Multiple choice question for engineering
Set 1
1. I. Zero winding resistance
II. Zero leakage flux
III. Constant core losses
Which of the above statements support the ideal transformer features?
a) I,II
b) III
c) I,III
d) I,II,III
Answer
Answer: a [Reason:] Ideal transformer has zero or very small core or copper losses.
2. I. Zero winding resistance
II. Zero leakage flux
III. Negligible core losses
Which of the above statements support the ideal transformer features?
a) I,II,III
b) III
c) I,III
d) I
Answer
Answer: a [Reason:] All the statements support the ideal nature of a transformer.
3. I. Zero winding resistance
II. Constant permeability
III. Constant core losses
Which of the above statements support the ideal transformer features?
a) I,II
b) III
c) I,III
d) I
Answer
Answer: a [Reason:] Ideal transformer has constant permeability so that the magnetization of the core remains linear.
4. I. Zero winding resistance
II. Constant permeability
III. Constant core losses
Iv. Zero leakage reactance
Which of the above statements support the ideal transformer features?
a) I,II,III,IV
b) III,IV
c) I,III
d) I,IV
Answer
Answer: a [Reason:] An ideal transformer should be free from all types of losses and must have a linear magnetization of the core material.
5. I. Variable winding reactance
II. Constant permeability
III. Constant core losses
Which of the above statements support the ideal transformer features?
a) I,II
b) III
c) II,III
d) I
Answer
Answer: c [Reason:] There should not be any leakage reactance of the winding so that leakage flux is zero and complete coupling takes place.
6. The voltage induced at the end of primary terminals of a two winding transformer consisting of N turns is
a) -N*dϕ/dt
b) N*dϕ/dt
c) -dϕ/dt
d) -N*dt/dϕ
Answer
Answer: a [Reason:] Emf is induced based on the electromagnetic induction principle for N turns winding.
7. Identify the phasor diagram for an ideal transformer at no load
a) 
b) 
c) 
d) 
Answer
Answer: a [Reason:] In an ideal transformer, the induced emf lags the flux by 90degrees.
8. Consider a 2-winding transformer as below. If the switch is open then the emf induced across the load ‘Zl’ having transformation ratio of 2 is
a) zero
b) V1
c) V1/2
d) 2*V1
Answer
Answer: a [Reason:] As the secondary switch is kept open, the voltage across load will be zero.
9. Consider a 2-winding transformer as below. If the switch is open then the emf induced across the secondary having transformation ratio of 2 is
a) zero
b) V1
c) V1/2
d) 2*V1
Answer
Answer: d [Reason:] V2/V1 = 2
S, V2 = 2*V1.
10. Consider a 2-winding transformer as below. If the switch is kept open then the emf
induced across the secondary having transformation ratio of ‘2’ is
a) zero
b) 2E1
c) E/2
d) E1
Answer
Answer: a [Reason:] The induced emf lags the flux by 90degrees.
11. I. Core flux in an ideal transformer remains constant.
II. Core flux is independent of the load current.
The correct statements is/are
a) I
b) II
c) I,II
d) none of the mentioned
Answer
Answer: a [Reason:] Any changes in the secondary circuit of the transformer is reflected automaticaly in the primary winding so the net core flux remains constant.
12. If it is happened to place an inductive load to a two winding transformer, then the phasor diagram for such a loading will be
a) 
b) 
c)
d) 
Answer
Answer: a [Reason:] I2 will lag behind the V2 by the angle θ .
13. I. Zero winding resistance
II. Constant permeability
III. Constant core losses
Iv. Zero leakage reactance
Which of the above assumptions about the transformer supports the fact that the instantaneous power input and output of a transformer is equal?
a) I,II,III,IV
b) III,IV
c) I,III
d) I,IV
Answer
Answer: c [Reason:] If the impedance overall is zero for a transformer, then the losses ocurring in a transformer will be zero. So the efficiency is 100%.
