Core loss (also called iron loss) is the term used to describe energy loss in
electro-magnetic circuits such as motors
, or inductors
which contain a ferromagnetic
core. There are two distinct phenomena that contribute to core losses, eddy current losses and hysteresis
Eddy Current Losses
When a current is passed through a wire it generates a magnetic field. When the current flowing through the wire is altered, the magnetic field changes. When a conductor is passed through a magnetic field, electricity is generated in the conducting material (thank you Michael Faraday). Conversely, if we hold a conducting object still in a moving magnetic field electricity is generated in the conductor. In the case of the core material these small electrical currents flow in circular paths and are called eddy currents. Energy induced into the core material in this manner will flow in circular paths until the energy is converted to heat because of the resistance of the core material. This is of course wasted energy, unless you wish to heat your domicile with a large inductor.
The energy lost to in this manner is proportional to the length of the path the eddy currents travel. To help reduce iron losses most low frequency transformers (like those used with the AC power grid are built with thin laminated steel. These laminates are glued together with a non-conductive adhesive to form a stack. By reducing the thickness of the conductor (to the thickness of a single lamina) the energy lost to this effect is reduced. Higher frequency transformers such as those used with Switch Mode Power Supplies use powdered iron cores, or a ceramic material called ferrite. By reducing the size of the conductive particles the energy lost through eddy current losses is reduced further.
The second mode of energy loss associated with core losses is hysteresis. Hysteresis losses are a bit more mysterious, and we must begin with a description of ferromagnetic materials in general. Conceptually ferromagnetic materials (like steel, basically any material that can be magnetized) are thought of as a matrix of magnetic domains. Try to picture each domain as having a magnetic alignment (like a compass arrow). In unmagnetized material these domains are unaligned and basically pointing in random directions.
When we pass a magnetic field through the material (by introducing a permanent magnet, or an electromagnetic field) some of the domains will begin to align along the magnetic lines of force. The stronger the magnetic field, the more domains that will align. When all of the domains are aligned, the material is said to be magnetically saturated. If we now remove the magnetic field, some of the domains will re-orient themselves but some of them will remain in their current position, or close to it. We have just created a permanent magnet, though probably not a very strong one. The strength of the magnet is determined by the number of domains that are aligned.
We have just changed the the behavior of the material and this took energy. If our goal was not to produce a permanent magnet this energy is wasted. If we maintain a constant magnetic field we will only suffer this initial energy loss, but what would happened if we were to introduce our core to an alternating magnetic field? As the magnetic field reverses, we will notice that many of the domains are aligned against the magnetic lines of force (the lines of force have reversed direction because we are changing the field). We must de-magnetized the material before we can magnetized it in the opposite direction. Both of these operations require energy, and again, that energy is more than likely wasted.
The amount of energy consumed by hysteresis losses is related to the area of the hysteresis loop (I will not attempt to describe this, please see references at bottom of write up). The closer we get to saturation, the higher our hysteresis losses are. Cores are usually operated well out of the saturation region for this reason.
Uses for Core Losses
As much as core losses have been described in a negative light in this article there are constructive uses for these effects. Core losses are used in a technique called inductive heating to rapidly heat or even melt metals. The metal is placed in a high frequency magnetic field. Energy is induced in the metal, which will eventually melt. Comercial inductive furnaces can melt small quantities of metal in a matter of a few seconds.
Chapman, Stephen J. Electric Machinery Fundamentals, Third Ed
Boston: WCB/McGraw-Hill, 1999