Measurement of Energy MCQ || Measurement of Energy Questions and Answers

1. The braking torque of induction type single-phase energy meter is:

  1. Directly proportional to the square of the flux
  2. Directly proportional to the flux
  3. Inversely proportional to the flux
  4. Inversely proportional to the square of the flux

Answer.1. A.C. measurements

Explanation:

Braking torque of induction type single-phase energy meter is:

${T_b} = k\frac{{{\phi ^2}}}{{{R_e}}}N \times R$

K = constant

ϕ = flux

N = speed in rpm

R = radius of the disc

Re = resistance in path of current (i.e. disc)

The braking torque of the induction type single-phase energy meter is directly proportional to the square of the flux. Supply voltage causes the shunt flux to induce an e.m.f in the disc. This is results in a self-braking torque proportional to the square of the voltage.

 

2. If an energy meter disc makes 10 revolutions in 100 sec when a load of 450 W is connected to it, the meter constant (in rev / kW-h) will be:

  1. 100
  2. 800
  3. 160
  4. 500

Answer.2. 800

Explanation:

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

Load power = 450 W, time = 100 sec, Number of revolutions = 10

Energy supplied in 100 seconds

= (450 × 100 × 10−3)/3600 = 12.25 × 10−3 kWh

Number of revolutions in 100 seconds = 10

Meter constant = number of revolutions / kWh

Meter constant = 10/12.25 × 10−3 = 800 rev/kWh

 

3. One single-phase energy meter operating on 230 V and 5 A for 5 hours makes 1940 revolutions. The Meter constant in revolution is 400. The power factor of the load will be:

  1. 1
  2. 0.8
  3. 0.7
  4. 0.6

Answer.2. 0.8

Explanation:

Given that,

Voltage (V) = 230 V

Current (I) = 5 A

Time (t) = 5 hours

Energy consumed or measured value,

E = VI cosϕ × t10-3

E = 230 × 5 × cosϕ × 5 × 10-3

E = 5.75 cosϕ kWh

Number of revolutions = 1940

Meter constant = 400

Meter Constant = Numbe of revolution/kWh

= 400 = 1940/5.75 cosϕ

cosϕ = 0.84

 

4. Watt-hour meter can be produced using:

  1. Heating effect
  2. Electromagnetic effect
  3. Induction effect
  4. Chemical effect

Answer.3. Induction effect

Explanation:

The operation of the secondary instruments depends on one of the following effects for producing deflecting torque.

1. Electromagnetic Effect: Using this effect, we can produce ammeters, wattmeters, and energy meters

2. Heating Effect: Using this effect we can produce ammeters and voltmeters.

3. Chemical Effect: Using chemical effect, we can produce D.C. ampere-hour meters.

4. Electrostatic Effect: Using electrostatic effect one can produce voltmeters (indirectly ammeters and watt-hour meters)

5. Induction Effect: Using the induction effect, we can produce voltmeters, ammeters, watt-hour meters, and energy meters.

Effect of Electricity Type of Instrument Suitable for
Magnetic Effect Ammeters, voltmeters, watt meters and energy meters D.C. and A.C Current
Electrodynamic effect Ammeters, voltmeters, watt meters D.C and A.C Current
Electromagnetic induction effect Ammeters, voltmeters watt meters and energy meters A.C Current only
Thermal effect Ammeters, voltmeters D.C and AC current
Chemical Effect Ampere-hour meters D.C only
Electrostatic effect Voltmeters D.C and A.C current

 

5. The meter constant of an energy meter will be given in:

  1. Revolutions/kWh
  2. kWh/Revolutions
  3. kW/Revolutions
  4. Revolutions/kW

Answer.1. Revolutions/kWh

Explanation:

  • 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 = No. of revolution by meter/Energyconsumed

 

6. The full-scale deflection in an induction instrument is about:

  1. 180°
  2. 270°
  3. 300°
  4. 360°

Answer.3. 300°

Explanation:

  • Induction type instrument can have full scale deflection of 300°
  • The operation of induction type instruments depends on the production of torque due to the reaction between two magnetic fluxes having some phase difference
  • These types of instruments are used only for AC measurements
  • Induction type instruments have good damping
  • Induction type instruments are less effect of stray magnetic fields as the operating fields are large

 

7. In a watt-hour meter, the creeping error is reduced by:

  1. Drilling two holes on opposite sides of the disc diameter
  2. Rotating the disk faster
  3. Increasing weight of the disk
  4. Using a copper disc

Answer.1. Drilling two holes on opposite sides of the disc diameter

Explanation:

  • Sometimes the disc of the energy meter makes slow but continuous rotation at no load i.e. when the potential coil is excited but with no current flowing in the load; This is called creeping.
  • This error may be caused due to overcompensation for friction, excessive supply voltage, vibrations, stray magnetic fields, etc
  • In order to prevent this creeping on no-load two holes or slots are drilled in the disc on opposite sides of the spindle.
  • This causes sufficient distortion of the field; The result is that the disc tends to remain stationary when one of the holes comes under one of the shunt magnets.

