September I S S O
Instrumentation New developments in the field of self-saturating magnetic amplifiers are described hg R d p h H. Muneh T ISdifficult to define the term
“amplifier” in unequivocal fashion. One possible definition is the following: “An amplifier is a device for increasing the magnitude of a variable.” The relationship between input and output may be linear or logarithmic, or it may follow some other law suited to the particular purpose for which the amplifier is intended. The output of an amplifier may be measured or the amplifier may be used as a null-point indicator. In the latter case, the amplifier is usually part of a servoloop. The amplifier in modern electronic recording potentiometers is used in this fashion.
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as projection systems and optical levers are not often used in planttype instruments although they are capable of great sensitivity. Hydraulic amplifiers are extremely versatile. They are most often used to operate heavy equipment. Pneumatic devices are frequently used as amplifiers too. For instance, the pneumatic relays used in many instruments make it possible for the minute m o u n t of power produced when a liquid expands in an industrial-type thermometer bulb to control the action of valves that would tax a man’s strength to open or close manually.
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Figure 1. Simple Form of Saturable Core Reactor
Many types of amplifiers are used in instruments. Mechanical devices for amplifying motion include simple lever and gear trains. For example, levers are used to amplify the motion of the float resting on the surface of the mercury in recording manometers. Gear trains are used to amplify the motion of the tip of the Bourdon tube in the common types of Bourdon tube pressure gages. Optical methods of amplification such
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To many people, the word amplifier connotes an electronic amplifier. Electronic amplifiers have many valuable characteristics. One of their important features is that high amplification ratios may be obtained. Another is that it is possible to secure an extremely linear relation between input and output or to make the relation follow other laws. In addition, speed of response is generally much higher than that of other types of amplifiers. One of the objectionable features of electronic amplifiers is their complexity. Because of the large number of components such as tubes, transformers, resistors, and capacitors involved in even a simple electronic amplifier, it is not easy for anyone without special training and test instruments to find the cause when such an amplifier fails to function. Magnettc amplU&rs
The last class of amplifiers which we wish to mention are magnetic amplifiers. A magnetic amplifier is a device using saturable reactors to permit control of a larger amount of power by a smaller amount. Figure 1 is a diagram of a simple form of saturable core reactor. It consists of a core of high permeability magnetic material with three windings. The two alternating current windings are connected so that the voltages which they induce in the direct current windkg are equal and opposite so that the alternating current component in the direct current winding is zero. When there is no current flowing
Instrumentation saturates the core, decreasing the inductance of the reactor. Thus, the current in the alternating current circuit can be controlled by varying the current in the direct current circuit. If the number of turns in the direct current winding is made large compared to that in the alternating current windings a large alternating current can be controlled with a small direct current. For this method of operation, it is necessary that the ampere turns in the direct current or control winding be equal to the ampere turns in the alternating current or output windings plus sufficient ampere turns to saturate the core. Self-saturating magnetic amplifiers are now more used than the simple type shown in Figure 1. Figure 2 shows a self-saturating magnetic amplifier. In this circuit, a rectifier, R, prevents flow of current in the load circuit in a direction which would desaturate the core. Therefore, assuming a perfect rectifier, only sufficient ampere turns to saturate the core are required. The use of high permeability core materials and gapless core construction makes the required magnetizing current very small. In the circuit shown in Figure 2 load current can pass only on one half cycle. In circuits used in practice, reactors are used in pairs as shown in Figure 3 so that full wave performance is obtained. The selfsaturating circuits require only one fiftieth as much control current to control a given load as is required with the circuit shown in Figure 1.
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Figure 3. Single-phase Self-saturating Magnetic Amplifier with Alternating Current Output
The modern magnetic amplifier is a much improved device. It is capable of power gains of 25,000 to 1 or more. Power from a fraction of a watt to several kilowatts can be controlled. It has the advantage that very few parts are required and that those few are not likely to deteriorate. A characteristic which might be a disadvantage in some cases is the response time. Time constants are approximately 3 seconds for 60cycle units. Operation a t 400 cycles gives time constants approximating 0.01 second. Magnetic amplifiers are manufactured by Vickers Electric Division, T’ickers Incorporated, St. Louis 3, Mo.
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