knowledge-18 EMI

• Electromagnetic induction is the process by which an e.m.f is induced in a conductor that experiences a change in magnetic flux.
• if conductor is part of a complete circuit the induced e.m.f will lead to an induced current.
• The change in magnetic flux can be due to relative movement between the conductor and the field , or due to a change in the strength of the field.
• Examples of changes in field strength:

  • moving a wire so that it cuts through lines of flux of a permanent magnet.
  • moving a permanent magnet into and out of a coil of wire
  • moving a conductor in and out of the field of a wire
  • spinning a coil of wire in a magnetic field
• Lenz's law states that the direction of induced e.m.f or current is always in the opposite direction to the change in flux producing it.
• lenz's law is due to the conservation of energy and can be used to reduce the direction of e.m.f or current induced in a conductor.
• Lenz's law can be used to explain why a magnet can cause a force on a non-magnet conductor if there is relative movement between the two.
• Faradays law states that the magnitude of an induced e.m.f in a conductor is equal to the rate of change in flux it experiences or the rate of change in flux linkage in a coil.
• Induced e.m.f = change in flux linkage / time taken
• For a straight conductor of length l moving at right angles through a magnetic field of strength B at a speed v , the e.m.f induced is :

  • E = BLv
• A simple a.c generator consists of a rectangular coil that spins in a uniform magnetic field. The flux linkage changes continuously , as does the rate of change in flux linkage so the induced e.m.f changes sinusoidally.
• The angular frequency of the coil is :

  • angular speed = 2 pie f
• An oscilloscope can be used as a voltmeter to show how voltage varies with time , it plots a graph of voltage against time on the screen in real time.
• oscilloscope:

  • the vertical axis represents voltage ,its scale can be adjusted using the y-grain control.
  • the horizontal axis represented time , its scale can be adjusted using time - base control.
• oscilloscopes (alternating current):

  • peak voltage = max voltage provided by supply in either direction
  • peak to peak voltage = twice the peak voltage
  • time period = time between two successive peaks
  • I rms = I0 /root (2)
  • V rms = V0 /root (2)
  • P = (I rms)^2 r = (V rms )^2 / R
• Transformers are a device that allows the size of an a.c voltage to be changed. A transformer consists of two coils of insulated wire around a laminated iron core.
• A step-up transformer :

  • has more turns on the secondary coil than on the primary coil; it steps up the voltage and steps down the current.
• A step-down transformer:

  • has fewer turns on the secondary coil than on the primary coil ; it steps down the voltage and steps up the current
• The iron core is a conductor and by changing the magnetic field it induces small currents in it called eddy currents.
• Transformers power loss:

  • heating of wires in coils - low resistance wires can improve this
  • reduction of flux and heating iron core by eddy currents induced in iron core - laminated iron core , has insulated sheets of iron glued together increasing resistance. reduces eddy currents
  • repeated heating effect can demagnetize the core - soft iron is easier to magnetize and demagnetize