Induction Type Energy Meter MCQ Quiz - Objective Question with Answer for Induction Type Energy Meter - Download Free PDF

Option 2 is NOT correct because eliminating high-voltage leads from PT bushings generally reduces complexity and potential points of failure, not increases them. So saying it increases short-circuit risk is incorrect.

Function of bushings in PT

• When a high-voltage conductor passes through a metal sheet or frame at earth potential, the necessary insulation is provided in the form of a bushing.
• The bushing's primary function is to prevent electrical breakdown between the enclosed conductor and the surrounding earthed metal work.
• The high voltage conductor passes through the bushing made of some insulating material (e.g., porcelain, steatite).
• In transformers, bushings are insulating devices that allow electrical conductors (such as transformer leads) to pass safely through the transformer tank or casing while preventing the high voltage from leaking to the grounded parts of the transformer.
• Their primary purpose is insulation, ensuring that the high-voltage leads are protected from the tank and the surrounding environment.
• Bushings provide a safe way to bring electrical connections out of the transformer without shorting the circuit or causing electrical faults.
• They are designed to handle high voltages and currents, while also preventing any potential dielectric breakdown between the conductor and the transformer tank.

Potential Transformer

A Potential Transformer (PT), also known as a Voltage Transformer, is an instrument transformer used to step down high voltage to a lower, measurable value for metering and protection in power systems.

PTs work on the same principle as a regular transformer but are designed for precision and safety, not power delivery.

Option 3 is NOT correct because:

• Voltage measurement accuracy in a PT depends primarily on the Ratio Error, not just the power angle (phase angle) error.
• Power angle error (or phase error) affects power and energy measurements (since they involve phase relationships), but not voltage measurement alone.
• Voltage measurement is mostly impacted by ratio error, which is the deviation from the ideal voltage transformation ratio.

Explanation:

Meter Constant and Speed of Induction Watt Hour Meter

Definition: The meter constant of a watt-hour meter indicates the number of revolutions of the meter's disc for a specific amount of energy consumed, typically expressed in revolutions per kilowatt-hour (rev/kWh). It helps in determining the speed of the disc based on the electrical load connected to the meter.

Problem Statement: In this problem, we are tasked with calculating the speed of the meter's disc for a single-phase 240V induction watt-hour meter with a meter constant of 400 revolutions per kWh. The current is 10A, and the power factor (pf) is 0.8 lagging.

Given Data:

• Meter constant: 400 revolutions per kWh
• Voltage (V): 240 V
• Current (I): 10 A
• Power factor (pf): 0.8 (lagging)

Step-by-Step Solution:

Step 1: Calculate the power consumed (P)

The power consumed in an electrical circuit is given by the formula:

P = V × I × pf

Substituting the given values:

P = 240 × 10 × 0.8

P = 1920 W or 1.92 kW

Step 2: Determine the number of revolutions per second

From the meter constant, we know that the meter completes 400 revolutions for every 1 kWh of energy consumed. To find the revolutions per second, we first calculate the energy consumed per second (in kWh) and then use the meter constant.

Energy consumed per second (in kWh):

The energy consumed per second in kWh is calculated as:

Energy (kWh) = Power (kW) × Time (hours)

For 1 second, time = 1/3600 hours:

Energy = 1.92 × (1/3600)

Energy = 0.000533 kWh

Revolutions per second:

The revolutions per second can be calculated using the meter constant:

Revolutions per second = Energy (kWh) × Meter constant

Revolutions per second = 0.000533 × 400

Revolutions per second = 0.2132 revolutions/second

Step 3: Convert revolutions per second to revolutions per minute (rpm)

To convert revolutions per second to revolutions per minute, multiply by 60:

Revolutions per minute (rpm) = Revolutions per second × 60

Revolutions per minute = 0.2132 × 60

Revolutions per minute = 12.8 rpm

Final Answer:

The speed of the meter disc is 12.8 rpm.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: 16.02 rpm

This option is incorrect. The value of 16.02 rpm is higher than the actual calculation. This might result from incorrectly assuming a higher energy consumption or misinterpreting the meter constant.

Option 3: 18.2 rpm

This option is also incorrect. It overestimates the revolutions per minute, likely due to a similar error in the calculation of power or energy consumed.

Option 4: 21.1 rpm

This option is incorrect as well. It significantly overestimates the speed of the meter disc, which is inconsistent with the given meter constant and load conditions.

