rotating field powerfactor meter

Rotating field power factor meter

  • A1,A2,A3 are three fixed coils which are displaced 120o from each other
  • These coils are connected to a 3-Φ R,Y and B power supply through current transformers as shown in figure.
  • Coil(P) is a fixed coil connected in series with a high resistance across on pair of 3-Φ lines and iron cylinder(c) is mouted inside coil(P).
  • To the iron cylinder(c) two sector shaped vanes(v) are provided which are 180o apart in space.
  • The damping vanes and apointer is mounted on spindle and there is no controlling system in this powerfactor meter.
Coils A1,A2 and A3 produce a rotating magnetic field.These field interacts with the coil(p) and iron system's alternating field. This causes an angular displacement in moving system which is determined by the phase angle of the current.
   Vanes v are magnetized by currnet Ip in coil(p) which is in phase with and proportional to the line voltage of the system.let hysteresis and eddy currnets are ignored,then iron cylinder, the vanes and the coil(p) are eqivalent electromagnetically to a rectangular moving coil pivoted with in coils A1,A2 and A3.
The center line of the moving coil being coincide with the axis of iron vanes.

torque acting on the moving system is given below

Td ∝{I1Ipcos(90o-Φ)sin(90o+Θ)+I2Ipcos(330o-Φ)sin(210o+Θ)+I3Ipcos(210o-Φ)sin(330o+Θ)}

for steady state deflection, the total torque nust be equal to zero
                                            Td=0
{I1Ipcos(90o-Φ)sin(90o+Θ)+I2Ipcos(330o-Φ)sin(210o+Θ)+I3Ipcos(210o-Φ)sin(330o+Θ)}=0

on soving the above equation, we get Θ = Φ
therefore deflection of iron vane from the reference axis is a direct measure of the phase angle between each line current and the corresponding phase voltage.

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