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Correct option is (b) preliminary LIGHT LOAD adjustment

To explain I would SAY: ENERGY meter can be tested for creeping using preliminary light load adjustment. Disc HOLES are so positioned that they aren’t under the poles of a series magnet.

Right ANSWER is (d) 10/\(\SQRT{3}\)

To EXPLAIN I WOULD say: For triangular WAVE- Average value = \(\frac{V_m}{3}\), rms value = \(\frac{V_m}{\sqrt{3}}\)

\(\frac{V_m}{3}\) = 10 V or, Vm = 30 V

So, rms = \(\frac{30}{\sqrt{3}}\) = 10\(\sqrt{3}\).

Right OPTION is (c) 12 rpm

For explanation: METER CONSTANT = \(\frac{Number \,of\, REVOLUTION}{Energy}\)

Number of revolution = \(\frac{600 × 100 × 15 × 0.8}{1000}\) = 720

∴ Speed in rpm = \(\frac{720}{60}\) = 12 rpm.

The correct CHOICE is (c) 0.2 % more

For EXPLANATION: Power indicated by watt-meter = 400 × 20 + 20^2 × 0.02 = 4008 W

Percentage increase = \(\frac{4008-4000}{4000}\) × 100 = 0.2 % more.

Correct option is (c) W1 = 0 W and W2 = 1000 W

To explain I WOULD SAY: W1 = 100 × 20 COS 90° = 0

W2 = \(\frac{100 × 20 × \sqrt{3}}{\sqrt{3}× 2}\) = 1000 W.

Correct option is (a) Zero

Easiest EXPLANATION: If we CONSIDER both the waves, we can see POSITIVE power and negative power in each CASE are equal. So, the NET resultant power is zero.

Right option is (d) 19 KW

Easiest explanation: w1 = 20.5 kW, W2 = -3.5 kW

∴ W = w1 + w2 = 20.5 – 3.5 = 17 kW.

Correct CHOICE is (c) 242 W

To elaborate: Watt-meter reading = Current through CC × Voltage ACROSS PC × cos (PHASE angle).

IBR = ICC = \(\frac{220∠120°}{100°}\) = 2.2∠120°

VYB = VPC = 220∠-120°

w = 2.2∠120°× 220∠-120° × cos 240° = 242 W.

Right option is (b) When the readings of the two watt-meters are EQUAL but opposite sign, the POWER factor is ZERO

The explanation is: When power factor is 0, we have ∅ = 90°

P1 = (3)^0.5 VIcos (30° – ∅) = (3)^0.5 VIcos (30° – 90°) = \(\frac{(3)^{0.5}}{2}\) × VI

P2 = (3)^0.5 VIcos (30° + ∅) = (3)^0.5 VIcos (30° + 90°) = \(– \frac{(3)^{0.5}}{2}\) × VI

∴Total powerP = 0

So, with zero power factor the readings of the two watt-meters are equal but of opposite sign.

Right ANSWER is (d) Unity

Best EXPLANATION: Power factor, cosθ = cos arc TAN (3)^0.5 \(\frac{W_1-W_2}{W_1+W_2}\)

W1 = W2 = cos 0° = 1.

Right choice is (c) 27.6 rpm

Best explanation: Meter CONSTANT = \(\frac{NUMBER \,of\, revolution}{Energy} = \frac{600 × 230 × 15 × 0.8}{1000}\) = 1656

∴ Speed in rpm = \(\frac{1656}{60}\) = 27.6 rpm.

Correct option is (d) Not affected by NEGATIVE or zero sequence voltages

For explanation I would say: If the PHASE voltage is unbalanced, then the power READING is not affected by both negative as well as zero sequence voltages. This is a characteristic property of the 2 watt-METER meter since it measures the active power on a three phase three wire system.

Correct choice is (a) 0.866 LAG

Easy explanation: Power factor, cosθ = COS arc tan (3)^0.5 \(\FRAC{W_1-W_2}{W_1+W_2}\)

Since, \(\frac{W_1}{W_2} = \frac{2}{1}\)

So, power factor = cos arc tan (3)^0.5 \(\frac{2-1}{2+1}\)

= cos 30° = 0.866 lag.

The CORRECT CHOICE is (b) – 168 W

Easiest EXPLANATION: VRY = 220∠30°

IRY = \(\frac{220∠30°}{100∠36.86°}\) = 2.2∠-6.68° A

Power = VBN IRY = COS∅ = \(\frac{220}{(3)^{0.5}}\)× 2.2 cos (126.86°)

= -167.6 W ≈ – 168 W.

Right CHOICE is (b) 668.6 μF

Explanation: P = P1 + P2 = 7000 – 2500 = 4500 W

∴ Power in each PHASE = 1500 W

Voltage of each phase = 231 V

So, cos∅ = 0.264

∴ Current in each phase = 24.6 A

∴ Z of each phase = 9.39 Ω

∴ R of each phase = 2.48 Ω

Reactance of each phase = 9.056 Ω

For one watt-meter to read zero, power factor should be 0.5

Now, tan∅ = \(\frac{X}{R}\) or, X = 4.29 Ω

∴ Capacitance reactance required = 9.056 – 4.29 = 4.76 Ω

∴ Capacitance C = \(\frac{1}{2π × 50 × 4.76}\) F = 668.6 μF.

Correct answer is (b) 0.5

To EXPLAIN: The load power factor is = 0.5. Since at this power factor ONE WATTMETER shows zero and the other shows a positive MAXIMUM value of power.

