Hysteresis in electrical engineering and electronics: what is it

In electrical engineering, there are various devices, the principle of which is based on electromagnetic phenomena. Where there is a core on which a coil of conductive material, such as copper, is wound, interactions are observed due to magnetic fields. These are relays, starters, contactors, motors and magnets. Among the characteristics of the cores there is such a characteristic as hysteresis. In this article we will look at what it is, as well as what are the benefits and harms of this phenomenon.

Definition of the concept

The word "Hysteresis" has Greek roots, it translates as lagging or lagging. This term is used in various fields of science and technology. In a general sense, the concept of hysteresis distinguishes the different behavior of the system under opposite influences.

This can also be said in simpler terms. Let's say there is some kind of system that can be influenced in several directions. If, when acting on it in the forward direction, after termination, the system does not return to its original state, but is established in an intermediate state - then, in order to return to its original state, it is necessary to act in a different direction with by some force. In this case, the system has hysteresis.

Sometimes this phenomenon is used for useful purposes, for example, to create elements that are triggered at certain threshold values ​​of the acting forces and for regulators. In other cases, hysteresis has a detrimental effect, let's consider this in practice.

Hysteresis in electrical engineering

In electrical engineering, hysteresis is an important characteristic for the materials from which the cores of electrical machines and apparatus are made. Before starting to explain, let's look at the magnetization curve of the core.

An image on a graph of this kind is also called a hysteresis loop.

Important! In this case, we are talking about the hysteresis of pheromagnets, here it is a nonlinear dependence of the internal the magnetic induction of the material on the value of the external magnetic induction, which depends on the previous state element.

When current flows through a conductor around the latter, a magnetic and electric field. If you wind the wire into a coil and pass a current through it, you get an electromagnet. If you put a core inside the coil, then its inductance will increase, as will the forces that arise around it.

What does hysteresis depend on? Accordingly, the core is made of metal; its characteristics and the magnetization curve depend on its type.

If you use, for example, hardened steel, then the hysteresis will be wider. When choosing so-called soft magnetic materials, the schedule will narrow. What does this mean and what is it for?

The fact is that when such a coil is operating in an alternating current circuit, the current flows in one or the other direction. As a result and magnetic force, the pole is constantly inverted. In a coil without a core, this happens in principle at the same time, but with a core it is different. It gradually magnetizes, its magnetic induction increases and gradually reaches an almost horizontal section of the graph, which is called the saturation section.

After that, if you start to change the direction of the current and the magnetic field, the core should be re-magnetized. But if you just turn off the current and thereby remove the source of the magnetic field, the core will still remain magnetized, although not so much. On the next chart, this is point "A". To demagnetize it to its original state, you need to create a negative magnetic field strength. This is point "B". Accordingly, the current in the coil must flow in the opposite direction.

The value of the magnetic field strength for complete demagnetization of the core is called the coercive force and the less it is, the better in this case.

The magnetization reversal in the opposite direction will proceed in the same way, but along the lower branch of the loop. That is, when operating in an alternating current circuit, part of the energy will be spent on reversing the magnetization of the core. This leads to the fact that the efficiency of the electric motor and transformer decreases. Accordingly, this leads to its heating.

Important! The lower the hysteresis and coercive force, the lower the core reversal losses.

In addition to the above, hysteresis is also characteristic for the operation of relays and other electromagnetic switching devices. For example, trip and close currents. When the relay is turned off, in order for it to work, a certain current must be applied. In this case, its holding current in the on state can be much lower than the turn-on current. It will only turn off when the current falls below the holding current.

Hysteresis in electronics

In electronic devices, hysteresis is mainly useful. Let's say this is used in threshold elements, for example, comparators and Schmidt triggers. Below you can see a graph of its states:

This is necessary in those cases for the device to work when the X signal is reached, after which the signal can begin to decrease and the device does not turn off until the signal drops to the Y level. This solution is used to suppress contact bounce, interference and random bursts, as well as in various regulators.

For example, a thermostat or temperature controller. Usually, its principle of operation is to turn off the heating (or cooling) device at the moment when the temperature in the room or other place has reached a predetermined level.

Let's consider two options for working briefly and simply:

1. No hysteresis. Switching on and off at a given temperature. However, there are nuances here. If you set the temperature regulator to 22 degrees and heat the room to this level, then as soon as the room is 22, it will turn off, and when it drops to 21 again, it will turn on. This is not always the right decision, because your controlled device will turn on and off too often. In addition, in most household and many industrial tasks, there is no need for such a clear temperature control.
2. With hysteresis. To make a certain gap in the permissible range of adjustable parameters, hysteresis is used. That is, if you set the temperature to 22 degrees, then as soon as it is reached, the heater will turn off. Suppose that the hysteresis in the regulator is set to a gap of 3 degrees, then the heater will start working again only when the air temperature drops to 19 degrees.

Sometimes this gap is adjusted at your discretion. In simple designs, bimetallic plates are used.

Finally, we recommend watching a useful video that explains what hysteresis is and how you can use it:

We examined the phenomenon and application of hysteresis in electrical engineering. The bottom line is the following: in an electric drive and transformers, it has a detrimental effect, and in electronics and various regulators it also finds useful application. We hope the information provided was useful and interesting for you!

Related materials:

• How does a magnetic starter work
• What are harmonics in the power grid
• How does the resistance of a conductor depend on temperature?