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. Thecontrol 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.