The spring constant of a 10-A dynamometer watt meter is 10.5 × 10− 6 N-m per radian. The variation of inductance with angular position of moving system is practically linear over the operating range, the rate of change being 0.078 mH per radian. If the full-scale deflection of the instrument is 83 degrees, calculate the current required in the voltage coil at full scale on d.c. circuit.
As seen from foot-note of Td = I1I2 dM/dθ N-m
Spring constant = 10.5 × 10− 6 N/m/rad
= 10.5 × 10−6 × π/180 N-m/degree
Td = spring constant × deflection = (10.5 × 10−6 × π/180) × 83
= 15.2 × 10−6 N-m
∴ 15.2 × 10−6 = 10 × I2 × 0.078 ; I2 = 19.5μA.
