Holes are drilled on the opposite sides of the spindles of an energy meter to
Right Answer is:
Avoid creep on load
Holes are drilled on the opposite sides of the spindles of an energy meter to avoid creep on load.
Error in single-phase Energy Meter
(1) Phase Errors: Normally, the flux due to the shunt magnet does not lag behind the supply voltage by exactly 90° due to the fact that the coil has some resistance. Therefore, the torque is not zero at the zero power factor. This is known as phase error and is compensated by means of an adjustable copper band placed over the central limb of the shunt magnet. Due to this reason, the shading ring is known as the power factor compensator.
(2) Speed Error: The speed error of the meter, when tested on a non-inductive load, can be eliminated by correctly adjusting the position of the braking magnet. Movement of the poles of the braking magnet towards the center of the disc reduces the braking torque and vice-versa.
(3) Friction Error: Frictional forces at the rotor bearings and in the register mechanism give rise to an unwanted braking torque on the disc rotor. This can be reduced by making the ratio of the shunt magnet flux and series magnet flux large with the help of two shading bands. Correct compensation is achieved when the rotor does not run on no-load with only supply voltage connected.
(4) Creeping Error: The slow but continuous rotation of the disc when only the pressure coils are excited but no current is flowing in the circuit is called “creeping”. It may be caused by various factors like incorrect friction compensation, vibration, stray magnetic fields, or due to the voltage supply being in excess of the normal.
To overcome this creeping effect on no-load, two holes are drilled in the disc on a diameter, i.e. on opposite sides of the spindle. This causes sufficient distortion of the field to prevent rotation when one of the holes comes under one of the poles of the shunt magnet.
(5) Temperature Error: The error due to temperature variations of the instruments are usually small because the various effects produced tend to neutralize one another.