Page 848 - Basic Electrical Engineering

the load. Normally, the current flowing through the pressure coil is kept very
low by connecting a high resistance in series with the pressure coil so as to
reduce error due to connections as described.

11.11 MEASUREMENT OF ENERGY

11.11.1 Introduction

We know that electrical energy is the product of electrical power and time. A
wattmeter gives the measure of power at a particular instant of time. If
electrical power consumed over a period of time is recorded, we can get the

measure of electrical energy consumed.
An energy meter is basically a wattmeter fitted with some recording

mechanism. An energy meter is also called a kWh meter, i.e., kilo Watt hour
meter. The meter which is installed at the main distribution board of power

supply at residences is a kWh meter. The energy consumed over a period of
time, say two months, is recorded in the form of digits. The electricity supply

authority prepares the energy consumption bill according to the consumption
made.
The most commonly used energy meter is the induction-type Watt hour

meter.

11.11.2 Constructional Details and Working Principle of Single-phase

Induction-type Energy Meter

The basic components of a kWh meter are as follows.

i. Pressure coil placed on a magnetic core.
ii. Current coils placed on a magnetic core; the mmf of these coils produce the driving torque.
iii. The moving system which is an aluminium disc placed between the pressure coil magnetic
core and the current coil magnetic core.
iv. The braking system which is a U-shaped permanent magnet called the brake magnet which
offers a braking effect to the rotating disc.
v. The registering or counting mechanism which consists of a gear train. The mechanism adds
together the revolutions made by the rotating aluminium disc. That is, the counting
mechanism integrates the instantaneous values of power consumed by the connected load over
a period of time.