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 Saravanan J

Assistant professor of EEE Department

Government College of Engineering

Tirunelveli, Tamil nadu

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Nominal 'π' method

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                       In the nominal pi model of a  medium transmission line,  the series impedance of the line is concentrated at the centre and half of each capacitance is placed at the centre of the line. The nominal Pi model of the line is shown in the diagram below. In this circuit, By Ohm’s law By KCL at node a, Voltage at the sending end By ohm’s law Sending-end current is found by applying KCL at node c or Equations can be written in matrix form as Also, Hence, the ABCD constants for nominal pi-circuit model of a medium line are Phasor diagram of nominal pi model The phasor diagram of a nominal pi-circuit is shown in the figure below. It is also drawn for a lagging power factor of the load. In the phasor diagram the quantities shown are as follows; OA = V r  – receiving end voltage. It is taken as reference phasor. OB = I r  – load current lagging V r  by an angle ∅ r . BE = I ab...
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Critical disruptive voltage and Visual critical voltage

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  Critical disruptive voltage:                       It is the minimum phase-neutral voltage at which corona occurs. Consider two conductors of radius  r  (cm) and spaced  d  (cm) apart.  If  V  is the phase-neutral potential, then potential gradient at the conductor surface is given by: g =[V/ r log e  (d/r)] volts / cm                In order that corona is formed, the value of g must be made equal to the breakdown strength of air.  The breakdown strength of air at  76 cm  pressure and temperature of  25ºC  is  30 kV/cm  (max) or  21·2 kV/cm  (r.m.s.) and is denoted by  g o . If  V C  is the phase-neutral potential required under these conditions, then, g o  =[V c / r log e  (d/r)] volts / cm where  g o  = breakdown strength of air at 76 cm of mercury an...
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LOADING CONDITIONS AND CLASSES OF DUTY

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TYPES OF LOAD:                1. Continues duty load                2. Short time duty load                3. Intermittent periodic duty load 1. Continues duty load:                On this duty the duration of load is for sufficiently long time such that all the parts of the motor attain thermal equilibrium. i.e. the motor attains its maximum final steady temperature rise. Drives continious duty are continiously running for long time. The continious rating of a motor may be defined as the load that may be carried by the machine for an indefinite time without the temperature rise of any part exceeding the maximum permissible value. Example: Ø   Fans Ø   Pumps   2. Short time duty load:                The motor operates at a constant loa...
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