Lecture 02Electro Mechanical System1 Assignment 1 Page 46, Problem 2.4, 2.5, 2.7 & 2.8 Due Date: Tuesday 1 st Feb, 2011

Lecture 02Electro Mechanical System2 Residual Flux  Metals that have a strong magnetic attraction can be modeled as being composed of many molecular size magnets. Orientation of the magnets are normally random. By applying an external magnetic field (e.g. using a coil with a current flow), the molecular size magnets will align themselves with the external field.

Lecture 02Electro Mechanical System3 Residual Flux  When the external magnetic field decreases, the magnetic domains tend to retain their original orientation. This is called residual induction.

Lecture 02Electro Mechanical System4 Hysteresis Loop  To eliminate the residual flux, a reverse coil current is required to generate a field H in the opposite direction. The magnetic domains gradually change their previous orientation until the flux density becomes zero. H C is the coercive force Energy is required to overcome the molecular friction of the domains to any changes in direction.

Lecture 02Electro Mechanical System5 Hysteresis Loop  AC magnets have ac flux changing continuously and will map out a closed curve call a hysteresis loop.  With an external ac flux, the B/H characteristics of a magnetic material traces out a curve from: (+Bm,+Hm) to (+Br,0) to (0, -Hc) to (-Bm,-Hm) to (-Br,0) to (0,+Hc)

Lecture 02Electro Mechanical System6 Hysteresis Losses  Magnetic material absorbs energy during each cycle and the energy is dissipated as heat.  The heat released per cycle [J/m3] is equal to the area [T-A/m] of the hysteresis loop.

Lecture 02Electro Mechanical System7 Hysteresis Losses caused by rotaiton  Hysteresis losses are also produced when piece of iron rotates in a permanent magnetic field  Consider an iron armature AB revolves in a field of permanent magnets N, S  As the armature rotates N pole of domain points towards A and then B  Reversal occur half cycle  Hysteresis losses occur as in an ac magnetic field

Lecture 02Electro Mechanical System8 Eddy Currents  An ac flux  linking a rectangular- shaped conductor induces an ac voltage E across its terminals  If the conductor terminals are shorted, a substantial current flows.  The same flux linking smaller coils induce lesser voltages and lower currents.  A solid metal plate is basically equivalent to a densely packed set of rectangular shaped coils.  The induced currents flowing inside the plate are eddy currents, and flow to oppose the change in flux.

Lecture 02Electro Mechanical System9 Eddy Current Losses  Eddy currents become a problem when iron must carry an ac flux  Eddy currents flow throughout the entire length of the iron core.  Resistance in the iron causes energy to be dissipated as heat.  Losses can be reduced by splitting the core into sections (lamination)  Subdividing causes the losses to decrease progressively  Varnish coatings insulate the laminates from current flows.  Silicon in the iron increases the resistance.