Magnetostriction

Why does the transformer hum?

You may have noticed the humming sound associated with a transformer or a fluorescent light ballast. For U.S. circuits, that hum will be at 120 Hz since the iron material associated with the transformer core responds mechanically to the magnetic field which is impressed upon it. The effect is called magnetostriction, and it is one of the magnetic properties which accompanies ferromagnetism. For 60 Hz applied magnetic fields in AC electrical devices such as transformers, the maximum length change happens twice per cycle, producing the familiar and sometimes annoying 120 Hz hum.

In formal treatments, a magnetostrictive coefficient L is defined as the fractional change in length as the magnetization increases from zero to its saturation value. The coefficient L may be positive or negative, and is usually on the order of 10^-5. There is an elastic strain energy associated with the deformation, leading to some dissipation of energy in transformer cores. If the magnetostriction acts to contract a specimen, then this will act against any tensile stress on the material and leads to a larger value for theYoung's modulus for the material. Two examples of measurements of this phenomena are included in the table below.

It is also observed that applied mechanical strain produces some magnetic anisotropy. If an iron crystal is placed under tensile stress, then the direction of the stress becomes the preferred magnetic direction and the domains will tend to line up in that direction. Ordinarily the direction of magnetization in iron is easily changed by rotating the applied magnetic field, but if there is tensile stress in the iron sample, there is some resistance to that rotation of direction. Bulk solid samples may have internal strains which influence the domain boundary movement.

Magnetostriction can be used to create vibrators, where usually some lever action is used in conjunction with the magnetic deformation to increase the resultant amplitude of vibration. Magnetostriction is also used to produce ultrasonic vibrations either as a sound source or as ultrasonic waves in liquids which can act as a cleaning mechanism in ultrasonic cleaning devices.