Induction-type single-phase Energy Meter MCQ || Energy Meter Questions and Answers

11. Moving system of the induction type single phase energy meter has _________

  1. Heavy aluminum disc
  2. Light aluminum disc
  3. Medium aluminum disc
  4. No aluminum disc

Answer.2. Light aluminum disc

Explanation:

Moving system of Induction Type Energy Meter:

  • It consists of a light aluminum disc mounted on a vertical spindle.
  • The aluminum disc is positioned in the air gap between the series and the shunt magnet.
  • The spindle is supported by a cup-shaped jeweled bearing at the bottom end and has a spring journal bearing at the top end. Since there is no control spring, the disc makes continuous rotation under the action of deflecting torque.

 

12. Creeping in the energy meter can be avoided by:

  1. Drilling two diametrically opposite holes in the disc.
  2. Using adjustable resistance.
  3. Providing a small shading loop between the central pole of the shunt magnet and the disc.
  4. Using shading bands around the central limb of the shunt magnet.

Answer.1. Drilling two diametrically opposite holes in the disc.

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.

 

13. Braking system consists of a single-phase Energy meter consisting of _____

  1. Bar magnet
  2. Temporary magnet
  3. Permanent magnet
  4. Super magnet

Answer.3. Permanent magnet

Explanation:

Braking system in single-phase induction type Energy meter

  • A permanent magnet positioned near the edge of the aluminum disc forms the braking system.
  • When an aluminum disc moves in the field of the braking magnet, flux is cut and eddy currents are induced in the disc.
  • Since the induced currents are proportional to the speed of the disc braking torque are proportional to the disc speed.
  • The position of the braking magnet is adjustable, and therefore, braking torque can be adjusted by shifting the magnet to different radial positions.

 

14. 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

Calculation;

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 %

 

15. What is the effect of eddy currents in the aluminum disc?

  1. Varies by a factor of twice the disc length
  2. Independent of the disc speed
  3. Varies by a factor of four times the disc size
  4. Proportional to the disc speed

Answer.4. Proportional to the disc speed

Explanation:

Braking system in single-phase induction type Energy meter

  • A permanent magnet positioned near the edge of the aluminum disc forms the braking system.
  • When an aluminum disc moves in the field of the braking magnet, flux is cut and eddy currents are induced in the disc.
  • Since the induced currents are proportional to the speed of the disc braking and torque are proportional to the disc speed.
  • The position of the braking magnet is adjustable, and therefore, braking torque can be adjusted by shifting the magnet to different radial positions.

 

16. Watt-hour meter can be produced using:

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

Answer.3. Induction effect

Explanation:

  • Watt-hour meter can be produced using in Induction Effect.
  • Induction-type instruments are most commonly used as energy meters for the measurement of energy in domestic and industrial ac circuits.
  • Induction-type meters have lower friction and higher torque/weight ratio; they are inexpensive, yet reasonably accurate and can retain their accuracy over a considerable range of loads and temperatures.

 

17. An energy meter produces a flux of ∅ when connected to a ______

  1. Supply Voltage
  2. Supply Current
  3. Spindle
  4. Electromagnet

Answer.1. Supply Voltage

Explanation:

An energy meter produces a flux  ∅ when connected to a supply voltage. When the energy meter is connected in the circuit, the current coil carries the load current and the pressure coil carries the current proportional to the supply voltage. Since the potential coil is highly inductive, current and flux lag the voltage by 90°.

 

18. 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

 

19. In an induction type Energy meter if the braking magnet is moved towards the center of the disc then the Braking torque _______

  1. Increase
  2. Decrease
  3. Remain same
  4. Either increase or decrease

Answer.2. Decrease

Explanation:

If the braking magnet is moved towards the center of the disc, flux cut by the disc is less. Further, this reduces the induced current, and thus, the braking torque is reduced.

 

20. A 230-V, single-phase domestic energy meter has a constant load of 4 A passing through it for 6 h at unity power factor. The meter disc makes 2208 revolutions during this period. What will be the energy consumed by the load if the meter disc completes 1240 revolutions?

  1. 2.5 kWh
  2. 2.8 kWh
  3. 3.5 kWh
  4. 3.1 kWh

Answer.4. 3.1 kWh

Explanation:

Meter constant K = R/E

R ∝ E

Given that,

Voltage = 230 V

Current = 4 A

cos ϕ = 1

Energy consumed in 6 hours is given as,

E1 = 230 × 4 × 6 × 1 = 5520 Wh

R1 = 2208

R1E1 = R2E2

Given, R2 = 1240

E2 = R2E1/R1

= (1240 × 5520)/2208 = 3100 Wh

Hence, the energy consumed in the 1240 revolution is 3.1 kWh.

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