In a single-phase energy meter, the speed of rotation will be proportional to the power, if and only if the phase angle between the supply voltage and the current of the pressure coil is ________.

Construction of energy meter
 

Driving System:

• The electromagnet is the main component of the driving system. The driving system has two coils: the current and the potential coil.
• The current coil is connected in series with the load and the potential coil is connected parallel to the supply.
   

Moving System:

• The moving system is the aluminum disc mounted on the shaft of the alloy. The eddy current is induced in the disc because of the change in the magnetic field.
• The interaction of the flux and the disc induces the deflecting torque. When the devices consume power, the aluminum disc starts rotating, and after some number of rotations, the disc displays the unit used by the load.
   

Braking System:

• The permanent magnet is used for reducing the rotation of the aluminum disc. The aluminum disc induces the eddy current because of its rotation.
• The eddy current cuts the magnetic flux of the permanent magnet and hence produces the braking torque. This braking torque opposes the movement of the disc, thus reducing its speed.
   

Registering System:

• The main function of the registration or counting mechanism is to record the number of rotations of the aluminum disc. The speed of rotation will be proportional to the power, provided the phase angle between the voltage coil and current coil fluxes is 90°.
• This ensures that the induced eddy currents interact properly with the magnetic fields, generating the correct torque.
• Hence, for accurate power measurement, the pressure coil current must lead or lag the supply voltage by 90°.