Correct option is D
Phase error:
The meter will read correctly only if the shunt magnet flux lags behind the supply voltage by exactly 90∘.
Since the shunt magnet coil has some resistance and is not completely reactive.
The shunt magnet flux does not lag the supply voltage by exactly 90∘.
The result is that the meter will not read correctly at all power factors.
This error occurs because the wattmeter's accuracy depends on the correct phase alignment between the voltage and current, and any deviation from the expected phase relationship results in incorrect power readings.
Creeping:
Sometimes the disc of the meter makes slow but continuous rotation at no load.
When potential coil is excited but with no current flowing in the load. This is called creeping.
This error may be caused due to overcompensation for friction, excessive supply voltage, vibrations, stray magnetic fields etc.
Speed error:
Sometimes the speed of the disc of the meter is either fast or slow, resulting in the wrong recording of energy consumption.
Frictional error:
Frictional forces at the rotor bearings and in the counting mechanism considerably to the braking torque.
Since friction torque is not proportional to the speed but is roughly constant, it can cause considerable error in meter reading.
