Electrodynamometer Wattmeter

Electrodynamometer Wattmeter

An electrodynamometer wattmeter consists of two fixed coils, F_A and F_B and a moving coil M as shown in figure 3.3. The fixed coils are connected in series with the load and hence carry the load current. These fixed coils form the current coil of the wattmeter. The moving coil is connected across the load and hence carries a current proportional to the voltage across the load. A highly non-inductive resistance R is put in series with the moving coil to limit the current to a small value. The moving coil forms the potential coil of the wattmeter.

 The fixed coils are wound with heavy wire of minimum number of turns. The fixed coils embrace the moving coil. Spring control is used for movement and damping is by air. The deflecting torque is proportional to the product of the currents in the two coils. Theses watt meters can be used for both DC and AC measurements. Since the deflection is proportional to the average power and the spring control torque is proportional to the deflection, the scale is uniform. The meter is free from waveform errors. However, they are more expensive.