New Electric Drives MCQs – Braking of DC Motors & Induction Motors MCQs ( Electric Drives ) MCQs
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Braking of DC Motors & Induction Motors MCQs ( Electric Drives ) MCQs – Electric Drives MCQs
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Regenerative Braking of DC Shunt Motors
1. A circuit consists of 3 F capacitor and 5 H inductor. Determine the order of the circuit.
a) 2
b) 1
c) 3
d) 0
Answer: a
Explanation: The order of the circuit is the number of memory/storing elements which are non-separable present in the circuit. In mathematics, the order is defined as the highest order derivate in the differential equation. The order of the circuit is 2.
2. The forced response is due to a source present in the circuit.
a) True
b) False
Answer: a
Explanation: The steady-state response is a part of the forced response. The forced response is due to the electrical source present in the circuit. Its mathematical equation involves the source present in the circuit.
3. Full form of DTC.
a) Direct torque control
b) Digital torque control
c) Discrete torque control
d) Distribution torque control
Answer: a
Explanation: DTC stands for Direct torque control. DTC and IFOC are the techniques used for controlling the speed and torque of a 3-phase induction motor.
4. The characteristics shown by an element in the I-V curve is V=I2. The nature of the element is _______
a) Non-linear, Bilateral, Passive
b) Linear, Unilateral, Active
c) Linear, Bilateral, Passive
d) Non-linear, Unilateral, Active
Answer: d
Explanation: The nature of the element is non-linear, unilateral and active. The shape of the characteristic is parabolic. For bilateral nature, it should be symmetrical in the first and third quadrant. Its slope is negative in the second quadrant which determines its active nature.
5. Transient response is a temporary response.
a) True
b) False
Answer: a
Explanation: Transient response is a primary response in the circuit. It is a temporary response which dies out at t=infinity. It consists of exponential decaying functions.
6. Calculate the steady state value for x(t)=4(1-e-3t).
a) 5
b) 4
c) 3
d) 2
Answer: b
Explanation: The steady state value is obtained at t=∞. The value of x(t) at t=∞ is 4(1-e-∞)=4(1-0)=4. The term e-3t is an exponentially decaying function.
7. The natural response is due to _________ conditions present in the circuit.
a) Initial
b) Final
c) Zero
d) Negative
Answer: a
Explanation: The natural response is due to the initial conditions present in the circuit. The natural response is a complete part of the transient response. It is mathematically represented as y(t)=yo×e-at.
8. Calculate the value of the coefficient of coupling for the isolated coils.
a) 0
b) 1
c) 5
d) 7
Answer: a
Explanation: The coefficient of coupling expresses how the two coils are magnetically coupled. It is mathematically represented as K=M÷√L1.L2. For the isolated coils, the value of the mutual inductance is 0. The value of the coefficient of coupling is 0.
9. Calculate the value of equivalent inductance for series aiding of two coils whose self inductances are 5 H, 2 H, and mutual inductance value is 4 H.
a) 15 H
b) 12 H
c) 13 H
d) 11 H
Answer: a
Explanation: The equivalent inductance for series aiding of two coils is Leq=L1+L2+2M=5+2+8=15 H. The value of equivalent inductance increases due to mutually induced e.m.f in case of the magnetically coupled circuit.
10. Calculate the value of equivalent inductance for series subtracting polarity of two coils whose self inductances are 14 H, 5 H, and mutual inductance value is 1 H.
a) 15 H
b) 17 H
c) 12 H
d) 10 H
Answer: b
Explanation: The equivalent inductance for series subtracting of two coils is Leq=L1+L2-2M=14+5-2=17 H. The value of equivalent inductance decreases due to negatively mutually induced e.m.f in case of the magnetically coupled circuit.
11. Calculate the value of the capacitive reactance during resonance if the value of the inductor is 5 H and supply frequency is 20 rad/sec.
a) 100 Ω
b) 200 Ω
c) 300 Ω
d) 700 Ω
Answer: a
Explanation: Resonance is defined as the phenomenon in which energy of any element changes from one form to another. During resonance condition Xc=Xl=ΩL=20×5=100 Ω.
12. Calculate the quality factor for the R-L circuit if R=28 Ω and L=2 H.
a) 14
b) 16
c) 10
d) 17
Answer: a
Explanation: The quality factor is defined as the ratio of the reactive power to the active power consumed. The resistor always absorbs active power and inductor absorbs the reactive power. Quality factor=R÷L=28÷2=14.
