UNBALANCED POLYPHASE SYSTEMS 864. EE Board Exam April 1982

Given a balanced 3-wire, three-phase system serving the following loads:

𝑉𝑎𝑏= 200∠0° 𝑉 𝑍𝑎𝑏= 10∠53.13° Ω 𝑉𝑏𝑐= 200∠240° 𝑉 𝑍𝑏𝑐= 10∠0° Ω 𝑉_𝑐𝑎= 200∠120° 𝑉 𝑍_𝑐𝑎= 10∠30° Ω Determine the current on line b

A. 20.34 A C. 24.36 A

B. 22.04 A D. 21.57 A

865. EE Board Exam April 1982

Given the following line voltages and two load impedances:

𝑉_𝑎𝑏= 117∠0° 𝑉 𝑍_𝑎𝑏= 5 + 𝑗12 Ω

𝑉_𝑏𝑐= 117∠240° 𝑉 𝑍_𝑏𝑐= 13 + 𝑗0 Ω 𝑉_𝑐𝑎= 117∠120° 𝑉

Solve for the current in line c.

A. 17.41 A C. 16.62 A

B. 17.95 A D. 18.46 A

866. EE Board Exam April 1988

Three unequal single-phase loads so connected across the lines of a balanced, 3-phase, 230 volts circuit. The first takes 106 A at 0.78 pf lagging and is connected across lines 1 & 2. The second takes 142 A, at 0.82 pf lagging and is connected across lines 2 & 3. And the third takes 28.4 kW at 0.77 pf lagging and is connected across lines 3 & 1. Find the three line currents.

A. 254.40 A, 211.38 A, 252 A B. 231.26 A, 215.20 A, 268 A C. 254.40 A, 215.20 A, 252 A D. 231.26 A, 211.38 A, 268 A

867. EE Board Exam October 1992

A 120-V per phase, three-phase Y-connected source delivers power to the following delta-connected load:

𝑃ℎ𝑎𝑠𝑒 1 = 40∠0°

𝑃ℎ𝑎𝑠𝑒 2 = 20∠ − 60°

𝑃ℎ𝑎𝑠𝑒 3 = 15∠45°

Solve for the three line currents.

A. 12.45 A, 9 A, 22.45 A B. 13.49 A, 9 A, 22.45 A C. 13.49 A, 10 A. 20.22 A D. 12.45 A, 10 A, 20.22 A

868. EE Board Exam October 1985 Given:

𝑉𝑎𝑏= 240∠0° 𝑉 𝑍𝑎𝑏= 6 + 𝑗8 Ω 𝑉𝑏𝑐= 240∠240° 𝑉 𝑍𝑏𝑐= 5 + 𝑗8.66 Ω 𝑉_𝑐𝑎= 240∠120° 𝑉 𝑍_𝑐𝑎= 10 + 𝑗0 Ω Solve for the three line currents Ia, Ib and Ic.

A. Ia = 45 A, Ib = 43 A, Ic = 20 A B. Ia = 48 A, Ib = 42 A, Ic = 24 A C. Ia = 45 A, Ib = 42 A, Ic = 20 A D. Ia = 48 A, Ib = 43 A, Ic = 24 A

869. EE Board Exam April 1985

A three phase 230-V circuit serves two single-phase loads, A and B. Load A is an induction motor

rated 8 hp, 230 V, 0.70 pf, 0.90 efficiency, which is connected across lines a and b. Load B draws 5 kW at 1.0 pf and is connected across lines b and c.

Assume a sequence of a-b-c, solve for the current on line b.

A. 42.19 A C. 41.08 A

B. 27.74 A D. 34.46 A

870. EE Board Exam April 1980

A factory is supplied by a three-phase, 3-wire system with the following characteristics:

𝑉_𝑎𝑏= 230∠0° 𝑉 𝐼_𝑎= 110∠ − 36.87° 𝐴 𝑉_𝑏𝑐= 230∠240° 𝑉 𝐼_𝑐= 125∠53.13° 𝐴 Find the line current Ib.

A. 145.3 A C. 184.6 A

B. 163.3 A D. 166.5 A

871. EE Board Exam April 1988

& 3. And the third takes 28.4 kW at 0.77 pf lagging.

Determine total apparent power.