14. I. Voltages – inverse ratio
II. Current – direct ratio
III. Impedance – direct ratio squared
Iv. Power – remains same
Mark the statements which are marked incorrect for an ideal transformer.
a) I,II,III,IV
b) III,IV
c) I,II
d) I,IV
Answer
Answer: c [Reason:] Voltages are transformed in direct ratio of the turns ratio. and current as inverse turns ratio.
15. I. Voltages – Direct ratio
II. Current – Inverse ratio
III. Impedance – Direct ratio squared
Iv. Power – Remains same
Mark the statements which are marked correct for an ideal transformer.
a) I,II,III,IV
b) III,IV
c) I,III
d) I,IV
Answer
Answer: a [Reason:] All the matches are correct.
16. I. Voltages – inverse ratio
II. Current – direct ratio
III. Impedance – direct ratio squared
Iv. KVA – remains same
Mark the statements which are marked correct for an ideal transformer.
a) I,II,III,IV
b) III,IV
c) I,III
d) I,IV
Answer
Answer: b [Reason:] Voltages are transformed in direct ratio of the turns ratio. and current as inverse turns ratio in a 2-winding transformer.
Set 2
1. A 3-phase induction motor can also be run as
a) synchronous generator
b) induction generator
c) dc shunt motor
d) any of the mentioned
Answer
Answer: b [Reason:] For the same machine if the slip is made negative by raising the speed above synchronous speed.
2. Induction generator
a) can not work in isolation
b) can work in isolation
c) should work in parallel with synchronous generators
d) any of the mentioned
Answer
Answer: c [Reason:] Induction generator needs the continuous flux and it can be either provided by the bus or the synchronous generator.
3. A self excited induction generator
a) must have static capacitor bank
b) can have static capacitor bank
c) should have static capacitor bank
d) any of the mentioned
Answer
Answer: a [Reason:] To provide the lagging reactive power, it is must that it should be have a source of lagging reactive power.
4. For an induction motor to work in isolation, it should have
a) lagging reactive power source
b) leading reactive power source
c) leading reactive power sink
d) any of the mentioned
Answer
Answer: a [Reason:] It must be connected to capacitors which can be the source for lagging reactive power.
5. The magnetization curve in the induction generator working in isolation has similar characteristic as
a) dc shunt generator
b) dc cumulative field generator
c) synchronous generator
d) all of the mentioned
Answer
Answer: a [Reason:] The voltage build up process of the induction generator is similar to that of the dc shunt generator.
6. If the residual flux is absent in the rotor iron of the isolated induction generator,
a) it will not run.
b) it will run as induction motor
c) it will burn the winding
d) it will not affect the characteristic of the generator
Answer
Answer: a [Reason:] If the residual flux of the isolated induction generator is not present, then voltage build up will not take place.
7. Voltage build up in an isolated induction generator depends on
A. Capacitor bank value
B. Residual magnetism of the rotor iron
C. Residual magnetism of the stator iron
a) A, B
b) A, B, C
c) B
d) A, C
Answer
Answer: a [Reason:] Voltage build up will depend on capacitance and the residual magnetism of the rotor iron.
8. I. Voltage of an externally excited induction generator can not be controlled.
II. Frequency of an externally excited induction generator can be controlled.
a) I is true, II is false
b) I is true, II is true
c) I is false, II is false
d) I is false, II is true
Answer
Answer: a [Reason:] Voltage and frequency of the externally excited induction generator can not be controlled, because they run in parallel with the existing 3-phase systems.
9. What is/are the shortcomings of an induction generator in real time system?
a) It needs considerable amount of reactive power
b) Efficiency is poor
c) It can work at leading power factor only
d) All of the mentioned
Answer
Answer: d [Reason:] All are the disadvantages of an induction generator.
10. Induction generator is self excited machine.
a) True
b) False
Answer
Answer: b [Reason:] No, induction machine is first run as motor and then as generator. So it needs the current to set up the flux continuously.
11. Induction generator can only work at leading reactive power.
a) True
b) False
Answer
Answer: a [Reason:] Because it can not supply any lagging power factor.
