Awakened to the
possibilities inherent in this design, Allied scientists began to push the development of magnetic amplifiers.
Before much progress had been made, though, the war was over. But the spark had been kindled, and a few years
later Vickers Inc. (now a division of Sperry Rand) came out with the first commercially produced magnetics.
By that time, interest had been aroused all over the world. In the following decade, hundreds of other
firms, including all the big names in electrical and electronic equipment, have added magnetics to their product
lines. And almost no branch of industry now operates without them. Flux Controls Current
A modern-day magnetic amplifier is, essentially, nothing more than an iron core with two or more coils of wire
wound around it. In construction and appearance, it is similar to a transformer. But there the similarity ends.
A magnetic amplifier - or saturable reactor, as it is sometimes called - is a true amplifier. Like a vacuum
tube, it uses a small signal to control a large one. But there are sharp differences. Where the vacuum tube
controls a current flowing to a d.c. power supply, the magnetic amplifier controls an a.c. flow. While the
vacuum tube is primarily a voltage amplifier, the magnetic is a power amplifier. And where the vacuum tube uses
voltage variations to control a flow of electrons, the magnetic amplifier controls current flow through a coil
by varying magnetic flux.
. Basic circuit for half-wave magnetic amplifier.
like the Vickers unit above allow fingertip control of elaborate lighting
systems in TV studios. (NBC photo)
magnetic amplifiers, this paper-making machine at West Tacoma Newsprint Corporation
in Tacoma, Washington, can operate at 5000 feet per second, many times faster than previously possible.
Magnetics continuously adjust the speed of the take-up rollers, slowing them down as the roll of paper gets
use magnetic amplifiers, too. Because the steel gets longer as it is
rolled, each set of rollers must turn at a slightly different speed. Magnetics keep all the rollers operating
at the proper speed relationship regardless of how fast the steel is fed in. (Pittsburgh Steel photo)
Magnetics come in half-wave and full-wave types, as do a.c. power supplies. First, let's look at the basic
half-wave circuit shown in Fig. 1.
A d.c. current flowing through the control winding will cause a
build-up of magnetic flux in the iron core. The greater the flux, the lower will be the impedance of the output
winding. With a lower impedance in the circuit, more current will flow from the a.c. power supply through the
output winding and the load.
When the current in the control winding reaches a certain point, the core is said to be saturated, which
means that it has all the flux it can hold. At this point, the impedance of the output winding is very low, and
the current through the load is very high. On the other hand, when there is no control current flowing, and
consequently no flux in the core, the output impedance is extremely high, and practically no current flows
through the output winding or the load. Thus, by controlling the current through the control winding, the output
winding impedance, and consequently the current through the load, is made continuously variable.
rectifier in series with the output winding keeps the constantly reversing polarity of the a.c. supply from
cancelling out the control winding flux. The direction of the current flow through the secondary is arranged so
that the magnetic fluxes created by the two windings reinforce each other rather than cancel each other out.
. Basic circuit for a full-wave magnetic amplifier.
A full-wave circuit is shown in Fig. 2. It works like the circuit in Fig. 1, except that it makes use of both
half cycles of the a.c. supply current. The two halves of the output winding are wound so that the direction of
the magnetic flux created by both of them in the center leg of the core is the same as the direction of the flux
created by the control winding.
The bias winding can be used to control the general range of the
amplifier's operation, just as the bias on a vacuum tube causes the tube to operate on a certain part of its
characteristic curve. In a magnetic amplifier, when a small bias current flows, a certain amount of flux is
continuously present in the core, even with no control voltage supplied. Thus, the impedance of the output
winding will never reach its maximum value, nor will the current through the load reach its minimum.
Many magnetic amplifiers have an additional control winding which is used for feedback. This winding taps a
certain amount of the output circuit's current and applies it back as a control current. As with a vacuum tube,
the feedback can be either negative or positive. In general, negative feedback improves the linearity of the
amplifier while positive feedback increases its gain.