Conclusion:

The correct answer is Option 1: 12.8 rpm. This result is derived using the meter constant and the actual power consumed under the given load conditions. Understanding the relationship between power, energy, and the meter constant is crucial for solving such problems accurately.

Construction of energy meter
 

Driving System:

• The electromagnet is the main component of the driving system. The driving system has two coils: the current and the potential coil.
• The current coil is connected in series with the load and the potential coil is connected parallel to the supply.
   

Moving System:

• The moving system is the aluminum disc mounted on the shaft of the alloy. The eddy current is induced in the disc because of the change in the magnetic field.
• 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.
   

Braking System:

• The permanent magnet is used for reducing the rotation of the aluminum disc. The aluminum disc induces the eddy current because of its rotation.
• The eddy current cuts the magnetic flux of the permanent magnet and hence produces the braking torque. This braking torque opposes the movement of the disc, thus reducing its speed.
   

Registering System:

• The main function of the registration or counting mechanism is to record the number of rotations of the aluminum disc. The speed of rotation will be proportional to the power, provided the phase angle between the voltage coil and current coil fluxes is 90°.
• This ensures that the induced eddy currents interact properly with the magnetic fields, generating the correct torque.
• Hence, for accurate power measurement, the pressure coil current must lead or lag the supply voltage by 90°.

Concept

The operating voltage for a fixed multimeter depends on the specific model and design of the device.

Generally, fixed multimeters (like bench or panel meters) are designed to operate within a certain range of input voltages provided by their power supply.

Here are some common scenarios:

Battery-Powered Multimeters:

• Typically operate on 9V batteries or sometimes AA/AAA batteries (1.5V each).

Mains-Powered Multimeters:

• Operate directly from the mains power supply, which could be 110V-120V AC or 220V-240V AC, depending on the region.
• Internally, these devices step down and regulate the voltage to lower levels (e.g., 5V or 12V DC) for operation.

Fixed Bench Multimeters:

• Use a regulated DC power supply, often requiring inputs like 5V, 12V, or 24V DC, depending on the design.

Concept:

In an energy meter, Meter constant (K) is given as,

\(K=\frac{R}{E}\)

And, \(R\propto E\)

Where,

R is the number of revolution and E is the energy consumed in kWh

And, E = VItcos ϕ

Calculation:

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

From above concept,

\(R\propto E\)

\(\therefore R_1E_2=R_2E_1\)

Given, R₂ = 1240

Hence,

\(E_2=\frac{R_2E_1}{R_1}=\frac{1240\times5520}{2208}=3100\ Wh\)

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

Concept:

Meter constant, \(K=\frac{R}{E}\)

Where R = revolution

E = Energy in kWh

Given actual revolution (K_A) = 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

Hence, E = 378/3600 kWh

Now, Measured revolution (K_M) = \(\frac{50}{378/3600}=476\)

Error = K_M - K_A = 476 - 500 = - 24

Now, % Error = \(\frac{-24}{500}\times 100=-4.8\%\)

Concept

The meter constant of a 1ϕ energy meter is given by:

\(Meter \space constant\space = {No. \space of \space revolutions \space in \space 1 \space hr\over No. \space of \space kW \space in \space 1 \space hr}\)

Calculation

Given, V = 230 volt

I = 20 A 

cos ϕ = 1

P = VI cos ϕ 

P = 230 × 20 × 1 = 4.6 kW

No. of revolutions in 1 hr = 2300/2 = 1150

\(Meter \space constant\space = {1150 \over 4.6}\)

Meter constant = 250 revolutions / kWh

• 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.
• 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.

Additional Information

• 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

Formula Used:

Meter Constant (K) = \(\frac{R}{E}\)

Here, R is revolution per hour

E is energy in kWh

Application:

We have,

V = 200 volts

K = 200

If the current is 20 A at 0.8 pf lagging

Hence,

P = 200 × 20 × 0.8 = 3.2 kW

Energy in 1 hour = 3.2 kWh

From the above concept,

R = EK = 3.2 × 200 = 640 revolution/hour

Now, revolution in minutes = 10.67 rpm

Energy meter: 

• The 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.''

Components:

Driving System:

• The components of this system are two silicon steel laminated electromagnet
• Upper magnet→ Shunt magnet→ behave as potential coil
• Lower magnet→ series magnet→ behave as current coil

Moving system: 

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

Registering system: 

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

Braking system: 

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

Solution:

Disc of the energy meter is made of aluminium.

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:

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.

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) = \(\frac{1500}{60}=25\)

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