Transformers Noise and magnetostriction phenomenon

1. Electrical Power DistributionEE543
Transformers Noise
and magnetostriction
phenomenon
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2. Name : Muhammad Fawaz Abdulhadi
ID : 11102667
Supervision By :Dr. Ahmed Anas El-wogood
Why do transformers hum?
Transformer noise is caused by a phenomenon which causes a piece of
magnetic sheet steel to extend itself when magnetized. When the
magnetization is taken away, it goes back to its original condition. This
phenomenon is scientifically referred to as magnetostriction. A transformer is
magnetically excited by an alternating voltage and current so that it becomes
extended and contracted twice during a full cycle of magnetization.
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According to Swiss scientists, the low-frequency hum produced by
transformers is often due to the phenomenon of magnetostriction. Basically,
when a ferromagnetic material is exposed to a magnetic field, it can actually
change shape, albeit very slightly (think microns). You see, at the
microscopic level, such materials consist of individual magnetic domains.
Think of these domains as tiny bar magnets. Whenever a magnetic field is
applied to the material, each domain actually rotates. This rotation can cause
the material to either expand or contract depending on the orientation of the
magnetic field.
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So, when you’ve got a transformer connected to a 50/60Hz AC line, it’s
dealing with a magnetic field that oscillates at 50/60Hz. However, the
transformer’s core actually undergoes magnetostriction twice during each
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3. electrical cycle, so the whole thing vibrates at 100/120Hz, thus producing
sound.
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The magnetization of any given point on the sheet varies, so the extension
and contraction is not uniform. A transformer core is made from many sheets
of special steel to reduce losses and moderate the ensuing heating effect.
The extensions and contractions are taking place erratically all over a sheet
and each sheet is behaving erratically with respect to its neighbor, so you can
see what a moving, writhing construction it is when excited. These extensions
are miniscule proportionally and therefore not normally visible to the naked
eye. However, they are sufficient to cause a vibration, and consequently
noise. Applying voltage to a transformer produces a magnetic flux, or
magnetic lines of force in the core. The degree of flux determines the amount
of magnetostriction and hence, the noise level.
Why not reduce the noise in the core by reducing the amount of flux?
Transformer voltages are fixed by system requirements. The ratio of these
voltages to the number of turns in the winding determines the amount of
magnetization. This ratio of voltage to turns is determined mainly for
economical soundness. Therefore the amount of flux at the normal voltage
is fixed. This also fixes the level of noise and vibration. Also, increasing (or
decreasing) magnetization does not affect the magnetostriction
equivalently. In technical terms the relationship is not linear.
Magnetostriction can be used to create vibrators, where usually some lever
action is used in conjunction with the magnetic deformation to increase the
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4. 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.
References:
http://www.nlvocables.com
http://hyperphysics.phy-astr.gsu.edu
http://www.mgmtransformer.com
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