• Using iron cores for the coils can considerably increase the operating torque.
• Ferrodynamic wattmeters employ cores of low loss iron so that there is a large increase in the flux density and consequently an increase in operating torque with little loss in accuracy.
• The fixed coil is wound on a laminated core having pole pieces designed to give a uniform radial field throughout the air gap.
• The moving coil is asymmetrically pivoted and is placed over a hook shaped pole piece.
• This type of construction permits the use of a long scale up to about 270° and gives a deflecting torque which is almost proportional to the average power.
• With this construction there is a tendency on the part of the pressure coil to creep (move further on the hook) when only the pressure coil is energized.
• This is due to the fact that a coil tries to take up a position where it links with maximum flux. The creep causes errors and a compensating coil is put to compensate for this voltage creep.
• The use of ferromagnetic core makes it possible to employ a robust construction for the moving element.
• Also the Instrument is less sensitive to external magnetic fields.
• On the other hand, this construction introduces non-linearity of magnetization curve and introduction of large eddy current & hysteresis losses in the core.
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