Academic freedom

From Wikipedia, the free encyclopedia

Academic freedom is the conviction that the freedom of inquiry by faculty members is essential to the mission of the academy as well as the principles of academia, and that scholars should have freedom to teach or communicate ideas or facts (including those that are inconvenient to external political groups or to authorities) without being targeted for repression, job loss, or imprisonment.

Academic freedom is a contested issue and, therefore, has limitations in practice. In the United States, for example, according to the widely recognized "1940 Statement on Academic Freedom and Tenure" of the American Association of University Professors, teachers should be careful to avoid controversial matters that are unrelated to the subject discussed. When they speak or write in public, they are free to express their opinions without fear from institutional censorship or discipline, but they should show restraint and clearly indicate that they are not speaking for their institution. Academic tenure protects academic freedom by ensuring that teachers can be fired only for causes such as gross professional incompetence or behavior that evokes condemnation from the academic community itself.

Historical background

Michael Polanyi argued that academic freedom was a fundamental necessity for the production of true knowledge.

 

Although the notion of academic freedom has a long implicit history (Leiden University, founded in 1575, birthplace of the modern concept), the idea was first clearly formulated in response to the encroachments of the totalitarian state on science and academia in general for the furtherance of its own goals. For instance, in the Soviet Union, scientific research was brought under strict political control in the 1930s. A number of research areas were declared "bourgeois pseudoscience" and forbidden, notably genetics and sociology. The trend toward subjugating science to the interests of the state also had proponents in the West, including the influential Marxist John Desmond Bernal, who published The Social Function of Science in 1939. 

In contrast to this approach, Michael Polanyi argued that a structure of liberty is essential for the advancement of science – that the freedom to pursue science for its own sake is a prerequisite for the production of knowledge through peer review and the scientific method.

In 1936, as a consequence of an invitation to give lectures for the Ministry of Heavy Industry in the USSR, Polanyi met Bukharin, who told him that in socialist societies all scientific research is directed to accord with the needs of the latest five-year plan. Demands in Britain for centrally planned scientific research led Polanyi, together with John Baker, to found the influential Society for Freedom in Science. The society promoted a liberal conception of science as free enquiry against the instrumental view that science should exist primarily to serve the needs of society.

In a series of articles, re-published in The Contempt of Freedom (1940) and The Logic of Liberty (1951), Polanyi claimed that co-operation amongst scientists is analogous to the way in which agents co-ordinate themselves within a free market. Just as consumers in a free market determine the value of products, science is a spontaneous order that arises as a consequence of open debate amongst specialists. Science can therefore only flourish when scientists have the liberty to pursue truth as an end in itself:

[S]cientists, freely making their own choice of problems and pursuing them in the light of their own personal judgment, are in fact co-operating as members of a closely knit organization.
Such self-co-ordination of independent initiatives leads to a joint result which is unpremeditated by any of those who bring it about.
Any attempt to organize the group ... under a single authority would eliminate their independent initiatives, and thus reduce their joint effectiveness to that of the single person directing them from the centre. It would, in effect, paralyse their co-operation.

Rationale

Proponents of academic freedom believe that the freedom of inquiry by students and faculty members is essential to the mission of the academy. They argue that academic communities are repeatedly targeted for repression due to their ability to shape and control the flow of information. When scholars attempt to teach or communicate ideas or facts that are inconvenient to external political groups or to authorities, they may find themselves targeted for public vilification, job loss, imprisonment, or even death. For example, in North Africa, a professor of public health discovered that his country's infant mortality rate was higher than government figures indicated. He lost his job and was imprisoned.

The fate of biology in the Soviet Union is also cited as a reason why society has an interest in protecting academic freedom. A Soviet biologist Trofim Lysenko rejected Western science – then focused primarily on making advances in theoretical genetics, based on research with the fruit fly (Drosophila melanogaster) – and proposed a more socially relevant approach to farming that was based on the collectivist principles of dialectical materialism. (Lysenko called this "Michurinism", but it is more popularly known today as Lysenkoism.) Lysenko's ideas proved appealing to the Soviet leadership, in part because of their value as propaganda, and he was ultimately made director of the Soviet Academy of Agricultural Sciences. Subsequently, Lysenko directed a purge of scientists who professed "harmful ideas", resulting in the expulsion, imprisonment, or death of hundreds of Soviet scientists. Lysenko's ideas were then implemented on collectivised farms in the Soviet Union and China. Famines that resulted partly from Lysenko's influence are believed to have killed 30 million people in China alone.

AFAF (Academics For Academic Freedom) of the United Kingdom is a campaign for lecturers, academic staff and researchers who want to make a public statement in favour of free enquiry and free expression. Their statement of Academic Freedom has two main principles:

1. that academics, both inside and outside the classroom, have unrestricted liberty to question and test received wisdom and to put forward controversial and unpopular opinions, whether or not these are deemed offensive, and
2. that academic institutions have no right to curb the exercise of this freedom by members of their staff, or to use it as grounds for disciplinary action or dismissal.

AFAF and those who agree with its principles believe that it is important for academics to be able not only to express their opinions, but also to put them to scrutiny and to open further debate. They are against the idea of telling the public Platonic "noble lies" and believe that people need not be protected from radical views.

For academic staff

The concept of academic freedom as a right of faculty members is an established part of most legal systems. Different from the United States, where academic freedom is derived from the guarantee of free speech under the First Amendment, constitutions of other countries (and particularly of civil law jurisdictions) typically grant a separate right to free learning, teaching, and research.

Canada

During the interwar years (cir. 1919–1939) Canadian academics were informally expected to be apolitical, lest they bring trouble to their respective universities which, at the time, were very much dependent upon provincial government grants. As well, many Canadian academics of the time considered their position to be remote from the world of politics and felt they had no place getting involved in political issues. However, with the increase of socialist activity in Canada during the Great Depression, due to the rise of social gospel ideology, some left-wing academics began taking active part in contemporary political issues outside the university. Thus, individuals such as Frank H. Underhill at the University of Toronto and other members or affiliates with the League for Social Reconstruction or the socialist movement in Canada who held academic positions, began to find themselves in precarious positions with their university employers. Frank H. Underhill, for example, faced criticism from within and without academia and near expulsion from his university position for his public political comments and his involvement with the League for Social Reconstruction and the Co-Operative Commonwealth Federation. According to Michiel Horn this era marked, "...a relaxation of the unwritten controls under which many Canadian professors had previously worked. The nature of the institutions, natural caution and professional pre-occupation had before the Depression inhibited the professoriate. None of these conditions changed quickly, but even at the provincial universities there were brave souls in the 1930s who claimed, with varying success, the right publicly to discuss controversial subjects and express opinions about them."

United Kingdom

The 1988 Education Reform Act established the legal right of academics in the UK 'to question and test received wisdom and to put forward new ideas and controversial or unpopular opinions without placing themselves in jeopardy of losing their jobs or the privileges they may have'. The principles of academic freedom are also articulated in the statutes of most UK universities. Concerns have been raised regarding threats to academic freedom in the UK, including the harassment of feminist academics. In response to such concerns, the Equality and Human Rights Commission has issued guidance.

