04-04-2012, 02:54 PM
Magnetic Amplifiers
seminar report on magnetic ampifiers.docx (Size: 103.98 KB / Downloads: 64)
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 The magnetic amplifier has now been largely superseded by the transistors-based amplifier, except in a few safety critical, high reliability or extremely demanding applications.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.
seminar report on magnetic ampifiers.docx (Size: 103.98 KB / Downloads: 64)
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.