A Dynamometer type wattmeter variably used for measurement of ac power as well as dc power.
It works on the dynamometer principle. This principle says a mechanical force exists when current flows through two current carrying conductor.
Constructional Features or Construction
There are two coils are used in this wattmeter. One is called current coil (CC) and the other is called potential coil (PC). Current coil is a fixed coil which is connected in series with the load and carries load current. While potential coil is a moving coil connected across the load through a series resistance R and carries a current which is proportional to the voltage across the load. The current coil or fixed coil is splitted into two parts. The controlling torque is provided by two spiral springs which also lead the current into and out of moving coil. Damping is provided by light aluminium vanes moving in an air dashpot. A pointer is attached to the movable coil that moves over a calibrated scale.Working
When the wattmeter is connected in the circuit to measure power. Current coil carries the load current I1 and produce a magnetic field. Potential coil carries current (I2) proportional to the load voltage and produces another magnetic field. The magnetic fields of the current and potential coils react on one another, causing the movable coil moves the pointer over the scale. The pointer comes to rest at a position where deflecting torque is equal to the controlling torque.The change of direction of current reverse current reverse current in both the current coils and potential or movable coil. So that, the direction of deflecting torque remains unchanged. Hence, these instrument suitable for the measurement of dc as well as ac power.
Deflecting torque Td α I1 I2
Deflecting torque, Td α VI1 α load power