 

8. An energy meter is designed to have 100 revolutions of the disc per unit of energy consumed. The energy consumed by the load-carrying 50 A at 230 V and 0.8 power factor. Find the percentage error if the meter actually makes 950 revolutions.

  1. 3.26% , Slow
  2. 3.15%, Slow
  3. 3.26%, Fast
  4. 3.15%, Fast

Answer.3. 3.26%, Fast

Explanation:

Meter constant K = R/E

Energy consumed or measured value = (VI cosϕ) (Time in hr)

Where, V = Voltage

I = current

cos ϕ = Power factor

Given-

Meter constant, K = 100 rev/kWh

Voltage (V) = 230 V

Current (I) = 50 A

Power factor (cos ϕ) = 0.8

Energy actually consumed in one hour is,

E = VI cos ϕ × 10-3 × t  kWh

= 230 × 50 × 0.8 × 10-3 × 1

= 9.20 kWh

Number of revolutions needed to made = 9.20 × 100 = 920

But it actually made 950 revolutions. So, it runs fast.

Percentage Error = (950 −920) × 100/920

= 3.26 %

 

9. The meter constant of a single-phase energy meter is 500 rev/kWh. The meter takes 86 seconds to make 50 revolutions while measuring a full load of 4.4 kW. The percentage error in the meter is

  1. – 2.43%
  2. 2.43%
  3. – 4.86%
  4. 4.86%

Answer.3. – 4.86%

Explanation:

Meter constant K = R/E

Where R = revolution

E = Energy in kWh

Given actual revolution (K = 500

If meter takes 86 sec to make 50 revolutions for measuring a full load of kilowatt or 1 kW

Now, Energy = Power × time = 4.4 kW × 86 sec = 378 kWsec

To convert sec to hour we have to divided by 3600

E = 378/3600 kWh

Measured revolution (KM) = 50/378/3600

(KM) = 476

Error = KM – KA = 476 – 500 = – 24

Now, % Error = (−24 × 100)/500

% Error = −4.8%

 

10. What does the reading of the first dial on the right of a 4-dial watt-hour meter indicate?

  1. Number of single units
  2. Total watts each hour
  3. Maximum value the meter can register
  4. Multiplier value of the preceding dial

Answer.1. Number of single units

Explanation:

A Watt-hour meter is a measuring device that can evaluate and records the electrical power passing through a circuit in a certain time.

  • Each dial on the meter is numbered from 0 to 9 and has a pointer like a hand on a clock that turns either clockwise or counterclockwise
  • The pointers advance only when electricity is being used. These dials measure the number of kilowatt-hours (kWh) used in 1s, 10s, 100s, 1,000s and 10,000s
  • The First (rightmost) dial indicates the number of single units in the watt-hour meter

 

11. Creep error may occur induction type energy meter due to

  1. Incorrect position of brake magnet
  2. Incorrect adjustment of position of shading band
  3. Overvoltage across voltage coil
  4. Increase in temperature

Answer.3. Overvoltage across voltage coil

Explanation:

  • 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.
  • 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.
  • The creeping error occurs because of excessive friction. The main driving torque is absent at no load. Hence the disc rotates because of the additional torque provided by the compensating vane.

 

12. An energy meter with a constant of 600 revolutions per unit used for energy measurement. It makes 500 revolutions in 30 seconds. Find the energy consumed by the load for an hour.

  1. 1000 units
  2. 100 units
  3. 10 unit
  4. 0.1 unit

Answer.2. 100 units

Explanation:

In an energy meter,

Meter constant = (Number of revolutions) / (Energy consumed in kWh)

Meter constant = 600

500 revolutions in 30 seconds

Number of revolutions in one hour = (500 × 3600)/30 = 6000

Now energy (E) consumed by the load in one hour is

E = 6000/600 = 100 kWh or units

 

13. Which energy meter system contains a rotating aluminum disc that is placed between the air gaps of series and shunt magnets and mounted on a shaft?

  1. Magnetic Flux
  2. Moving System
  3. Registering System
  4. Driving System

Answer.2. Moving System

Explanation:

In the moving system, a thin aluminum disk is placed in the air gap between the two electromagnets and mounted on a vertical shaft that is free to rotate.

In the driving system, the components of this system are two silicon steel laminated electromagnets. Upper magnet→ Shunt magnet→ behaves as the potential coil. The lower magnet→ series magnet→ behaves like the current coil.

Registering System registers the number of rotations of the disk which is proportional to the energy consumed directly in kilowatt-hour.

Braking system has a permanent magnet called a brake magnet. It is located near the disk so that eddy currents are induced and braking toque to the disk.

