2dix-The Student Choice
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To calibrate the given Energy Meter by phantom loading.

 

OBJECT: To calibrate the given Energy Meter by phantom loading.

APPARATUS REQUIRED:

Energy Meter, Auto Transformer, U.P.F. Wattmeter, Voltmeter, Ammeter, Resistive Load, Stop Watch, Connecting Wires.

 

THEORY:

Induction type of energy meters are universally used for measurement of energy in domestic and industrial a.c. circuits. Induction type of meters possesses lower friction and higher torque/weight ratio. Also they are inexpensive and accurate, and retain their accuracy over a wide range of loads and temperature conditions. There are four main parts of the operating mechanism:

(i)Driving system

(ii)Moving system

(iii)Braking system and

(iv) Registering system.

 

 Driving System: The driving system of the meter consists of two electro-magnets. The core of the se electromagnets is made up of silicon steel laminations. The coil of one of the electromagnets is excited by the load current. This coil is called the ‘current coil’. The coil of second electromagnet is connected across the supply and, therefore, carries a current proportional to the supply voltage. This coil is called the ‘pressure coil’. Consequently the two electromagnets are known as series and shunt magnets respectively. Copper shading bands are provided on the central limb. The position of these banks is adjustable. The function of these bands is to bring the flux produced by the shunt magnet exactly in quadrature with the applied voltage.

Moving System: This consists of an aluminum disc mounted on a light alloy shaft. This disc is positioned in the air gap between series and shunt magnets.

Braking System: A permanent magnet positioned near the edge of the aluminum disc forms the braking system. The aluminum disc moves in the field of this magnet and thus provides a braking torque. The position of the permanent magnet is adjustable, and therefore, braking torque can be adjusted by shifting the permanent magnet to different radial positions as explained earlier.

Registering (counting) Mechanism: The function of a registering or counting mechanism is to record continuously a number which is proportional to the revolutions made by the moving system. In all induction instruments we have two fluxes produced by currents flowing in the windings of the instrument. These fluxes are alternating in nature and so they produce emf s in a metallic disc or a drum provided for the purpose. These emfs in turn circulate eddy currents in the metallic disc or the drum. The breaking torque is produced by the interaction of eddy current and the field of permanent magnet. This torque is directly proportional to the product of flux of the magnet, magnitude of eddy current and effective radius ‘R’ from axis of disc. The moving system attains a steady speed when the driving torque equals braking torque. The term testing includes the checking of the actual registration of the meter as well as the adjustments done to bring the errors of the meter with in prescribed limits. AC energy meters should be tested for the following conditions:1.At 5% of marked current with unity pf.2.At 100% (or) 125% of marked current.3.At one intermediate load with unity pf.4.At marked current and 0.5 lagging pf.

The supply voltage is applied across the pressure coil. The pressure coil winding is highly inductive as it has very large number of turns and the reluctance of its magnetic circuit is very small owing to presence of air gaps of very small length. Thus the current Ip through the pressure coil is proportional to the supply voltage and lag it by a few degrees less the 90 degrees. This is because the winding has a small resistance and there are iron losses in the magnetic circuit. Current input produces a flux. This flux divides itself into two pars фg and фp. The major portion фg flows across the side gaps as reluctance of this path is small. The reluctance to the path of flux фp is large and hence its magnitude is small. This flux фp goes across aluminium disc and hence is responsible for production of driving torque. Flux фp is in phase with current Ip and is proportional to it. Therefore flux фp is proportional to voltage V and lags it by an angle a few degrees less than 90 degrees since flux фp is alternating in nature, it induces an eddy emf in the disc which in turn produces eddy current, The load current I flows through he current coil and produces a flux фg. This flux is proportional to the load current and is in phase with it. This flux produces eddy current is in disc. Now the eddy current is interacts with flux фp to produce a torques and eddy current is interacts with фg to produce another torque. Here two torques are in the opposite direction and the net torque is the different of these.

 

 

 

 

CIRCUIT DIAGRAM:

Fig. 9.1 Energy Meter by phantom loading

PROCEDURE:

1. Connections are made as per the circuit diagram.

2. Set Auto Transformer at zero voltage position before switching on the supply.

3. Gradually increase the voltage using the auto-transformer till the voltmeter reads 230V.

4. Now apply the Load at certain value (i.e. 2A )

5. Time taken for 25 rev. of the disc of the energy meter in the forward direction is noted

6. Record the Voltmeter, Ammeter, & Wattmeters are noted.

7. The experiment is repeated for different values of current (i.e. 4A, 6A,8A) at constant voltage.

8. After noting the values slowly decrease the auto transformer till

Voltmeter comes to zero voltage position and switch off the supply.

OBSERVATION TABLE:

 

 

Sl

No.

Voltage

(V)

(Volts)

Load

Current

(IL)

(Amps)

Wattmeter

Reading

W (Watts)

Time

for

10 rev

T(Sec)

Energy

meter

Reading

(E1)

Actual

Energy

(E2) = W X T

% Error =

E1-E2/E2 x 100

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

RESULT: The given single phase energy meter is tested at different loads and calibration curve is plotted.

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