12. Choose the correct match for the corresponding characteristics of a 3-phase induction motor.
x-axis y-axis
A. stator current power output (P) Curve I
B. torque slip (Q) Curve II
C. speed power output (R) Curve III
a) A-II, B-III,C-I
b) A-III, B-II,C-I
c) A-III, B-I,C-II
d) A-I, B-II,C-III
Answer
Answer: a [Reason:] Speed will remain constant for a fair amount of operating range. Torque will be linear in small slip regions.
13. The most acceptable application field where an induction generator cana be used is
a) wind power system
b) steam power system
c) hydro power system
d) nuclear power station
Answer
Answer: a [Reason:] Induction generator is widely acceptable in low speed and low power operation like wind power systems.
14. A 3-phase induction motor has full load efficiency of 0.7 and a maximum efficiency of 0.85. It is operated at a slip of 0.6 by reduced voltage method. The efficiency of the motor at this operating point is
a) less than 0.35
b) greater than 0.35
c) in the range of 0.8 to 0.2
d) none of the mentioned
Answer
Answer: a [Reason:] For the slip more than 40%, the efficiency reduces more than 50% for an induction motor.
15. Two three phase squirrel-cage induction generator are identical in all respects except the fact that the slot depths in machine B is more than that of A.
Machine B as compared to machine A, will have
a) less pull out torque but better power factor
b) more pull out torque but poor power factor
c) less pull out torque and poor power factor
d) more pull out torque and better power factor
Answer
Answer: a [Reason:] As the slot depth in machine B is more than that of A, B will have higher reactance as the leakage flux will be lesser.
Set 3
1. The rotor of the slip ring induction motor is connected to an ac source and stator is short circuited. If the rotating magnetic field is rotates clockwise, the rotor rotates in
a) anticlockwise
b) clockwise
c) remains stationary
d) any of the mentioned
Answer
Answer: a [Reason:] Rotor will rotate in anticlockwise direction as the net speed w.r.t. stator should be stationary.
2. Rotor leakage impedance at starting is different from its value at normal running conditions.
a) True
b) False
Answer
Answer: a [Reason:] The rotor leakage impedance depends on the slip of the machine. Slip varies with the running conditions.
3. If the hysteresis is neglected in the induction machine, then the the air gap flux _______ the resultant mmf.
a) is in phase with
b) lags
c) leads by a small angle
d) can not decide the nature of the operation
Answer
Answer: a [Reason:] When the hysteresis loss is neglected, then the air gap flux will be in phase with the resultant mmf.
4. On increasing the starting torque of the induction motor, the maximum torque is also increased.
a) False
b) True
Answer
Answer: a [Reason:] It is false, as the starting torque and the maximum torque are independent of each other.
5. Approximately the efficiency of the rotor of the induction motor depends on the
a) rotor speed
b) synchronous speed
c) rotor speed and synchronous speed
d) magnetic field speed
Answer
Answer: c [Reason:] Efficiency of the rotor is approximately equal to ratio of speed of rotor to synchronous speed.
6. A 400, 3-phase, 50 Hz, 4 pole induction motor takes a line current of 10 A with 0.86 pf lagging. What is the stator input?
a) 5.95 kW
b) 6.95 kW
c) 4.45 kW
d) 8.38 kW
Answer
Answer: a [Reason:] P = 1.73*V*I*cosθ = 1.73*400*10*0.86 = 5.958 kW.
7. The power input to the 3-phase induction motor is 60 kW. The stator losses total 1 kW. Find the mechanical power developed at the slip of 3%.
a) 57.23 kW
b) 58 kW
c) 56.8 kW
d) 59 kW
Answer
Answer: a [Reason:] Air gap power = 60-1 = 59 kW
Pm = (1-s)Pag = 0.97*59 = 57.23 kW
8. Choose the most appropriate.
The torque slip characteristic of the induction motor at the slip of 3-6 %.
a)
b)
c)
d)
Answer
Answer: a [Reason:] At low slip regions, T α s*V^2/R2.
