Multiple choice question for engineering
1. A 3-phase induction motor is fed at the stator at a frequency f. If the rotor speed is Nr and synchronous speed is Ns,
then the absolute speed of the resultant flux in space is
b) (Ns+Nr) if the flux rotates in the same same direction as the rotor
c) (Ns-Nr) if the flux rotates in the same same direction as the rotor
2. I. Leakage reactance is dependent on supply current.
II. Magnetizing reactance is dependent on the air gap flux.
Which of the following is correct?
d) None of the mentioned
3. I. Rotor resistance is dependent on the speed.
II. Core loss does not depend on the supply frequency and voltage.
d) None of the mentioned
4. The stator of a 3-phase, six pole induction motor’s stator is connected to 50 Hz supply while its rotor is fed from a 20 Hz supply.
The possible speeds at which the machine will run will be
(i) 1600rpm (ii) 600rpm (iii) 1400rpm (iv) 400 rpm
5. For a 3-phase 4-pole induction motor has open circuited slip rings. The frequency across the slip rings is 75 Hz. Then we can conclude that the motor is run at a speed of
(i) 750rpm (ii) 1500rpm (iii) 2250rpm (iv) 3750 rpm
6. A 3-phase slip ring IM is has its copper bars replaced by the aluminium bars in the machine. With the above change,
a) starting torque increases, maximum torque remains same, smT increase
b) starting torque increases, maximum torque remains same, smT remains same
c) starting torque increases, maximum torque remains increases, smT increase
d) starting torque decreases, maximum torque remains same, smT decrease
7. A 3-phase slip ring IM is has its copper bars changed by the aluminium bars in the machine. With the above change,
a) starting torque remains same
b) starting torque increase
c) starting torque decrease
d) maximum torque increase
8. A center ammeter connected to the rotor end circuit of a 6-pole,50 Hz, induction motor makes 45 complete oscillations in a minute.
Then the rotor speed and the speed of stator field w.r.t. rotor is
a) 985, 15
b) 970, 1000
c) 985, 985
d) 985, 970
9. Choose from the below which can be obtained by the equivalent circuit of an electrical machine?
a) Complete performance characteristics of the machine
b) Temperature coefficients of the machine components
c) Type of protection to be used in the machine
d) Design parameters of the windings
10. If one of the supply fuses blows off and gets open circuited, then for the connected induction machine continues to run with increased slip.
11. Due to the line to ground fault, phase ‘a’ in the induction motor phase supply gets cut, then machine
a) continues to run with excessive supply current
b) continues to run with reduced slip
d) all of the mentioned
12. For 3-phase induction motor, as load increases from no load towards the full load,
a) power factor improves
b) power factor remains same
c) power factor varies linearly
d) power factor increases till 40% load and then decreases
13.For 3-phase induction motor, as load increases from no load towards the full load, torque _______ slip.
a) increases in proportion to
b) decreases in proportion to
c) remains constant to
d) increases hyperbolically to
14. As a 3-phase induction motor, as load increases from no load towards the full load,
A. power factor improves
B. torque increases
C. air gap flux falls steeply
a) A, B
b) B, C
c) A, B, C
15. Torque-slip characteristic of an induction motor is linear in the smaller slip values, because
a) effective rotor resistance is very large compared to reactance
b) rotor resistance is equal to stator resistance
c) rotor resistance is equal to rotor reactance
d) rotor reactance almost equal to stator reactance
16. If a 3-phase induction motor is fed from the rotor short circuiting the stator terminals, frequency of the current flowing in the short-circuited stator is
a) slip frequency
b) supply frequency
d) frequency corresponding to the rotor speed
17. A constant torque variable power drive of induction machine has
a) constant air gap flux.
b) constant slip
c) variable air gap flux, constant slip
d) constant slip, constant air gap flux
18. If an emf at slip frequency is injected at an angle α with existing rotor induced emf, then
a) power factor only improves
b) speed changes
c) power factor improves and speed changes
d) power factor worsens and speed changes
1. A 315V, 3-phase 400 MVA alternator is running at no load at constant excitations. The most appropriate representation in space domain is
2. A 315V, 3-phase 400 MVA alternator is running at lagging power factor at constant excitations. The most appropriate representation in space domain is
3. A 315V, 3-phase 400 MVA alternator is running at leading power factor load at constant excitations. The most appropriate representation in space domain is
4. A 315V, 3-phase 400 MVA alternator is running at unity power factor at constant excitations. The most appropriate representation in space domain is
5. A 315V, 3-phase 400 MVA alternator is running at zero power factor lagging at constant excitations. The most appropriate representation in space domain is
6. A 315V, 3-phase 400 MVA alternator is running at zero power factor leading at constant excitations. The most appropriate representaion in space domian is
d) none of the mentioned
7. With the alternator running at rated terminal voltage Vt, at rated current Ia = 1 per unit. What happens if load current is reduced to zero, if it was operating at lagging power factor?
8. With the alternator running at rated terminal voltage Vt, at rated current Ia = 1 per unit. What happens if load current is reduced to zero, if it was operating at leading power factor?
