Magnetic Hysteresis & Autoinduction Explained

Magnetic Hysteresis in Ferromagnetic Materials

When a magnetic material is subjected to a changing magnetic field intensity (H), the magnetic induction (B) lags behind. This phenomenon is known as magnetic hysteresis. (See Figure 1). When a ferromagnetic substance is subjected to a cyclical (alternating positive and negative) magnetic field intensity, it traces a hysteresis loop.

Key points on the hysteresis curve (See Figure 1):

• O-B: Magnetization curve.
• O-R: Residual magnetization.
• O-D: Coercive force.

When applying an alternating magnetization intensity (+ and -) to a ferromagnetic substance, the resulting hysteresis loop is shown in the image. The magnetic induction (B) lags behind the magnetic field intensity (H). At point B, even when H = 0, the substance retains some magnetization (residual magnetization). This property is commonly used to magnetize certain tools.

To remove the residual magnetization (point D), a reverse magnetic field intensity (coercive force, H) must be applied. The cycle continues through points E, F, and G. The curve does not pass through the origin (0) due to hysteresis.

Autoinduction

Autoinduction occurs when a changing electric current in a solenoid causes a change in the magnetic flux through the same solenoid. This change in flux induces an electromotive force (EMF) that opposes the change in current. This phenomenon is called autoinduction, and the induced EMF is called the autoinduced electromotive force.

For example, the spark created when opening a circuit with a coil (e.g., engine ignition, ballast, transformer) is evidence of this phenomenon.

Foucault Currents (Eddy Currents)

Foucault currents are induced currents that oppose the change in magnetic flux. When a changing magnetic flux passes through a metallic mass, electric currents are induced within the mass. Two key aspects of Foucault currents should be considered:

• Energy is lost due to the Joule effect (heat) when these currents are induced.
• The induced currents create their own magnetic fields.

The most damaging effect is usually the heat generated.

Foucault Current Flow

• Harmful effects: In transformers, the core is not a single piece but rather a grouping of several isolated plates to minimize eddy currents.
• Beneficial effects: Electric induction furnaces and induction cooking utilize Foucault currents for heating.