Single Phase Energy Meter Error MCQ || Errors in Single Phase Energy Meter Questions and Answers

Answer.1. Abnormal currents and voltages

Explanation:

There are various errors present in the single-phase induction type energy meter. The driving system can cause errors due to inaccurate phase angles, abnormal frequencies, the effect of temperature on the resistance, and an unsymmetrical magnetic circuit.

Abnormal load currents and voltages may lead to the wrong magnitude of fluxes.

Answer.3. Small iron pieces

Explanation:

In some meters, a slow but continuous rotation of the disc can be observed even when there is no current flowing through the current coil, and only the pressure coil is energized. This is called creeping. The primary reason for creeping is due to over-compensation for friction.

Prevention of Creeping

1. In order to prevent creeping on no-load, two holes are drilled in the disc on the opposite side of the spindle at the same radius. This causes sufficient distortion of the field to prevent continuous rotation. The disc remains stationary when one of the holes comes under one of the poles of the shunt magnet.
2. Creeping can also be avoided by attaching a tiny piece of iron to the edge of the disc. The brake magnet in such a case can lock the iron piece to itself and prevent the creeping of the disc.

Answer.3. Less than 90° Lagging

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.

Answer.3. Over-Friction Compensation

Explanation:

In some meters, the disc rotates slowly and continuously when there is no load. The rotation of the disc without any current through the current coil and only due to excitation of the pressure coil is called Creeping. This is due to friction overcompensation, vibrations, the excessive voltage across the voltage coil, and stray magnetic fields. Among all these, the compensation for friction is the major one.

Answer.2. Prevent Creeping

Explanation:

In some meters, a slow but continuous rotation of the disc can be observed even when there is no current flowing through the current coil, and only the pressure coil is energized. This is called creeping. The primary reason for creeping is due to over-compensation for friction.

Prevention of Creeping

1. In order to prevent creeping on no-load, two holes are drilled in the disc on the opposite side of the spindle at the same radius. This causes sufficient distortion of the field to prevent continuous rotation. The disc remains stationary when one of the holes comes under one of the poles of the shunt magnet.
2. Creeping can also be avoided by attaching a tiny piece of iron to the edge of the disc. The brake magnet in such a case can lock the iron piece to itself and prevent the creeping of the disc.

Answer.1. Constant

Explanation:

Generally, the supply voltage and frequency are constant in the induction type energy meter. The main requirement of an induction-type energy meter is that it should record the actual energy supplied to the load. The supply can fluctuate due to unavoidable reasons leading to errors in the reading of the energy meter.

Answer.4. Both 1 and 2

Explanation:

Due to an increase in temperature, the resistance of all aluminum and copper parts gets increased which may cause the following errors.

• Torques produced by all shading bands get decreased.
• The potential coil flux gets reduced.
• A decrease in the phase angle between the potential coil flux and supply voltage occurs.
• The phase angle of eddy currents also gets reduced.

Answer.1. Flux is proportional to square of voltage

Explanation:

Breaking 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

R_e = 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 results in a self-braking torque proportional to the square of the voltage.

Answer.1. Inductive Load

Explanation:

The Temperature errors in the energy meter occur due to the rise in temperature tend to neutralize one another’s effects and hence, are very small.

These errors are negligible with non-inductive loads. However, for low power factor loads, these errors become serious and cause the meter to register high and run fast.

The temperature rise effects can be compensated by using a temperature shunt on the brake magnet. Some special magnetic materials such as Mutemp can also be used whose permeability decreases with an increase in temperature.

Answer.2. By a temperature shunt

Explanation:

The Temperature errors in the energy meter occur due to the rise in temperature tend to neutralize one another’s effects and hence, are very small.

These errors are negligible with non-inductive loads. However, for low power factor loads, these errors become serious and cause the meter to register high and run fast.

The temperature rise effects can be compensated by using a temperature shunt on the brake magnet. Some special magnetic materials such as Mutemp can also be used whose permeability decreases with an increase in temperature.

