Question

The full-scale range of the wattmeter shown is 100 W. The turns ratios are as indicated. The RMS source voltage is $V_s=200$ V. The wattmeter reading will be _____ W (rounded off to the nearest integer).

Hint:

Wattmeters measure true power: if current and voltage are $90^\circ$ out of phase, power is zero even if both quantities are nonzero.

Answer 1

Correct Answer: 0

The wattmeter’s current coil is in series with the load current, while its potential coil is connected across an {open-circuited} transformer winding that only carries magnetizing flux. The voltage across that winding is induced in quadrature (90°) with the current in the series path (ideal transformers assumed, no winding resistance). Hence the phase angle between the potential-coil voltage and the current-coil current is $90^\circ$. A dynamometer wattmeter reads \[ P = V I \cos\phi, \] so with $\phi=90^\circ$, $\cos\phi=0$ and the wattmeter reads $0\ \text{W}$.