4S6GGS: Watt meter

Watt meter

Induction type wattmeter consists of two laminate electromagnets known as shunt electromagnet and series electromagnet respectively. Shunt magnet is excited by the current proportional to the voltage across load flowing through the pressure coil and series magnet is excited by the current, proportional to the voltage across the load flowing through the pressure coil and series magnet is excited by the load current flowing through the current coil. A thin disc made of Cu or Al, pivoted at its centre, is placed between the shunt and series magnets so that it cuts the flux from both of the magnets. The deflection torque is produced by interaction of eddy current induced in the disc and the inducing flux in order to cause the resultant flux in shunt magnet to lag in phase by exactly 90° behind the applied voltage, one or more copper rings, known as copper shading bond are provided on one limb at the shunt magnet. Correct disppointed between shunt and series magnet fluxes may be attained by adjusting the position of copper shading bonds.

The pressure coil circuit of introduction type instrument is made as inductive as possible so that the flux of the shunt magnet may lag by 90° behind the applied voltage.

Volt amperes and with some modifications for measuring power factor and frequency in an circuit. Hence it can be used for measuring as well as dc.

Dynamometer type wattmeter

This instrument is similar in design and principle to the dynamometer ammeter and voltmeter. When the instrument of this type is used as a wattmeter, the fixed coil, which is divided into two equal portions in order to provide uniform field, is employed as current coil and the moving coil is used as pressure coil i.e.

the fixed coil carries the current flowing through the circuit and the moving coil carries the current proportional to the voltage across the circuit. A high non-inductive resistance is connected in series with the moving coil in order to limit

the current. The magnetic fields of the fixed and moving coils react on one another causing the moving coil to turn about its axis. The movement is controlled by hair-springs which also lead the current into and out of moving element. Damping is provided by light aluminum vanes moving in an air dash pot. The pointer is fixed to the moving – coil spindle, and moves over a suitably calibrated scale.

Dynamometer type wattmeter has the advantages of (i) uniform scale and (ii) high degree of accuracy and disadvantages of (i) errors due to stray field acting on the moving coil and (ii) serious error due to inductance of pressure coil unless special precautions are taken to reduce this effect.