Generator Set

A generator set, also known as a genset, is the combination of an electrical generator and an engine mounted together to form a single piece of equipment that produces electrical power. In the generator, referred to as an alternator, the mechanical work of the engine generates electrical power. Generation sets are used in sites that are not connected to the power grid or to supply emergency power when the grid fails. Generator sets may also be employed to produce energy during peak usage hours when the energy costs are the highest, to decrease the amount of electricity that must be purchased from the utility company, in a power-saving application called peak shaving.

Diesel genset

Caterpiller diesel genset at MNSU [129].


An electrical generator is a device that converts mechanical energy from an external source to electrical energy. It is important to understand that a generator does not actually "create" electrical energy, but uses a prime mover to force the movement of electric charges in the wire of its windings through a circuit. Thus, the electric current supplied by the generator is actually the flow of electric charges.

The generator works using the process of electromagnetic induction. During electromagnetic induction, a source moves an electrical conductor, or wire, in a magnetic field, which induces the flow of electric charges. The movement of the wire creates a voltage difference between the two ends of the wire, causing electric charges to flow and generating current.

Generator at MNSU
Generator at MNSU [129].
The generator’s interior components, including the stator and the rotor, work in combination to generate electricity by generating movement between the magnetic and electric fields. The stator is a stationary component that contains a set of wires wound in coils around an iron core. The rotor or armature, connected to the output crankshaft of the engine, produces the rotating magnetic field. The rotor generates a moving magnetic field around the stator, which induces a difference between the windings of the stator. A specific output voltage of the generator can be produced by varying the winding patterns. This produces alternating current (AC).

Generators can produce a tremendous amount of heat, and the interior components must be cooled constantly so the generator is not damaged. The cooling method is a critical design element of a generator. Most generators are air cooled, hydrogen cooled, or liquid cooled. The size and type of generator determines the type of cooling system that is needed. Air cooling systems are usually used for smaller generators (which cannot reject heat as quickly as with liquid or hydrogen). Hydrogen cooling, which is usually used for large generators, is the most efficient method.

Diesel Genset

Caterpillar genset cylinders
Caterpillar generator cylinders at MNSU [129].
The combination of a diesel engine, an electrical generator, and various other components—such as an excitation system, control system, fuel, and the housing—is referred to as a diesel generating set, or diesel genset. These components work together to produce an AC electrical current. The engine produces mechanical energy that is transformed into electrical energy by the generator. Depending on the size of the components, a diesel generator can produce anywhere from 8 kW for a home application to 2000 kW for an industrial complexes [56].

There are many reasons that diesel gensets are commonly chosen for power generation. Diesel fuel is safer to store because it does not ignite as readily as gasoline, natural gas, or propane. Diesel fuel is difficult to ignite intentionally at atmospheric pressure and almost impossible to ignite accidentally. Diesel engines typically burn about 30–50% less fuel than a gasoline engine on a per kW basis. Diesel engines running at 1800 RPM operate on average for 12,000 to 30,000 hours before major maintenance is required [57]. This is two to three times longer than the lifespan for a gaseous generator set.

Minnesota State University, Mankato

The Minnesota State campus currently purchases its bulk electricity from a local utility. However, the utility plant on campus provides emergency and stand-by power generation. Emergency generators must have the capacity to supply power within 10 seconds of a power failure. At the campus plant the emergency power is generated by two model D600FRX4T1 Katolight diesel generators.

Katolight Generators at MNSU
Katolight generators at MNSU [129].
Running at full capacity, the Katolight diesel generators produce 600 kW of electrical power at 60 Hz, which is used to power the emergency lights, exit signs, and fire alarms. These generators use 12 cylinders under a four-stroke cycle, meaning the piston moves up and back down the cylinder two times to produce one power cycle. At 100% power capacity, an individual engine consumes 47.4 Gal/hr of fuel. The cylinders within the emergency generator have a bore of 5.1 inches, and a displacement of 1458 cubic inches, with a compression ratio of 14:1. The generator is designed for a shaft speed of 1800 RPM. The two generators together are equivalent to the combined horsepower of about five diesel pick-up trucks.

