Electric Drives Questions and Answers – Dynamic Braking of DC Shunt Motors

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Dynamic Braking of DC Shunt Motors”.

1. The sine function is an odd function.
a) True
b) False

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

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

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.
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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

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.

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.
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6. Calculate the steady state value for x(t)=7e-9t.
a) 0
b) 8
c) 3
d) 1

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

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

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

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

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

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.

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

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

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.

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

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.

Sanfoundry Global Education & Learning Series – Electric Drives.

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