13. The quality factor is calculated for ___________
a) Power factor loads
b) Inductive coils
c) Lagging loads
d) Leading loads
Answer: b
Explanation: Quality factor is calculated for inductive coils, not for power factor loads. Its value determines the quality of the inductor or capacitor. It shows how good an inductor or capacitor can absorb reactive power.
14. Calculate the equivalent resistance when two resistances are connected in parallel of values 8 Ω, 8 Ω.
a) 3 Ω
b) 2 Ω
c) 4 Ω
d) 7 Ω
Answer: c
Explanation: When two resistances are connected in parallel their equivalent resistance is equal to the harmonic mean of the individual resistances. Req=R1.R2÷(R1+R2)=8×8÷(8+8)=4 Ω.
15. Calculate the quality factor for the R-C circuit if R=2 Ω and C=1 F.
a) 0.2
b) 0.4
c) 0.6
d) 0.5
Answer: d
Explanation: The quality factor is defined as the ratio of the reactive power to the active power consumed. The resistor always absorbs active power and capacitor absorbs the reactive power. Quality factor=1÷RC=1÷2=.5.
Regenerative Braking of DC Series Motors
1. What is the condition for maximum power transfer theorem in DC circuits?
a) RL=2Rth
b) RL≠Rth
c) RL≫Rth
d) RL=Rth
Answer: d
Explanation: The condition for maximum power transfer theorem is load should be variable and RL=Rth. During maximum power transfer condition voltage across load becomes half of Thevenin voltage and efficiency becomes 50 %.
2. Instantaneous power in the 3-φ system is constant.
a) True
b) False
Answer: a
Explanation: The 3-φ system is more economical than 2-∅ system because of no vibrations in power waveform. The instantaneous power in the 3-∅ system is constant. P=3VpIpcos(φ)=√3VLILcos(∅).
3. The length of phasor is ___________
a) R.M.S
b) Average
c) Peak to Peak
d) Minimum
Answer: a
Explanation: Phasors are the rotating values which rotate with some angular frequency Ω. The length of the phasor value is r.m.s value.
4. Transient analysis is only applicable to bounded systems.
a) True
b) False
Answer: a
Explanation: Bounded systems are those systems which have some finite maximum value and are decaying functions. Transient analysis is only applicable to bounded signals as they achieve steady state after some time.
5. Calculate the velocity of the ball if the angular speed is 7 rad/s and radius is .2 m.
a) 2.5 m/s
b) 1.4 m/s
c) 4.5 m/s
d) 1.0 m/s
Answer: b
Explanation: The velocity of the ball can be calculated using the relation V=Ω×r. The velocity is the vector product of angular speed and radius. V = Ω×r = 7×.2 = 1.4 m/s.
6. Calculate the value of the time period if the frequency of the signal is .001 Hz.
a) 1000 sec
b) 2000 sec
c) 5000 sec
d) 1500 sec
Answer: a
Explanation: The time period is defined as the time after the signal repeats itself. It is expressed in second. T = 1÷F=1÷.001=1000 sec.
7. Calculate the value of the frequency of the 440 V DC supply.
a) 100 Hz
b) 0 Hz
c) 200 Hz
d) 500 Hz
Answer: b
Explanation: The frequency is defined as the number of oscillations per second. It is reciprocal of the time period. DC supply magnitude is constant. It does not change with time so the frequency of DC supply is 0 Hz.
8. Calculate the value of power factor if the values of R and Z are 2 Ω and 10 Ω.
a) 0.8
b) 0.5
c) 0.2
d) 0.4
Answer: c
Explanation: The power factor is defined as the ratio of active power to the apparent power. Cos(φ)=R÷Z=2÷10=.2. It has no unit.
9. Calculate the value of capacitor voltage during resonance condition if the value of supply voltage is 20 V and the quality factor is 3.
a) 60 V
b) 50 V
c) 10 V
d) 30 V
Answer: a
Explanation: During the resonance condition XL=Xc. The value of the capacitor voltage is Q×Vs. The power factor of the circuit is one. Vc=20×3=60 V.
