magnetic amplifier

Magnetic amplifierFrom Wikipedia, the free encyclopedia

The magnetic amplifier (colloquially known as a "mag amp") is an electromagnetic device for amplifyingelectrical signals. The magnetic amplifier was invented early in the 20th century, and was used as analternative to vacuum tube amplifiers where robustness and high current capacity were required. World WarII Germany perfected this type of amplifier, and it was used in the V-2 rocket. The magnetic amplifier wasmost prominent in power control and low-frequency signal applications from 1947 to about 1957, when thetransistor began to supplant it.[1] The magnetic amplifier has now been largely superseded by thetransistor-based amplifier, except in a few safety critical, high reliability or extremely demandingapplications. Combinations of transistor and mag-amp techniques are still used.

Contents

1 Strengths2 Limitations3 Principle of operation4 Applications5 History

5.1 Early development5.2 Usage in radio5.3 Usage in aircraft5.4 Usage in computing

6 Misnomer uses7 References8 External links

Strengths

The magnetic amplifier is a static device with no moving parts. It has no wear-out mechanism and has a goodtolerance to mechanical shock and vibration. It requires no warm-up time.[2] Multiple isolated signals maybe summed by additional control windings on the magnetic cores. The windings of a magnetic amplifier havea higher tolerance to momentary overloads than comparable solid-state devices. The magnetic amplifier isalso used as a transducer in applications such as current measurement and the flux gate compass.

Limitations

The gain available from a single stage is limited and low compared to electronic amplifiers. Frequencyresponse of a high gain amplifier is limited to about one-tenth the excitation frequency, although this is oftenmitigated by exciting magnetic amplifiers with currents at higher than utility frequency.[1] Solid-stateamplifiers can be more compact and efficient than magnetic amplifiers. The bias and feedback windings arenot unilateral, and may couple energy back from the controlled circuit into the control circuit. Thiscomplicates the design of multistage amplifiers when compared with electronic devices.[1]

Principle of operation

Visually a mag amp device may resemble a transformer but the operating principle is quite different from a

Magnetic amplifier - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Magnetic_amplifier

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A saturable reactor, illustrating theprinciple of a magnetic amplifier

transformer - essentially the mag amp is a saturable reactor. It makesuse of magnetic saturation of the core, a non-linear property of acertain class of transformer cores. For controlled saturationcharacteristics the magnetic amplifier employs core materials thathave been designed to have a specific B-H curve shape that is highlyrectangular, in contrast to the slowly-tapering B-H curve of softlysaturating core materials that are often used in normal transformers.

The typical magnetic amplifier consists of two physically separate butsimilar transformer magnetic cores, each of which has two windings -a control winding and an AC winding. A small DC current from a lowimpedance source is fed into the series-connected control windings.The AC windings may be connected either in series or in parallel, theconfigurations resulting in different types of mag amps. The amountof control current fed into the control winding sets the point in theAC winding waveform at which either core will saturate. Insaturation, the AC winding on the saturated core will go from a high impedance state ("off") into a very lowimpedance state ("on") - that is, the control current controls at which voltage the mag amp switches "on".

A relatively small DC current on the control winding is able to control or switch large AC currents on the ACwindings. This results in current amplification.

Two magnetic cores are used because the AC current will generate high voltage in the control windings. Byconnecting them in opposite phase, the two cancel each other so no current is induced in the control circuit.

Applications

Magnetic amplifiers were important as modulation and control amplifiers in the early development of voicetransmission by radio.[2] A magnetic amplifier was used as voice modulator for a 2 kilowatt Alexandersonalternator, and magnetic amplifiers were used in the keying circuits of large high-frequency alternators usedfor radio communications. Magnetic amplifiers were also used to regulate the speed of Alexandersonalternators to maintain the accuracy of the transmitted radio frequency.[2]

The ability to control large currents with small control power made magnetic amplifiers useful for control oflighting circuits, for stage lighting and for advertising signs. Saturable reactor amplifiers were used for controlof power to industrial furnaces.[2] Small magnetic amplifiers were used for radio tuning indicators, control ofsmall motor and cooling fan speed, control of battery chargers.

Magnetic amplifiers were used extensively as the switching element in early switched-mode (SMPS) powersupplies,[3] as well as in lighting control. Semiconductor based solid-state switches have largely supersededthem, though recently there has been some regained interest in using mag amps in compact and reliableswitching power supplies. PC ATX power supplies often use mag amps for secondary side voltage regulation.

Magnetic amplifiers are still used in some arc welders.

Magnetic amplifier transformer cores designed specifically for switch mode power supplies are currentlymanufactured by several large electromagnetics companies, including Metglas and Mag-Inc.

Magnetic amplifiers can be used for measuring high DC-voltages without direct connection to the highvoltage and are therefore still used in the HVDC-technique.

Magnetic amplifiers were used by locomotives to detect wheel slip, until replaced by Hall Effect currenttransducers. The cables from two traction motors passed through the core of the device. During normal


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operation the resultant flux was zero as both currents were the same and in opposite directions. The currentswould differ during wheel slip, producing a resultant flux that acted as the Control winding, developing avoltage across a resistor in series with the AC winding which was sent to the wheel slip correction circuits.

History

Early development

A voltage source and a series connected variable resistor may be regarded as a direct current signal sourcefor a low resistance load such as the control coil of a saturable reactor which amplifies the signal. Thus, inprinciple, a saturable reactor is already an amplifier, although before 20th century they were used for simpletasks, such as controlling lighting and electrical machinery as early as 1885.[4][5][6]

In the early 20th Century, the General Electric Company, under the direction of engineer E. F. W.Alexanderson, developed a system of transoceanic radio communications, using continuous wavetransmission over great distances. Alexanderson drew upon the work of Nikola Tesla and ReginaldFessenden as the inspiration for his system.

