Magnetic
Amplifiers
A magnetic amplifier is a device which controls the power delivered from an a.c. source by employing a controllable non linear reactive elements or circuit generally interposed in series with the load. The power required to control the reactive element or circuit is made for less than the amount of power controlled; and hence power amplification is achieved. The non-linear reactive element is a saturable reactor. When used in a combination with a set of high-grade rectifiers, it exhibits power amplification properties in the sense that small changes in control power result in considerable changes in output power. The basic component of a magnetic amplifier, as mentioned above, is the saturable reactor. It consists of a laminated core of some magnetic material. The hysteresis loop of the reactor core is a narrow and steep one. A schematic diagram of a simple saturable core reactor with control winding and a.c. winding wound on two limbs. The control winding having a number of turns, Na.c. is fed with d.c. supply. By varying the control current, it is possible to largely vary the degree of saturation of the core. The other winding, called the a.c. winding or gate winding having a number of turns, Na.c. is fed from an a.c. source, the load being connected in series with it. The property of the reactor which makes it behave as a power amplifier is its ability to change the degree of saturation of the core when the control winding mmf (magneto motive force i.e., ampere turns), established by d.c. excitation, is changed. The a.c. power supply will have high impedance if the core is unsaturated and the varying values of lower impedances as the core is increasingly saturated. When the core is completely saturated, the impedance of the a.c. winding becomes negligibly small and the full a.c. voltage appears across the load. Small values of current through the control winding, which has a large number of turns, will determine the degree of saturation of the core and hence change the impedance of the output circuit and control the flow of current through the load. By making the ratio of control winding turns to the a.c. winding turns large, an extremely high value of output current can be controlled by a very small amount of control current, The saturable core reactor circuit shown in Fig. has certain serious disadvantages. The core gets partially desaturated in the half-cycle in which the a.c. winding mmf opposes the control winding mmf. This difficulty is overcome by employing a rectifier in the output circuit as shown in Fig. Here the desaturating (damagnetising) effect by the half-cycle of the output current is blocked by the rectifier. On the other hand, the output and control winding mmfs aid each other to effect saturation in the half-cycle in which current passes through the load, thus making the reactor a self-saturating magnetic amplifier. Another difficulty that is experienced is that a high voltage is induced in the control winding due to transformer action. In order that this voltage is unable to send current to the d.c. circuit a high inductance should be connected in series with the control winding. This, however, slows down the response of the control system and hence the overall system. The saturable core is generally made of a saturable ferromagnetic material. For magnetic amplifiers of lower ratings usual transformer type construction using silicon steel (3 to 3.5 per cent Si) is used. Use of high quality nickel-iron alloy materials, however , makes possible much higher performance amplifiers of smaller size and weight. In order to realize the advantages of these materials, use is made of toroidal core configuration.
The magnetic
amplifier (colloquially known as the "mag amp") is an electromagnetic
device for amplifying electrical signals. The magnetic amplifier was invented
early in the 20th century, and was used as an alternative to vacuum tube
amplifiers where robustness and high current capacity were required. World War
II Germany
perfected this type of amplifier, and it was used for instance in the V-2 rocket.
The magnetic amplifier has now been largely superseded by the transistor-based
amplifier, except in a few safety critical, high reliability or extremely
demanding applications.
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[edit] Principle of operation
Visually a
mag amp device may resemble a transformer but the operating principle is quite
different from a transformer - essentially the mag amp is a saturable
reactor. It makes use of magnetic saturation of the core, a non-linear
property of a certain class of transformer cores. For controlled saturation
characteristics the magnetic amplifier employs core materials that have been
designed to have a specific B-H curve shape that is highly rectangular, in contrast to
the slowly tapering B-H curve of softly saturating core materials that are
often used in normal transformers.
The typical
magnetic amplifier consists of two physically separate but similar transformer magnetic
cores, each of which has two windings - a control winding and an AC
winding. A small DC current from a low impedance source is fed into the
series-connected control windings. An AC voltage is fed into one AC winding,
with the other AC winding connected to the load. The AC windings may be
connected either in series or in parallel, the configurations resulting in
different types of mag amps. The amount of control current fed into the control
winding sets the point in the AC winding waveform at which either core will
saturate. In saturation, the AC winding on the saturated core will go from a
high impedance state ("off") into a very low impedance 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 AC windings. This results in current amplification.
[edit] Applications
Magnetic
amplifiers were used extensively as the switching element in early
switched-mode (SMPS)
power supplies[1],
as well as in lighting control. They have been largely superseded by
semiconductor based solid-state switches, though recently there has been some
regained interest in using mag amps in compact and reliable switching 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 currently manufactured 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 high voltage and are therefore still used in the HVDC-technique.
Another
small book on the subject of Magnetic amplifiers by the US Navy (1951)[2]
[edit] Misnomer
uses
Late in the
20th century, 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 designs with an
unusual power supply circuit. They were not magnetic amplfiers in the sense of
this article.