10. When 30 A current flows into the positive terminal of current source 8 V. Calculate the power delivered by the source.
a) -240 W
b) 360 W
c) -430 W
d) 500 W
Answer: a
Explanation: When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power. Power delivered is -30×8=-240 W.
11. The slope of the V-I curve is 17.587o. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
a) .322 Ω
b) .360 Ω
c) .316 Ω
d) .778 Ω
Answer: c
Explanation: The slope of the V-I curve is resistance. The slope given is 17.587o so R=tan(17.587o)=.316 Ω. The slope of the I-V curve is reciprocal of resistance.
12. Calculate the equivalent inductance when two inductors are connected in parallel of values 12 H and 12 H.
a) 6 H
b) 10 H
c) 12 H
d) 8 H
Answer: a
Explanation: When two inductors are connected in parallel their equivalent inductance is equal to the harmonic mean of the inductances. Leq=L1×L2÷(L1+L2)=6 H.
13. Calculate the active power in an 0 Ω resistor with 0 current flowing through it.
a) inf MW
b) 0 MW
c) 2 MW
d) 18 MW
Answer: b
Explanation: The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the angle between V & I is 0o. P=I2R=0×0×0=0 MW.
14. When 1 A current flows out of the positive terminal of voltage source 6 V. Calculate the power delivered by the source.
a) 6 W
b) 7 W
c) -9 W
d) -5 W
Answer: a
Explanation: When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power. Power delivered by the source is 1×6=6 W.
15. Calculate the value of inductor voltage during resonance condition if the value of supply voltage is 7 V and the quality factor is 9.
a) 59 V
b) 63 V
c) 73 V
d) 33 V
Answer: b
Explanation: During the resonance condition XL=Xc. The value of the inductor voltage is Q×Vs. The power factor of the circuit is one. Vc=7×9=63 V.
Dynamic Braking of DC Shunt Motors
1. The sine function is an odd function.
a) True
b) False
Answer: a
Explanation: The odd functions are those functions which are symmetric about the origin. The sine function is an odd function whose time period is 2π.
2. The cosine function is an even function.
a) True
b) False
Answer: a
Explanation: The even functions are those functions which are a mirror image of the y-axis. The cosine function is an even function whose time period is 2π.
3. Full form of NENO.
a) Neither even nor odd
b) Neither energy nor odd
c) Neither even nor original
d) Neither even nor orthogonal
Answer: a
Explanation: NENO stands for Neither even nor odd. The functions which are not the mirror image of the y-axis and nor symmetric about the origin are NENO functions.
4. The characteristics shown by an element in the V-I curve is V=Is(1-e-V/K). The nature of the element is _______
a) Non-linear, Bilateral, Passive
b) Linear, Unilateral, Active
c) Linear, Bilateral, Passive
d) Non-linear, Unilateral, Passive
Answer: d
Explanation: The nature of the element is non-linear, unilateral and passive. The shape of the characteristic is exponential rising. For bilateral nature, it should be symmetrical in the first and third quadrant. Its slope is positive in the first quadrant which determines its passive nature.
5. Calculate the resonant frequency if the values of the capacitor and inductor are 2 F and 2 H.
a) .5 rad/sec
b) .6 rad/sec
c) .8 rad/sec
d) .9 rad/sec
Answer: a
Explanation: During resonance condition XL=Xc. The value of the resonant frequency is 1÷√LC=1÷√4=.5 rad/sec. The Voltage across the capacitor and inductor becomes equal.
6. Calculate the steady state value for x(t)=7e-9t.
a) 0
b) 8
c) 3
d) 1
Answer: a
Explanation: The steady state value is obtained at t=∞. The value of x(t) at t=∞ is 7e-∞=4(0)=0. The term e-9t is an exponentially decaying function.
7. The maximum Voltage across the capacitor Vc(t)=Vo(1- e-t) is __________
a) Vo
b) 2Vo
c) 3Vo
d) -Vo
Answer: a
Explanation: The Vc(t)=Vo(1- e-t) is Vo is the transient equation of the capacitor Voltage. At the steady state (t=∞) Vc(t)=Vo(1- e-∞) is Vo. The maximum Voltage across the capacitor is Vo.
8. Calculate the value of the coefficient of coupling for the tightly coupled coils.
a) 0
b) 1
c) 3
d) 2
Answer: b
Explanation: The coefficient of coupling expresses how the two coils are magnetically coupled. It is mathematically represented as K=M÷√L1.L2. For tightly coupled coils, the value of the mutual inductance is √L1.L2. The value of the coefficient of coupling is 1.