The correct answer is (a) True

The best I can explain: A rotating MAGNETIC field is PRODUCED when the three coils are excited by a supply of three phase. As a result, eddy e.m.f circulates in the DISC. A torque is produced as a result of the interaction between the eddy currents and the rotating magnetic field.

Right choice is (a) star connected

Explanation: In a rotating type phase sequence indicator, the THREE COILS are connected in the FORM of a star and EXCITED by a three-phase SUPPLY.

The correct choice is (d) 3 coils

The best I can explain: A rotating type of phase sequence indicator basically consists of three coils. These are mounted at 120º to each other is space. The ENDS of the coils are CONNECTED to the terminals R, Y, and B RESPECTIVELY.

Right choice is (c) 2

To explain: Generally, two types of phase sequence indicators are used. The FIRST ONE is the ROTATING type while the second type is the STATIC type. Usually the rotating type of phase sequence INDICATOR is used.

Right choice is (b) same phase sequence

The EXPLANATION is: During synchronization, the phase sequences of any two GIVEN SUPPLIES are maintained equal. This is ENSURED by MAKING use of a phase sequence indicator.

Right option is (a) True

The EXPLANATION is: The phase sequence for a three phase supply indicates the order in which MAXIMUM values of phase voltages ER, EY, and EB occur. A three phase supply can be in EITHER RYB or RBY configuration.

Right option is (c) only applying rated voltage to PRESSURE coil

To explain: Rated voltage is applied to the pressure coil. The current coil is provided with a current at UNITY p.f. ERRORS are kept minimum and the POSITION of the BRAKE magnet is so adjusted that disc rotates at the correct speed.

The correct CHOICE is (b) applying rated voltage to pressure coil and a p.f. of 0.5 for current coil

Best EXPLANATION: Rated voltage is applied to the pressure coil. The current coil is provided with a current at 0.5 p.f. lagging. TILL the disc rotates at correct speed, LAG device is ADJUSTED.

Right option is (a) applying RATED VOLTAGE across the pressure coil

Explanation: Disc rotation is adjusted in such a WAY that CORRECT speed is maintained. The pressure coil is supplied with the rated voltage and the current coil is PROVIDED with only about 5 % of the full load at u.p.f.

The correct choice is (d) multiplies alternating VOLTAGE and CURRENT

Explanation: A multiplier basically performs the multiplication of the alternating voltage and the current. Multiplier also PROVIDES the current in the form of INSTANTANEOUS power to a voltage CONTROLLED oscillator.

The correct option is (a) True

To explain: An ELECTRONIC energy meter does not have frictional losses, creeping is not NEEDED irrespective of the nature of the load such as LOW load, full load POWER factor, etc and the accuracy in the reading is of the ORDER of ±1%.

The correct ANSWER is (b) into DIGITAL

The explanation is: The analog signal OBTAINED in an electronic energy meter is converted into digital by MAKING use of a digital circuit. By making use of a seven-segment display, energy is MENTIONED in watt-hours.

Right answer is (c) PRODUCT of instantaneous voltage and current

For explanation: The AVERAGE power is COMPUTED as the product of the instantaneous voltage ACROSS the load and the instantaneous current flowing through the load. A scaling device is used to bring the supply voltage to a proper LEVEL.

Right answer is (b) power consumption and time duration

Explanation: An ELECTRONIC energy meter is used in two stages. Energy measurement basically involves the measurement of power and the time duration. In the first stage, it is used as a WATTMETER while in the SECOND stage it is used MONITORING the power CONSUMED in a time interval.

Correct option is (a) IC

For EXPLANATION: BASICALLY, integrated circuits ALSO known as IC’s are used for the operation of an ELECTRONIC energy meter.

The correct ANSWER is (c) ATTACHING small iron pieces

For explanation: By attaching some iron pieces to the edge of the disc, CREEPING can be limited in some energy meters. Force of attraction that is EXPERIENCED by the brake magnet as a result of the iron piece is ENOUGH to eliminate the creeping.

Right answer is (B) drilling two diametrically opposite holes

Easiest explanation: In an energy meter, creeping causes the disc to rotate even when there is no current FLOWING. By drilling two diametrically opposite holes under the edge of the poles of a shunt magnet, rotation of the disc is limited to a MINIMUM VALUE.

Right option is (a) True

Easiest explanation: Even when there is no current FLOW through the ENERGY meter, disc rotates SLOWLY. This is known as creeping. This is occurs as a result of the over compensation PROVIDED for friction.

Right option is (c) due to ASYMMETRY in MAGNETIC circuit

Explanation: In an ENERGY meter, when the magnetic circuit is ASYMMETRICAL, a driving TORQUE is produced. As a result of this driving torque, the energy meter creeps.

The correct ANSWER is (a) flux is PROPORTIONAL to SQUARE of voltage

The best I can EXPLAIN: In an energy meter, the supply voltage may fluctuate as a result of unavoidable reasons leading to errors in the reading. 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 square of the voltage.

Correct ANSWER is (b) False

The explanation: In an energy meter, phase angle errors occur as a RESULT of improper adjustments of lag condition, abnormal FREQUENCIES etc. Due to temperature, changes in resistance VALUES ALSO lead to error in the phase angle.

Correct option is (a) DUE to ABNORMAL currents and voltages

For explanation: In the driving system of an ENERGY meter, magnitude of FLUX can be INCORRECT as a result of abnormal values of currents and voltages. This occurs due to a change in the resistance of the pressure coil circuit.