A. 94 kVA C. 78 kVA

B. 83 kVA D. 101 kVA

872. REE Board Exam October 1996

The following information is given for a delta-connected load of three numerically equal

impedances that differ in power factor. Line voltage

= 120 volts. 𝑍_𝑎𝑏= 15∠30° Ω, 𝑍_𝑏𝑐= 15∠0° Ω and 𝑍_𝑐𝑎= 15∠ − 30° Ω. Phase sequence of voltages is a-b-c. Using the phase sequence as a guide, calculate the total power drawn by the load.

A. 2,624 W C. 2,564 W

B. 2,472 W D. 2,731 W

873. EE Board Exam April 1993

In AC circuit, find the total power in kW in an unbalanced three-phase circuit loaded as follows:

Phase I = 120 V, 100 A, unity pf. Phase II = 100 V, 230 A, 80% pf and phase III = 110 V, 85 A, 77% pf.

A. 37.6 kW C. 32.8 kW

B. 35.3 kW D. 38.2 kW

874. EE Board Exam April 1983

Two single-phase transformers are connected in V (open delta) and serving a delta connected

impedance load. Each impedance is equal to 16∠36.87° Ω. If the transformer voltages impressed on the impedances are 𝑉_𝑎𝑏= 240∠0° 𝑉, 𝑉_𝑏𝑐= 240∠240° 𝑉, 𝑉_𝑐𝑎= 240∠120° 𝑉. Solve for the total kVA drawn by the load.

A. 6.23 C. 10.8

B. 8.31 D. 11.3

875. EE Board Exam October 1980, October 1982 Three impedances Zan = 20 + j0, Zbn = 16 + j12, Zcn

= 5 – j15 ohms, are connected in wye across a 230 V (line to line), 3-phase, 4-wire source. The phase sequence is a-b-c, counterclockwise. Determine the current passing thru the neutral.

A. 7.54 A C. 8.81 A

B. 9.12 A D. 8.02 A

876. EE April 1981

A wye-connected transformer with neutral connection has balanced voltages of 265 V between lines and neutral. The transformer is serving two single phase motors. Motor A (rated 4 hp, 0.90 efficiency, 0.80 power factor lagging) is connected across line a and neutral. Motor B (rated 3 hp, 0.85 efficiency, 0.85 power factor lagging) is connected across line b and neutral. Solve for the neutral current, using Van as reference vector.

A. 20.42 A C. 22.45 A

B. 25.37 A D. 23.14 A

877. REE Board Exam October 1998

The loads of a wye connected transformer are: Ia = 10 cis (-30°); Ib = 12 cis 215°; Ic = 15 cis 82°. What is the neutral current?

A. 1.04 cis 72.8° C. 0.92 cis 62.5°

B. 2.21 cis (-30°) D. 3.11 cis 72.6°

878. EE Board Exam April 1980

A factory is supplied by a three-phase, 3-wire system with the following characteristics:

𝑉_𝑎𝑏= 230∠0° 𝑉 𝐼_𝑎= 110∠ − 36.87° 𝐴 𝑉_𝑏𝑐= 230∠240° 𝑉 𝐼_𝑐= 125∠53.13° 𝐴 Determine the power consumed by the load.

A. 42.75 kW C. 40.23 kW

B. 48.78 kW D. 45.12 kW

879. EE Board Exam April 1981

The following voltages and line currents were measured to a 3-phase, 3-wire feeder serving a commercial building:

𝑉_𝑎𝑏= 2,400∠0° 𝑉 𝐼_𝑎= 85∠330° 𝐴 𝑉_𝑏𝑐= 2,400∠240° 𝑉 𝐼_𝑐= 100∠80° 𝐴 Solve for the real power in kW drawn by the commercial building.

A. 402.2 C. 419.5

B. 404.5 D. 421.5

880. EE Board Exam October 1981

A 3-phase, 3-wire load draws the following line currents: 𝐼_𝑎= 60∠330° 𝐴, 𝐼𝑏= 78.4∠214° 𝐴 and 𝐼𝑐= 75∠80° 𝐴. If the voltages impressed on the load are balanced 3-phase, having a magnitude of 4140 volts line to line, solve for the total power in kW.

