1. The braking torque of induction type single-phase energy meter is:
Directly proportional to the square of the flux
Directly proportional to the flux
Inversely proportional to the flux
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:
100
800
160
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
0.8
0.7
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:
Heating effect
Electromagnetic effect
Induction effect
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 the 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:
Revolutions/kWh
kWh/Revolutions
kW/Revolutions
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/Energy consumed
6. The full-scale deflection in an induction instrument is about:
180°
270°
300°
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:
Drilling two holes on opposite sides of the disc diameter
Rotating the disk faster
Increasing the weight of the disk
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.
3.26% , Slow
3.15%, Slow
3.26%, Fast
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
– 2.43%
2.43%
– 4.86%
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?
Number of single units
Total watts each hour
The maximum value the meter can register
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
Incorrect position of brake magnet
Incorrect adjustment of the position of the shading band
Overvoltage across voltage coil
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 is used for energy measurement. It makes 500 revolutions in 30 seconds. Find the energy consumed by the load for an hour.
1000 units
100 units
10 unit
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?
Magnetic Flux
Moving System
Registering System
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-hours.
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?
Incorrect position of brake magnets.
Incorrect adjustment of the position of shading bands.
Slow but continuous rotation of the aluminum disc.
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
Accuracy variation due to test load
Need continuous Monitoring
The Meter constant can’t be determined
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 are connected back to back in the test circuit.
The disadvantages of this method are
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
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 the unity power factor. If the meter disc makes 2300 revolutions for 2 hours, the meter constant will be:
0.25 revolution/kWh
250 revolution/kWh
0.30 revolution/kWh
270 revolution/kWh
Answer.2. 250 revolution/kWh
Explanation:
Meter constant = (Number of revolutions) / (Energy consumed in kWh)
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:
Indicating
Deflecting
Digital
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?
0.67%
0.76%
1%
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
800 rev/kWh
222 rev/kWh
147 rev/kWh
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:
100
600
800
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 a short time test of the energy meter the other meter used is
Substandard type
Standard type
Either 1 and 2
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.