Caterpillar Genertors at MNSU
Caterpillar generators at MNSU [129].
Three Caterpillar stand-by generators were installed in 2005. The installed system consists of three 3516B LOW BSFC 2250 kW Caterpillar generators. Each stand-by unit consists of two main parts: the engine and the electrical generator. The engine is a 16-cylinder diesel engine that rotates a shaft connected to the generator. The engine uses a four-stroke cycle, meaning the piston travels up and down in the cylinder twice to produce one power cycle to rotate the engine shaft. The cylinders in the Caterpillar engines at the utility plant have a total piston displacement of 69 liters. An average car, in contrast, has approximately 3 liters of total displacement. The bore, or diameter, of the Caterpillar cylinders is 6.69 in. (170 mm), and the stroke on the cylinders is 7.48 in. (190 mm). The compression ratio on a diesel engine is the ratio of the initial volume contained within the cylinder to the volume after the air is compressed before fuel is added. The compression ratio on these engines is 14:1. Each generator has a maximum power output of 2 MW electrical power at a frequency of 60 Hz and 1,800 rpm. The amount of fuel consumed by each engine is a function of the load on the generator. At full load, fuel consumption is 120.9 Gal/hr.

Other Fuel Options

Gensets are usually powered by diesel fuel, but other fuels can be used as well.


Portable Gas Generator Set
Portable gas generator [28].

Gasoline-fueled generator sets are the second most commonly used type of genset. Gasoline generator can be purchased at a lower cost than diesel generators, whose components are more expensive. Given their lower purchase price, gasoline generators are cost-effective when used intermittently because they are not as fuel-efficient as diesel generators. Gasoline generators are particularly well suited for portable use because gasoline is readily available at gas stations all across the U.S.

Natural Gas

Natural gas burns more cleanly than diesel or gasoline, with fewer negative impacts on the environment. Because gaseous gensets have a shorter lifespan than liquid fuel gensets, the purpose of the genset must be evaluated. Natural gas gensets are practical because the many buildings and homes that already have access to natural gas would not require external storage.


Propane also burns more cleanly than diesel or gasoline, but a propane genset can consume up to three times as much fuel as a diesel generator. Furthermore, propane is extremely flammable, which makes it dangerous to work with. For emergency home use, however, a propane-fueled generator is more reliable because propane can be stored virtually indefinitely without gumming up the engine's fuel system [59].

Generator Set Applications

Generator sets are designed differently for different applications; some are designed for continuous loads, while others are designed to be operated only for shorter periods of time, such as for backup generation, peak shaving or portable use.

Continuous Load

Generator sets that are used continuously are usually employed in remote locations or sites where there is no access to a power grid, such as mining operations, construction sites, and ships. In rare cases, even where there is access to a power grid it may be cheaper to generate power with a genset.

Backup Generation

Backup generators usually sit idle in the basements of hospitals, schools, communication facilities, and military bases, but when a power grid fails, they are critical to maintaining operations at these facilities, with potentially life-saving impacts. They start automatically during power outages in order reduce the amount of downtime of the facilities.

Peak Shaving

Gensets used for peak shaving are used only when the electricity demands are highest, typically between 3:00 pm and 8:00 pm, when people are just getting home from work, turning on the lights and television, cranking up the air conditioning, and starting to cook dinner.

Generator at MNSU
Generator at MNSU [129].

Sometimes an institution will make arrangements with an electric company to receive a lower rate in exchange for allowing the company to cut off power from the institution during peak demand periods. During those periods the utility company notifies the institution that power will be cut for a certain amount of time, during which the institution will create its own power using the gensets.

Portable Use

In some instances, electricity is needed but cannot be obtained from the grid because the site is mobile (for example recreational vehicles (campers, RV's, boats) and commercial mobile vehicles (fire and emergency trucks, utility and telecom fleets). In response to this need, many businesses have manufactured a variety of portable gensets that are smaller and easy to use. But these come with lower outputs because of the smaller size.

Cogeneration Application

Cogeneration is the simultaneous generation of electricity and heat using a single fuel source. In a generator set system, cogeneration recaptures waste heat that is discharged through the exhaust from the engine. Cogeneration prevents the loss of energy in the exhaust exiting a stationary diesel engine, which can be as hot as 1200°F [60].

The electricity production of cost-effective cogeneration systems that use diesel engine generators area can range from 30 kW to more than 100 MW. Making continuous use of both electricity and thermal energy can save up to 35% on overall energy costs [61].


Physical sites that include Generator Sets:
Minnesota State University, Mankato


Components on Engaged that are included in Generator Sets:
Diesel Engine

Video on electromagnetic induction.

Want to read about Siemens hydrogen cooled generators with water cooled stator windings? Click here.

Excitation systems provide variable DC current, control terminal voltage, ensure stable operation with other components, add to the system’s transient stability, and to keep the machine within acceptable operating range.

Want to learn about diesel genset maintenance? Cummins discusses this here.

Comparison of different fuel types.

How do continuous generators compare to backup generators? Read this.
Want to learn more about peak shaving? Baldor discusses this here.