9. The torque and the slip characteristic is most correctly related by which of the following below in the region of 30-50% operation of slip.
a)
b)
c)
d)
Answer
Answer: c [Reason:] At the higher values of the slip, T α 1/s.
10. An induction motor X operating at the slip of 3%.
An induction motor Y operating at slip of 40%.
a) Machine X is only stable
b) Machine Y is only stable
c) Both the machine X and Y are stable
d) Neither X nor Y is stable
Answer
Answer: a [Reason:] Induction motor operates stable in the low slip regions as the losses are reduced.
11. A 3-phase induction motor had the maximum rated torque of 40 N-m while the load torque applied to it is 55 N-m.
a) The machine will become unstable
b) The machine will operate in stable region
c) The machine will get heated up
d) The machine will run at dangerously high speed
Answer
Answer: a [Reason:] Induction motor is unstable when the load torque is more the the maximum torque of the machine.
12. I. Maximum torque can be achieved by inserting an additional resistance at desired slip in the rotor.
II. Maximum torque can be altered by adding the external resistance in the rotor.
a) Only I is true
b) Both I and II are true
c) Neither I nor II is true
d) Only II is true
Answer
Answer: a [Reason:] Maximum torque is independent of the resistance.
13. If the frequency is reduced by 50% for a 3-phase, 50 Hz induction motor having 4 poles. The slip at which the maximum torque occurs becomes
a) 2*s
b) s
c) 1/2s
d) 1/s
Answer
Answer: a [Reason:] s=r/x
s α 1/f.
If the frequency is reduced to half, slip increases to twice.
14. If the voltage of a three phase induction motor if increased to 200%, then the slip if the machine changes to
a) 1/s
b) s
c) 1/2s
d) 2*s
Answer
Answer: c [Reason:] The slip will remain independent of the operating voltage.
15. If the voltage and frequency of the induction machine is reduced to 50% of the operating region,the slip of the machine will be
a) 2*s
b) s
c) 1/2s
d) 1/s
Answer
Answer: a [Reason:] Slip α 1/f; Even though the flux remains constant as the v/f ratio is constant.
16. If the operating voltage of the induction motor is increased by twice while keeping the frequency of the operation constant.
Then the new starting torque will be
a) 4*T
b) 1/4*T
c) T
d) 2*T
Answer
Answer: a [Reason:] Tst α V^2.
T2/T1 = (2/1)^2
T2 = 4*T1.
17. The phasor diagram for the induction motor is
a)
b)
c)
d)
Answer
Answer: a [Reason:] Relative space angle between the two rotating magnetic fields is 90+θ.
18. The phasor diagram for the induction generator is best represented by
a)
b)
c)
d)
Answer
Answer: c [Reason:] Relative space angle between the two rotating magnetic fields is 90-θ.
Set 4
1. An induction motor can be said analogous to
a) transformer
b) synchronous motor
c) universal motor
d) stepper motor
Answer
Answer: a [Reason:] An induction motor has similar operation as a transformer.
2. A 3-phase induction motor with its rotor blocked behaves similar to a
a) transformer under short circuit of secondary terminals
b) transformer under open circuit of secondary
c) synchronous motor under slip test
d) synchronous motor under open circuit
Answer
Answer: a [Reason:] It is analogous to transformer under the shorted terminals of the secondary.
3. No load current in induction motor is 10-20% of full load current and the no load current of transformer is 2-6%.
a) True
b) False
Answer
Answer: a [Reason:] The air gap in induction motor is more prominent than transformer and so the flux requirement will also be more.
4. The rated current in induction motor for a three phase system is 100A. What can be the no load estimated current for the machine
a) 12 A
b) 20 A
c) 30 A
d) 5 A
Answer
Answer: a [Reason:] No load current in induction motor is 10-20% of full load current.
5. The no load current of the transformer is very less due to
a) mutual flux having low reluctance iron core
b) mutual flux having high reluctance iron core
c) leakage flux having low reluctance iron core
d) leakage flux having high reluctance iron core
Answer
Answer: a [Reason:] It is due to the low reluctance path, the flux requirement is low in the transformer.