The result of this work was the 2 kW Alexanderson alternator, which produced radio frequencies from 50 to100 kHz and which critics had previously denounced as impractical. Later, Guglielmo Marconi took a vestedinterest in the project and, in 1915, witnessed a demonstration of a new, 50 kW, 50 kHz alternator.

The experimental telegraphy and telephony demonstrations made during 1917 attracted the attention of theUS Government, especially in light of partial failures in the transoceanic cable across the Atlantic Ocean.The 50 kW alternator was commandeered by the US Navy and put into service in January 1918 and wasused until 1920, when a 200 kW generator-alternator set was built and installed.

Usage in radio

Magnetic amplifiers were used to control large high-power alternators by turning them on and off fortelegraphy or to vary the signal for voice modulation. The alternator's frequency limits were rather low towhere a frequency multiplier had to be utilized to generate higher radio frequencies than the alternator wascapable of producing. Even so, early magnetic amplifiers incorporating powdered-iron cores were incapableof producing radio frequencies above approximately 200 kHz. Other core materials, such as ferrite cores andoil-filled transformers, would have to be developed to allow the amplifier to produce higher frequencies.

Usage in aircraft

Magnetic amplifiers were used in aircraft systems (avionics) before the advent of high reliabilitysemiconductors. They were important in implementing early autoland systems and Concorde made use of thetechnology for the control of its engine air intakes before development of a system using digital electronics.

Usage in computing

Magnetic amplifiers were widely studied during the 1950s as a potential switching element for mainframecomputers. Mag amps could be used to sum several inputs in a single core, which was useful in thearithmetic logic unit (ALU). Custom tubes could do the same, but transistors could not, so the mag amp wasable to combine the advantages of tubes and transistors in an era when the latter were expensive andunreliable.

That era was short, lasting from the mid 1950s to about 1960, when new fabrication techniques producedgreat improvements in transistors and dramatically lowered their cost. Only one large-scale mag amp


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machine was put into production, the UNIVAC Solid State, but a number of contemporarylate-1950's/early-1960s computers used the technology, like the Ferranti Orion.

Misnomer uses

In the 1970s, Robert Carver designed and produced several high quality high-powered audio amplifiers,calling them magnetic amplifiers. In fact, they were in most respects conventional audio amplifier designswith unusual power supply circuits. They were not magnetic amplifiers as defined in this article.

References

^ a b c H. P. Westman et al, (ed), Reference Data for Radio Engineers, Fifth Edition, 1968, Howard W. Samsand Co., no ISBN, Library of Congress Card No. 43-14665 chapter 14

1.

^ a b c d H. F. Storm, Magnetic Amplifiers, John Wiley and Sons, New York, 1955 page 3832.^ Abraham I. Pressman (1997). Switching Power Supply Design. McGraw-Hill. ISBN 0-07-052236-7.3.^ Electronics Design and Development Division, ed. (May 1954) [1951]. "History". Magnetic Amplifiers – ARising Star in Naval Electronics. 18th and Constitution Avenue, Washington 25, D.C.: Bureau of Ships,Department of the Navy. p. 2. NAVSHIPS 900,172. "The magnetic amplifier is not new – the principles of thesaturable core control were used in electrical machinery as early as 1885 although they were not identified assuch."

4.

^ Mali, Paul (August 1960). "Introduction" (http://www.pmillett.com/Books/mag_amp.pdf) (PDF). MagneticAmplifiers – Principles and Applications. New York: John F. Rider Publisher. p. 1. Library of Congress CatalogNumber 60-12440. http://www.pmillett.com/Books/mag_amp.pdf. Retrieved 2010-09-19. "Magnetic amplifierswere developed as early as 1885 in the United States. At that time they were known as saturable reactors andwere used primarily in electrical machinery and in theater lighting."

5.

^ Kemp, Barron (August 1962). "Magnetic Amplifiers". Fundamentals of Magnetic Amplifiers. Indianapolis,Indiana: Howard W. Sams & Co.. p. 7. Library of Congress Catalog Card Number: 62-19650. "The use ofmagnetic forces for amplification is not new; a survey of its history shows that although the device was notknown as a magnetic amplifier at the time, it was used in electrical machinery as early as 1885."

6.

Alexanderson, E. F. W., "Transoceanic Radio Communication," General Electric Review, October1920, pp. 794–797.Cheney, Margaret, "Tesla: Man Out of Time," 1981, New York: Simon & Schuster, Inc.Chute, George M., "Magnetic Amplifiers," Electronics in Industry, 1970, New York: McGraw-Hill,Inc., pp. 344–351.Trinkaus, George, "The Magnetic Amplifier: A Lost Technology of the 1950s," Nuts & Volts, February2006, pp. 68–71.Trinkaus, George, editor, "Magnetic Amplifiers: Another Lost Technology," 1951: Electronics Designand Development Division, Bureau of Ships, United States Navy.

External links

Magnetic Amplifier Control for Simple, Low-Cost, Secondary Regulation - B. Mammano, TexasInstruments Seminar 500 (http://focus.ti.com/lit/ml/slup129/slup129.pdf)MAG-AMP Magnetic Amplifiers Intro, Butlerwinding (http://web.archive.org/web/20090603071406/http://www.butlerwinding.com/elelectronic-transformer/mag-amp.html)

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