Excitation (magnetic)

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In electromagnetism, excitation is the process of generating a magnetic field by means of an electric current.

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An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate (excite) the field, otherwise no power is transferred to or from the rotor. Field coils yield the most flexible form of magnetic flux regulation and de-regulation, but at the expense of a flow of electric current. Hybrid topologies exist, which incorporate both permanent magnets and field coils in the same configuration. The flexible excitation of a rotating electrical machine is employed by either brushless excitation techniques or by the injection of current by carbon brushes (static excitation).

A 100 kVA direct-driven power station AC alternator with a separate belt-driven exciter generator, date c. 1917 Murray Alternator with Belt-Driven Exciter.jpg
A 100 kVA direct-driven power station AC alternator with a separate belt-driven exciter generator, date c. 1917

Excitation in generators

A self-excited shunt-wound DC generator is shown on the left, and a magneto DC generator with permanent field magnets is shown on the right. The shunt-wound generator output varies with the current draw, while the magneto output is steady regardless of load variations. Self Excited vs Magneto.png
A self-excited shunt-wound DC generator is shown on the left, and a magneto DC generator with permanent field magnets is shown on the right. The shunt-wound generator output varies with the current draw, while the magneto output is steady regardless of load variations.
A separately-excited DC generator with bipolar field magnets. Separately-excited generators like this are commonly used for large-scale power transmission plants. The smaller generator can be either a magneto with permanent field magnets or another self-excited generator. Separately Excited.png
A separately-excited DC generator with bipolar field magnets. Separately-excited generators like this are commonly used for large-scale power transmission plants. The smaller generator can be either a magneto with permanent field magnets or another self-excited generator.
A field coil may be connected in shunt, in series, or in compound with the armature of a DC machine (motor or generator). Serie Shunt Coumpound.png
A field coil may be connected in shunt, in series, or in compound with the armature of a DC machine (motor or generator).

For a machine using field coils, as is the case in most large generators, the field must be established by a current in order for the generator to produce electricity. Although some of the generator's own output can be used to maintain the field once it starts up, an external source of current is needed for starting the generator. In any case, it is important to be able to control the field since this will maintain the system voltage.

Amplifier principle

Except for permanent magnet generators, a generator produces output voltage proportional to the magnetic flux, which is the sum of flux from the magnetization of the structure and the flux proportional to the field produced by the excitation current. If there is no excitation current the flux is tiny and the armature voltage is almost nil.

The field current controls the generated voltage allowing a power system’s voltage to be regulated to remove the effect of increasing armature current causing increased voltage drop in the armature winding conductors. In a system with multiple generators and a constant system voltage the current and power delivered by an individual generator is regulated by the field current. A generator is a current to voltage, or transimpedance amplifier. To avoid damage from progressively larger over-corrections, the field current must be adjusted more slowly than the effect of the adjustment propagates through the power system.

Separate excitation

Alternator of 1930s diesel generating set, with excitation dynamo above Alternator from Allen generating set, ex-Malvern.jpg
Alternator of 1930s diesel generating set, with excitation dynamo above

For large, or older, generators, it is usual for a separate exciter dynamo to be powered in parallel with the main power generator. This is a small permanent-magnet or battery-excited dynamo that produces the field current for the larger generator.

Self excitation

Modern generators with field coils are usually self-excited; i.e., some of the power output from the rotor is used to power the field coils. The rotor iron retains a degree of residual magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field induces a weak current in the rotor coils, which in turn creates an initial field current, increasing the field strength, thus increasing the induced current in the rotor, and so on in a feedback process until the machine "builds up" to full voltage.

Starting

Self-excited generators must be started without any external load attached. Any external load will sink the electrical power from the generator before the capacity to generate electrical power can increase.