France

Professors at public French universities and researchers in public research laboratories are expected, as are all civil servants, to behave in a neutral manner and to not favor any particular political or religious point of view during the course of their duties. However, the academic freedom of university professors is a fundamental principle recognized by the laws of the Republic, as defined by the Constitutional Council; furthermore, statute law declares about higher education that "teachers-researchers (university professors and assistant professors), researchers and teachers are fully independent and enjoy full freedom of speech in the course of their research and teaching activities, provided they respect, following university traditions and the dispositions of this code, principles of tolerance and objectivity". The nomination and promotion of professors is largely done through a process of peer review rather than through normal administrative procedures.

Germany

The German Constitution (German: Grundgesetz) specifically grants academic freedom: "Art and science, research and teaching are free. Freedom of teaching does not absolve from loyalty to the constitution" (Art. 5, para. 3). In a tradition reaching back to the 19th century, jurisdiction has understood this right as one to teach (Lehrfreiheit), study (Lernfreiheit), and conduct research (Freiheit der Wissenschaft) freely, although the last concept has sometimes been taken as a cover term for the first two. Lehrfreiheit embraces the right of professors to determine the content of their lectures and to publish the results of their research without prior approval.

Since professors through their Habilitation receive the right to teach (Latin: venia docendi) in a particular academic field, academic freedom is deemed to cover at least the entirety of this field. Lernfreiheit means a student's right to determine an individual course of study. Finally, Freiheit der Wissenschaft permits academic self-governance and grants the university control of its internal affairs.

Mauritius

In Mauritius the academic staff have the following rights, which are stated in the Chapter II Constitution of Mauritius: the protection of Freedom of Conscience, Protection of Freedom of Expression, Protection of Freedom of Assembly and Association, Protection of Freedom to Establish schools and the Protection from Discrimination. In a 2012 paper on the University of Mauritius the author states that although there are no records of abuse of human rights or freedom of the state "subtle threats to freedom of expression do exist, especially with regard to criticisms of ruling political parties and their leaders as well as religious groups." The government of Mauritius endorses the practice of academic freedom in the tertiary institutions of the country. Academic freedom became a public issue in May 2009 when the University of Mauritius spoke out against the previous vice chancellor Professor I. Fagoonee, who had forwarded a circular sent by the Ministry of Education to academics. This circular targeted public officers and required them to consult their superiors before speaking to the press. According to the paper, academics were annoyed by the fact that the vice chancellor had endorsed the circular by sending it to them when it was addressed to public officers. In an interview the vice- chancellor stated that while academics were free to speak to the press they should not compromise university policy or government policy. An academic spoke to the prime minister and the issue was eventually taken up to parliament. The vice chancellor was then required to step down. In return the government publicly endorsed the practice of academic freedom.

The institutional bureaucracy and the dependence on the state for funds has restricted the freedom of academics to criticize government policy. An interview with Dr. Kasenally an educator at the University of Mauritius expresses her views on academic freedom in the university. The professor states that in 1970s to 1980s the university was at the forefront of debates. But in the 1990s the university stepped away from controversial debates. In 1986, the rights of academics to engage in politics was removed to curtail academic freedom. Academics at the University of Mauritius have thus been encouraged to not express their views or ideas especially if the views oppose those of the management or government. While there have been no cases of arrests or extreme detention of academics, there is a fear that it would hinder their career progress especially at the level of a promotion thus, the academics try to avoid participating in controversial debates.

Netherlands

In the Netherlands the academic freedom is limited. In November 1985 the Dutch Ministry of Education published a policy paper titled Higher Education: Autonomy and Quality. This paper had a proposal that steered away from traditional education and informed that the future of higher education sector should not be regulated by the central government. In 1992 the Law of Higher Education and Research (Wet op het hoger onderwijs en wetenschappelijk onderzoek, article 1.6) was published and became effective in 1993. However, this law governs only certain institutions. Furthermore, the above provision is part of an ordinary statute and lacks constitutional status, so it can be changed anytime by a simple majority in Parliament.

Philippines

The 1987 Philippine Constitution states that, "Academic Freedom shall be enjoyed in all institutions of higher learning." Philippine jurisprudence and courts of law, including the Philippine Supreme Court tend to reflexively defer to the institutional autonomy of higher institutions of learning in determining academic decisions with respect to the outcomes of individual cases filed in the courts regarding the abuse of Academic Freedom by professors, despite the individual merits or demerits of any cases. A closely watched case was the controversial case of University of the Philippines at Diliman Sociology Professor Sarah Raymundo who was not granted tenure due to an appeal by the minority dissenting vote within the faculty of the Sociology Department. This decision was sustained upon appeal by the dissenting faculty and Professor Raymundo to the University of the Philippines at Diliman Chancellor Sergio S. Cao; and though the case was elevated to University of the Philippines System President Emerlinda R. Roman, Roman denied the appeal which was elevated by Professor Raymundo to the university's board of regents for decision and the BOR granted her request for tenure. A major bone of contention among the supporters of Professor Raymundo was not to question the institutional Academic Freedom of the department in not granting her tenure, but in asking for transparency in how the Academic Freedom of the department was exercised, in keeping with traditions within the University of the Philippines in providing a basis that may be subject to peer review, for Academic decisions made under the mantle of Academic Freedom.

South Africa

The South African Constitution of 1996 offers protection of academic freedom and the freedom of scholarly research. Academic freedom became a main principle for higher education by 1997. Three main threats are believed to jeopardize academic freedom: government regulations, excessive influence of private sector sponsor on a university, and limitations of freedom of speech in universities.

There have been an abundance of scandals over the restricted academic freedom at a number of universities in South Africa. The University of KwaZulu-Natal received fame over its restricted academic freedom and the scandal that occurred in 2007. In this scandal a sociology lecturer, Fazel Khan was fired in April 2007 for "bringing the university into disrepute" after he released information to the news media. According to Khan he had been airbrushed from a photograph in a campus publication because of his participation in a staff strike last February. In light of this scandal the South African Council on Higher Education released a report stating that the state is influencing academic freedom. In particular, public universities are more susceptible to political pressure because they receive funds from the public.

New Zealand

Academic freedom pertains to forms of expression by academic staff engaged in scholarship and is defined by the Education Act 1989 (s161(2)) as: a) The freedom of academic staff and students, within the law, to question and test received wisdom, to put forward new ideas and to state controversial or unpopular opinions; b) The freedom of academic staff and students to engage in research; c) The freedom of the university and its staff to regulate the subject matter of courses taught at the university; d) The freedom of the university and its staff to teach and assess students in the manner they consider best promotes learning; and e) The freedom of the university through its council and vice-chancellor to appoint its own staff. 

United States

In the United States, academic freedom is generally taken as the notion of academic freedom defined by the "1940 Statement of Principles on Academic Freedom and Tenure", jointly authored by the American Association of University Professors (AAUP) and the Association of American Colleges (AAC, now the Association of American Colleges and Universities). These principles state that "Teachers are entitled to freedom in the classroom in discussing their subject." The statement also permits institutions to impose "limitations of academic freedom because of religious or other aims", so long as they are "clearly stated in writing at the time of the appointment". The Principles have only the character of private pronouncements, not that of binding law. 

Seven regional accreditors work with American colleges and universities, including private and religious institutions, to implement this standard. Additionally, the AAUP, which is not an accrediting body, works with these same institutions. The AAUP does not always agree with the regional accrediting bodies on the standards of protection of academic freedom and tenure. The AAUP lists (censures) those colleges and universities which it has found, after its own investigations, to violate these principles. There is some case law in the United States that holds that teachers are limited in their academic freedom.