Answer.4. Both 1 and 2

Explanation:

In an energy meter, a certain amount of variation from its specified value of supply voltage is permitted and thus, the meters should be compensated. The errors occur due to voltage variations because of two reasons:

1. On account of saturation in iron parts, the relationship between the supply voltage and shunt magnet flux is non-linear.
2. A dynamically induced emf is produced in the disc by the shunt magnet flux which further generates a self-braking torque in proportion to the square of the supply voltage.

Answer.3. Diverting Flux

Explanation:

• In an energy meter, a certain amount of variation from its specified value of supply voltage is permitted and thus, the meters should be compensated.
• This variation in voltage is compensated by diverting a large portion of the flux into the active path, whenever a rise in voltage occurs.
• This diversion is performed by a saturable magnetic shunt. Another method is to cut the holes in the side limbs of the shunt magnet so that its reluctance can be increased.

Answer.2. Light Load compensation

Explanation:

• The errors occurring due to friction at the top and bottom of the spindle due to its bearing equipment are very serious and cannot be removed merely by careful design.
• These errors hold greater importance at low loads and, thus, some arrangements must be made to ensure accurate readings.
• Friction compensation is also known as light load compensation.

Answer.4. Short circuited band

Explanation:

• Friction error is introduced due to friction at the rotor bearing and in the register mechanism. Because of this error, an unwanted braking torque acts on the moving system and the meter registers less energy than the actual energy passing through it.
• This error is compensated by placing two short-circuited bands on the outer limbs of the shunt magnet. These bands embrace the flux contained in the two outer limbs of the shunt magnet.
• Due to this strip, an additional torque independent of the load is produced which acts on the disc in the direction of rotation. This compensates for the friction and the meter can be made to read accurately.

Answer.4. Short circuited band

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.

In order to remove this error, the flux produced by the shunt magnet should be made to lag behind the supply voltage exactly by 90°.

This is accomplished by adjusting the position of the copper shading band provided on the central limb of the shunt magnet. An error on the fast side under these conditions can be eliminated by bringing the shading band near to the disc and vice versa.

Answer.2. Speed Error

Explanation:

1. The speed of the disc of an energy meter is either faster or slower when tested on a load having a moderate power factor.
2. Therefore, the energy meter registers either more or less energy than the actual energy passing through it, and an error is introduced.
3. In order to remove this error, the radial position of the braking magnet is adjusted. The movement of the braking magnet, away from the center of the disc, increases the braking torque that decreases the speed of the disc, and vice versa.

Answer.1. Overload compensation

Explanation:

• The dynamically induced emf (responsible for the production of braking torque) can be made small in comparison to statistically induced emf (responsible for producing the driving torque) by making the current coil flux small in comparison to the pressure coil flux.
• Usually, an overload compensating device is added to compensate for overload. It is a magnetic shunt for the series magnets core and approaches saturation at overloads to reduce its permeability.
• Thus, only a small portion of the series magnetic flux gets diverted at large values of currents and the remaining larger portion appears in the air gap of the disc contributing to driving torque.

Answer.2. Braking Torque

Explanation:

A dynamically induced emf is produced in the disc due to its continuous revolution in the field of the series magnets when a load is applied. Due to this emf, eddy currents are produced and interact with the field of the series magnet to generate braking torque.

This self-braking torque is in proportion to the square of the load current and increases with an increase in the load. This results in a lower registration when the load current is high. This self-braking action can be minimized by keeping the speed of the disc as low as possible, approximately equal to 40 rpm at full load.

Answer.2. Eddy Current

Explanation:

In the energy meter, a thin disc made of copper or aluminum pivoted at the center is placed between the shunt and series magnets so that it cuts the flux from both of the magnets. The deflecting torque is produced by the interaction of eddy currents induced in the discs and the flux inducing it.

In order to cause the resultant flux in the shunt magnet to lag in phase by exactly 90° behind the applied voltage. one or more copper rings, known as copper-shading bands are provided on one limb of the shunt magnet.

Answer.2. Temperature Error

Explanation:

The Temperature errors in the energy meter occur due to the rise in temperature tend to neutralize one another’s effects and hence, are very small. These errors are negligible with non-inductive loads. However, for low power factor loads, these errors become serious and cause the meter to register high and run fast.