Variants

Multiple versions of self-exitation exist: [1]

  • a shunt, the simplest design, uses the main winding for the excitation power;
  • an excitation boost system (EBS) is a shunt design with a separate small generator added to temporarily provide an energy boost when the main coil voltage drops (for example, due to a fault). The boost generator is not rated for permanent operation;
  • an auxiliary winding is not connected to the main one and thus is not subject to voltage changes caused by the change of the load.

Field flashing

If the machine does not have enough residual magnetism to build up to full voltage, usually a provision is made to inject current into the field coil from another source. This may be a battery, a house unit providing direct current, or rectified current from a source of alternating current power. Since this initial current is required for a very short time, it is called field flashing. Even small portable generator sets may occasionally need field flashing to restart.

The critical field resistance is the maximum field circuit resistance for a given speed with which the shunt generator would excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. It is a tangent to the open circuit characteristics of the generator at a given speed.

Brushless excitation

Brushless excitation creates the magnetic flux on the rotor of electrical machines without the need of carbon brushes. It is typically used for reducing the regular maintenance costs and to reduce the risk of brush-fire. It was developed in the 1950s, as a result of the advances in high-power semiconductor devices. [2] The concept was using a rotating diode rectifier on the shaft of the synchronous machine to harvest induced alternating voltages and rectify them to feed the generator field winding. [3] [4] [5]

Brushless excitation has been historically lacking the fast flux de-regulation, which has been a major drawback. However, new solutions have emerged. [6] Modern rotating circuitry incorporates active de-excitation components on the shaft, extending the passive diode bridge. [7] [8] [9] Moreover, their recent developments in high-performance wireless communication [10] [11] have realized fully controlled topologies on the shaft, such as the thyristor rectifiers and chopper interfaces. [12] [13] [14] [15] [16] [17] [18]

Related Research Articles

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An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. An electric generator is mechanically identical to an electric motor, but operates in reverse, converting mechanical energy into electrical energy.

<span class="mw-page-title-main">Electric generator</span> Device that converts other energy to electrical energy

In electricity generation, a generator is a device that converts motion-based power or fuel-based power into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, gas turbines, water turbines, internal combustion engines, wind turbines and even hand cranks. The first electromagnetic generator, the Faraday disk, was invented in 1831 by British scientist Michael Faraday. Generators provide nearly all the power for electrical grids.

<span class="mw-page-title-main">Alternator</span> Device converting mechanical into electrical energy

An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually, the term refers to small rotating machines driven by automotive and other internal combustion engines.

<span class="mw-page-title-main">Rotating magnetic field</span> Resultant magnetic field

A rotating magnetic field (RMF) is the resultant magnetic field produced by a system of coils symmetrically placed and supplied with polyphase currents. A rotating magnetic field can be produced by a poly-phase (two or more phases) current or by a single phase current provided that, in the latter case, two field windings are supplied and are so designed that the two resulting magnetic fields generated thereby are out of phase.

<span class="mw-page-title-main">Synchronous motor</span> Type of AC motor

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<span class="mw-page-title-main">DC motor</span> Motor which works on direct current

A DC motor is an electrical motor that uses direct current (DC) to produce mechanical force. The most common types rely on magnetic forces produced by currents in the coils. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor.

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A rotary converter is a type of electrical machine which acts as a mechanical rectifier, inverter or frequency converter.

<span class="mw-page-title-main">Field coil</span> Electromagnet used to generate a magnetic field in an electro-magnetic machine

A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows.

<span class="mw-page-title-main">AC motor</span> Electric motor driven by an AC electrical input

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<span class="mw-page-title-main">Rotor (electric)</span> Non-stationary part of a rotary electric motor

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<span class="mw-page-title-main">Dynamo</span> Electrical generator that produces direct current with the use of a commutator

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In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity to mechanical power while an electric generator converts mechanical power to electricity. The moving parts in a machine can be rotating or linear. While transformers are occasionally called "static electric machines", since they do not have moving parts, generally they are not considered "machines", but as electrical devices "closely related" to the electrical machines.