Academic freedom for colleges and universities (institutional autonomy)

A prominent feature of the English university concept is the freedom to appoint faculty, set standards and admit students. This ideal may be better described as institutional autonomy and is distinct from whatever freedom is granted to students and faculty by the institution.

The Supreme Court of the United States said that academic freedom means a university can "determine for itself on academic grounds:

1. who may teach,
2. what may be taught,
3. how it should be taught, and
4. who may be admitted to study."

In a 2008 case, a federal court in Virginia ruled that professors have no academic freedom; all academic freedom resides with the university or college. In that case, Stronach v. Virginia State University, a district court judge held "that no constitutional right to academic freedom exists that would prohibit senior (university) officials from changing a grade given by (a professor) to one of his students." The court relied on mandatory precedent of the U.S. Supreme Court case of Sweezy v. New Hampshire and a case from the fourth circuit court of appeals. The Stronach court also relied on persuasive cases from several circuits of the courts of appeals, including the first, third, and seventh circuits. That court distinguished the situation when a university attempts to coerce a professor into changing a grade, which is clearly in violation of the First Amendment, from when university officials may, in their discretionary authority, change the grade upon appeal by a student. The Stronach case has gotten significant attention in the academic community as an important precedent.

Relationship to freedom of speech

Academic freedom and free speech rights are not coextensive, although this widely accepted view has been recently challenged by an "institutionalist" perspective on the First Amendment. Academic freedom involves more than speech rights; for example, it includes the right to determine what is taught in the classroom. The AAUP gives teachers a set of guidelines to follow when their ideas are considered threatening to religious, political, or social agendas. When teachers speak or write in public, whether via social media or in academic journals, they are able to articulate their own opinions without the fear from institutional restriction or punishment, but they are encouraged to show restraint and clearly specify that they are not speaking for their institution. In practice, academic freedom is protected by institutional rules and regulations, letters of appointment, faculty handbooks, collective bargaining agreements, and academic custom.

In the U.S., the freedom of speech is guaranteed by the First Amendment, which states that "Congress shall make no law... abridging the freedom of speech, or of the press...." By extension, the First Amendment applies to all governmental institutions, including public universities. The U.S. Supreme Court has consistently held that academic freedom is a First Amendment right at public institutions. However, The United States' First Amendment has generally been held to not apply to private institutions, including religious institutions. These private institutions may honor freedom of speech and academic freedom at their discretion.

Controversies

Evolution debate

Academic freedom is also associated with a movement to introduce intelligent design as an alternative explanation to evolution in US public schools. Supporters claim that academic institutions need to fairly represent all possible explanations for the observed biodiversity on Earth, rather than implying no alternatives to evolutionary theory exist.

Critics of the movement claim intelligent design is religiously motivated pseudoscience and cannot be allowed into the curriculum of US public schools due to the First Amendment to the United States Constitution, often citing Kitzmiller v. Dover Area School District as legal precedent. They also reject the allegations of discrimination against proponents of intelligent design, of which investigation showed no evidence.

A number of "academic freedom bills" have been introduced in state legislatures in the United States between 2004 and 2008. The bills were based largely upon language drafted by the Discovery Institute, the hub of the Intelligent Design movement, and derive from language originally drafted for the Santorum Amendment in the United States Senate. According to the Wall Street Journal, the common goal of these bills is to expose more students to articles and videos that undercut evolution, most of which are produced by advocates of intelligent design or biblical creationism. The American Association of University Professors has reaffirmed its opposition to these bills, including any portrayal of creationism as a scientifically credible alternative and any misrepresentation of evolution as scientifically controversial. As of June 2008, only the Louisiana bill has been successfully passed into law.

Communism

In the 20th Century and particularly the 1950s during McCarthyism, there was much public date in print on Communism's role in academic freedom, e.g., Sidney Hook's Heresy, Yes–Conspiracy, No and Whittaker Chambers' "Is Academic Freedom in Danger?" among many other books and articles.

The "Academic bill of rights"

Students for Academic Freedom (SAF) was founded and is sponsored by the David Horowitz Freedom Center to advocate against a perceived liberal bias in U.S. colleges and universities. The organization collected many statements from college students complaining that some of their professors were disregarding their responsibility to keep unrelated controversial material out of their classes and were instead teaching their subjects from an ideological point of view. SAF drafted model legislation, called the Academic Bill of Rights, which has been introduced in several state legislatures and the U.S. House of Representatives. The Academic Bill of Rights is based on the Declaration of Principles on Academic Freedom and Academic Tenure as published by the American Association of University Professors in 1915, and modified in 1940 and 1970. 

According to Students for Academic Freedom, academic freedom is "the freedom to teach and to learn." They contend that academic freedom promotes "intellectual diversity" and helps achieve a university's primary goals, i.e., "the pursuit of truth, the discovery of new knowledge through scholarship and research, the study and reasoned criticism of intellectual and cultural traditions, the teaching and general development of students to help them become creative individuals and productive citizens of a pluralistic democracy, and the transmission of knowledge and learning to a society at large." They feel that, in the past forty years, the principles as defined in the AAUP Declaration have become something of a dead letter, and that an entrenched class of tenured radical leftists is blocking all efforts to restore those principles. In an attempt to override such opposition, the Academic Bill of Rights calls for state and judicial regulation of colleges. Such regulation would ensure that:

• students and faculty will not be favored or disfavored because of their political views or religious beliefs;
• the humanities and social sciences, in particular, will expose their students to a variety of sources and viewpoints, and not present one viewpoint as certain and settled truth;
• campus publications and invited speakers will not be harassed, abused, or otherwise obstructed;
• academic institutions and professional societies will adopt a neutral attitude in matters of politics, ideology or religion.

Opponents claim that such a bill would actually restrict academic freedom, by granting politically motivated legislators and judges the right to shape the nature and focus of scholarly concerns. According to the American Association of University Professors, the Academic Bill of Rights is, despite its title, an attack on the very concept of academic freedom itself: "A fundamental premise of academic freedom is that decisions concerning the quality of scholarship and teaching are to be made by reference to the standards of the academic profession, as interpreted and applied by the community of scholars who are qualified by expertise and training to establish such standards." The Academic Bill of Rights directs universities to implement the principle of neutrality by requiring the appointment of faculty "with a view toward fostering a plurality of methodologies and perspectives," an approach they claim is problematic because "It invites diversity to be measured by political standards that diverge from the academic criteria of the scholarly profession." For example,"no department of political theory ought to be obligated to establish 'a plurality of methodologies and perspectives' by appointing a professor of Nazi political philosophy." Concurring, the president of Appalachian Bible College in West Virginia fears that the Academic Bill of Rights "would inhibit his college's efforts to provide a faith-based education and would put pressure on the college to hire professors... who espouse views contrary to those of the institution."

Pontifical universities

Pontifical universities around the world such as The Catholic University of America, the Pontifical University of Saint Thomas Aquinas, Angelicum in Rome, the Université catholique de Louvain in Belgium, and the Pontifical Catholic University of Peru depend for their status as pontifical universities and for the terms of academic freedom on the Pope through the Congregation for Catholic Education. The terms of academic freedom at ecclesiastical institutions of education are outlined in the apostolic constitution Sapientia Christiana.

Specific cases

While some controversies of academic freedom are reflected in proposed laws that would affect large numbers of students through entire regions, many cases involve individual academics that express unpopular opinions or share politically unfavorable information. These individual cases may receive widespread attention and periodically test the limits of, and support for, academic freedom. Several of these specific cases are also the foundations for later legislature.

