GATE Electrical Engineering (EE) Test: Measurement of Power & Energy

An energymeter is an induction and an integrating type instrument.

To reduce the error in the wattmeter reading due to pressure coil inductance, the wattmeters are compensated by connecting a capacitor in parallel with a portion of series resistance (multiplier) as shown below.

 

At AF (audiofrequency) and RF (radio frequency) it is easier to measure voltage, current and impedance, than to measure- power. Direct measurement of power is not done in this range. Here, power is calculated from the equation 
P = E²IR or P = I² R

For the energy meter to record true energy the pressure coil should be highly inductive (i.e, Δ = 90°). Thus, for the angle between the shunt magnet flux (φρ) and the supply voltage to be 90°, a lagging coil is placed between the potential coil and the rotating aluminium disc. Lag adjustment sometimes is called power factor or quadrature or inductive load adjustment.

From two-wattmeter method of measurement of power, we have:

When the energy meter is tested under high current rating, there is lot of waste of power. To reduce this unwanted power loss, Phantom or Fictitious loading is done.

Braking torque provided by the permanent magnet is given by

Where,
N = speed of the disc in rpm
φm = maximum value of flux of the permanent magnet 
d = distance of the permanent magnet from the centre of the revolving disc

Given, energy meter constant,
K = 1800 revolutions per kWh Actuai energy or true energy consumed by the energy meter at half load during 138 seconds is:

Also, measured energy or energy recorded is