Generators come in all shapes and sizes, can be classified on the basis of its purpose of use, based on the type of power output it generates both in terms of wattage and type of current (A.C or D.C).Even A.C generators can be single-phase or multi-phase generators. They can be categorized by their size which ranges from household and portable to mid-range to large industrial ones.

Although there is a myriad of ways of classifying the different categories of generators, we shall follow a simple path to categorize as many types of generators as possible.

Even though generators can be categorized into many types, they all have one singular thing in common. The basic working principle behind every generator is the same. A winding coil(armature) rotating inside a permanent magnetic field or a stationary coil inside a variable(rotating) magnetic field (also known as an Alternator).

This principle which is common to every type of generator in the world with the sole exception of a magnetohydrodynamic generator(MHD) which are not used widely at all due to the lack of efficiency. They work on the principle of the Lorentz force ( F= Q.(V x B)) which basically means that a moving charged particle will experience a force when placed inside a magnetic field. This type of generators like all others relies on moving a conductive relies on moving a conductor through a magnetic field to generate electric current. The MHD generator uses hot conductive ionized gas (a plasma) as the moving conductor.

These type of generators are quite expensive, difficult to maintain and hence not used widely. So we shall preclude from discussing this type of generator any further. The only reason it is being mentioned is due to its unique mechanism of operation.

All Generators can be broadly classified into:
A.C Generators
D.C Generators

A.C Generators: Also known as alternators or Synchronous generators, these type of generators are the main sources of power supply in this day and age as Alternating electrical power is the preferred and the predominant source of power throughout the world. These are of two types one is an induction generator and another one is a  synchronous generator or the alternator.

The induction generator requires no separate DC excitation, regulator controls, frequency control or governor. It works by generating an electromotive force by driving a conductor(winding coil or armature ) through a magnetic field.

Synchronous generators are large size generators mainly used in power plants. These may be rotating field type or rotating armature type. In rotating armature type, the armature is at rotor and field is at the stator. Rotor armature current is taken through slip rings and brushes. They suffer from high energy losses and are mainly used for low-power requirements. The rotating field(magnetic) type is widely used because of high power generation capability and absence of slip rings and brushes.

A.C generator can generate both  2-phase or 3-phase current or poly-phase(6 or more). 2-phase and 3-phase currents in the simplest sense means that the polarity of the current changes twice and thrice every cycle of operation respectively.

DC Generators: D.C generators are typically found in off-grid applications. These generators give a seamless power supply directly into electric storage devices and DC power grids without novel equipment. The stored power is carried to loads through dc-ac converters. The DC generators could be controlled back to an unmoving speed as batteries tend to be stimulating to recover considerably more fuel.

D.C Generators are classified according to the way their magnetic field is developed in the stator of the machine.

1.Permanent-magnet DC generators: These do not require external field excitation because it employs permanent magnets to produce the magnetic flux. They are used for low power applications.
2.Separately-excited DC generators: These require external field excitation to produce the magnetic flux. The excitation can be modulated to get variable output power.
3.Self-excited DC generators: These can produce their own magnetic fields once they get started due to due to residual magnetism present in the poles of their stators.

Generators can be classified on the basis of fuel used to rotate the coil inside the magnetic field. Regardless of the size, generators may run on gasoline, diesel, natural gas, propane, bio-diesel, water, sewage gas or hydrogen. Most of the smaller units are built to use gasoline (petrol) as a fuel, and the larger ones have various fuel types, including diesel, propane and natural gas. All these fuels have the same function- to turn the turbine shaft which drives the rotor inside the generator.

They can be classified according to their size and hence power output The generator voltage (volts), frequency (Hz) and power (watts) ratings are selected to suit the load that will be connected.Engine-driven generators fueled on natural gas fuel often form the heart of small-scale (less than 1,000 kW) combined heat and power installations.

They range from small to mid-size stationary engine-generators which can deliver anything between 1-1000 KW or even more.Large scale Generator sets produce enough kilowatts of power to run anything from a full-size building to a large hospital. Large generators are also used onboard ships that utilize a diesel-electric powertrain.

Classification based on application and power rating :
Generators in this classification system are categorized into:
●Standby power generators:   These generators are by far the most common and its easy to see why as these are back-up or emergency generators. The most important thing to understand about standby generators is that they are designed to be run for a limited amount of time as a standalone machine. They don’t have any overload capacity and should never be run in conjunction with the local grid power.
●Prime power generators: These are used when power is not available from the local utility. These types of generators fall into two categories:
●Indefinite Run-time Generators: Indefinite running time means the generator can be safely run at maximum power and variable load for an unlimited number of hours each year. However, there may be restrictions on load based on how many hours each year the unit is needed to run.
●Limited Run-time Generators: Limited running time means the unit can provide maximum power for a limited number of hours at variable loads. An excellent example of when to use limited run time generators is during a planned power outage
●Continuous power generators: As the name implies, these kind of generator can be run continuously proving 100% power for unlimited hours per year. These are generally used in places where the local power grid is absent such as in remote places like mines, military installations etc.

Broadly speaking we have covered all our bases for the different types of generators out there in the world. They may be classified into a few other categories also such as specialized generators serving special needs or on the basis of portability the most common example being that of the self-contained unit or ‘Genset’ which can be widely seen in most buildings and hospitals often several units in combination.

However, the main categories or classifications of generators have been discussed above.