The Bassett Affair at Duke University

The Bassett Affair at Duke University in North Carolina in the early 20th century was an important event in the history of academic freedom. In October 1903, Professor John Spencer Bassett publicly praised Booker T. Washington and drew attention to the racism and white supremacist behavior of the Democratic party. Many media reports castigated Bassett, and several major newspapers published opinion pieces attacking him and demanding his termination. On December 1, 1903, the entire faculty of the college threatened to resign en masse if the board gave in to political pressures and asked Bassett to resign. He resigned after "parents were urged to withdraw their children from the college and churchmen were encouraged not to recommend the college to perspective students." President Teddy Roosevelt later praised Bassett for his willingness to express the truth as he saw it.

Professor Mayer and DeGraff of The University of Missouri

In 1929, Experimental Psychology Professor Max Friedrich Meyer and Sociology Assistant Professor Harmon O. DeGraff were dismissed from their positions at the University of Missouri for advising student Orval Hobart Mowrer regarding distribution of a questionnaire which inquired about attitudes towards partners' sexual tendencies, modern views of marriage, divorce, extramarital sexual relations, and cohabitation. The university was subsequently censured by the American Association of University Professors in an early case regarding academic freedom due a tenured professor.

Professor Rice of Rollins College

In a famous case investigated by the American Association of University Professors, President Hamilton Holt of Rollins College in March 1933 fired John Andrew Rice, an atheist scholar and unorthodox teacher, whom Holt had hired, along with three other "golden personalities" in his push to put Rollins on the cutting edge of innovative education. Holt then required all professors to make a "loyalty pledge" to keep their jobs. The American Association of University Professors censured Rollins. Rice and the three other "golden personalities", who were all dismissed for refusing to make the loyalty pledge, founded Black Mountain College.

William Shockley

In 1978, a Nobel prize-winning physicist, electronics inventor, and electrical engineering professor, William Shockley, was concerned about relatively high reproductive rates among people of African descent, because he believed that genetics doomed black people to be intellectually inferior to white people. He stated that he believed his work on race to be more important than his work leading to the Nobel prize. He was strongly criticized for this stance, which raised some concerns about whether criticism of unpopular views of racial differences suppressed academic freedom.

President Summers of Harvard

In 2006, Lawrence Summers, while president of Harvard University, led a discussion that was intended to identify the reasons why fewer women chose to study science and mathematics at advanced levels. He suggested that the possibility of intrinsic gender differences in terms of talent for science and mathematics should be explored. He became the target of considerable public backlash. His critics were, in turn, accused of attempting to suppress academic freedom. Due to the adverse reception to his comments, he resigned after a five-year tenure. Another significant factor of his resignation was several votes of no-confidence placed by the deans of schools, notably multiple professors in the Faculty of Arts and Sciences.

Duke Lacrosse Scandal

The 2006 scandal in which several members of the Duke Lacrosse team were falsely accused of rape raised serious criticisms against exploitation of academic freedom by the university and its faculty to press judgement and deny due process to the three players accused. This case was very controversial due to the rape culture it pertains to.

Professor Khan of the University of KwaZulu-Natal

In 2006 trade union leader and sociologist Fazel Khan was fired from the University of KwaZulu-Natal in Durban, South Africa after taking a leadership role in a strike. In 2008 international concern was also expressed at attempts to discipline two other academics at the same university – Nithiya Chetty and John van der Berg – for expressing concern about academic freedom at the university.

Author J Michael Bailey of Northwestern University

J. Michael Bailey wrote a popular science-style book, The Man Who Would Be Queen, which promotes Ray Blanchard's theory that trans women are motivated by sexuality, and dismisses the "woman trapped in a man's body" concept of transsexuality. In 2007 in an effort to discredit his book, some activists filed formal complaints with Northwestern University accusing Bailey of conducting regulated human research. They also filed a complaint with Illinois state regulators, requesting that they investigate Bailey for practicing psychology without a license. Regulators dismissed the complaint. Other academics have also accused him of sexual misconduct.

Professor Li-Ann of New York University School of Law

In 2009 Thio Li-ann withdrew from an appointment at New York University School of Law after controversy erupted about some anti-gay remarks she had made, prompting a discussion of academic freedom within the law school. Subsequently, Li-ann was asked to step down from her position in the NYU Law School.

Professor Robinson of the University of California at Santa Barbara

In 2009 the University of California at Santa Barbara charged William I. Robinson with anti-Semitism after he circulated an email to his class containing photographs and paragraphs of the Holocaust juxtaposed to those of the Gaza Strip. Robinson was fired from the university, but after charges were dropped after a worldwide campaign against the management of the university.

The Diliman Affair of the University of the Philippines

The University of the Philippines at Diliman affair where controversy erupted after Professor Gerardo A. Agulto of the College of Business Administration was sued by MBA graduate student Chanda R. Shahani for a nominal amount in damages for failing him several times in the Strategic Management portion of the Comprehensive Examination. Agulto refused to give a detailed basis for his grades and instead invoked Academic Freedom while Shahani argued in court that Academic Freedom could not be invoked without a rational basis in grading a student.

Professor Salaita of the University of Illinois at Urbana-Champaign

In 2013 the University of Illinois at Urbana–Champaign offered Steven Salaita a faculty position in American Indian studies but then withdrew the offer in 2014, after reviewing some of his comments on Twitter about Israel.

Professor Guth of Kansas University

Professor David Guth of Kansas University was persecuted by the Kansas Board of Regents due to his tweet, from a personal account linked to the university, regarding the shootings which stated, "#NavyYardShooting The blood is on the hands of the #NRA. Next time, let it be YOUR sons and daughters. Shame on you. May God damn you." Following the controversial comments, Kansas University suspended, but ultimately allowed him to come back. Because of this incident and the moral qualms it raised, the Kansas Board of Regents passed a new policy regarding social media. This new legislature allowed universities to discipline or terminate employees who used social media in ways "contrary to the best interests of the university."

Commutator (electric)

From Wikipedia, the free encyclopedia

 

Commutator in a universal motor from a vacuum cleaner. Parts: (A) commutator, (B) brush, (C) rotor (armature) windings, (D) stator (F) (field) windings, (E) brush guides

 

A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine. Two or more electrical contacts called "brushes" made of a soft conductive material like carbon press against the commutator, making sliding contact with successive segments of the commutator as it rotates. The windings (coils of wire) on the armature are connected to the commutator segments. 

Commutators are used in direct current (DC) machines: dynamos (DC generators) and many DC motors as well as universal motors. In a motor the commutator applies electric current to the windings. By reversing the current direction in the rotating windings each half turn, a steady rotating force (torque) is produced. In a generator the commutator picks off the current generated in the windings, reversing the direction of the current with each half turn, serving as a mechanical rectifier to convert the alternating current from the windings to unidirectional direct current in the external load circuit. The first direct current commutator-type machine, the dynamo, was built by Hippolyte Pixii in 1832, based on a suggestion by André-Marie Ampère. 

Commutators are relatively inefficient, and also require periodic maintenance such as brush replacement. Therefore, commutated machines are declining in use, being replaced by alternating current (AC) machines, and in recent years by brushless DC motors which use semiconductor switches.

