Measurement of Energy MCQ Quiz in मल्याळम - Objective Question with Answer for Measurement of Energy - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Concept:

Meter constant \(= \frac{{no\;of\;revolution\;in\;1\;hr}}{{no\;of\;kw\;in\;1\;hr\;}}\)

Calculation:

Given, V = 230 volt

I = 20 amp at unity power factor

Power = VIcosϕ = 230 × 20 × 1 = 4600 watt = 4.6 kW

No of revolution in 1 hr = 2300/2 = 1150 revolution

Energy meter: 

• Energy meter measures electrical energy in a kilowatt hour.
• It is also known as kilowatt-hour meter or Induction type watt-hour meter.
• It is a type of ''integrating instruments.''

Energy meter

The meter which is used for measuring the energy utilized by the electric load is known as the energy meter. 

It is a type of integrating instrument.

Construction of Energy Meter

1.) Driving System 

• The electromagnet is the main component of the driving system.
• The driving system has two electromagnets. The upper one is called the shunt electromagnet, and the lower one is called the series electromagnet.
• The series electromagnet is excited by the load current flow through the current coil.
• The coil of the shunt electromagnet is directly connected with the supply and hence carries the current proportional to the shunt voltage. This coil is called the pressure coil.
• The center limb of the magnet has a copper band. These bands are adjustable. The main function of the copper band is to align the flux produced by the shunt magnet in such a way that it is exactly perpendicular to the supplied voltage.

2.) Moving System

• The moving system is the aluminum disc mounted on the shaft of the alloy.
• The disc is placed in the air gap of the two electromagnets.
• The eddy current is induced in the disc because of the change in the magnetic field. This eddy current is cut by the magnetic flux. 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.

​3.) Braking system

• The permanent magnet is used for reducing the rotation of the aluminum disc. 
• The braking torque opposes the movement of the disc, thus reducing its speed.
• The permanent magnet is adjustable due to which the braking torque is also adjusted by shifting the magnet to the other radial position.

4.) Registration (Counting Mechanism)

• The main function of the registration or counting mechanism is to record the number of rotations of the aluminum disc.
• Their rotation is directly proportional to the energy consumed by the loads in the kilowatt hour.

Friction Compensation (or) Friction Error:

• Frictional forces at the rotor bearings and in the counting (or register) mechanism cause a noticeable error, especially at light loads.
• At light loads, the torque due to friction adds considerably to the braking torque on the disc rotor.
• Since friction torque is not proportional to the speed but is roughly constant it can cause a considerable error in meter reading.

Adjustment:

• This error can be reduced by making the ratio of the shunt magnet flux φ2 and series magnet flux φ1 large with the help of two shading rings (or shading bands).
• These bands embrace the flux contained in the two outer limbs of the shunt magnet and thus eddy currents are induced in them which cause a phase displacement between the enclosed flux and the main gap flux.
• As a result, small driving torque is exerted on the disc rotor, this torque being adjusted by variation of the positions of these bands to compensate for friction in the instrument.
• The correctness of friction compensation is achieved by running the meter at a high load of about 8 to 10% of full load when the disc should rotate correctly.
• Overcompensation leads to creep. This adjustment is known as light load adjustment.

Energy meter:

In an induction type energy meter, a braking magnet is used to control the speed of the rotating disc. The braking magnet interacts with the disc by producing eddy currents, which create an opposing torque to slow down the disc. If the energy meter slows down unexpectedly on a specified load, adjustments can be made by:

• Adjusting the position of the braking magnet and moving it away from the center of the disc: 
  Correct: Moving the braking magnet away from the center reduces the braking torque, allowing the disc to rotate faster and compensate for the slowdown.
  Also, 
  \({T_B} \propto N\phi _m^2d\) ⇒ N ∝ 1 / d

• Adjusting the position of the braking magnet and moving it closer to the center of the disc:
  Incorrect: Moving the magnet closer to the center increases the braking torque, further slowing down the disc, which is not desired if the meter is already running slow.
  ​

• Adjusting the load:
  Incorrect: Adjusting the load does not compensate for the internal slowdown of the energy meter. The meter's performance should not depend on changing the external load conditions.

In an AC energy meter, there are 4 types of system

Driving system:

Performed by 2 silicon steel laminated electromagnets.

Moving system:

Performed by aluminum disc, placed between these 2 electromagnets.

Braking system:

• The main part of this system is a permanent magnet called brake magnet. 
• It is located near the disk so that eddy currents are induced in it due to the movement of the rotating disk through the magnetic field.
• This eddy current reacts with the flux and exerts a braking torque which opposes the motion of the disk.
• The speed of the disk can be controlled by changing flux.

Registering system: 

It registers the number of rotations of the disk which is proportional to the energy consumed.

Formula Used:

Meter Constant (K) = \(\frac{R}{E}\)

Here, R is revolution per hour

E is energy in kWh

Application:

We have,

V = 250 volts

I = 15 A

In one hour,

Energy consumed (E) = (250 × 15 × 1) Wh = (250 × 15 × 10^-3) kWh

K = 400

From above concept,

R = KE = 400 × (250 × 15 × 10^-3) = 1500 rev/hour

Here question has concern with revolution in rpm.

Hence, R_(rpm) = \(\frac{1500}{60}=25\)

Given that, current (I) = 50 A

Voltage (V) = 230 V

Time (t) = 37 seconds

Energy \(E = VIt = 230 \times 50 \times \frac{{37}}{{3600}} \times {10^{ - 3}} = 0.1182\;kWh\)

Meter constant, 1 kWh = 520 revolutions

Energy recorded by meter during the test period = 61/520 = 0.1173 kWh

Percentage error \(= \frac{{0.1173 - 0.1182}}{{0.1182}} \times 100 = -0.76\;\%\)

Creeping in the induction type energy meter is the phenomenon in which the aluminum disc rotates continuously when only the voltage is supplied to the pressure coil and no current flows through the current coil. It is the kind of error in which the energy meter consumes a very small amount of energy even when no load is attached to the meter.

The creeping increases the speed of the disc even under the light load condition which increases the meter reading. Vibration, stray magnetic field and the extra voltage across the potential coil are also responsible for the creeping.

Concept:

• The number of revolutions made by the energy meter per kilowatt-hour is known as the meter constant of an energy meter.
• Unit of meter constant is revolution per kilowatt-hour (rev/kWh)
• It is constant for a particular energy meter.

Meter constant (K) = No. of revolution by meter / Energy consumed (E)

E (in kWh) = voltage x current x cos ϕ × time x 10-3 = Load × time x 10-3

Where, cos ϕ = power factor

Calculation:

Load power = 225 W, time = 100 sec, Number of revolutions = 5

Energy supplied in 100 seconds \(= 225 \times \frac{{100}}{{3600}} \times {10^{ - 3}} = 6.25 \times {10^{ - 3}}\;kWh\)

Number of revolutions in 100 seconds = 5

Meter constant = number of revolutions / kWh

Meter constant \( = \frac{5}{{6.25 \times {{10}^{ - 3}}}} = 800\;rev/kWh\)