This sound is familiar to everyone: a short, sharp metallic click. For a car owner — it is the rhythm of turn signals; for a process control engineer — confirmation of circuit switching; for an ordinary person — the characteristic noise of an old refrigerator.
But behind the familiar «click» lies a complex physical process in which electrical energy turns into mechanical motion. The heart of this process is the magnetic circuit — a system of soft magnetic material that guides magnetic flux.
To understand why one relay works «forever» while another loses its characteristics after only thousands of cycles, one must start not with the contacts or the coil, but with the metal from which its «soul» is made. As a practical example we will look at materials that PZPS produces — a plant specializing in alloys for tasks where ordinary steel is no longer enough.
The operating principle of any electromagnetic relay (contactor, starter) is based on Ampere’s law: an electric current flowing through a conductor creates a magnetic field.
When we apply current to the coil, a magnetic field arises around the turns of wire. But there is an important problem: air is an extremely poor conductor of magnetic flux. In magnetic-circuit terms this means that the magnetic reluctance of air (reactive reluctance) is very high.
To keep the flux from dissipating and to perform useful work, a core is placed inside the coil — a magnetic circuit made of soft magnetic material.
Simplified, the process looks like this:
the coil creates a magnetic field;
the magnetic circuit forms a directed magnetic flux;
the armature (moving part) becomes magnetized;
the armature overcomes the return-spring force and is attracted to the core;
the contacts close — and we hear the click.
And here the main engineering question arises: why does one core operate once, while another operates a million times without loss of pull force and without «sticking»?
The answer lies in the magnetic properties of the metal and in how it behaves under repeated magnetization cycles.
In the coil, under the action of electric current, magnetomotive force is created equal to the product of the number of turns and the current flowing through that conductor.
Magnetic flux closes through the core and armature. The most vulnerable place is the air gap. Even a micron-scale gap sharply increases magnetic reluctance. Therefore geometric accuracy, surface quality, and dimensional stability after processing are no less important than the chemical composition of the alloy.
One typical relay failure is a situation where after power is removed the armature does not return fully. This is called «sticking».
The physical cause is residual magnetization linked to magnetic hysteresis.
If the material has high coercive force, it:
demagnetizes poorly;
retains residual induction;
continues to hold the armature even without power.
Therefore soft magnetic materials with minimal hysteresis losses and high reversibility of the magnetic state are critically important for relay magnetic circuits.
A magnetic circuit is not simply a «piece of iron». It is a functional element that must combine several parameters at once.
For the relay magnetic system to work predictably and stably, the material must provide a balance of the following characteristics:
High magnetic permeability (μ). The higher μ, the less current is required to create the needed magnetic flux. This directly affects energy consumption.
Low coercive force (Hc). The material must demagnetize easily. Otherwise «sticking» due to residual magnetization is possible.
High saturation induction (Bs). The higher Bs, the greater the magnetic flux that can pass through the core without saturation. This is especially important for small magnetic-circuit dimensions.
Low remagnetization losses. For AC relays, hysteresis and eddy-current losses matter because they cause heating and reduce pull force.
Processability. The material must withstand stamping, cutting, machining, and heat treatment without degrading magnetic properties.
It is the combination of these parameters that determines material selection — from ordinary steels to nickel and cobalt precision alloys.
The St. Petersburg Precision Alloys Plant specializes in materials used where ordinary steel is no longer enough. Let us consider the main material groups used in relay magnetic circuits.
Low-carbon steels are used in powerful and relatively inexpensive DC relays. Essentially this is iron with a minimal impurity content (usually less than 0.04% carbon).
Their advantage is high saturation induction. The core can pass a large magnetic flux and create significant pull force. Such materials suit power circuits, contactors, and simple electromagnets.
There are also limitations that become critical for high-precision equipment:
sensitivity to corrosion;
less stable characteristics under long cyclic operation.
If a relay must operate in fractions of a millisecond and work at low currents, ordinary steel is not enough.
Precision alloys make it possible to:
reduce operating current;
reduce coil size;
increase sensitivity;
increase life in number of cycles.
Permalloys (Fe–Ni): ultra-high magnetic permeability
Permalloys — iron–nickel alloys considered the «gold standard» of soft magnetic materials for sensitive magnetic systems. PZPS produces grades 50Н, 79НМ, 80НХС.
A high nickel content (up to 80%) changes the iron crystal lattice and reduces magnetic anisotropy. As a result magnetic permeability is hundreds of times higher than that of ordinary steels.
A permalloy relay can:
operate at a substantially lower current;
work from weak power sources;
provide high repeatability of operation.
Such a relay can truly respond to currents at the level of low-power batteries — not as a «trick», but as a consequence of the physics of the material.
But there are also downsides:
sensitivity to mechanical deformation (stamping, bending, impact);
the need for special heat treatment after machining to restore magnetic properties.
Permendur (Fe–Co): when power in a small volume matters most
If permalloy is the material for sensitivity, then permendur is for maximum magnetic power.
Iron–cobalt alloys have a record saturation induction — up to 2.4 T. PZPS produces grades 49КФ, 49К2Ф, 49К2ФА-ВИ.
When it is necessary to provide maximum magnetic flux in a compact core — for example in miniature reed switches, polarized relays, or high-speed control systems — such alloys are used. They make it possible to reduce structure mass, raise specific power, and retain characteristics at small dimensions.
Even with the same design, relays can differ in life by several times.
And the cause is often not assembly quality, but how the magnetic circuit:
enters saturation;
retains residual magnetization;
responds to mechanical stresses;
changes properties when heated.
From an engineering viewpoint, material selection is a balance among energy efficiency, speed, life, dimensions, and cost.
A relay click is not just a sound. It is the acoustic trace of what, in fractions of a second:
the coil created magnetomotive force;
the magnetic circuit closed the flux;
the armature overcame the spring force;
the contacts switched the circuit.
And if this click must be repeated a million times with equal confidence, the decisive role belongs not only to the electrical circuit, but also to the metal — its structure, magnetic parameters, and process stability.
That is why in modern equipment the magnetic circuit is a precision element where PZPS precision alloys become the foundation of reliability.
