing for increased engine life as well. Recent developments to integrate an exhaust scrubber have impacted the efficiency of diesel generators while they still outperform other gensets in energy efficiency.

Gasoline Gensets

Gasoline is a lightweight, volatile conventional fuel that quickly evaporates. Gasoline generators have a lower heat of combustion and produce less energy on a volume normalized basis when compared to diesel generators. In most cases, gasoline generators are selected over diesel gensets due to lower air pollutants and fuel availability.

Propane (LPN) and Natural Gas Gensets

Propane and natural gas are considered environmentally friendly fuels. Most gensets that use propane or natural gas can be converted to burn either fuel, while some tri-fuel gensets are capable of being adjusted to burn gasoline as well. Propane is the energy efficient alternative while natural gas is a cheaper fuel source. Both fuels are used in lean-burn gensets that use a mixture of fuel and air in order to reduce nitric oxides (NOx) emissions and increase fuel efficiency.

Specifications

Electrical power generators supply either a single or three phase AC voltage. This is the direct output from the alternator while voltage governors, frequency converters, and rectifiers are used to condition the output in order to supply a fixed voltage and frequency. The frequency of the AC voltage from the alternator is dependent on the rotational speed of the combustion engine and the number of magnetic poles. The following formula illustrates the relationship between the frequency, number of magnetic poles, and rotational speed of the alternator:

Where:

     Frequency (Hz)

   Rotational Speed of Engine (rpm)

    Number of Magnetic Poles (#)

As the load on the generator increases, the torque exerted on the rotor increases, requiring an additional EMF force to maintain the specified voltage and frequency. The load on the generator should not affect the rotational speed of the engine, but rather affects the fuel consumption.

Output Power

Electrical power generators contain a nominal power rating. An important specification includes AC prime power rating, which is expressed in volt-amperes (VA). For DC devices, an important specification includes DC power, which is expressed in watts (W). As a general principle, both AC and DC electrical power generators should not be operated at maximum power output for more than 30 minutes or for periods of time exceeding manufacturer recommendations.

Portability

Generators are designed for either permanent installation or packaged for ease of portability. Typically, portable gensets are smaller units while large units are mounted in a fixed location. Portable generators may be frame mounted, include an integral trailer, lifting hook, or otherwise include hardware that allows the unit to be transported.

Standards

Industrial standards and regulations specify accepted practices for installation, operation, and maintenance of electrical power generators. Standards or regulations may be specific to government agencies (e.g., Department of Defense (DoD)), or may outline state and local codes that mandate specific requirements for industrial, commercial, and residential use (e.g. National Electrical Installation Standards (NEIS)).

National Electrical Installation Standards (NEIS). One of the most important developments that NECA works on is the National Electrical Installation Standards (NEIS), which are developed by NECA and in partnership with other industry organizations. NEIS are the standards for electrical construction. In addition to the basic safety requirements available in the National Electrical Code (NEC), the National Electrical Installation Standards clearly define installation instructions of products and systems. It is important to mention that the NEIS are submitted for approval by the American National Standards Institute (ANSI).

U.S. Department of Defense (DoD). The U.S. Department of Defense (DOD) promotes standardization of material, facilities, and engineering practices for the purpose of improving military operational readiness, and reduce total ownership costs and acquisition cycle time. DOD standards state requirements in performance terms in order to make maximum use of non-government standards and commercial technologies, products, and practices. The Defense Standardization Program manages this process and provides a uniform series of specifications, standards, and related documents.

Related Information

Engineering360—Pencil and Paper as Thermoelectric Power Generators

Resources

Converting Generator Frequencies to Supply 60Hz and 50Hz Power

Electronics Tutorial about Electromagnetic Induction

Generator Fuel—What Generator Fuel Is Best?

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Tutorials.ws | Briggs & Stratton | Cummins Power Generation | Caterpillar Electric Power