# In an energy meter, the steady speed of disc can be achieved when

In an energy meter, the steady speed of disc can be achieved when

### Right Answer is: Operating torque is equal to the braking torque

#### SOLUTION

In an energy meter, the steady speed of the disc can be achieved when operating torque is equal to the braking torque.

## Induction-type Single-phase Energy Meter

An induction-type instrument can be used as an ammeter, voltmeter, or wattmeter, the induction-type energy meters are more popular. Induction-type single-phase energy meter is used invariably to measure the energy consumed in an AC circuit in a prescribed period where supply voltage and frequency are constant. The energy meter is an integrating instrument that measures the total quantity of electrical energy supplied to the circuit in a given period.

Principle

The basic principle of an induction-type energy meter is electromagnetic induction. When AC flows through two suitably located coils (current coil and potential coil), they produce the rotating magnetic field that is cut by the metallic disc suspended between the coils, and thus, an emf is induced in the disc that circulates eddy currents in it. By the interaction of rotating magnetic field and eddy currents, electromagnetic torque is developed that causes the disc to rotate. This is the same principle that is applied in single-phase induction motors.

A single-phase energy meter has four essential parts:

1. Operating system
2. Moving system
3. Braking system
4. Registering system

Braking System

• The braking system consists of a braking device which is usually a permanent magnet positioned near the edge of the aluminum disc.
• The emf induced in the aluminum disc due to relative motion between the rotating disc and the fixed permanent magnet (brake magnet) induces an eddy current in the disc.
• When the disc rotates in the air gap, eddy currents are induced in the disc which opposes the cause producing them i.e. relative motion of the disc with respect to the magnet. This eddy current, while interacting with the brake magnet flux, produces a retarding or braking torque.
• This braking torque is proportional to the speed of the rotating disc. When the braking torque becomes equal to the operating torque, the disc rotates at a steady speed.

The braking torque is proportional to the flux of the braking magnet and the eddy current induced in the moving system due to its rotation in the field of the braking magnet.

TB ∝ φi

where φ is the flux of the braking magnet and ‘i’ the induced current. Now i = e/R where e is the induced e.m.f. and R the resistance of the eddy current path. Also, e ∝ φn where n is the speed of the moving part of the instrument.

∴ TB ∝ φ × φn/R

TB ∝ φ2n/R

The braking torque Tb opposes the rotation of the disc.

When the driving and braking torques are equal, the disc rotates at a steady speed and at the steady speed, the speed of the disc.

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