Principle of operation

A commutator consists of a set of contact bars fixed to the rotating shaft of a machine, and connected to the armature windings. As the shaft rotates, the commutator reverses the flow of current in a winding. For a single armature winding, when the shaft has made one-half complete turn, the winding is now connected so that current flows through it in the opposite of the initial direction. In a motor, the armature current causes the fixed magnetic field to exert a rotational force, or a torque, on the winding to make it turn. In a generator, the mechanical torque applied to the shaft maintains the motion of the armature winding through the stationary magnetic field, inducing a current in the winding. In both the motor and generator case, the commutator periodically reverses the direction of current flow through the winding so that current flow in the circuit external to the machine continues in only one direction.

Simplest practical commutator

Practical commutators have at least three contact segments, to prevent a "dead" spot where two brushes simultaneously bridge only two commutator segments. Brushes are made wider than the insulated gap, to ensure that brushes are always in contact with an armature coil. For commutators with at least three segments, although the rotor can potentially stop in a position where two commutator segments touch one brush, this only de-energizes one of the rotor arms while the others will still function correctly. With the remaining rotor arms, a motor can produce sufficient torque to begin spinning the rotor, and a generator can provide useful power to an external circuit.

Ring/segment construction

Cross-section of a commutator that can be disassembled for repair.

 

A commutator consists of a set of copper segments, fixed around the part of the circumference of the rotating machine, or the rotor, and a set of spring-loaded brushes fixed to the stationary frame of the machine. Two or more fixed brushes connect to the external circuit, either a source of current for a motor or a load for a generator. 

Commutator segments are connected to the coils of the armature, with the number of coils (and commutator segments) depending on the speed and voltage of the machine. Large motors may have hundreds of segments. Each conducting segment of the commutator is insulated from adjacent segments. Mica was used on early machines and is still used on large machines. Many other insulating materials are used to insulate smaller machines; plastics allow quick manufacture of an insulator, for example. The segments are held onto the shaft using a dovetail shape on the edges or underside of each segment. Insulating wedges around the perimeter of each segment are pressed so that the commutator maintains its mechanical stability throughout its normal operating range. 

In small appliance and tool motors the segments are typically crimped permanently in place and cannot be removed. When the motor fails it is discarded and replaced. On large industrial machines (say, from several kilowatts to thousands of kilowatts in rating) it is economical to replace individual damaged segments, and so the end-wedge can be unscrewed and individual segments removed and replaced. Replacing the copper and mica segments is commonly referred to as "refilling". Refillable dovetailed commutators are the most common construction of larger industrial type commutators, but refillable commutators may also be constructed using external bands made of fiberglass (glass banded construction) or forged steel rings (external steel shrink ring type construction and internal steel shrink ring type construction). Disposable, molded type commutators commonly found in smaller DC motors are becoming increasingly more common in larger electric motors. Molded type commutators are not repairable and must be replaced if damaged. In addition to the commonly used heat, torque, and tonnage methods of seasoning commutators, some high performance commutator applications require a more expensive, specific "spin seasoning" process or over-speed spin-testing to guarantee stability of the individual segments and prevent premature wear of the carbon brushes. Such requirements are common with traction, military, aerospace, nuclear, mining, and high speed applications where premature failure can lead to serious negative consequences.

Friction between the segments and the brushes eventually causes wear to both surfaces. Carbon brushes, being made of a softer material, wear faster and may be designed to be replaced easily without dismantling the machine. Older copper brushes caused more wear to the commutator, causing deep grooving and notching of the surface over time. The commutator on small motors (say, less than a kilowatt rating) is not designed to be repaired through the life of the device. On large industrial equipment, the commutator may be re-surfaced with abrasives, or the rotor may be removed from the frame, mounted in a large metal lathe, and the commutator resurfaced by cutting it down to a smaller diameter. The largest of equipment can include a lathe turning attachment directly over the commutator. 

A tiny 5-segment commutator less than 2 mm in diameter, on a direct-current motor in a toy radio control ZipZaps car.

Brush construction

Various types of copper and carbon brushes.

 

Early machines used brushes made from strands of copper wire to contact the surface of the commutator. However, these hard metal brushes tended to scratch and groove the smooth commutator segments, eventually requiring resurfacing of the commutator. As the copper brushes wore away, the dust and pieces of the brush could wedge between commutator segments, shorting them and reducing the efficiency of the device. Fine copper wire mesh or gauze provided better surface contact with less segment wear, but gauze brushes were more expensive than strip or wire copper brushes. 

Modern rotating machines with commutators almost exclusively use carbon brushes, which may have copper powder mixed in to improve conductivity. Metallic copper brushes can be found in toy or very small motors, such as the one illustrated above, and some motors which only operate very intermittently, such as automotive starter motors.

Motors and generators suffer from a phenomenon known as 'armature reaction', one of the effects of which is to change the position at which the current reversal through the windings should ideally take place as the loading varies. Early machines had the brushes mounted on a ring that was provided with a handle. During operation, it was necessary to adjust the position of the brush ring to adjust the commutation to minimise the sparking at the brushes. This process was known as 'rocking the brushes'. 

Various developments took place to automate the process of adjusting the commutation and minimizing the sparking at the brushes. One of these was the development of 'high resistance brushes', or brushes made from a mixture of copper powder and carbon. Although described as high resistance brushes, the resistance of such a brush was of the order of milliohms, the exact value dependent on the size and function of the machine. Also, the high resistance brush was not constructed like a brush but in the form of a carbon block with a curved face to match the shape of the commutator. 

The high resistance or carbon brush is made large enough that it is significantly wider than the insulating segment that it spans (and on large machines may often span two insulating segments). The result of this is that as the commutator segment passes from under the brush, the current passing to it ramps down more smoothly than had been the case with pure copper brushes where the contact broke suddenly. Similarly the segment coming into contact with the brush has a similar ramping up of the current. Thus, although the current passing through the brush was more or less constant, the instantaneous current passing to the two commutator segments was proportional to the relative area in contact with the brush. 

The introduction of the carbon brush had convenient side effects. Carbon brushes tend to wear more evenly than copper brushes, and the soft carbon causes far less damage to the commutator segments. There is less sparking with carbon as compared to copper, and as the carbon wears away, the higher resistance of carbon results in fewer problems from the dust collecting on the commutator segments.

The ratio of copper to carbon can be changed for a particular purpose. Brushes with higher copper content perform better with very low voltages and high current, while brushes with a higher carbon content are better for high voltage and low current. High copper content brushes typically carry 150 to 200 amperes per square inch of contact surface, while higher carbon content only carries 40 to 70 amperes per square inch. The higher resistance of carbon also results in a greater voltage drop of 0.8 to 1.0 volts per contact, or 1.6 to 2.0 volts across the commutator.

Brush holders

Compound carbon brush holder, with individual clamps and tension adjustments for each block of carbon.

 

A spring is typically used with the brush, to maintain constant contact with the commutator. As the brush and commutator wear down, the spring steadily pushes the brush downwards towards the commutator. Eventually the brush wears small and thin enough that steady contact is no longer possible or it is no longer securely held in the brush holder, and so the brush must be replaced.

It is common for a flexible power cable to be directly attached to the brush, because current flowing through the support spring would cause heating, which may lead to a loss of metal temper and a loss of the spring tension. 

When a commutated motor or generator uses more power than a single brush is capable of conducting, an assembly of several brush holders is mounted in parallel across the surface of the very large commutator. This parallel holder distributes current evenly across all the brushes, and permits a careful operator to remove a bad brush and replace it with a new one, even as the machine continues to spin fully powered and under load. 

