Dept of E&E ELE101/102 1

Measurement of Energy

• Estimation of power consumed over a given period of time

• Energy meter: Measures the total energy consumed in a given time period.

• Also called, Watt-hour meter

• 1KWh = 1000 Watts for 1 hour = 1 unit

Dept of E&E ELE101/102 2

Single phase Energy Meter

Construction:

Dept of E&E ELE101/102 3

Construction:

Driving system: Shunt Magnet (Pressure Coil): Supply Voltage Series Magnet (Current Coil): Load current Shading Bands: to obtain 90o displacement between flux set up by

shunt magnet and supply voltage

Moving system: Aluminium disc mounted on a spindle Interaction between the fluxes of shunt and series magnet produces

the rotating force on disc.

Braking system: Permanent Magnet Edge of the disc

Registering system: Train of gear mechanism Turns the display

Dept of E&E ELE101/102 4

The braking torque Tb α speed of disc

The driving torque Td α power

For steady speed of rotation,Td=Tb

Power α speed

Energy α No of revolutions in time t

The total number of revolutions of the disc in a given time period is proportional to the energy consumed during that time.

Single phase Energy Meter

Working:

E α VI cosΦ t

Dept of E&E ELE101/102 5

Exercise

A single phase energy meter is used to measure the energy consumed in a 230 V, 50 Hz, single phase load drawing a current of 10A at 0.8 lagging pf and makes 1020 revolutions for 30 minutes. If the energy meter constant is 1200 revolutions per unit of energy, find the percentage error. Also, find the speed of the disc in rpm.

Ans:

1104 1200*60

30

1000

0.8*10*230 made be to srevolution ofNumber

7.6%1104

11041020 100*

made be to srevolution

made be to srevolutionmade sRevolutionerror Percentage

rpm 3430

1020 Speed