It works on a very simple principle which is stated as "when any current carrying conductor is placed inside a magnetic field, it experiences a mechanical force and due to this mechanical force, deflection of conductor takes place."
It consists of the following parts:
Scale is uniform up to a certain limit.
They can be used for both to measure ac as well dc quantities as scale is calibrated for both. Errors/Demerits :
Errors in the pressure coil inductance
Errors may be due to pressure coil capacitance
Errors may be due to mutual inductance effects
Errors may be due connections (i.e. pressure coil is connected after current coil)
Error due to Eddy currents
In an ideal dynamo-meter type wattmeter the current in the pressure coil is in phase with the applied voltage. But practically the pressure coil of wattmeter has an inductance and current in it will lag behind the applied voltage. If there is no inductance the current in pressure coil will be in phase with the applied voltage. In the absence of inductance in pressure coil of wattmeter, it will read correctly in all power factors and frequency. The wattmeter will read high when the load power factor is lagging ,as in that case the effect of pressure coil inductance is to reduce the phase angle between load current and pressure coil current . Hence the wattmeter will read high. This is very serious error. The wattmeter will read low when the load power factor is leading as in that case the effect of pressure coil inductance is to increase the phase angle between load current and pressure coil current. Hence the wattmeter will read low. Compensation for inductance of pressure coil. Inductance of pressure coil can be reduced by means of capacitor connected in parallel with a portion of multiplier (series resistance).
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