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
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 larger.
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.
A 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
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
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 transistor-manufacturing processes.
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.
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 vital.
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
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.
a 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.