6. The no load current of the induction motor is high due to
a) long and high reluctance path between stator and rotor
b) mutual flux having moderate reluctance path between stator and rotor
c) leakage flux having low reluctance iron core
d) leakage flux having high reluctance iron core
Answer
Answer: a [Reason:] It is due to the higher reluctance path, the flux requirement is low in the induction motor.
7. At no load induction motor has possible power factor as
a) 0.2
b) 0.5
c) 0.65
d) 0
Answer
Answer: a [Reason:] At no load the lower power factor is low and lagging in nature.
8. Mechanically air gaps in induction motor are kept very low to avoid
a) lower power factor
b) lagging nature
c) magnetizing current
d) all of the mentioned
Answer
Answer: d [Reason:] Air gap is kept lower to avoid the low value power factor, lagging behaviour and to reduce the magnetizing current.
9. The low no load power factor
a) reduces full load operating pf
b) increases full load operating pf
c) reduces full load excitation voltage
d) increases full load excitation voltage
Answer
Answer: a [Reason:] The low no-load power factor reduces full load operating pf.
10. An induction motor when started on load,it does not accelerate up to full speed but runs at 1/7th of the rated speed.
The motor is said to be
a) Locking
b) Plumming
c) Crawling
d) Cogging
Answer
Answer: c [Reason:] Running stably at low speed around 1/7th of normal motor speed is known as crawling.
11. The great advantage of the double squirrel-cage induction motor over single cage rotor is that its
a) efficiency is higher
b) power factor is higher
c) slip is larger
d) starting current is lower
Answer
Answer: d [Reason:] The outer cage has higher resistance and low reactance while vice versa for the inner cage.So at starting the current is confined to outer cage with the reduction in starting current improving the torque.
12. The rotor of 3-phase slip ring induction motor is fed from a 3-phase supply with its stator winding short circuited having rotor rotating
clockwise at a speed of Nr, then
a) speed of air gap field w.r.t. stator is Ns-Nr anticlockwise
b) speed of air gap field w.r.t. stator is Ns-Nr clockwise
c) speed of airgap field w.r.t rotor is Ns clockwise
d) speed of airgap field w.r.t. stator is Ns-Nr clockwise
Answer
Answer: a [Reason:] The air gap field of stator = -(Ns-Nr).
13. (I) Even at no load a large 3-phase squirrel- cage induction motor is started at reduced voltage than rated.
(II) If a large 3–phase squirrel-cage induction motor with no load is started at full voltage, it will be damaged.
a) I is true, II is false
b) I is true and II is also true
c) I is false, II is true
d) I and II are false
Answer
Answer: a [Reason:] Large current of short duration are not harmful to induction motor but it may cause voltage drop in the power supply.
Set 5
1. Insertion of external resistance in the squirrel cage induction motor
a) increases torque
b) decreases torque
c) increases torque for some seconds and then decreases
d) does not affect
Answer
Answer: a [Reason:] Adding the external resistance reduces the starting current and increases the current.
2. Insertion of external resistance in the squirrel cage induction motor _________ the current, ________ the starting current.
a) reduces, increases
b) increases, reduces
c) increases, increases
d) reduces, reduces
Answer
Answer: a [Reason:] Adding the external resistance reduces the starting current and increases the current.
3. Rotor ohmic loss is ________ slip.
a) proportional to
b) inverse of
c) inversely proportional to
d) hyperbolic to
Answer
Answer: a [Reason:] Rotor ohmic loss is direct proportional to slip.
4. A 3-phasle, 6-pole, 50 Hz induction motor has a full load speed of 960 rpm with its slip rings short circuited. The motor drives the constant load. If the speed is reduced to 800 rpm, then ohmic losses will
a) reduce
b) increase
c) reduce first then becomes constant
d) remains same
Answer
Answer: a [Reason:] The ohmic losses will increase as the slip will increase due to reduction of slip.