A. 556.16 C. 536.54

B. 506.85 D. 520.18

881. EE Board Exam October 1983

Given the following load impedances in delta and the impressed voltages as follows:

𝑉𝑎𝑏= 220∠0° 𝑉 𝑍𝑎𝑏= 8 + 𝑗6 Ω 𝑉_𝑏𝑐= 220∠240° 𝑉 𝑍_𝑏𝑐= 8.66 − 𝑗5 Ω 𝑉_𝑐𝑎= 220∠120° 𝑉 𝑍_𝑐𝑎= 10 + 𝑗0 Ω What will be the reading of the two wattmeters connected to measure total power. Use line a as the common potential point.

A. 3.869 kW, 9.031 kW

C. 3.125 kW, 6.778 kW

B. 2.546 kW, 8.357 kW

D. 4.055 kW, 9.848 kW

882. EE Board Exam October 1984

The 3-phase power supply to a factory has the following measurements:

𝑉_𝑎𝑏= 240∠0° 𝑉 𝐼_𝑎= 120∠330° 𝐴 𝑉_𝑏𝑐= 240∠240° 𝑉 𝐼_𝑏= 157∠214° 𝐴 𝑉𝑐𝑎= 240∠120° 𝑉 𝐼𝑐= 150∠80° 𝐴 Solve for the total power drawn.

A. 60.2 kW C. 58.8 kW

B. 56.5 kW D. 62.4 kW

883. EE Board Exam April 1985

A three-phase 230-V circuit serves two single-phase loads, A and B. Load A is an induction motor rated 8 hp, 230 V, 0.70 pf, 0.90 efficiency, which is connected across lines a and b. Load B draws 5 kW at 1.0 pf and is connected across lines b and c.

Assume a sequence of a-b-c, solve for the total power factor of the load.

A. 0.907 C. 0.864

B. 0.704 D. 0.886

884. EE Board Exam October 1987

A wound rotor motor, 7.5 HP, 230 volts, 3-phase takes a line current of 18.4 ampere, when operating at rated output at an efficiency of 88%. Calculate the indication on the wattmeter when this is inserted to measure power by the T-method.

A. 3.179 kW C. 3.361 kW

B. 4.401 kW D. 4.042 kW

885. EE Board Exam October 1994

A wattmeter with its current coil in line 2 and potential coil across lines 2 and 3 is connected to a balanced 3-phase system. The only load supplied is a single phase one connected to lines 1 and 2. This load is known to be inductive. If the wattmeter reads zero watts, determine the power factor of the single-phase load.

A. 0.707 C. 0.800

B. 0.866 D. 0.900

886. EE Board Exam April 1984

A balanced 3-phase load draws 75 amperes line current at 230 volts line to line and 0.848 lagging power factor. If the two-wattmeter is used, solve for the readings of the two wattmeters.

A. 15.32 kW, 10.02 kW

C. 16.42 kW, 8.92 kW

B. 17.86 kW, 7.48 kW

D. 17.24 kW, 8.10 kW

887. EE Board Exam October 1980

Three equal impedances, each having a resistance of 8 ohms and an inductive reactance of 7 ohms are connected in delta to lines a, b and c of a 240 V, 3-phase, 3-wire line, phase sequence a-b-c. What is the reading of a single-phase wattmeter connected with its current coil in line a and the potential coil across lines b and c?

A. 6,180 W C. 6,561 W

B. 6,324 W D. 6,004 W

888. EE Board Exam October 1992

A 3-phase feeder carries two lagging balanced loads. The power observed by each is measured by two wattmeter method, giving the following

readings:

First Load: W1 = 160 kW W2 = 96 kW Second Load: W1 = 90 kW W2 = 48 kW What is the combined kVA load on the feeder?

A. 434.68 C. 504.35

B. 462.35 D. 420.12

889. EE Board Exam October 1992

National Power Corporation used two wattmeters to measure 3-phase power of a balanced Y-connected lagging power factor motor loads. Each wattmeter indicates 15.4 kW. The voltage coils are connected across lines 2 and 3, and across lines 1 and 3, respectively. The line to line voltages are 230 volts with V12 leading V23 and the line currents are each 120 A. Calculate the total power supplied.