Single-stage magnetics can be built with gains of about 200,000, far beyond the capabilities of the
vacuum tube. With a gain on this order, a few milliwatts or power in the control winding - an amount that could
be supplied by one or two flashlight cells - may control a load of 25,000 watts in the output circuit.
Rugged and Reliable
. Magnetics are extremely rugged. They can be - and frequently are -
completely potted and sealed in airtight containers. They thrive on extremes of heat, dust, moisture, vibration,
and other adverse conditions that would put vacuum tubes and transistors out of operation. Their efficiency is
high, as with transformers and other magnetic devices. In addition, no filament current is required. So little
heat is generated by magnetics that they can be packed into extremely small containers which need practically no
ventilation or cooling.
Because magnetics can handle large amounts of current easily, they are a natural
choice for electric furnace control. A Reynolds Aluminum Company furnace in Corpus Christi, Texas, uses such a
control system. Precise furnace control by magnetics also helps to "grow" transistors in the latest types of
Magnetics have recently begun to invade the field of entertainment,
too. NBC's two big color television studies - one in Burbank, California, the other in Brooklyn, N. Y. - have
magnetic amplifier lighting-control systems. With this setup, the lighting man has fingertip control over each
of the hundreds of lights throughout the studio. He can control them individually or in banks, as he desires,
working from a small keyboard that looks something like an organ console. Unlike older types of theatre lighting
devices - autotransformers and rheostats-magnetics present no fire hazard.
Since magnetic amplifiers
have no moving parts and no delicate components, they last for years with virtually no maintenance. For this
reason, they are used in such critical applications as the control of the atomic pile in nuclear subs and in
missile-guidance systems, where reliability under adverse conditions of vibration, heat, and acceleration is
Reliability is also the reason magnetics were chosen to monitor and control the critical voltages
and currents of the transatlantic cable. If a voltage begins to change, a magnetic compensates for the change,
and, at the same time, sounds an alarm so an operator can check to find the reason for the change. If the
current drawn by the underwater repeater amplifier tubes begins to rise, once again the alarm is given, and
corrective action is taken automatically. By insuring that the current does not rise to dangerous levels, the
magnetics prolong the lives of the submerged tubes. This is important because lifting the cable to replace a
damaged tube costs thousands of dollars. Long-Life Switching
. Basic magnetic amplifier
circuits can be modified to give special effects. For example, a magnetic to which excessive positive feedback
has been applied becomes "bistable." This means that it is stable in only two states of operation: maximum
output or minimum output. There is no in-between. The amplifier is adjusted so that the core is normally in a
non-saturated state. But even the tiniest input signal - perhaps only a few microamperes - will throw it into
complete saturation. Thus it becomes the equivalent of an extremely sensitive switch, or relay.
magnetic amplifier is a switch without moving parts or contacts, and it is virtually indestructible. The
bistable magnetic is beginning to find widespread use as a replacement for relays where long, reliable service
is of great importance.
Several automotive companies - the Ford Motor Co., for example - are now using
magnetics to control the flow of parts in the engine assembly line. First, proximity switches containing
magnetic amplifiers sense the presence or absence of necessary parts on an automated line. Other magnetics, cued
by the proximity switch, supply the parts as needed. Since there are no moving components and no contacts, these
magnetics show no signs of wear after millions upon millions of operations - long after normal relay contacts
would have worn out. Another series of magnetics controls the speed of the engine assembly conveyor, to
determine the proper production rate.
The uses for magnetic amplifiers are almost limitless. They serve
as memory units in computers and as speed regulators in steel, paper, and textile mills; they control gun
turrets and radar antennas on navy ships; they regulate the voltage output of huge turbine generators; they
control automatic elevators, mine hoists, power shovels, cranes, and printing presses. In short, wherever the
considerations of precise, reliable, trouble-free control are important - from jet aircraft to atomic submarines
- you'll find magnetic amplifiers working silently and efficiently. Posted