9. The maximum current in the inductor IL(t)=Io(1 – e-t/α) is __________
a) Io e-t/α
b) Io
c) 2Io
d) -Io
Answer: b
Explanation: The IL(t)=Io(1 – e-t/α) is Io is the transient equation of the inductor current. At the steady state (t=∞) IL(t)=Io(1- e-∞) is Io. The maximum current in the inductor is Io.
10. 20 V, 10 A, 10 rpm separately excited dc motor with armature resistance (Ra) equal to .8 ohms. Calculate back emf developed in the motor when it operates on the full load. (Assume rotational losses are neglected)
a) 12 V
b) 14 V
c) 13 V
d) 11 V
Answer: a
Explanation: Back emf developed in the motor can be calculated using the relation Eb = Vt – I×Ra. In question, it is asking for a full load. 20 V is terminal Voltage it is fixed so Eb = 20-10×.8 = 12 V.
11. Speed of DC shunt motor is directly proportional to___________
a) Eb
b) Φ
c) Vt
d) Ia.Ra
Answer: a
Explanation: The back e.m.f in case of DC shunt motor is Eb=Vt-Ia.Ra. The speed in DC shunt motor is
Eb÷Kv. The speed is directly proportional to Eb.
12. Calculate the value of the angular acceleration of the DC shunt motor using the given data: J = 1 kg-m2, load torque = 1 N-m, motor torque = 2 N-m.
a) 1 rad/s2
b) 2 rad/s2
c) 3 rad/s2
d) 5 rad/s2
Answer: a
Explanation: Using the dynamic equation of motor J×(angular acceleration) = Motor torque – Load torque: 1×(angular acceleration) = 2-1=1, angular acceleration=1 rad/s2.
13. Calculate the quality factor for the R-L circuit if R=16 Ω and XL=8 Ω with supply frequency is 1 rad/sec.
a) 2
b) 6
c) 0
d) 7
Answer: a
Explanation: The quality factor is defined as the ratio of the reactive power to the active power consumed. The resistor always absorbs active power and inductor absorbs the reactive power. ΩL=8, L=8 Henry and quality factor=R÷L=16÷8=2.
14. Calculate the equivalent resistance when two armature resistances are connected in parallel of values 6 Ω, 3 Ω.
a) 3 Ω
b) 2 Ω
c) 4 Ω
d) 7 Ω
Answer: b
Explanation: When two resistances are connected in parallel their equivalent resistance is equal to the harmonic mean of the individual resistances. Req=R1.R2÷(R1+R2)=6×3÷(6+3)=2 Ω.
Speed Control Of Direct Current And Induction Motors MCQs
15. Calculate the quality factor for the R-C circuit if R=1 Ω and C=1 F.
a) 2
b) 4
c) 1
d) 5
Answer: c
Explanation: The quality factor is defined as the ratio of the reactive power to the active power consumed. The resistor always absorbs active power and capacitor absorbs the reactive power. Quality factor=1÷RC=1÷1=1.
Dynamic Braking of DC Series Motors
1. Dynamic braking requires a strong _________
a) Magnetic field
b) Electrostatic field
c) Nuclear field
d) Gravitational field
Answer: a
Explanation: Dynamic braking requires a strong magnetic field to convert energy from the magnetic domain into the electrical domain. The kinetic energy is wasted at a higher rate.
2. In DC series machine ∅ is directly proportional to Ia.
a) True
b) False
Answer: a
Explanation: In series DC machine, the field winding is in series with the armature winding. The armature current flows through the field winding. F=NIa=R∅. Ia∝∅.
3. For faster braking, the series field is separated from the armature circuit.
a) True
b) False
Answer: a
Explanation: For faster braking, the series field is separated from the armature circuit. The series field is excited by a different voltage source.
4. At the starting, the field is very strong but it decreases with time because of the ________
a) Reduction in armature current
b) Increase in armature current
c) Reduction in speed
d) Increase in speed
Answer: a
Explanation: At the starting, the field is very strong but it decreases with time because of the reduction in armature current. ∅∝Ia.