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<span class="mw-page-title-main">Magneto</span> Electricity-producing machine

A magneto is an electrical generator that uses permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a commutator to produce direct current. It is categorized as a form of alternator, although it is usually considered distinct from most other alternators, which use field coils rather than permanent magnets.

<span class="mw-page-title-main">Alternator (automotive)</span> Devices in automobiles to charge the battery and power the electrical system

An alternator is a type of electric generator used in modern automobiles to charge the battery and to power the electrical system when its engine is running.

<span class="mw-page-title-main">Synchronverter</span> Type of electrical power inverter

Synchronverters or virtual synchronous generators are inverters which mimic synchronous generators (SG) to provide "synthetic inertia" for ancillary services in electric power systems. Inertia is a property of standard synchronous generators associated with the rotating physical mass of the system spinning at a frequency proportional to the electricity being generated. Inertia has implications towards grid stability as work is required to alter the kinetic energy of the spinning physical mass and therefore opposes changes in grid frequency. Inverter-based generation inherently lacks this property as the waveform is being created artificially via power electronics.

Electromagnetically induced acoustic noise, electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials vibrating under the excitation of electromagnetic forces. Some examples of this noise include the mains hum, hum of transformers, the whine of some rotating electric machines, or the buzz of fluorescent lamps. The hissing of high voltage transmission lines is due to corona discharge, not magnetism.

The reactances of synchronous machines comprise a set of characteristic constants used in the theory of synchronous machines. Technically, these constants are specified in units of the electrical reactance (ohms), although they are typically expressed in the per-unit system and thus dimensionless. Since for practically all machines the resistance of the coils is negligibly small in comparison to the reactance, the latter can be used instead of (complex) electrical impedance, simplifying the calculations.

