# 3-phase Energy Meter MCQ || Polyphase Energy Meter Questions and Answers

1. Bondel’s Theorem is applicable to

1. Wattmeter Only
2. Energy Meter Only
3. Both wattmeter and Energy Meter
4. Voltmeter

Answer.3. Both wattmeter and Energy Meter

Explanation:

Blondel’s Theorem states that if a network is supplied through N conductors, the total power is measured by summing the readings of N wattmeters so arranged that a current element of a wattmeter is in each line and the corresponding voltage element is connected between that line and a common point.

Similar to wattmeters, Blondel’s theorem is applicable to energy meters as well. Thus, a three-element energy meter is required for a three-phase four-wire system, and for a three-phase three-wire system, a two-element energy meter is required.

2. The type of arrangement used to measure the power through the energy meter is

1. Three-phase Four-wire system
2. Three-phase Three-wire System
3. Both 1 and 2
4. DC System

Explanation:

Similar to wattmeters, Blondel’s theorem is applicable to energy meters as well. Thus, a three-element energy meter is required for a three-phase four-wire system, and for a three-phase three-wire system, a two-element energy meter is required.

Therefore, these meters are categorized as three-element energy meters and two-element energy meter

3. A three-element energy meter is used for _______

1. Three-phase Four-wire system
2. Three-phase Three-wire System
3. Both 1 and 2
4. DC System

Explanation:

A three-element energy meter is used for the three-phase four-wire system. The fourth wire is a neutral wire. The construction of an individual element is similar to that of a single-phase energy meter.

One unit of three-element, the three-phase element is always cheaper than three units of a single-phase energy meter.

4. The additional wire i.e fourth wire in the three-element energy meter is known as

1. Ground Wire
2. Neutral Wire
3. Phase Wire
4. Supply wire

Explanation:

A three-element energy meter is used for the three-phase four-wire system. The fourth wire is a neutral wire. Each element has the same constructional details as that of a single-phase energy meter. Three discs are provided for each element, thus, it is a multi-disc type wattmeter.

5. The pressure coil in the three-element energy meter is connected in _____

1. Series with Neutral only
2. Parallel with Neutral only
3. Series with Neutral and Line
4. Parallel with Neutral and Line

Answer.4. Parallel with Neutral and Line

Explanation:

A three-element energy meter is used for the three-phase four-wire system. The fourth wire is a neutral wire. Each element has the same constructional details as that of a single-phase energy meter. Three discs are provided for each element, thus, it is a multi-disc type wattmeter. The pressure and current coils are labeled as P1, P2, P3, and C1, C2, C3, respectively.

The pressure coils are connected across a neutral and a line whereas the current coils are connected in series with the lines.

The coils are connected in such a manner that the net torque produced is the sum of the torques due to all three elements.

6. One unit of three-element, the three-phase element is always _____ than three units of single-phase energy meter.

1. Cheaper
2. Costlier
3. Beneficial
4. Harder

Explanation:

A three-element energy meter is used for the three-phase four-wire system. The fourth wire is a neutral wire. The construction of an individual element is similar to that of a single-phase energy meter.

One unit of three-element, the three-phase element is always cheaper than three units of a single-phase energy meter. But due to the interaction between eddy currents produced by one element with the flux produced by another element, there may be errors in the measurement by the three-phase energy meter. Such errors may be reduced by suitable adjustments.

7. The pressure coil in the three-element energy meter is connected in _____

1. Series with Neutral only
2. Parallel with Neutral only
3. Series with Line
4. Parallel with Neutral and Line

Explanation:

A three-element energy meter is used for the three-phase four-wire system. The fourth wire is a neutral wire. Each element has the same constructional details as that of a single-phase energy meter. Three discs are provided for each element, thus, it is a multi-disc type wattmeter. The pressure and current coils are labeled as P1, P2, P3, and C1, C2, C3, respectively.

The pressure coils are connected across a neutral and a line whereas the current coils are connected in series with the lines.

The coils are connected in such a manner that the net torque produced is the sum of the torques due to all three elements.

8. Two element energy meter is also known as _______

1. Single Disc type wattmeter
2. Multi-Disc type wattmeter
3. Single Disc type Voltmeter
4. Multi-Disc type Ammeter

Explanation:

Two element energy meter is also called a multi-disc type wattmeter as two separate discs are provided for each element. It is used to measure energy for a three-phase three-wire system.

The connections of the meter are similar to that of two wattmeter method of power measurement. The pressure coils, carried by the shunt magnet are connected in parallel and the current coils, carried by the series magnet are connected in series.

The total torque generated is equal to the sum of the torques generated by each element and the method of torque generation is similar to the single-phase energy meter.

9. A two-element energy meter is used for _______

1. Three-phase Four-wire system
2. Three-phase Three-wire System
3. Both 1 and 2
4. DC System

Explanation:

Two element energy meter is also called a multi-disc type wattmeter as two separate discs are provided for each element. It is used to measure energy for a three-phase three-wire system.

The connections of the meter are similar to that of two wattmeter method of power measurement. The pressure coils, carried by the shunt magnet are connected in parallel and the current coils, carried by the series magnet are connected in series.

10. In a two-element energy meter the pressure coil Is carried by ____

1. Shunt Magnet
2. Series Magnet
3. Both shunt and Series Magnet
4. None of the above

Explanation:

Two element energy meter is also called a multi-disc type wattmeter as two separate discs are provided for each element. It is used to measure energy for a three-phase three-wire system.

The connections of the meter are similar to that of two wattmeter method of power measurement. The pressure coils, carried by the shunt magnet are connected in parallel and the current coils, carried by the series magnet are connected in series.

11. In a two-element energy meter, the current coil Is carried by ____

1. Shunt Magnet
2. Series Magnet
3. Both shunt and Series Magnet
4. None of the above

Explanation:

Two element energy meter is also called a multi-disc type wattmeter as two separate discs are provided for each element. It is used to measure energy for a three-phase three-wire system.

The connections of the meter are similar to that of two wattmeter method of power measurement. The pressure coils, carried by the shunt magnet are connected in parallel and the current coils, carried by the series magnet are connected in series.

12. The conditions for which energy meters are tested are _____

1. Power factor Testing
2. Starting Test
3. Creep test
4. All of the above

Explanation:

Before energy meters are released for measuring purposes, their actual registration is checked. Also, the errors are kept within reasonable limits by doing some adjustments. This is known as testing the energy meter. Some of the conditions for which energy meters are tested are given below.

1. The power factor is unity and the current is 5% of the marked value.
2. Power factor is unity and the current is 100% or 125% of marked value.
3. Current is equal to the marked value and 0.5 lagging power factor.
4. Power factor is unity with one intermediate load.

Starting test: This test does not check the accuracy of the meter, rather it is performed to check if the meter starts and runs properly. According to this test, the meter should start and run with normal voltage and a current of 0.5% of the marked value.

Creep test: This test states that the meter should not creep, that is, it must not undergo more than one revolution for a voltage of 110% of its marked value and open current circuit.

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