5. A 3-phase, 6-pole, 50 Hz induction motor has a full load speed of 960 rpm with its slip rings short circuited. The motor drives the constant load. If the speed is reduced to 800 rpm, then ohmic losses will be
a) 5 times that at full load
b) 0.2 times as that at full load
c) 15 times as that at full load
d) same as that at full load
Answer
Answer: a [Reason:] slip at full load = (1000-960)/1000 = 0.04
slip at 800 rpm = 0.2
ohmic loss at 800 rpm = (slip at 800/slip at 960 rpm)*ohmic loss at full load = 5
6. A 3-phase, 6-pole, 50 Hz induction motor has a full load speed of 960 rpm with its slip rings short circuited. The motor drives the constant load. If the speed is reduced to 400 rpm, then ohmic losses will be
a) 15
b) 1/15
c) 5
d) 1/5
Answer
Answer: a [Reason:] slip at full load = (1000-960)/1000 = 0.04
slip at 400 rpm = 0.6
ohmic loss at 800 rpm = (slip at 400/slip at 960 rpm)*ohmic loss at full load = 15
7. The starting torque of the squirrel-cage induction motor is
a) slightly more than full load torque
b) slightly less than full load torque
c) equal to full load torque
d) zero
Answer
Answer: a [Reason:] To overcome the inertia the starting torque of the induction motor is slightly more than full load torque to start.
8. For achieving high starting torque and high operating efficiency an induction motor should have _______ rotor circuit resistance and _______ circuit resistance under operating condition.
a) high, low
b) low, high
c) low, low
d) high, high
Answer
Answer: a [Reason:] At starting the rotor circuit resistance must be small and while operating under normal conditions the resistance should be low to reduce losses.
9. In a 3-phase slip ring induction motor high starting torque is achieved by
a) increasing supply voltage
b) increasing supply frequency
c) connecting a capacitor across the motor terminals
d) connecting a star connected resistance across slip ring terminals of motor
Answer
Answer: d [Reason:] To increase starting torque, we need to increase the rotor resistance.
10. In a three-phase induction motor, the starting torque is maximum when
a) r2 = x2
b) r2 = 1/x2
c) r2 = (x2)2
d) r2 = 1 pu
Answer
Answer: a [Reason:] The maximum slip is obtained at the condition of start is when r2 = x2.
11. The losses that occur in induction motor are
a) stator copper loss
b) rotor iron loss
c) rotor copper loss
d) all of the mentioned
Answer
Answer: a [Reason:] All of the losses mentioned occur in an induction motor.
12. Iron loss in the rotor is negligible. This is due to
a) very low frequency of emf induced in the rotor
b) very low emf induced in the rotor
c) very low flux density in rotor parts
d) laminated core
Answer
Answer: a [Reason:] Due to the low frequency of the emf induced in the rotor, the iron losses are reduced.
13. In an induction motor, what is the ratio of the rotor copper loss to rotor input?
a) s
b) 1/s
c) 1-s
d) s/(1-s)
Answer
Answer: a [Reason:] P(cu)/P(in)= slip
14. The rotor power input of 3-phase induction motor is 15 kW at a slip of 5%. The mechanical power developed at the shaft is
a) 14.25 kW
b) 7.5 kW
c) 4 kW
d) 700 W
Answer
Answer: a [Reason:] Pm = (1-s)Pin = 0.95*15 = 14.25 kW
15. The synchronous watt is
a) the torque under which synchronous speed would develop a power of 1 W or power input to rotor in watts
b) shaft output in watts at sub synchronous speed
c) input power to stator at synchronous speed
d) unit rating of synchronous machine
Answer
Answer: a [Reason:] It is the torque under which synchronous speed would develop a power of 1 W or power input to rotor in watts.
16. If the rotor circuit resistance is increased in an induction motor, maximum torque will occur at
a) lower speed
b) higher speed
c) same speed
d) none of the mentioned
Answer
Answer: a [Reason:] Increasing the resistance will reduce the speed at which maximum torque will occur.