A. 37.44 kW C. 39.67 kW

B. 30.72 kW D. 34.88 kW

890. EE Board Exam April 1980

A factory is supplied by a three-phase, 3-wire system with the following characteristics:

𝑉_𝑎𝑏= 230∠0° 𝑉 𝐼_𝑎= 110∠ − 36.87° 𝐴

𝑉𝑏𝑐= 230∠240° 𝑉 𝐼𝑐= 125∠53.13° 𝐴 A. 0.934 lagging C. 0.892 lagging B. 0.908 lagging D. 0.866 lagging

891. EE Board Exam October 1981

A 3-phase, 3-wire load draws the following line currents:

𝐼_𝑎= 60∠330° 𝐴, 𝐼_𝑏= 78.4∠214° 𝐴 and 𝐼_𝑐=

75∠80° 𝐴. If the voltages impressed on the load are balanced 3-phase, having a magnitude of 4140 volts line to line, solve for the power factor of the load.

A. 0.976 C. 0.982

B. 0.999 D. 0.906

892. EE Board Exam April 1995

Three unequal single-phase induction motor loads are connected across the lines and neutral conductor of a balanced, 3-phase, 350 volts circuit.

The line to neutral voltages is each 202 volts. The first load takes 20 kW at 0.82 power factor, the second takes 28 kW at 0.75 power factor, and the third takes 36 kW at 0.80 power factor. What is the current in the neutral conductor?

A. 105.5 amps C. 125.4 amps B. 86.6 amps D. none of these

893. For an unbalanced load which connection is suitable A. 3 wire open delta

B. 4 wire star connection C. 3 wire delta connection D. 3 wire star connection

894. What is the minimum number of wattmeters required for measuring power of a three phase balanced load?

A. two C. one

B. four D. three

895. The power is to be measured for a balanced delta connected load whose terminals cannot be opened.

How many wattmeters do you need?

A. four C. two

B. one D. three

896. What is the minimum number of wattmeters required to measure unbalanced power for a three-phase system?

A. two C. three

B. four D. one

897. In two wattmeter method, the readings of the wattmeter will be identical when

A. load in one of the phases is zero B. power factor is unity

C. power factor is 0.5 D. neutral is earthed

898. Two wattmeters can be used to measure 3-phase for a

A. balanced and unbalanced load B. unbalanced load only

C. balanced load only D. unity power factor only

899. In 2 wattmeter method, the reading of one of the wattmeter will be zero when

A. power factor is unity B. power factor is 0.5

C. load in one of the phases is zero D. a neutral wire is not provided 900. For a 3 phase unbalanced load

A. the power factor of each phase will be in proportional to the load

B. the power factor of each phase will be the same C. the power factor of at least one of the phase

must be leading

D. the power factor of each phase may be different 901. A wattmeter is installed in a balanced 3-phase

system. The wattmeter will measure

Find the total power.

A. 1,553 W C. 1,883 W

B. 2,589 W D. 2,104 W

903. A 100 V, balanced three-phase source has two single-phase loads. The first load has an impedance of (5 + jX) ohms and connected across lines A and B. The second load is connected across B and C and has an impedance of (R – j2) ohms. Determine the values of R and X, if the current in line B is 25.93∠ − 102.37° A and the ratio of X to R is 1.5.

A. 2 Ω, 3 Ω C. 4 Ω, 6 Ω

B. 3 Ω, 4.5 Ω D. 5 Ω, 7.5 Ω 904. Three – single phase loads are connected between

lines of a 280 V, balanced three phase source. The currents measured in lines B and C are: 𝐼_𝑏= 85.22∠ − 72.5° A, 𝐼_𝑐= 60.71∠170° A. What is the negative sequence component of the currents?

A. 39.21∠105.2° A C. 37.64∠ − 104.5° A

B. 𝟑𝟑. 𝟗𝟑∠𝟏𝟔𝟏. 𝟔° A D. 41.82∠72.8° A 905. Two of the three unbalanced currents are given for

an unbalanced, three-phase system. Find the positive sequence of phase B current of the neutral current is 32∠39.2° A.