5. Calculate the time period of the waveform x(t)=1cot(πt+7π).
a) 0.5 sec
b) 1 sec
c) 3 sec
d) 5 sec
Answer: b
Explanation: The fundamental time period of the cot wave is π. The time period of x(t) is a π÷π=1 sec. The time period is independent of phase shifting and time shifting.
6. In dynamic braking, DC series motor behaves as a ____________
a) Separately excited motor
b) Transformer
c) Induction motor
d) Thyristor
Answer: a
Explanation: In dynamic braking, DC series motor behaves as a separately excited motor as the motor armature is disconnected from the source and connected across resistance RB.
7. Calculate the flux produced by the DC series field winding using the following data: Ia=5 A, N=23, R=23.
a) 5 Wb
b) 6 Wb
c) 9 Wb
d) 8 Wb
Answer: a
Explanation: In DC series motor, field windings are connected in series with the armature circuit. F=NIa=R∅. Ia∝∅. ∅=F÷R=5 wb.
8. The shape of the speed-armature current characteristics in DC series motor are __________
a) Hyperbolic
b) Parabola
c) Circle
d) Ellipse
Answer: a
Explanation: The shape of the speed-armature current characteristics in DC series motor is hyperbolic. The motor equation is Eb=Vt-IaRa=Km∅ωm. N=(Vt/KmIa)-Ra/Km. N∝1÷Ia.
9. The shape of the speed-torque current characteristics in DC series motor are __________
a) Rectangular Hyperbolic
b) Parabola
c) Circle
d) Ellipse
Answer: a
Explanation: The shape of the speed-torque characteristics in the DC series motor is a rectangular hyperbola. The motor equation is Eb=Vt-IaRa=Km∅ωm. N=(Vt/KmIa)-Ra/Km. Ia∝√T .N∝1÷√T.
10. DC series motors are used where the high ________ torque is required.
a) Maximum
b) Minimum
c) Starting
d) Stalling
Answer: c
Explanation: DC series motors are used where the high starting torque is required. At starting the torque produced by DC series motor is very high. To start a motor torque should be greater than load torque.
11. The shape of the current-torque current characteristics in DC series motor is a __________
a) Rectangular Hyperbolic
b) Parabola
c) Circle
d) Ellipse
Answer: b
Explanation: The shape of the current-torque characteristics in the DC series motor is a parabola. The motor equation is Eb=Vt-IaRa=Km∅ωm. T=Km∅Ia. ∅∝Ia. T∝Ia2.
12. Calculate the current in DC series field winding using the following data: Ia=3 A.
a) 4 A
b) 2 A
c) 3 A
d) 6 A
Answer: c
Explanation: In DC series motor, field windings are connected in series with the armature circuit. The armature current will also flow through the series winding. If=Ia=3 A.
13. For a given torque, increasing diverter resistance of a DC series motor causes the ________
a) decrease in speed and less armature current
b) decrease in speed but armature current remains the same
c) increase in speed and more armature current
d) increase in speed but armature current remains the same
Answer: a
Explanation: In DC series motor, increasing diverter resistance increases the field current and field flux and also decreases the armature current and therefore decreases the speed.
14. The field diverter resistance for a DC series motor is kept ________
a) High
b) Low
c) zero
d) one
Answer: b
Explanation: The field diverter resistance for a DC series motor is kept low in order to increase the speed of the motor. This will reduce the field flux and increase the armature current.
Countercurrent Braking of DC Shunt Motors
1. Plugging is suitable for __________ type loads.
a) Gravitational
b) Electrostatic
c) Weightless
d) Leading
Answer: a
Explanation: Plugging is suitable for gravitational type loads. It halts the motor operation. It can reverse the direction of rotation of the motor. It can hold the motor at zero speed.
2. In DC shunt machine ∅ is directly proportional to Ia.
a) True
b) False
Answer: b
Explanation: In shunt DC machine, the field winding is in parallel with the armature winding. The armature current flows through the armature winding. IF∝∅∝Vt.
3. Braking resistance should be selected in order to limit the braking current.
a) True
b) False
Answer: a
Explanation: During braking, a high current flows in the armature circuit. To limit the braking current a proper value of braking resistance should be selected.
4. Torque developed in the motor is ___________
a) KmΦIa
b) KmΦIa3
c) 2KmΦIa
d) KmΦIa2
Answer: a
Explanation: The torque developed in the motor is directly proportional to the flux, machine constant, armature current. It is mathematically represented as T= KmΦIa.