High power, high current commutated equipment is now uncommon, due to the less complex design of alternating current generators that permits a low current, high voltage spinning field coil to energize high current fixed-position stator coils. This permits the use of very small singular brushes in the alternator design. In this instance, the rotating contacts are continuous rings, called slip rings, and no switching happens. 

Modern devices using carbon brushes usually have a maintenance-free design that requires no adjustment throughout the life of the device, using a fixed-position brush holder slot and a combined brush-spring-cable assembly that fits into the slot. The worn brush is pulled out and a new brush inserted.

Brush contact angle

Different types of brushes have different brush contact angles

 

Commutator and brush assembly of a traction motor; the copper bars can be seen with lighter insulation strips between the bars. Each dark grey carbon brush has a short flexible copper jumper lead attached. Parts of the motor field winding, in red, can be seen to the right of the commutator.

 

The different brush types make contact with the commutator in different ways. Because copper brushes have the same hardness as the commutator segments, the rotor cannot be spun backwards against the ends of copper brushes without the copper digging into the segments and causing severe damage. Consequently, strip/laminate copper brushes only make tangential contact with the commutator, while copper mesh and wire brushes use an inclined contact angle touching their edge across the segments of a commutator that can spin in only one direction. 

The softness of carbon brushes permits direct radial end-contact with the commutator without damage to the segments, permitting easy reversal of rotor direction, without the need to reorient the brush holders for operation in the opposite direction. Although never reversed, common appliance motors that use wound rotors, commutators and brushes have radial-contact brushes. In the case of a reaction-type carbon brush holder, carbon brushes may be reversely inclined with the commutator so that the commutator tends to push against the carbon for firm contact.

The commutating plane

Commutating plane definitions.

 

The contact point where a brush touches the commutator is referred to as the commutating plane. To conduct sufficient current to or from the commutator, the brush contact area is not a thin line but instead a rectangular patch across the segments. Typically the brush is wide enough to span 2.5 commutator segments. This means that two adjacent segments are electrically connected by the brush when it contacts both.

Compensation for stator field distortion

Centered position of the commutating plane if there were no field distortion effects.

 

Most introductions to motor and generator design start with a simple two-pole device with the brushes arranged at a perfect 90-degree angle from the field. This ideal is useful as a starting point for understanding how the fields interact but it is not how a motor or generator functions in actual practice. 

On the left is an exaggerated example of how the field is distorted by the rotor. On the right, iron filings show the distorted field across the rotor.

In a real motor or generator, the field around the rotor is never perfectly uniform. Instead, the rotation of the rotor induces field effects which drag and distort the magnetic lines of the outer non-rotating stator. 

Actual position of the commutating plane to compensate for field distortion.

 

The faster the rotor spins, the further this degree of field distortion. Because a motor or generator operates most efficiently with the rotor field at right angles to the stator field, it is necessary to either retard or advance the brush position to put the rotor's field into the correct position to be at a right angle to the distorted field. 

These field effects are reversed when the direction of spin is reversed. It is therefore difficult to build an efficient reversible commutated dynamo, since for highest field strength it is necessary to move the brushes to the opposite side of the normal neutral plane. These effects can be mitigated by a Compensation winding in the face of the field pole that carries armature current. 

The effect can be considered to be analogous to timing advance in an internal combustion engine. Generally a dynamo that has been designed to run at a certain fixed speed will have its brushes permanently fixed to align the field for highest efficiency at that speed.

Further compensation for self-induction

Brush advance for Self-Induction.

 

Self-induction – The magnetic fields in each coil of wire join and compound together to create a magnetic field that resists changes in the current, which can be likened to the current having inertia.

In the coils of the rotor, even after the brush has been reached, currents tend to continue to flow for a brief moment, resulting in a wasted energy as heat due to the brush spanning across several commutator segments and the current short-circuiting across the segments. 

Spurious resistance is an apparent increase in the resistance in the armature winding, which is proportional to the speed of the armature, and is due to the lagging of the current. 

To minimize sparking at the brushes due to this short-circuiting, the brushes are advanced a few degrees further yet, beyond the advance for field distortions. This moves the rotor winding undergoing commutation slightly forward into the stator field which has magnetic lines in the opposite direction and which oppose the field in the stator. This opposing field helps to reverse the lagging self-inducting current in the stator. 

So even for a rotor which is at rest and initially requires no compensation for spinning field distortions, the brushes should still be advanced beyond the perfect 90-degree angle as taught in so many beginners textbooks, to compensate for self-induction.

Limitations and alternatives

Low voltage dynamo from late 1800s for electroplating. The resistance of the commutator contacts causes inefficiency in low voltage, high current machines like this, requiring a huge elaborate commutator. This machine generated 7 volts at 310 amps.

 

Although direct current motors and dynamos once dominated industry, the disadvantages of the commutator have caused a decline in the use of commutated machines in the last century. These disadvantages are:

• The sliding friction between the brushes and commutator consumes power, which can be significant in a low power machine.
• Due to friction, the brushes and copper commutator segments wear down, creating dust. In small consumer products such as power tools and appliances the brushes may last as long as the product, but larger machines require regular replacement of brushes and occasional resurfacing of the commutator. So commutated machines are not used in low particulate or sealed applications or in equipment that must operate for long periods without maintenance.
• The resistance of the sliding contact between brush and commutator causes a voltage drop called the "brush drop". This may be several volts, so it can cause large power losses in low voltage, high current machines. Alternating current motors, which do not use commutators, are much more efficient.
• There is a limit to the maximum current density and voltage which can be switched with a commutator. Very large direct current machines, say, more than several megawatts rating, cannot be built with commutators. The largest motors and generators are all alternating-current machines.
• The switching action of the commutator causes sparking at the contacts, posing a fire hazard in explosive atmospheres, and generating electromagnetic interference.

With the wide availability of alternating current, DC motors have been replaced by more efficient AC synchronous or induction motors. In recent years, with the widespread availability of power semiconductors, in many remaining applications commutated DC motors have been replaced with "brushless direct current motors". These don't have a commutator; instead the direction of the current is switched electronically. A sensor keeps track of the rotor position and semiconductor switches such as transistors reverse the current. Operating life of these machines is much longer, limited mainly by bearing wear.

Repulsion induction motors

These are single-phase AC-only motors with higher starting torque than could be obtained with split-phase starting windings, before high-capacitance (non-polar, relatively high-current electrolytic) starting capacitors became practical. They have a conventional wound stator as with any induction motor, but the wire-wound rotor is much like that with a conventional commutator. Brushes opposite each other are connected to each other (not to an external circuit), and transformer action induces currents into the rotor that develop torque by repulsion. 

One variety, notable for having an adjustable speed, runs continuously with brushes in contact, while another uses repulsion only for high starting torque and in some cases lifts the brushes once the motor is running fast enough. In the latter case, all commutator segments are connected together as well, before the motor attains running speed. 

Once at speed, the rotor windings become functionally equivalent to the squirrel-cage structure of a conventional induction motor, and the motor runs as such.

Laboratory commutators

Commutators were used as simple forward-off-reverse switches for electrical experiments in physics laboratories. There are two well-known historical types:

Ruhmkorff commutator

This is similar in design to the commutators used in motors and dynamos. It was usually constructed of brass and ivory (later ebonite).