A. 32∠39.2° A C. 106.23∠61.5°

908. As the poles of a network shift away from the axis, the response

A. remain constant C. becomes more oscillating

B. becomes less oscillating D. none of these 909. The response of a network is decided by the location

A. Its zeros C. both zeros &

poles

B. Its poles D. neither zeros nor poles

910. The pole-zero configuration of a network function is shown. The magnitude of the transfer function will A. Decrease with frequency

B. increase with frequency

C. Initially increase and then decreases with frequency

D. Be independent of frequency

911. Given I1 = 2V1 + V2 and I2 = V1 + V2 the Z-parameters are given by

A. 2, 1, 1, 1 C. 1, 1, 1, 2 B. 1, -1, -1, 2 D. 2, -1, 1, 1 912. The short – circuit admittance matrix of a two-port

network is as shown The two-port network is A. Non reciprocal & passive B. Non-reciprocal & active C. Reciprocal & passive D. reciprocal & active

913. If the two port network is reciprocal, then A. Z12 / Y12 = Z122 – Z11 Z12 B. Z12 = 1/Y22

C. h12 = -h21 D. AD-BC = 0

914. Two networks are cascaded through an ideal buffer.

If tr1 & tr2 are the rise times of two networks, then the over-all rise time of the two networks together will be

A. √ tr1 tr2 C. tr1 + tr2

B. √ (tr12 +tr22) D. (tr1 + tr2 )/ 2 915. Which one of the following combinations of open

circuit voltage and Thevenin’s equivalent resistance represents the Thevenin’s equivalent of the circuit shown in the given figure?

A. 1 V, 10 Ω C. 1 mV, 1 kΩ B. 1 V, 1 kΩ D. 1 mV, 10 Ω D. Symmetrical Components

916. REE Board Exam October 1998

If the loads of a wye-connected transformer are:

Ia = 10 cis (-30°) Ib = 12 cis 215°

Ic = 15 cis 82°

What is the phase b positive sequence component?

A. 13.4 cis (-32.2°) C. 12.27 cis 208.4°

B. 10.2 cis 240° D. 12.27 cis (-31.6°)

917. REE Board Exam March 1998, September 2001 The three unbalanced currents are:

Ia = 10 cis (-30°) Ib = 0

Ic = 10 cis 150°

Find the negative sequence current of phase a.

A. 8.66 cis 30° C. -5.77 B. 5.77 cis (-60°) D. 5.77 918. EE Board Exam October 1984

Given the following currents:

Ia = 60 + j0 A Ib = -36 – j48 A Ic = -48 + j36 A

Solve for the negative sequence component Ia.

A. 8.641 – j1.543 C. 9.751 – j1.464 B. 9.436 + j1.346 D. 8.354 + j1.034 919. REE Board Exam October 1998

The three unbalanced currents are:

Ia = 10 cis (-30°) Ib = 0

Ic = 10 cis 150°

Find the zero sequence current.

A. 3.33 cis 30° C. 5.77

B. 0 D. 3.33

920. EE Board Exam October 1984 Given the following currents:

Ia = 60 + j0 A Ib = -36 – j48 A Ic = -48 + j36 A

Solve for the zero component of Ia.

A. 10 + j4 C. -8 – j4

B. 8 – j6 D. 12 – j6

921. REE Board Exam October 1997

The sequence currents of phase a current are as follows:

Zero sequence current = 14.13∠17.34°

Positive sequence current = 2.98∠10.06 Negative sequence current = 708.26∠ − 31°

Determine the phase a current.

A. 𝟕𝟐𝟎∠ − 𝟑𝟎° C. 710∠88°

B. 730∠ − 15.2° D. 695∠15.2°

922. REE Board Exam October 1998

The sequence components of phase a current are:

Zero sequence current = 0.47 + j1.49 Positive sequence current = 18.4 cis (-31.6°) Negative sequence current = 3.23 cis 168.2°

Determine the phase b current.

A. 18 cis 215° C. 19 cis 220°

B. 15 cis 240° D. 20 cis 225°

923. REE Board Exam March 1998

The sequence components of phase a current are:

Zero sequence current = 0.47 + j1.49 Positive sequence current = 18.4 cis (-31.6°) Negative sequence current = 3.23 cis 168.2°

Determine the phase c current.

A. 17.5 cis 91° C. 22.5 cis 82°

B. 18 cis 215° D. 15 cis 100°

924. EE Board Exam April 1992

Determine the symmetrical components of the line current in line ‘a’ if one of the in-phase impedance of its delta connected load connected across lines ‘ca’

is removed. The delta load with impedance of 10∠0°

ohms per phase is supplied from a 220 volts, 60 cycle, 3-phase source. Assume a phase sequence of a-b-c.

