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Assignment 1

Assignment 1. Page 46, Problem 2.4, 2.5, 2.7 & 2.8 Due Date: Tuesday 1 st Feb, 2011. 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.

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Assignment 1

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  1. Assignment 1 • Page 46, Problem 2.4, 2.5, 2.7 & 2.8 • Due Date: Tuesday 1st Feb, 2011 Electro Mechanical System

  2. 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. Electro Mechanical System

  3. Residual Flux • When the external magnetic field decreases, the magnetic domains tend to retain their original orientation. • This is called residual induction. Electro Mechanical System

  4. 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. • HC is the coercive force • Energy is required to overcome the molecular friction of the domains to any changes in direction. Electro Mechanical System

  5. 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) Electro Mechanical System

  6. 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. Electro Mechanical System

  7. 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 Electro Mechanical System

  8. 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. Electro Mechanical System

  9. 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. Electro Mechanical System

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