Moving-iron Power Factor Meter
Moving Iron Power Factor meters are the three phase power
factor meters. These power factor meters consist of a moving iron pointer.
It consists of a three fixed coils R, Y and B. These coils are connected to
three phase supply on which it is to be used.
Construction
These power factor meters have three fixed coils as mentioned
above and it has three terminals of R,Y and B with axes and these fixed coils
are connected mutually at 120 degrees. These coils intersects on the center
line of the instrument. A fixed coil C is located at the center of the three
fixed coils. This coil C is connected in series with high resistance
across one of the lines. The coil B is threaded by the spindle. It carries an
iron cylinder and the sector shaped iron vanes are fixed on it. The spindle
has no control springs and it carries a pointer and damping vanes.
Working
The coil C produces alternating flux with the three current
coils and it acts on a moving system for the deflection of a pointer. The
angular deflection of the iron vanes from the line M, N is equal to the phase
angle φ. The rotating field of the coils of R, Y and B turns the moving iron
continuously in a direction of a rotating flux. The high resistance for the
moving iron is used to reduce eddy current in it.
Advantages
These meters are strong and cheap.
Their scales are up to 360 degrees.
There are no electrical connections in the moving parts.
Disadvantages
These instruments have so many errors.
These instruments are not accurate.
These instruments have hysteresis losses and eddy current losses
in their iron parts.