5. Full form of TVR is _________
a) Terminal voltage reversal
b) Total voltage reversal
c) Terminal voltage redirect
d) Total voltage reversal
Answer: a
Explanation: TVR stands for terminal voltage reversal. It is one of the methods of countercurrent braking of the DC shunt motor. It halts the motor rapidly and reverses its direction of rotation.
6. Calculate the current in DC shunt field winding using the following data: Ia=15 A, IL=21, R=22, N=484.
a) 4 A
b) 2 A
c) 7 A
d) 6 A
Answer: d
Explanation: In DC shunt motor, field windings are connected in parallel with the armature circuit. IF=IL-Ia=11-5=6 A. Motor current is the sum of field and armature current.
7. Calculate the flux produced by the DC shunt field winding using the following data: Ia=5 A, IL=11, R=2, N=4.
a) 14 Wb
b) 12 Wb
c) 17 Wb
d) 18 Wb
Answer: b
Explanation: In DC shunt motor, field windings are connected in parallel with the armature circuit. IF=IL-Ia=11-5=6 A. F=NIF=R∅. IF∝∅. ∅=F÷R=12 wb.
8. The shape of the speed-armature current characteristics in DC shunt motor is __________
a) Hyperbolic
b) Parabola
c) Negative slope linear line
d) Ellipse
Answer: c
Explanation: The shape of the speed-armature current characteristics in DC shunt motor is negative slope linear line. The motor equation is Eb=Vt-IaRa=Km∅ωm. N=(Vt-IRa)/Km. N vs Ia is a negative slope line with slope = -Ra/Km.
9. The shape of the speed-torque characteristics in DC shunt motor is __________
a) Rectangular Hyperbolic
b) Whole x-y plane
c) Circle
d) Ellipse
Answer: a
Explanation: The shape of the speed-torque characteristics in the DC shunt motor is a rectangular hyperbola. The motor equation is Eb=Vt-IaRa=Km∅ωm. N=(Vt/KmIa)-Ra/Km. Ia∝√T .N∝1÷√T.
10. DC shunt motors are used where the high _________ is required.
a) Starting torque
b) Maximum torque
c) Minimum torque
d) Breakdown torque
Answer: a
Explanation: DC series motors are used where the high starting torque is required. At starting the torque produced by DC series motor is very high.
11. The shape of the current-torque characteristics in DC shunt motor is __________
a) Rectangular Hyperbola
b) Parabola
c) Straight line
d) Ellipse
Answer: c
Explanation: The shape of the current-torque characteristics in the DC shunt motor is a straight line. The motor equation is Eb=Vt-IaRa=Km∅ωm. T=Km∅Ia. ∅∝If. Torque ∝ Ia.
12. The relationship between the torque(T) and power(P) developed in the DC shunt motor is ___________ (Neglecting all the losses)
a) T ∝ √P
b) T ∝ P2
c) T ∝ ∛P
d) T ∝ P
Answer: d
Explanation: The torque developed in the DC shunt motor is the ratio of power developed and angular
speed of the motor. T=EbIa÷ωm=P÷ωm. T ∝ P.
13. The value of current at time of starting is ___________
a) High
b) Low
c) Very low
d) 0
Answer: a
Explanation: At the time of starting the motor is at rest. The e.m.f developed in the motor is zero. Ia×Ra = Vt-Eb. Ia=Vt÷Ra. The starting current is very high about 10-15 times of full load current.
15. Calculate the bandwidth in series RLC circuit if the frequency is 20 Hz and the quality factor is 5.
a) 2 Hz
b) 4 Hz
c) 6 Hz
d) 8 Hz
Answer: b
Explanation: Bandwidth is defined as the range of frequencies for which the signal exists. Selectivity is inversely proportional to the bandwidth. B.W(Hz)=f÷Q=20÷5=4 Hz.
Induction Motors – Regenerative Braking (3-phase IM)
1. Power input to an induction motor is given by _________
a) 3VpIpcosΦ
b) 2VpIpcosΦ
c) VpIpcosΦ
d) 5VpIpcosΦ
Answer: a
Explanation: Power input to a 3-Φ induction motor is 3VpIpcosΦ. The power remains the same for Delta and star connection. The power is constant and has no vibrations.
