A. Choose the Correct Answer:
1. In two wattmeters method of power measurement, one of the wattmeters will show negative reading when the load power factor angle is strictly
(a) than 30°
(b) less than 60°
(c) greater than 30°
(d) greater than 60°
2. Three identical impedances are connected in delta to a 3-phase supply of 400 V. The line-current is 34.65 A and the total power taken is 14.4 kW. The resistance of the load in each phase (in ohm) is
(a) voltage and line current
(b) phase voltage and phase current
(c) line voltage and phase current
(d) phase voltage and line current.
4. Which of the following statements associated with 3-phase delta connected circuits is true?
(a) voltage is equal to phase voltage
(b) Line current is equal to phase current
(c) Line voltage is √3 time of phase voltage
(d) Line currents are 60° apart.
5. In a 3-phase supply, floating neutral is undesirable because it may result in …. across the load.
(a) line voltages
(b) high voltage
(c) low voltage
6. Phase reversal in a 4-wire unbalanced load supplied from a balanced 3-phase supply causes change in
(a) power consumed
(b) magnitude of phase currents
(c) only the magnitude of the neutral current
(d) magnitude as well as phase angle of the neutral current.
7. A 3-phase star connected symmetrical load consumes P watts of power from a balanced supply. If the same load is connected in delta to the same supply, the power consumption will be
(d) not determined from the given data.
8. Three unequal impedances are connected in star to a 3-phase system. The sum of three line currents will be
(a) equal to the each line current
(c) none of these
9. Three equal impedances are first connected in delta across a 3 – ɸ balanced supply. If the same impedances are connected in star across the same supply
(a) phase currents will be one-third
(b) line currents will be one-third
(c) power consumed will be one-third
(d) none of the above.
10. Which of the following is a four wire system?
(c) Both delta and star
(d) Neither delta nor star.
11. In a 3-phase balanced star-connected load, neutral current is equal to
12. Which of the following equations is valid for a 3-phase 4-wire balanced star-connected load?
(a) IR + IY + IR = IN = O
(b) IN + IY – IB = IN
(c) IR – IY + IB = IN
(d) VB + VR + VY ⁄ Z =N
13. Three unequal impedances are connected in delta to a 3-phase, 3-wire system.
a) The voltages across the three phases will be different
b) Both of the phase currents and line currents. will be unbalanced
c) Phase currents will be unbalanced but the line currents will be balanced
d) None of the above.
14. The relationship between the line and phase voltages of a delta-connected circuit is given by
a) VL = VP
b) VL =√3 VP
15. In case of a delta connected load, if one resistor is removed, the power will become
d) none of the above.
16. Readings of 1154 and 577 watts are obtained when two wattmeters method was used on a balanced load. The delta connected load impedance for a system of 100 V will be
a) 15 ± 30°
b) 15 +30°
c) 15 -30°
d) 15 +90°.
17. Which of the following statements is true about two-wattmeters method for power measurement in 3-phase circuit?
a) Power can be measured using two wattmeters method only for star-connected 3-phase circuits
b) When two wattmeters show identical readings, the power factor is 0.5
c) When power factor is unity, one of the wattmeter reads zero
d) When the readings of the two wattmeters are equal but of opposite sign, the power factor is zero.
18. The minimum number of wattmeters to measure power in a 3-phase unbalanced star-connected load is
19. While measuring power in a three phase load by two-wattmeters method, the readings of two wattmeters will be equal and opposite when
a) pf unity
b) load is balanced
c) phase angle is between 60° and 90°
d) the load is purely inductive.
20. In the measurement of 3-phase power by two-wattmeters method, if the two wattmeter will be equal the power factor of the circuit is
a) 0.8 lagging
b) 0.8 leading
21. Which of the following are the necessary conditions for an entire 3-Φ system to be balanced?
- The line voltages are equal in magnitude
- The phase differences between successive line voltages are equal
- The impedances in each of the phases are identical.
Select the correct answer using the codes given below:
a) 1, 2 and 3
b) 1 and 3
c) 1 and 2
d) 2 and 3.
