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22-09-2008, 12:23 AM
Post: #1
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.

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14-07-2009, 09:47 PM
Post: #2
RE: Magnetic Amplifiers
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25-07-2009, 10:14 PM
Post: #3
RE: Magnetic Amplifiers
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25-07-2009, 10:19 PM
Post: #4
RE: Magnetic Amplifiers
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04-04-2012, 02:54 PM
Post: #5
RE: Magnetic Amplifiers
Magnetic Amplifiers


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Magnetic amplifier
Various different amplifiers are used for the amplification of current and voltage in electronic devices. A magnetic amplifier uses the principle of transformers along with the concept of magnetic saturation of the core.
A magnetic amplifier is a device used for controlling the flow of power to a load by means of saturating a magnetic core. They are widely used in recent years for both high and low power applications.
The magnetic amplifier (colloquially known as a "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 2 Germany perfected this type of amplifier, and it was used for instance in the V-2 rocket. The magnetic amplifier was most prominent in power control and low-frequency signal applications from 1947 to about 1957, when the transistors began to supplant it. The magnetic amplifier has now been largely superseded by the transistors-based amplifier, except in a few safety critical, high reliability or extremely demanding applications. Combinations of transistor and mag-amp techniques are still used.
The magnetic amplifier has certain advantages over other types of amplifiers. These include
(1) High efficiency (up to 90 percent);
(2) Reliability (long life, freedom from maintenance, reduction of spare parts inventory);
(3) Ruggedness (shock and vibration resistance, high overload capability, freedom from effects of moisture); and (4) no warm-up time. The magnetic amplifier has no moving parts and can be hermetically sealed within a case similar to the conventional dry-type transformer.



Working of a Magnetic Amplifier

Magnetic materials suitable for magnetic amplifiers have a rectangular hysteresis loop with sharp saturation and low hysteresis loses. The excitation level is adjusted such that the flux swing in the core occurs between the knees of the B-H curve.
The typical magnetic amplifier circuit shown above has the following components:
• R1 - load resistance
• G - gate winding
• C - control winding
• Ec - excitation
• Es - input from AC source
• Is - amplified AC current
• Ic - control DC current


Applications
• Magnetic amplifiers were important as modulation and control amplifiers in the early development of voice transmission by radio.
• A magnetic amplifier was used as voice modulator and magnetic amplifiers were used in the keying circuits of large high-frequency alternators used for radio communications.
• Magnetic amplifiers were also used to regulate the speed of Alexanderson alternators to maintain the accuracy of the transmitted radio frequency.
• The ability to control large currents with small control power made magnetic amplifiers useful for control of lighting circuits, for stage lighting and for advertising signs.
• Saturable reactor amplifiers were used for control of power to industrial furnaces.
• Small magnetic amplifiers were used for radio tuning indicators, control of small motor and cooling fan speed, and control of battery chargers.
• Magnetic amplifiers were used extensively as the switching element in early switched-mode (SMPS) power supplies, as well as in lighting control.


History & Early development
A voltage source and a series connected variable resistor may be regarded as a direct current signal source for a low resistance load such as the control coil of a saturable reactor which amplifies the signal. Thus, in principle, a saturable reactor is already an amplifier, although before 20th century they were used for simple tasks, 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 wave transmission over great distances. Alexanderson drew upon the work of Nikola Tesla and Reginald Fessenden as the inspiration for his system.

Limitations
The gain available from a single stage is limited and low compared to electronic amplifiers. Frequency response of a high gain amplifier is limited to about one-tenth the excitation frequency, although this is often mitigated by exciting magnetic amplifiers with currents at higher than utility frequency. Solid-state amplifiers can be more compact and efficient than magnetic amplifiers. The bias and feedback windings are not unilateral, and may couple energy back from the controlled circuit into the control circuit. This complicates the design of multistage amplifiers when compared with electronic devices.
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