2. For motoring mode, the phase angle between the stator phase voltage and stator phase current should be _______
a) < 90°
b) > 90°
c) < 40°
d) 180°
Answer: a
Explanation: For motoring mode, the phase angle between the stator phase voltage and stator phase current should be < 90°. The rotor magnetic field tries to catch up the stator magnetic field.
3. The relative speed between the stator magnetic field and rotor in the induction motor is ______
a) Ns
b) Ns-Nr
c) Ns+Nr
d) 0
Answer: b
Explanation: In induction motor rotor tries to catch up the stator magnetic field in order to satisfy Lenz law. Effect opposes the cause. Stator magnetic field rotates at the speed of synchronous speed.
4. The relative speed between the stator magnetic field and rotor in the synchronous generator is ______
a) Ns
b) Ns-Nr
c) Ns+Nr
d) 0
Answer: d
Explanation: In synchronous generator rotor and stator magnetic field rotates at speed of synchronous speed in order to produce the steady state torque.
5. The relative speed between the stator magnetic field and stator in the induction motor is ______
a) Ns
b) Ns-Nr
c) Ns+Nr
d) 0
Answer: a
Explanation: In Induction motor stator magnetic field rotates at the speed of synchronous speed. The stator is made up of Silicon steel and remains at rest. The relative speed is Ns-0 = Ns.
6. The relative speed between the rotor and stator in the induction motor is ______
a) Nr
b) Ns-Nr
c) Ns+Nr
d) 0
Answer: a
Explanation: In Induction motor rotor tries to catch up the stator magnetic field in order to satisfy Lenz law. Effect opposes the cause. Stator remains at rest. The relative speed is Nr-0=Nr.
7. What will happen if we increase the air gap in the induction motor?
a) Power factor will reduce
b) Power factor will increase
c) Reduction in harmonics
d) Speed will increase
Answer: a
Explanation: If we increase the air gap in the induction motor, the machine will draw more magnetizing current to maintain flux in the air gap. The power factor of the machine will reduce.
8. The ratio of the rotor copper loss and air gap power is _______
a) s
b) 1-s
c) 1+s
d) 0
Answer: a
Explanation: The ratio of the rotor copper loss and air gap power is s. The air gap power in the induction motor is Pg and the rotor copper loss is s×Pg. The ratio is s.
9. Magnetizing current in the induction motor is ______
a) more than the transformer
b) equal as a transformer
c) less than the transformer
d) 0
Answer: a
Explanation: The magnetizing current in the induction motor is (23-25)% of the full load current. The magnetizing current in the transformer is 3-5% of the full load.
10. Single phase induction motor is self-starting.
a) True
b) False
Answer: b
Explanation: The single-phase induction motor is not self-starting because of the equal and opposite forward & backward magnetic field. The net torque developed is zero.
11. What is the speed of the 6th order time harmonics in the induction motor?
a) Ns
b) Ns/6
c) 6×Ns
d) 0
Answer: c
Explanation: The time harmonics are present in the supply voltage. The speed of the 6th order time harmonics in the induction motor is Ns/6.
12. What is the speed of the 11th order harmonics in the induction motor?
a) Ns
b) Ns/11
c) 2×Ns
d) Nr
Answer: b
Explanation: The speed of the 6k±1 is Ns÷6k±1. The speed of the 11th order time harmonics in the induction motor is Ns/11.
13. A 3 – phase, 4 pole IM is supplied from a 40 Hz source. Calculate rotor frequency when the rotor runs 100 rpm.
a) 35 Hz
b) 36.6 Hz
c) 9 Hz
d) 8 Hz
Answer: b
Explanation: The slip of the induction motor can be calculated using the relation s = Ns-Nr÷ Ns. Ns=120F÷P=1200 rpm. The value of slip is .91. The rotor frequency is 36.6 Hz.
14. Calculate the three-phase power using the following data: Vp=23 V, Ip=23 A, ∅=60.
a) 793.5 W
b) 790 W
c) 792 W
d) 791 W
Answer: a
Explanation: Power input to a 3-Φ induction motor is 3VpIpcosΦ. The power remains the same for Delta and star connection. P=3×23×23×co60°=793.5 W.