 

14. Which of the following is the cause of meter phase error in induction type energy meter?

  1. Incorrect position of brake magnets.
  2. Incorrect adjustment of the position of shading bands.
  3. Slow but continuous rotation of aluminium disc.
  4. Temperature variations.

Answer.2. Incorrect adjustment of the position of shading bands.

Explanation:

  • The phase error in the energy meter is introduced because the shunt magnetic flux does not lag behind the supply voltage by exactly 90° due to some resistance of the coil and iron losses.
  • The angle of lag is slightly less than 90°.
  • Because of this error, the torque is not zero at the zero power factor of the load, and therefore, the energy meter registers some energy even though the actual energy passing through the meter is zero at the zero power factor.
  • Incorrect adjustment of the position of these shading bands causes phase error.

 

15. The disadvantage of time measurement testing in energy meters is

  1. Accuracy variation due to test load
  2. Need continuous Monitoring
  3. Meter constant can’t be determined
  4. All of the above

Answer.4. All of the above

Explanation:

In the time measurement testing method a sub-standard wattmeter and the energy meter under test is connected back to back in the test circuit.

The disadvantages of this method are

  1. The accuracy of the method depends on the testing load being maintained constant during the test period. This needs continuous vigilance and also careful regulation of the test load during the test period. So, an additional observer, other than that actually measuring the time interval, is required
  2. The test merely checks the accuracy of the meter disc. It does not verify that the actual meter constant is the same as the nominal constant.

 

16. A 230 V single-phase energy meter has a constant load current of 20 A at unity power factor. If the meter disc makes 2300 revolutions for 2 hours, the meter constant will be:

  1. 0.25 revolution/kWh
  2. 250 revolution/kWh
  3. 0.30 revolution/kWh
  4. 270 revolution/kWh

Answer.2. 250 revolution/kWh

Explanation:

Meter constant = (Number of revolutions) / (Energy consumed in kWh)

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

Meter constant = 1150/4.6 = 250 revolution/kwh

 

17. The kWh meter can be classified as a / an instrument:

  1. Indicating
  2. Deflecting
  3. Digital
  4. Integrating

Answer.4. Integrating

Explanation:

Integrating Instruments: These instruments record the consumption of the total quantity of electricity, energy etc. during a particular period of time. These instruments give reading for a specific period of time but no indication of reading for a particular instant of time.

Example: Ampere-hour meter, Energy (kWh) meter, kilovolt ampere-hour meter.

 

18. A 230-V, 50-A energy meter on full load test makes 61 revolutions in 37 seconds. If the meter constant is 520 rev/kwh, what is the percentage error?

  1. 0.67%
  2. 0.76%
  3. 1%
  4. 0.24%

Answer.2. 0.76%

Explanation:

Given that, current (I) = 50 A

Voltage (V) = 230 V

Time (t) = 37 seconds

E = VI × T = $230 \times 50 \times \frac{{37}}{{3600}} \times {10^{ – 3}}$

E = 0.1182 kWh

Meter constant, 1 kWh = 520 revolutions

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

Percentage error = 100(0.1173 − 0.1182)/0.1182

= −0.76%

 

19. If an energy meter makes 5 revolutions in 100 seconds when a load of 225 W is connected, the meter constant is

  1. 800 rev/kWh
  2. 222 rev/kWh
  3. 147 rev/kWh
  4. 13 rev/kWh

Answer.1. 800 rev/kWh

Explanation:

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

Given

Load power = 225 W

time = 100 sec

Number of revolutions = 5

Energy supplied in 100 seconds

= (225 × 100 × 10−3)/3600 = 6.25 × 10−3 kWh

Number of revolutions in 100 seconds = 5

Meter constant = number of revolutions / kWh

Meter constant = 5/6.25 × 10−3 = 800 rev/kWh

 

20. If an energy meter disc makes 10 revolutions in 10 minutes when a load of 600 W is connected to it, the meter constant in rev / kWh is:

  1. 100
  2. 600
  3. 800
  4. 200

Answer.3. 800

Explanation:

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

Load power = 450 W, time = 100 sec, Number of revolutions = 10

Energy supplied in 100 seconds

= (450 × 100 × 10−3)/3600 = 12.25 × 10−3 kWh

Number of revolutions in 100 seconds = 10

Meter constant = number of revolutions / kWh

Meter constant = 10/12.25 × 10−3 = 800 rev/kWh

 

21. In short time test of energy meter the other meter used is

  1. Substandard type
  2. Standard type
  3. Either 1 and 2
  4. Voltmeter

Answer.1. Substandard type

Explanation:

In the Short time test method of testing, the energy meter under test and sub-standard energy, meter measure the energy consumed in a predetermined time by a test circuit.  Both the energy meters record the energy for a predetermined number of revolutions of the energy meter under test.

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