Pohl commutator

This consisted of a block of wood or ebonite with four wells, containing mercury, which were cross-connected by copper wires. The output was taken from a pair of curved copper wires which were moved to dip into one or other pair of mercury wells. Instead of mercury, ionic liquids or other liquid metals could be used.

Electric generator

From Wikipedia, the free encyclopedia

 

U.S. NRC image of a modern steam turbine generator (STG).

 

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

The reverse conversion of electrical energy into mechanical energy is done by an electric motor, and motors and generators have many similarities. Many motors can be mechanically driven to generate electricity and frequently make acceptable manual generators.

Terminology

Early Ganz Generator in Zwevegem, West Flanders, Belgium

 

Electromagnetic generators fall into one of two broad categories, dynamos and alternators.

• Dynamos generate pulsing direct current through the use of a commutator.
• Alternators generate alternating current.

Mechanically a generator consists of a rotating part and a stationary part:

Rotor

The rotating part of an electrical machine.

 

Stator

The stationary part of an electrical machine, which surrounds the rotor.

One of these parts generates a magnetic field, the other has a wire winding in which the changing field induces an electric current:

Field winding or field (permanent) magnets

The magnetic field producing component of an electrical machine. The magnetic field of the dynamo or alternator can be provided by either wire windings called field coils or permanent magnets. Electrically-excited generators include an excitation system to produce the field flux. A generator using permanent magnets (PMs) is sometimes called a magneto, or permanent magnet synchronous generators (PMSMs).

 

Armature

The power-producing component of an electrical machine. In a generator, alternator, or dynamo, the armature windings generate the electric current, which provides power to an external circuit. The armature can be on either the rotor or the stator, depending on the design, with the field coil or magnet on the other part.

History

Before the connection between magnetism and electricity was discovered, electrostatic generators were invented. They operated on electrostatic principles, by using moving electrically charged belts, plates, and disks that carried charge to a high potential electrode. The charge was generated using either of two mechanisms: electrostatic induction or the triboelectric effect. Such generators generated very high voltage and low current. Because of their inefficiency and the difficulty of insulating machines that produced very high voltages, electrostatic generators had low power ratings, and were never used for generation of commercially significant quantities of electric power. Their only practical applications were to power early X-ray tubes, and later in some atomic particle accelerators.

Faraday disk generator

The Faraday disk was the first electric generator. The horseshoe-shaped magnet (A) created a magnetic field through the disk (D). When the disk was turned, this induced an electric current radially outward from the center toward the rim. The current flowed out through the sliding spring contact m, through the external circuit, and back into the center of the disk through the axle.

The operating principle of electromagnetic generators was discovered in the years of 1831–1832 by Michael Faraday. The principle, later called Faraday's law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux. 

He also built the first electromagnetic generator, called the Faraday disk; a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage. 

This design was inefficient, due to self-cancelling counterflows of current in regions of the disk that were not under the influence of the magnetic field. While current was induced directly underneath the magnet, the current would circulate backwards in regions that were outside the influence of the magnetic field. This counterflow limited the power output to the pickup wires, and induced waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction. 

Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher, more useful voltages. Since the output voltage is proportional to the number of turns, generators could be easily designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs.

Jedlik and the self-excitation phenomenon

Independently of Faraday, Ányos Jedlik started experimenting in 1827 with the electromagnetic rotating devices which he called electromagnetic self-rotors. In the prototype of the single-pole electric starter (finished between 1852 and 1854) both the stationary and the revolving parts were electromagnetic. It was also the discovery of the principle of dynamo self-excitation, which replaced permanent magnet designs. He also may have formulated the concept of the dynamo in 1861 (before Siemens and Wheatstone) but didn't patent it as he thought he wasn't the first to realize this.

Direct current generators

Hippolyte Pixii's dynamo. The commutator is located on the shaft below the spinning magnet.

 

This large belt-driven high-current dynamo produced 310 amperes at 7 volts. Dynamos are no longer used due to the size and complexity of the commutator needed for high power applications.

 

A coil of wire rotating in a magnetic field produces a current which changes direction with each 180° rotation, an alternating current (AC). However many early uses of electricity required direct current (DC). In the first practical electric generators, called dynamos, the AC was converted into DC with a commutator, a set of rotating switch contacts on the armature shaft. The commutator reversed the connection of the armature winding to the circuit every 180° rotation of the shaft, creating a pulsing DC current. One of the first dynamos was built by Hippolyte Pixii in 1832.

The dynamo was the first electrical generator capable of delivering power for industry. The Woolrich Electrical Generator of 1844, now in Thinktank, Birmingham Science Museum, is the earliest electrical generator used in an industrial process. It was used by the firm of Elkingtons for commercial electroplating.

The modern dynamo, fit for use in industrial applications, was invented independently by Sir Charles Wheatstone, Werner von Siemens and Samuel Alfred Varley. Varley took out a patent on 24 December 1866, while Siemens and Wheatstone both announced their discoveries on 17 January 1867, the latter delivering a paper on his discovery to the Royal Society.

The "dynamo-electric machine" employed self-powering electromagnetic field coils rather than permanent magnets to create the stator field. Wheatstone's design was similar to Siemens', with the difference that in the Siemens design the stator electromagnets were in series with the rotor, but in Wheatstone's design they were in parallel. The use of electromagnets rather than permanent magnets greatly increased the power output of a dynamo and enabled high power generation for the first time. This invention led directly to the first major industrial uses of electricity. For example, in the 1870s Siemens used electromagnetic dynamos to power electric arc furnaces for the production of metals and other materials. 

The dynamo machine that was developed consisted of a stationary structure, which provides the magnetic field, and a set of rotating windings which turn within that field. On larger machines the constant magnetic field is provided by one or more electromagnets, which are usually called field coils. 

Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution. Before the adoption of AC, very large direct-current dynamos were the only means of power generation and distribution. AC has come to dominate due to the ability of AC to be easily transformed to and from very high voltages to permit low losses over large distances.

Synchronous generators (alternating current generators)

Ferranti alternating current generator, c. 1900.

 

Through a series of discoveries, the dynamo was succeeded by many later inventions, especially the AC alternator, which was capable of generating alternating current. It is commonly known to be the Synchronous Generators (SGs). The synchronous machines are directly connected to the grid and need to be properly synchronized during startup. Moreover, they are excited with special control to enhance the stability of the power system.

Alternating current generating systems were known in simple forms from Michael Faraday's original discovery of the magnetic induction of electric current. Faraday himself built an early alternator. His machine was a "rotating rectangle", whose operation was heteropolar - each active conductor passed successively through regions where the magnetic field was in opposite directions.

Large two-phase alternating current generators were built by a British electrician, J.E.H. Gordon, in 1882. The first public demonstration of an "alternator system" was given by William Stanley, Jr., an employee of Westinghouse Electric in 1886.

Sebastian Ziani de Ferranti established Ferranti, Thompson and Ince in 1882, to market his Ferranti-Thompson Alternator, invented with the help of renowned physicist Lord Kelvin. His early alternators produced frequencies between 100 and 300 Hz. Ferranti went on to design the Deptford Power Station for the London Electric Supply Corporation in 1887 using an alternating current system. On its completion in 1891, it was the first truly modern power station, supplying high-voltage AC power that was then "stepped down" for consumer use on each street. This basic system remains in use today around the world. 

A small early 1900s 75 kVA direct-driven power station AC alternator, with a separate belt-driven exciter generator.

 

After 1891, polyphase alternators were introduced to supply currents of multiple differing phases. Later alternators were designed for varying alternating-current frequencies between sixteen and about one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.

