The braking system of an energy meter

Energy meter:

• Energy meter or Watt-hour meter is used to measure the energy in kWh.
• It is an integrating type instrument.
• Its working principle is similar to the transformer.
• There are three essential mechanisms required in the energy meter named Driving torque, Braking torque, and registered mechanism.

Driving torque:

• This torque is required to revolve the disc or rotate the disc.
• It is obtained by the electromagnetic induction effect.

Braking torque:

• It is required to rotate the disc at a constant speed.
• It is obtained by using a permanent magnet placed inside the energy meter near the Aluminum disc.
• Eddy currents have induced in the magnet due to the movement of the rotating disc through the magnetic field. This eddy current reacts with the flux and exerts a braking torque which opposes the motion of the disk. The speed of the disk can be controlled by changing flux.
• Breaking torque of induction type single-phase energy meter is:

\({T_b} = k\frac{{{\phi ^2}}}{{{R_e}}}N \times R\)

K = constant

ϕ = flux

N = speed in rpm

R = radius of the disc

R_e = resistance in path of current (i.e. disc)

The braking torque of induction type single-phase energy meter is directly proportional to the square of the flux.

Registered mechanism:

• It registers the no. of rotations or revolutions of the disc which is proportional to the energy consumed in kWh.
• Meter constant = (No. of revolutions / kWh)

 Important Points

Creeping:

• Sometimes the disc of the energy meter makes the slow but continuous rotation at no load i.e. when the potential coil is excited but with no current flowing in the load called creeping error
• This error may be caused due to overcompensation for friction, excessive supply voltage, vibrations, stray magnetic fields, etc
• It can be reduced by making two opposite holes on the disc.