Chapter 12
Fundamentals of Electricity & Electronics
Some of the disadvantages of the magnetic amplifier are:
- Incapacity to handle low-voltage signals
- Not usable in high-frequency applications
- Time delay associated with magnetic affects
- Poor fidelity
The basic operating principles of the magnetic amplifier are fairly simple. Keep in mind that all amplifiers are current control devices. In this particular case, power that is delivered iron core transformer, the iron core is magnetized and demagnetized at the same frequency as that of the applied voltage. This, in turn, induces a voltage in the transformers secondary winding. The output voltage across the terminals of the secondary depends on the relationship of the number of turns in the primary and the secondary of the transformer.
The iron core of the transformer has a saturation point after which the application of a greater magnetic force produces no change in the intensity of magnetization. Hence, there is no change in transformer output, even if the input is greatly increased. The magnetic amplifier circuit in Figure 12-247 is used to explain how a simple magnetic amplifier functions.
- Assume that there is 1 ampere of current in coil A, which has 10 turns of wire. If coil B has 10 turns of wire, an output of 1 ampere is obtained if coil B is properly loaded.
- By applying direct current to coil C, the core of the magnetic amplifier coil can be further magnetized. Assume that coil C has the proper number of turns and, upon the application of 30 milliamperes, that the core
is magnetized to the point where 1 ampere on coil A results in only 0.24 ampere output from coil B.
- By making the DC input to coil C a continuous variable from 0 to 30 milliamperes and by maintaining an input of 1 ampere on coil A, it is possible to control the output of coil B to any point between 0.24 ampere and 1 ampere in this example.
The term “amplifier” is used for this arrangement because, by use of a few milliamperes, control of an output of 1 or more amperes is obtained.
Saturable-Core Reactor
The same procedure can be used with the circuit shown in Figure 12-248. A saturable-core reactor is a magnetic-core coil whose reactance is controlled by changing the permeability of the core. Varying the unidirectional flux controls the permeability of the core.
By controlling the extent of magnetization of the iron ring, it is possible to control the amount of current flowing to the load, since the amount of magnetization controls the impedance of the AC input winding. This type of magnetic amplifier is called a simple saturable reactor circuit.
Adding a rectifier to such a circuit would remove half the cycle of the AC input and permit DC to flow to the load. The amount of DC flowing in the load circuit is controlled by a DC control winding (sometimes referred to as bias). This type of magnetic amplifier is referred to as being self-saturating.
To use the full AC input power, a circuit such as that shown in Figure 12-249 may be used. This circuit uses a full-wave bridge rectifier. The load receives a controlled DC by using the full AC input. This type of circuit is known as a self-saturating, full-wave magnetic amplifier.
In Figure 12-250, it is assumed that the DC control winding is supplied by a variable source, such as a sensing circuit. To winding, magnetizes the magnetic core in one direction.
The DC control winding, acting in opposition to the reference winding, either increases (degenerative) or decreases (regenerative) the magnetization of the core to change the amount of current flowing through the load. This is essentially a basic preamplifier.