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Chapter: Measurements and Instrumentation - Electrical and Electronics Instruments

Ferrodynamic Wattmeters

Ferrodynamic Wattmeters
The operating torque can be considerably increased by using iron cores for the coils.

Ferrodynamic Wattmeters

 

The operating torque can be considerably increased by using iron cores for the coils.

 

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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