22. The power delivered to a 3-phase load can be measured by the use of 2 wattmeters only when the
a) load is balanced
b) load is unbalanced
c) 3-phase load is connected to the source through 3-wires
d) 3-phase load is connected to the source through 4-wires
23. The ratio of the readings of two wattmeters connected to measure power in a balanced 3-phase load is 5:3 and the load is inductive. The power factor of the load is’
a) 0.917 lead
b) 0.917 lag
d) 0.6 lag.
24. In two phase system phase voltages differ by
25. W1 and W2 are the readings of two wattmeters used to measure power of a 3-phase balanced load. The reactive power drawn by the load is
c) √3(WI + W2)
d) (W1 – W2).
26. In a two wattmeters method of measuring power in a 3-phase system one of the wattmeters reads negative implying
a) wattmeter connection is faulty
b) load is unbalanced
c) power flow is in the reverse direction
d) power factor is less than 0.5.
27. URN, UYN and UBN are the instantaneous line to neutral voltages and iR. iY and iB are instantaneous line currents in a balanced three phase circuit, the computation URN(iY – iB)-(uYN – uBN) iR will yield a quantity proportional to the
a) active power
b) power factor
c) reactive power
d) complex power.
28. If the of a R-phase of a 3-phase star connected alternator becomes reversely connected by mistake, it will affect
a) ERY and EYB
b) EYB and EBR
c) ERY and EBR
d) ERY, EYB and EBR.
29. When phase sequence at the 3-phase load is reversed
a) phase currents are changed in magnitude
b) phase currents change in phase angle but not in magnitude
c) total power consumed is changed
d) phase powers are changed.
30. Lamp load is being supplied from a 3-phase, 4-wire, 230/400 V ac supply system. If a 3-phase motor is now switched on across the same supply, then neutral current will
c) remain unchanged
31. Lamps of 40W, 100 W and 200 W ratings are connected in different phases of a 3-phase, 4-wire supply. If the neutral wire breaks, then the lamp likely to fuse first will be
a) 200 W
b) 100 W
d) all lamps
32. The instantaneous values of currents in phases R and Y of a 3-phase system are 25 A each. For a phase sequence of BRY, the instantaneous value of current in phase B is
a) 25 A
b) 50 A
c) 12.5 A
d) 43.3 A
33. The power measurement in balanced 3-ɸ circuit can be done by
a) one wattmeter method only
b) two wattmeters method only
c) three wattmeters method only
d) anyone of the above.
[B.T.E. M.P., 1998]
B. Say ‘Yes’ or ‘No’:
1.Polyphase system means the system which consists of numerous windings or circuits.
3. A load circuit is said to be balanced when the loads (impedances) connected in various phases are same in magnitude as well as in phase.
4.Rotating magnetic field can be set up by passing polyphase current through stationary coils.
5.Polyphase system is less capable and reliable than single phase system.
6.Parallel operation of polyphase alternators is simple as compared to that of single phase alternators because of pulsating reaction in single-phase alternators.
7.Double script notation is usually preferred in A.C. electrical circuits.
8.Phase sequence is the order or sequence in which the currents or voltages in different phases attain their maximum values one after the other.
9. If the phase sequence is reversed by interchanging any two terminals of the 3-phase supply, the motor would rotate in the same direction.
10. Star-connected system is obtained by joining together similar ends either the start or the finish, the other ends are joined to the line wires.
11. The common point at which similar (start or finish) ends, are connected is called the neutral or star point.
12. In a balanced star-connected system line voltage = phase voltage.
13. In a balanced delta-connected system line current is equal to phase current.
14. Delta-connected system is best suited for rotary converters.
15. Three wattmeters method is useful for measuring power in 3-phase, 3-wire load circuits.
16. Power in a 3-phase circuit = √3ELIL cos ɸ
17. In the case of balanced three-phase circuit the reactive power can be determined by use of one wattmeter.
18. A load having a low lagging power factor gives a poor voltage regulation.
19. Static capacitors are installed to improve the power factor of a group of AC. motors.
20. Static capacitors are practically loss free.