Moving iron Power Factor Meter
Construction of Moving iron Power Factor Meter
One type of power factor meter suitable for 3-phase balanced circuits is Moving iron Power Factor Meter shown in Figure (A). It consists of three fixed coils R, Y and B with axes mutually at 120° and intersecting on the centre line of the instrument. These coils are connected respectively in R, Y and B lines of the 3-phase supply through current transformers. When so energised, the three coils produce a synchronously rotating flux.
There is a fixed coil B at the centre of three fixed coils and is connected in series with a high resistance across one of the pair of lines, say, across R and Y lines as shown. Coil B is threaded by the instrument spindle which carries an iron cylinder C [Figure (B)] to which are fixed sector-shaped iron vanes V1 and V2. The same spindle also carries damping vanes and pointer (not shown in the figure) but there are no control springs. The moving system is shown separately in Figure (B).
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Working of Moving iron Power Factor Meter
The alternating flux produced by coil B interacts with the fluxes produced by the three current coils and causes the moving system to take up a position determined by the power factor angle of the load. However, the instrument is calibrated to read the power factor cos φ directly instead of φ. In other words, the angular deflection φ of the iron vanes from the line M N in Figure (A) is equal to the phase angle φ.
Because of the rotating field produced by coils R, Y and B, there is a slight induction-motor action which tends to continuously turn the moving iron in the direction of the rotating flux. Hence, it becomes essential to design the moving iron as to make this torque negligibly small i.e. by using high-resistance metal for the moving iron in order to reduce eddy currents in it.
Merits and Demerits of Moving iron Power Factor Meter
Moving iron p.f. meters are more commonly used as compared to the electrodynamic type power factor meter because
(i) They are robust and comparatively cheap
(ii) They have scales upto 360° and
(iii) In their case, all coils being fixed, there are no electrical connections to the moving parts.
On the other hand, they are not as accurate as the electrodynamic type of instruments and, moreover, suffer from erros introduced by the hysteresis and eddy-current losses in the iron parts–these losses varying with load and frequency.
Read article – electrodynamic type power factor meter
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