Induction Motors – Dynamic Braking(IM)
1. Order of the circuit is the number of memory elements present in the circuit.
a) True
b) False
Answer: a
Explanation: The order of the circuit is the number of memory/storing elements which are non-separable present in the circuit. The examples of memory elements are capacitor, inductor.
2. In Induction motor rotor laminations are thicker than stator laminations.
a) True
b) False
Answer: a
Explanation: The lamination thickness basically depends upon the frequency. The rotor frequency is s×f where f is stator supply frequency. The rotor laminations are thicker than stator laminations because rotor frequency is less.
3. Choose the Induction motor with peak speed.
a) 10 Pole
b) 12 Pole
c) 14 Pole
d) 16 Pole
Answer: a
Explanation: Synchronous speed of Induction motor is Ns=120×f÷P. Ns∝(1÷P). The synchronous speed is inversely proportional to the pole. The maximum speed will be achieved with a minimum number of poles.
4. Which starting method is the best method in Induction motor?
a) Direct online starting
b) Autotransformer starting
c) Reactance starting
d) Star-Delta starting
Answer: d
Explanation: The Star-Delta starting method is one of the best methods for the starting of the Induction motor. This method is cheaper and the line current in the star is 1÷3 times of in delta connection.
5. Induction motor is analogous to __________
a) Transformer
b) DC machine
c) Synchronous motor
d) Synchronous generator
Answer: a
Explanation: The Induction motor is analogous to the transformer. It is also known as a rotating transformer. Both works on the principle of electromagnetic induction.
6. Which type of Induction motor is best for pole changing method?
a) SCIM
b) WRIM
c) Single-phase IM
d) Linear IM
Answer: a
Explanation: Pole changing method is used to control the speed of Induction motor. It is only applicable for SCIM because SCIM rotor is made up of aluminum, copper bars.
7. _________ motor operates at high power factor.
a) Capacitor start
b) Capacitor run
c) Shaded pole
d) Reluctance
Answer: b
Explanation: In the capacitor run, the capacitor is permanently connected with auxiliary windings. The capacitor is used for reactive power compensation. It improves the power factor of the machine.
8. The use of the capacitor banks in the 3-phase Induction motor is ________
a) To increase the power factor
b) To decrease the speed
c) To increase the speed
d) To decrease the power factor
Answer: a
Explanation: Capacitor banks are used with 3-phase Induction motor in order to improve the power factor of the machine. The inductor motor generally operates with .6-.8 power factor range.
9. The concept of the slip is used in ___________
a) Synchronous machine
b) Induction machine
c) DC machine
d) Transformer
Answer: b
Explanation: The slip in the Induction motor is s = Ns-Nr÷ Ns. The slip existence is due to the relative speed between the stator magnetic field and rotor.
10. Which type of Induction motor is best for e.m.f injection method?
a) SCIM
b) WRIM
c) Single-phase IM
d) Linear IM
Answer: b
Explanation: E.M.F injection method is used to control the speed of Induction motor. It is only applicable for WRIM because WRIM rotor consists of copper windings connected in star connection. The external e.m.f can be connected with it using the slip rings.
11. The slots used in the Induction motor are ___________
a) Semi-closed
b) Closed
c) Vacuum
d) Open
Answer: a
Explanation: The slots used in the Induction motor are semi-closed. Open slots are used in the synchronous machine. Semi-closed slots have moderate air gap and lesser flux leakage.
12. The slots used in the Synchronous machines are ___________
a) Closed
b) Semi-open
c) Vacuum
d) Open
Answer: d
Explanation: The slots used in the Synchronous machine are open. Open slots have higher air gap and minimum flux leakage. The maximum power transfer capability increase due to a decrease in the leakage reactance.
13. The unbalanced set of 3-phase voltages causes ________
a) Positive sequence current
b) Negative sequence current
c) Zero sequence current
d) Half sequence current
Answer: b
Explanation: The unbalanced set of 3-phase voltages causes a negative sequence current. The magnetic field generated due to negative sequence current is in the opposite direction to the fundamental magnetic field. This induces double the frequency current in the rotor side which causes overheating of the motor.
14. The ratio of the rotor gross power and air gap power is _______
a) s
b) 1-s
c) 1+s
d) 0
Answer: b
Explanation: The ratio of the rotor gross power and air gap power is 1-s. The air gap power in the induction motor is Pg and the rotor gross power is (1-s)×Pg. The ratio is 1-s.