Understanding Externally Excited Generators: How They Work and How They Are Used

June 28, 2024 Luciano Bertene

Generator saturation curve DC25255b525255d-1965032

Instead of using permanent magnets to form the magnetic field, we use a combination of electromagnets called field poles, shown in the figure. The field current in such a generator is supplied by a separate source (such as a battery or another generator called an exciter). The generator is energized individually (separately). Thus, the DC power supply connected to terminals a and b produces an excitation current I. When the armature coil is driven by a motor or diesel engine, a voltage EO appears to be applied between the brush terminals x and y.

Idle and saturation curve

When a single/externally excited DC generator is inactive (i.e., armature circuit open), a change in field current causes a corresponding change in induced voltage. We are concerned with the connection between the field flow and the excitation current.

Ferroresonant Transformer – CVT

Field flow vs. excitation current

Let's gradually increase the excitation current Ix so that the mmf of the field increases, which in turn increases the flux (f per pole). If we plot f as a function of Ix, we tend to obtain the saturation curve below. This curve is obtained regardless of whether the generator is firing or not.

When the excitation current is relatively small, the flux is small, so the iron in the machine is unsaturated. Little or no mmf is needed to detect the change through the iron, with the result that the mmf produced by the field coils is almost entirely present to drive the flux through the air gap. Due to the constant air permeability, the variation increases in direct proportion to the excitation current, which is shown on the curve by the linear section 0a of the saturation curve.

Principle and functionality of synchronous machine in wind energy

However, if we increase the excitation current, the iron in the field and, therefore, the coil begins to saturate. An excessive increase in mmf is now necessary to achieve a small increase in flow, as shown in the curve through sections b and c. The machine is now considered saturated. Iron saturation becomes crucial when we reach the so-called “knee” of the saturation curve.

Below is the output voltage that varies with field current for a single excited DC generator.

Curva de saturação do gerador DC25255b525255d-1965032-6923292

What are cryogenic engines and cryogenic fuels?

What is a cryogenic engine?

A cryogenic jet engine is a jet engine that consumes cryogenic or oxidizing fuels; their fuels or oxidizers (or both) are liquid and kept at very low temperatures. These engines were one of the factors in the worldwide success of landing on the Moon with the Saturn V rocket.

The cryogenic engine consists of the combustion chamber, fuel injector, pyrotechnic igniter, oxidation cryopumps, fuel cryopumps, gas turbine, cryogenic valves, regulators, engine nozzle and fuel tanks. When it comes to supplying propellants to the combustion chamber, cryogenic engines are pressure- or pump-fed, and pump-fed engines additionally feature a gas generator cycle, a staged combustion cycle, or an expander cycle.

Considering voltage when designing a transformer

These powerful rocket engines were initially considered individually by American, German, and Soviet engineers; everyone recognized that rocket engines require a high mass flow of oxidizer and fuel to produce sufficient thrust. Chemical elements (oxygen) and hydrocarbons with low relative molecular mass were then used as chemical substances and fuels. Both are gases in temperature and pressure. In theory, the size and mass of the fuel tanks would greatly reduce rocket performance if the propellants were stored as pressurized gases. To achieve the required mass flow, the only option was to cool the propellants to cryogenic temperatures (below -150°C, -238°F) and convert them into liquid form. Therefore, all cryogenic rocket engines are, by definition, liquid propellants or hybrid rocket engines.

Types of protection relays and their functions

What are cryogenic fuels?

Cryogenic fuels must be stored at very low temperatures to remain liquid. These fuels are used in machines that operate internally (e.g. rockets, satellites, etc.). Therefore, normal fuel cannot be used there as the environment does not support ignition. Refrigerants are most commonly liquefied gases such as liquid hydrogen.

Harmonics in the transformer

In addition to operating sine wave transformers, harmonic behavior becomes critical as power transformer size and power increase. The effects of harmonic currents are:

Additional copper losses due to harmonic currents.
Increase in main losses.
There was an increase in electromagnetic interference in communication circuits.

Transformer harmonic voltages, on the other hand, cause:

Increased dielectric load on insulation
Electrostatic interference in communication circuits.
Resonance between electrical winding phenomenon and power capacity.

Properties and types of insulators

What is an insulator?

An insulator is a solid material whose internal electrical charge does not circulate freely and therefore does not conduct electricity under the influence of an electric field. Insulators are non-conductive materials that resist electricity. They have a high resistance value, usually in the megohm range.

Properties of insulators

It has high strength and resistivity.
Great dielectric strength.
High mechanical resistance
I can't stand high temperatures.
It is not possible for temperature to change in nature.
It must not absorb water.
It can be done in any format.
There simply cannot be a fire.

Features and types of stairs

What do you call a conductor?

The conductor is the wire that transfers electricity from the utility to the load. The resources are operated by the victims. This is called load.

For example, TV, refrigerator, mill, heater, lighting, etc.

Generally, all types of metals are used for conduction purposes; some metals simply allow current to flow. This type of metal is called a “good conductor”.

Stair properties

Simply conduct electricity.
There would be less resistance.
High tensile stress would occur.
More elasticity.
It will not be corroded by air, rain or heat.
When a current flows through the conductor, it heats up. Therefore, it will not be flooded with heat.
Easy to paste.
The cost is low and definitely worth buying.