A. Ia1 = 11 A, Ia2 = 11 A, Ia0 = 0 A B. Ia1 = 7.33 A, Ia2 = 7.33 A, Ia0 = 7.33 A C. Ia1 = 22 A, Ia2 = 22 A, Ia0 = 22 A D. Ia1 = 25.4 A, Ia2 = 12.7 A, Ia0 = 0 A 925. EE Board Exam April 1991

A star-connected balanced load takes 75 A from a balanced 3-phase, 4-wire supply. If the two supply lines of the fuses are removed determine the symmetrical components of the lines after the fuses are removed.

A. I1 = 25 A, I2 = 25 A, I3 = 25 A B. I1 = 25 A, I2 = 50 A, I3 = 0 A C. I1 = 75 A, I2 = 75 A, I3 = 75 A D. I1 = 75 A, I2 = 0 A, I3 = 0 A 926. REE Board Exam September 2000

If the loads of a wye-connected transformer are:

Ia = 10 cis (-30°)

A. solving unbalanced polyphase circuits

B. analyzing the performance of 3-phase electrical machinery

C. calculating currents resulting from unbalanced faults

D. all of the above

928. An unbalanced system of 3-phase voltages having RYB sequence actually consists of

A. a positive-sequence component B. a negative-sequence component C. a zero-sequence component D. all of the above

929. The zero-sequence component of the unbalanced 3-phase system of vectors VA, VB and VC is of their load currents will consists of

A. positive-sequence components B. negative-sequence components C. zero-sequence components D. only A and B

931. In symmetrical components, what is the vector sum of 1 + a + a²?

A. 1 C. -1

B. 0 D. infinity

932. REE Board Exam October 1997

The sequence currents of a three phase current are:

Zero sequence current = 14.13 cis 17.34°

Positive sequence current = 708.26 cis (-31°) Negative sequence current = 2.98 cis 10.06°

Determine the phase a current.

A. 720 cis (-30°) C. 710 cis 88°

B. 730 cis (-15.2°) D. 695 cis 15.2°

933. REE Board Exam April 2001 The three unbalanced currents are:

Ia = 10 cis (-30°) Ib = 0

Ic = 10 cis 150°

Find the phase B positive sequence current.

A. 8.66 A C. 5.77 A

B. 5.77 cis 240° A D. 8.66 cis 120° A 934. REE Board Exam September 2002

The phase currents of a three-phase system are:

Ia = 100 cis 0°

Ib = 80 cis 240°

Ic = 91.8 cis 130.9°

Find the zero sequence current.

A. 90.23 cis 3.68° A B. 270.7 cis 3.68° A C. 34.68 cis (-30.24°) A D. none of the above

935. Given three unbalanced three-phase voltages:

Va = 150 + j0 V cm on sides. Find the force in Newton?

The correct answer is: 15.5 x 10^(-10)

2. In a cable capacitor, voltage gradient is maximum at the surface of the

The correct answer is: conductor

3. Which of the following voltmeter has the most inaccurate reading?

The correct answer is: Low sensitive, 900 kohms

4. Electrostatic instruments are exclusively used as

The correct answer is: voltmeters

5. The input impedance of your test equipment should exceed the impedance of the circuit under test by what ratio?

The correct answer is: 10 to 1

6. Part of the which produces bright spot through collection of electrons

The correct answer is: Aquadag coating

7. An isolated sphere 10 cm in radius is charged in air to 500 Volts. How much charge is required?

The correct answer is: 5.563 nC

8. A 3-phase, 3-wire load draws the following line currents: Ia = 60 cis (330 deg) A, Ib = 78.4 cis (214 deg) A, Ic = 75 cis (80 deg) A. If the voltages impressed on the load are balanced 3-phase, having a magnitude of 4140 volts line to line, solve for the power factor of the load.

The correct answer is: 0.999

9. When will an ac voltage, v = 120 sin 120*pi*t reach its first peak?

The correct answer is: 4.167 ms

10. A balanced delta load with impedances of 15 - j9 ohms is connected to a three phase source by three wires each of which has 2 + j5 ohms impedance.

The load phase voltage is 120 V. Find the line voltages at the source.