9. If the machine was operating at upf, then the new excitation voltage will be
a) greater than 1 pu
b) less than 1 pu
c) 1 pu
10. The variation of Vt vs Ia is for the alternator is given below.
Choose the appropriate
a) A-zpf lead; B-0.8 lead; C-0.8 lag; D-zpf lag
b) A-zpf lag; B-0.8 lead; C-0.8 lag; D-zpf lead
c) A-zpf lead; B-0.8 lag; C-0.8 lead; D-zpf lag
d) A-zpf lead; B-0.8 lag; C-0.8 lead; D-zpf lag
11. Mark the plot for terminal voltage vs armature current for the alternator.
12. Alternator compounding characteristic is obtained by
a) keeping Vt constant
b) keeping Ef constant
c) keeping Ia constant
d) varying Vt
13. At zpf lag, the excitation should be increased in order to maintain the armature terminal voltage constant.
14. The rating of the alternator is decided by
c) armature current
15. The core losses in synchronous machine is _______ dependent.
1. In the design of a single mode step index optical fiber close to upper cut off, the single mode operation is NOT preserved if
a) Radius as well as operating wavelength are halved
b) Radius as well as operating wavelength are doubled
c) Radius is halved and operating wavelength is doubled
d) Radius is doubled and operating wavelength is halved
2. Which parameters cannot be computed from the Smith chart?
c) Reflection coefficient and VSWR
d) Intrinsic impedance
3. Consider a transmission line of characteristic impedance 50 ohm. Let it be terminated at one end by +j50 ohm. The VSWR produced by it in the transmission line will be
4. A transmission line whose characteristic impedance is a pure resistance
a) Must be a lossless line
b) Must be a distortionless line
c) May not be a lossless line
d) May be a lossless and may not be a distortionless line
5. A two wire transmission line of characteristic impedance Z is connected to a load impedance ZL(ZL and Zo are not equal). Impedance matching cannot be achieved with
a) A quarter wavelength transformer
b) A half wavelength transformer
c) An open circuited parallel stub
d) A short circuited parallel stub
6. A transmission line of pure resistance characteristic impedance is terminated with an unknown load. The measured value of VSWR on the line is 2 and a voltage minimum point is found to be at the load. The load impedance is then
b) Purely capacitive
c) Purely resistive
d) Purely inductive
7. Which one of the following field patterns represents a TEM wave travelling in the positive x direction?
a) E = +8y, H = -4z
b) E = -2y, H = -3z
c) E = +2z, H = +2y
d) E = -3y, H = +4z
8. The modes in a rectangular waveguide are denoted by TEmn / TMmn, where m and n are the Eigen values along the larger and smaller dimensions of the waveguide respectively. Which one of the following statements is true?
a) The TM10 mode does not exist
b) The TE10 mode does not exist
c) The TE10 and TM10 both exist and have same cut off frequencies
d) When m and n are increases, the cut off frequency decreases
9. In a microwave test bench, why is the microwave signal amplitude modulated at 1 kHz?
a) To increase the sensitivity of measurement
b) To transmit the signal to a far off place
c) To study amplitude modulation
d) Because crystal detector fails at microwave frequencies
10. Consider a lossless antenna with a directive gain of +6 decibel. If 1 milliwatt of power is fed to it, the total power radiated by the antenna will be (in milliwatt)
1. The Smith chart is a polar chart which plots
a) R vs Z
b) R vs Znorm
c) T vs Z
d) T vs Znorm
2. The Smith chart is graphical technique used in the scenario of transmission lines. State true/false.
3. The Smith chart consists of the
a) Constant R and variable X circles
b) Variable R and constant X circles
c) Constant R and constant X circles
d) Variable R and variable X circles
4. The circles in the Smith chart pass through which point?
5. Moving towards the clockwise direction in the Smith chart implies moving
a) Towards generator
b) Towards load
c) Towards stub
d) Towards waveguide
6. The centre of the point having a normalised resistance of 1.2 ohm and reactance of 1.5 ohm is
7. The normalised load impedance of the transmission line 50 ohm with a load of 30 ohm is
8. The radius of the point having a normalised resistance of 1 ohm is
9. The best stub selection for the transmission line will be
a) Series open
b) Series short
c) Shunt open
d) Shunt short
10. The centre and radius of a line with normalised load impedance of 1 + 0.5j is
a) (1,2) and 2
b) (2,1) and 2
c) (1,2) and 1
d) (2,1) and 1
1. The source voltage of a 75ohm transmission line is given by 150V. Find the load current.
2. The guided terminations are used to
a) Increase reflection
b) Increase transmission
c) Eliminate reflection loss
d) Eliminate attenuation
3. Which type of wave does the resonant cavity produce?
a) Standing waves
b) Guided waves
c) Transmitted waves
d) Attenuated waves
4. Which of the following parameter cannot be calculated from the standing waves?
a) Peak voltage and peak current
c) Reflection and transmission coefficients
d) Attenuation constant
5. For efficient transmission, the characteristic impedance of the transmission line has to be
a) 50 ohm
b) 75 ohm
c) Either 50 or 75 ohm
d) Neither 50 nor 75 ohm
6. The cavity resonators are used in the klystron amplifiers for
a) Amplifying RF signals
b) Amplifying microwave signals
c) Attenuating RF signals
d) Attenuating microwave signals
7. The cavity resonators used in the reflex klystron oscillators are for
a) Generating RF signals
b) Generating microwave signals
c) Amplifying RF signals
d) Amplifying microwave signals
8. One of the applications of the cavity resonators is duplexer in RADAR systems. State true/false.
9. Cavity wave meters are used to measure which parameter of the wave?
b) Reflection factor
10. The waveguides are terminated by the procedure of