References

  1. ↑ Noland et al. 2019, p. 109708.
  2. ↑ Fenwick, D.R.; Wright, W.F. (1976). "Review of trends in excitation systems and possible future developments". Proceedings of the Institution of Electrical Engineers. 123 (5): 413. doi:10.1049/piee.1976.0093. ISSN   0020-3270.
  3. ↑ Salah, Mohamed; Bacha, Khmais; Chaari, Abdelkader; Benbouzid, Mohamed El Hachemi (September 2014). "Brushless Three-Phase Synchronous Generator Under Rotating Diode Failure Conditions" (PDF). IEEE Transactions on Energy Conversion. 29 (3): 594–601. Bibcode:2014ITEnC..29..594S. doi:10.1109/tec.2014.2312173. ISSN   0885-8969. S2CID   1386715.
  4. ↑ Zhang, YuQi; Cramer, Aaron M. (December 2017). "Numerical Average-Value Modeling of Rotating Rectifiers in Brushless Excitation Systems". IEEE Transactions on Energy Conversion. 32 (4): 1592–1601. Bibcode:2017ITEnC..32.1592Z. doi:10.1109/tec.2017.2706961. ISSN   0885-8969. S2CID   20095186.
  5. ↑ Nuzzo, Stefano; Galea, Michael; Gerada, Chris; Brown, Neil (April 2018). "Analysis, Modeling, and Design Considerations for the Excitation Systems of Synchronous Generators". IEEE Transactions on Industrial Electronics. 65 (4): 2996–3007. doi:10.1109/tie.2017.2756592. ISSN   0278-0046. S2CID   2108726.
  6. ↑ Nøland, Jonas Kristiansen (2017). "A New Paradigm for Large Brushless Hydrogenerators : Advantages Beyond the Static System". DIVA.
  7. ↑ Rapid de-excitation system for synchronous machines with indirect excitation, 2010-02-11, retrieved 2018-05-28
  8. ↑ Rebollo, Emilio; Blazquez, Francisco; Blanquez, Francisco R.; Platero, Carlos A.; Redondo, Marta (2015-07-01). "Improved high-speed de-excitation system for brushless synchronous machines tested on a 20 MVA hydro-generator". IET Electric Power Applications. 9 (6): 405–411. doi: 10.1049/iet-epa.2014.0313 . ISSN   1751-8660.
  9. ↑ Rebollo, Emilio; Platero, Carlos A.; Blazquez, Francisco; Granizo, Ricardo (2017-04-01). "Internal sudden short-circuit response of a new HSBDS for brushless synchronous machines tested on a 15 MVA generator". IET Electric Power Applications. 11 (4): 495–503. doi:10.1049/iet-epa.2016.0525. ISSN   1751-8660. S2CID   113771801.
  10. ↑ Pang, Zhibo; Luvisotto, Michele; Dzung, Dacfey (September 2017). "Wireless High-Performance Communications: The Challenges and Opportunities of a New Target". IEEE Industrial Electronics Magazine. 11 (3): 20–25. doi:10.1109/mie.2017.2703603. ISSN   1932-4529. S2CID   36317354.
  11. ↑ Llano, Danilo X.; Abdi, Salman; Tatlow, Mark; Abdi, Ehsan; McMahon, Richard A. (2017-09-09). "Energy harvesting and wireless data transmission system for rotor instrumentation in electrical machines" (PDF). IET Power Electronics. 10 (11): 1259–1267. doi:10.1049/iet-pel.2016.0890. ISSN   1755-4535. S2CID   55831511.
  12. ↑ Rotating electrical machine, 2014-05-28, retrieved 2018-05-28
  13. ↑ Systems and methods concerning exciterless synchronous machines, 2017-10-06, retrieved 2018-05-28
  14. ↑ Noland, Jonas Kristiansen; Hjelmervik, Karina Bakkelokken; Lundin, Urban (March 2016). "Comparison of Thyristor-Controlled Rectification Topologies for a Six-Phase Rotating Brushless Permanent Magnet Exciter". IEEE Transactions on Energy Conversion. 31 (1): 314–322. Bibcode:2016ITEnC..31..314N. doi:10.1109/tec.2015.2480884. ISSN   0885-8969. S2CID   40426107.
  15. ↑ Noland, Jonas Kristiansen; Evestedt, Fredrik; Perez-Loya, J. Jose; Abrahamsson, Johan; Lundin, Urban (May 2017). "Design and Characterization of a Rotating Brushless Outer Pole PM Exciter for a Synchronous Generator". IEEE Transactions on Industry Applications. 53 (3): 2016–2027. doi:10.1109/tia.2017.2669890. ISSN   0093-9994. S2CID   37649499.
  16. ↑ Noland, Jonas Kristiansen; Evestedt, Fredrik; Perez-Loya, J. Jose; Abrahamsson, Johan; Lundin, Urban (March 2018). "Testing of Active Rectification Topologies on a Six-Phase Rotating Brushless Outer Pole PM Exciter". IEEE Transactions on Energy Conversion. 33 (1): 59–67. Bibcode:2018ITEnC..33...59N. doi:10.1109/tec.2017.2746559. ISSN   0885-8969. S2CID   3347183.
  17. ↑ Noland, Jonas Kristiansen; Evestedt, Fredrik; Perez-Loya, J. Jose; Abrahamsson, Johan; Lundin, Urban (February 2018). "Comparison of Thyristor Rectifier Configurations for a Six-Phase Rotating Brushless Outer Pole PM Exciter". IEEE Transactions on Industrial Electronics. 65 (2): 968–976. doi:10.1109/tie.2017.2726963. ISSN   0278-0046. S2CID   25027522.
  18. ↑ Noland, Jonas Kristiansen; Evestedt, Fredrik; Lundin, Urban (2018). "Failure-Modes Demonstration and Redundant Postfault Operation of Rotating Thyristor Rectifiers on Brushless Dual-Star Exciters". IEEE Transactions on Industrial Electronics. 66 (2): 842–851. doi:10.1109/tie.2018.2833044. ISSN   0278-0046. S2CID   52913506.

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