Self-excitation

As the requirements for larger scale power generation increased, a new limitation rose: the magnetic fields available from permanent magnets. Diverting a small amount of the power generated by the generator to an electromagnetic field coil allowed the generator to produce substantially more power. This concept was dubbed self-excitation. 

The field coils are connected in series or parallel with the armature winding. When the generator first starts to turn, the small amount of remanent magnetism present in the iron core provides a magnetic field to get it started, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field which generates a larger armature current. This "bootstrap" process continues until the magnetic field in the core levels off due to saturation and the generator reaches a steady state power output. 

Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger. In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.

Specialized types of generator

Direct current (DC)

An important class of direct-current generators are the dynamos - these are electrical machines with commutators to produce (DC) direct current, and are self excited - their field electromagnets are powered by the machine's own output. Other types of DC generator use a separate source of direct current to energize their field magnets.

Homopolar generator

A homopolar generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim (or ends of the cylinder), the electrical polarity depending on the direction of rotation and the orientation of the field. 

It is also known as a unipolar generator, acyclic generator, disk dynamo, or Faraday disc. The voltage is typically low, on the order of a few volts in the case of small demonstration models, but large research generators can produce hundreds of volts, and some systems have multiple generators in series to produce an even larger voltage. They are unusual in that they can produce tremendous electric current, some more than a million amperes, because the homopolar generator can be made to have very low internal resistance.

Magnetohydrodynamic (MHD) generator

A magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. The first practical design was the AVCO Mk. 25, developed in 1965. The U.S. government funded substantial development, culminating in a 25 MW demonstration plant in 1987. In the Soviet Union from 1972 until the late 1980s, the MHD plant U 25 was in regular commercial operation on the Moscow power system with a rating of 25 MW, the largest MHD plant rating in the world at that time. MHD generators operated as a topping cycle are currently (2007) less efficient than combined cycle gas turbines.

Alternating current (AC)

Induction generator

Induction AC motors may be used as generators, turning mechanical energy into electric current. Induction generators operate by mechanically turning their rotor faster than the synchronous speed, giving negative slip. A regular AC asynchronous motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as minihydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls. They do not require an exciter circuit because the rotating magnetic field is provided by induction from the stator circuit. They also do not require speed governor equipment as they inherently operate at the connected grid frequency.

To operate, an induction generator must be excited with a leading voltage; this is usually done by connection to an electrical grid, or sometimes they are self-excited by using phase correcting capacitors.

Linear electric generator

In the simplest form of linear electric generator, a sliding magnet moves back and forth through a solenoid - a spool of copper wire. An alternating current is induced in the loops of wire by Faraday's law of induction each time the magnet slides through. This type of generator is used in the Faraday flashlight. Larger linear electricity generators are used in wave power schemes.

Variable-speed constant-frequency generators

Many renewable energy efforts attempt to harvest natural sources of mechanical energy (wind, tides, etc.) to produce electricity. Because these sources fluctuate in power applied, standard generators using permanent magnets and fixed windings would deliver unregulated voltage and frequency. The overhead of regulation (whether before the generator via gear reduction or after generation by electrical means) is high in proportion to the naturally-derived energy available. 

New generator designs such as the asynchronous or induction singly-fed generator, the doubly-fed generator, or the brushless wound-rotor doubly-fed generator are seeing success in variable speed constant frequency applications, such as wind turbines or other renewable energy technologies. These systems thus offer cost, reliability and efficiency benefits in certain use cases.

Common use cases

Vehicular generators

Roadway vehicles

Motor vehicles require electrical energy to power their instrumentation, keep the engine itself operating, and recharge their batteries. Until about the 1960s motor vehicles tended to use DC generators with electromechanical regulators. Following the historical trend above and for many of the same reasons, these have now been replaced by alternators with built-in rectifier circuits.

Bicycles

Bicycles require energy to power running lights and other equipment. There are two common kinds of generator in use on bicycles: bottle dynamos which engage the bicycle's tire on an as-needed basis, and hub dynamos which are directly attached to the bicycle's drive train. The name is conventional as these they are small permanent-magnet alternators, not self-excited DC machines as are dynamos. Some electric bicycles are capable of regenerative braking, where the drive motor is used as a generator to recover some energy during braking.

Sailboats

Sailing boats may use a water- or wind-powered generator to trickle-charge the batteries. A small propeller, wind turbine or impeller is connected to a low-power generator to supply currents at typical wind or cruising speeds.

Electric scooters

Electric scooters with regenerative braking have become popular all over the world. Engineers use kinetic energy recovery systems on the scooter to reduce energy consumption and increase its range up to 40-60% by simply recovering energy using the magnetic brake, which generates electric energy for further use. Modern vehicles reach speed up to 25-30 km/h and can run up to 35-40 km.

Genset

An engine-generator is the combination of an electrical generator and an engine (prime mover) mounted together to form a single piece of self-contained equipment. The engines used are usually piston engines, but gas turbines can also be used. And there are even hybrid diesel-gas units, called dual-fuel units. Many different versions of engine-generators are available - ranging from very small portable petrol powered sets to large turbine installations. The primary advantage of engine-generators is the ability to independently supply electricity, allowing the units to serve as backup power solutions.

Human powered electrical generators

A generator can also be driven by human muscle power (for instance, in field radio station equipment). 

Protesters at Occupy Wall Street using bicycles connected to a motor and one-way diode to charge batteries for their electronics

 

Human powered direct current generators are commercially available, and have been the project of some DIY enthusiasts. Typically operated by means of pedal power, a converted bicycle trainer, or a foot pump, such generators can be practically used to charge batteries, and in some cases are designed with an integral inverter. An average "healthy human" can produce a steady 75 Watts (0.1 horsepower) for a full eight hour period, while a "first class athlete" can produce approximately 298 Watts (0.4 horsepower) for a similar period. At the end of which an undetermined period of rest and recovery will be required. At 298 Watts the average "healthy human" becomes exhausted within 10 minutes. The net electrical power that can be produced will be less, due to the efficiency of the generator. Portable radio receivers with a crank are made to reduce battery purchase requirements, see clockwork radio. During the mid 20th century, pedal powered radios were used throughout the Australian outback, to provide schooling (School of the Air), medical and other needs in remote stations and towns.

Mechanical measurement

Designed to measure shaft speed, a tachogenerator is a device which produces an output voltage proportional to that speed. Tachogenerators are frequently used to power tachometers to measure the speeds of electric motors, engines, and the equipment they power. speed. With precise construction and design, generators can be built to produce very precise voltages for certain ranges of shaft speeds.

Equivalent circuit

Equivalent circuit of generator and load.

• G, generator
• V_G, generator open-circuit voltage
• R_G, generator internal resistance
• V_L, generator on-load voltage
• R_L, load resistance

An equivalent circuit of a generator and load is shown in the adjacent diagram. The generator is represented by an abstract generator consisting of an ideal voltage source and an internal resistance. The generator's ${\displaystyle V_{\text{G}}}$ and ${\displaystyle R_{\text{G}}}$ parameters can be determined by measuring the winding resistance (corrected to operating temperature), and measuring the open-circuit and loaded voltage for a defined current load. 

This is the simplest model of a generator, further elements may need to be added for an accurate representation. In particular, inductance can be added to allow for the machine's windings and magnetic leakage flux, but a full representation can become much more complex than this.