The correct answer is: 259 V

11. A three-phase motor takes 10 kVA at 0.67 pf lagging from a source of 230 volts. It is in parallel with a balanced delta load having 16 ohms resistance and 12 ohms capacitive reactance in series in each phase. Determine the total power factor.

The correct answer is: 0.967 lagging

12. To roughly check his kWh meter an electrician hook-up a wattmeter, he switched on the load, he counted 5 rev in 18 sec, the meter constant is 3.2.

The wattmeter reads 368 Watts. What is the error of the meter?

The correct answer is: 8% slower

13. Two single-phase transformers are connected in V (open delta) and serving a delta connected

impedance load. Each impedance is equal to 16 cis (36.87 deg) ohms. If the transformer voltages impressed on the impedances are Vab = 240 cis (0 deg) V, Vbc = 240 cis (240 deg) V, Vca = 240 cis (120 deg) V. Solve for the total kVA drawn by the load.

The correct answer is: 10.8

14. Three resistance are connected in series has a total resistance of 120 ohms. If Rx = 20 ohms and Ry is twice greater than Rz. Find Ry and Rz.

The correct answer is: Ry = 75 ohms& Rz = 25 ohms

15. The resistance of a coil of wire is 1 k-ohms at 20 degrees C. If the coil is immersed into oil, the resistance falls to 880 ohms. If the wire has a temperature coefficient of 0.006 at 20 degrees C, how much is the temperature of the liquid?

The correct answer is: 0 degrees C 16. Which of the following ammeters is the most

sensitive?

The correct answer is: 0 - 1 micro-ammeter 17. A plate capacitor is made up of 501 sheet of

aluminum 25 cm x 30 cm with a dielectric of paraffin paper 0.0015 cm thick. What is the capacitance in microfarads of the condenser (K for paraffin paper is 2.3)?

The correct answer is: 25

18. A three-phase balanced load is connected across 220 V, three-phase, ACB source. A wattmeter with its current coil in line A and voltage coil across lines A and B reads 800 W. The potential coil is then connected across lines A and C with the current coil in the same line. What is the power factor of the load if the meter reads -800 W?

The correct answer is: 0.87 lagging

19. Two electrons in a vacuum experience a force of 2 x 10^-15 N. How far apart are the electrons?

The correct answer is: 3.39 x 10^-7 m

20. Calculate the capacitance between two parallel plates each of which is 100 cm^2 and 2 mm apart in air.

The correct answer is: 0.443 uuF

21. The hot resistance of a 100 watt, 250 V incandescent lamp would be

The correct answer is: 625 ohms

22. A dc motor draws 200 Amp is located at 100 ft from the supply line. The wire has a resistance of 0.01 ohm per 100 ft. What is the voltage at the motor terminals if the supply voltage is 120 Volts?

The correct answer is: 116 volts

23. Find the Thevenin impedance equivalent across R2 of a linear close circuit having 10 volt supply in series with two resistors (R1= 50 ohms and R2 = 200 ohms).

The correct answer is: 40 ohms

24. A 10 ohms R1, 30 ohms XL and 60 ohms XL are connected in parallel across a 220 volts 60 Hz source. What is the current in R1 in ampere?

The correct answer is: 22

25. A load of 10 ohms was connected to a 12-volt battery. The current drawn was 1.18 amperes. What is the internal resistance of the battery?

The correct answer is: 0.20 ohm 26. What is a dielectric?

The correct answer is: an insulator between two metal plates in a capacitor

27. The time constant of an R-C circuit is defined as the time during which capacitor charging current becomes ____ percent of its ____ value.

The correct answer is: 37, initial 28. The unit of electric intensity is

The correct answer is: both B and C

29. For medium, electric flux density is related to electric density E by the equation

The correct answer is: D = (permittivity of free space*relative permittivity)E

30. Which part of the oscilloscope emits electrons?

The correct answer is: Heater cathode

31. A coil has a resistance of 100 ohms at 90 degrees C. At 100 degrees C, its resistance is 101 ohms.

What is the temperature coefficient of the wire at 90 degrees C?

The correct answer is: 0.001

32. When the grid bias of the oscilloscope is varied, what is being controlled?

The correct answer is: Amplitude

33. An electric water heater has a rating of 1 kW, 230

In document ACDC MCQ (Page 112-128)