31. In the chopper circuit, commutation times are _______
A. Current commutation is equal to voltage commutation
B. Current commutation is less as compared to that of voltage commutation
C. Current commutation is more as compared to that of voltage commutation
D. Both commutation techniques are not comparable
Answer: B
In the current commutation, commutation time=CVr÷Io.
In voltage commutation, commutation time= CVs÷Io.
Hence, the commutation time of the current commutation is less as compared to the voltage commutation.
32. The generated e.m.f from 4-pole armature having 1 conductor driven at 1 rev/sec having flux per pole as 10 Wb, with wave winding is ___________
A. 30 V
B. 40 V
C. 70 V
D. 20 V
Answer: D
The generated e.m.f can be calculated using the formula
Eb = Φ×Z×N×P÷60×A
Φ represents flux per pole
Z represents the total number of conductors
P represents the number of poles
A represents the number of parallel paths
N represents speed in rpm. In wave winding number of parallel paths is 2.
Eb = 10×4×1×60÷60×2 = 20 V.
33. The unit of voltage is Pascal.
A. True
B. False
Answer: B
The voltage is equal to one volt when 1 A of current flows through a 1 Ω resistor. It is mathematically represented as I×R. It is expressed in terms of a volt(V).
34. Calculate the moment of inertia of the sphere having a mass of 8.4 kg and a radius of 61 cm.
A. 3.124 kgm2
B. 3.125 kgm2
C. 4.545 kgm2
D. 5.552 kgm2
Answer: B
The moment of inertia of the egg can be calculated using the formula
I=Σmiri2.
The mass of the egg and radius is given
I=(8.4)×(.61)2=3.125 kgm2.
It depends upon the orientation of the rotational axis.
35. The most suitable control-motor application is __________
A. AC shunt motor
B. DC separately motor
C. AC one-phase induction motor
D. DC shunt motor
Answer: B
DC separately motor has definite full-load speed, so they don’t ‘run away’ when the load is suddenly thrown off provided the field circuit remains closed. The speed for any load within the operating range of the motor can be readily obtained.
36. In a DC series motor, the e.m.f developed is proportional to _______
A. N×Ia
B. N×Ia2
C. N×Ia3
D. N×Ia5
Answer: A
In a DC series motor, the e.m.f developed is equal to KmΦN. In a DC series, the motor field winding is connected in series with the armature so the flux in the field winding is proportional to the current
Eb = KmΦN α Ia×N.
37. Calculate the value of the time period if the frequency of the signal is .07 sec.
A. 14.28 sec
B. 14.31 sec
C. 14.23 sec
D. 14.78 sec
Answer: A
The time period is defined as the time after the signal repeats itself. It is expressed in second.
T = 1÷F=1÷.07=14.28 sec.
38. The slope of the V-I curve is 13.89°. Calculate the value of resistance.
A. .247 Ω
B. .345 Ω
C. .231 Ω
D. .222 Ω
Answer: A
The slope of the V-I curve is resistance.
The slope given is 13.89° so R=tan(13.89°)=.247 Ω.
It behaves like a normal resistor.
39. In a DC shunt motor, the e.m.f developed is proportional to ___________
A. Ia
B. Ia2
C. Ia3
D. Iao
Answer: D
In a DC shunt motor, the e.m.f developed is equal to KmΦN. In a DC shunt, the motor field windings are connected separately and excited by a constant DC voltage.
E = KmΦN α Ia°.
40. Calculate the power factor angle during the resonance condition.
A. 0°
B. 10°
C. 80°
D. 90°
Answer: D
During the resonance condition, the reactive power generated by the capacitor is completely absorbed by the inductor. Only active power flows in the circuit. Net reactive power is equal to zero and Φ=0°.
41. Calculate the value of the duty cycle if the system is on for 5 sec and off for inf sec.
A. 0
B. .4
C. .2
D. .1
Answer: A
The duty cycle is Ton÷Ttotal. It is the ratio of the time for which the system is active and the time taken by the signal to complete one cycle. D = Ton÷Ttotal=5÷inf=0.
42. Calculate the value of the frequency of the AC supply in India.
A. 0 Hz
B. 50 Hz
C. 49 Hz
D. 60 Hz
Answer: B
The frequency is defined as the number of oscillations per second. It is reciprocal to the time period. AC supply magnitude is variable. It changes with time so the frequency of the AC supply is 50 Hz.
43. DC series motor cannot run under no load.
A. True
B. False
Answer: A
DC series motor cannot be run under no-load condition because at a no-load speed of the motor is very which can damage the shaft of the motor. There should be some load that should be connected to it.
44. Calculate the value of the frequency if the time period of the signal is .2 sec.
A. 5 Hz
B. 4 Hz
C. 2 Hz
D. 3 Hz
Answer: A
The frequency is defined as the number of oscillations per second. It is reciprocal to the time period. It is expressed in Hz.
F = 1÷T = 1÷.2 = 5 Hz.
45. 40 V, 7 A, 70 rpm DC separately excited motor having a resistance of 0.3 ohms excited by an external dc voltage source of 4 V. Calculate the torque developed by the motor on half load.
A. 18.10 N-m
B. 4.24 N-m
C. 40.45 N-m
D. 52.64 N-m
Answer: A
Back emf developed in the motor during the full load can be calculated using the equation
Eb = Vt-I×Ra = 37.9 V
Machine constant Km = Eb÷Wm which is equal to 5.17.
Torque can be calculated by using the relation
T = Km× I = 5.17×3.5 = 18.10 N-m.
46. Calculate the active power developed by a motor using the given data: Eb = 5.5 V and I = .5 A.
A. 2.75 W
B. 2.20 W
C. 5.30 W
D. 5.50 W
Answer: A
Power developed by the motor can be calculated using the formula
P = Eb×I = 5.5×.5 = 2.75 W.
If rotational losses are neglected, the power developed becomes equal to the shaft power of the motor.
47. Calculate the value of the angular acceleration of the motor using the given data: J = .1 kg-m2, load torque = 45 N-m, motor torque = 55 N-m.
A. 100 rad/s2
B. 222 rad/s2
C. 300 rad/s2
D. 400 rad/s2
Answer: A
Using the dynamic equation of motor J×(angular acceleration)
= Motor torque – Load torque: .1×(angular acceleration)
= 55-45=10, angular acceleration = 100 rad/s2.
48. Calculate the moment of inertia of the tennis ball having a mass of 7 kg and a diameter of 152 cm. [/bg_collapse]
A. 3.55 kgm2
B. 4.47 kgm2
C. 2.66 kgm2
D. 1.41 kgm2
Answer: C
The moment of inertia of the tennis ball can be calculated using the formula
I=mr2×.5.
The mass of the ball and diameter is given
I=(7)×.5×(.76)2=2.66 kgm2.
It depends upon the orientation of the rotational axis.
49. Calculate the moment of inertia of the thin spherical shell having a mass of 7.8 kg and diameter of 145.6 cm.
A. 2.72 kgm2
B. 5.96 kgm2
C. 5.45 kgm2
D. 2.78 kgm2
Answer: A
The moment of inertia of the thin spherical shell can be calculated using the formula
I=mr2×.66.
The mass of the thin spherical shell and diameter is given
I=(7.8)×.66×(.728)2=2.72 kgm2.
It depends upon the orientation of the rotational axis.
50. Calculate the time period of the waveform y(t)=87cos(πt+289π÷4).
A. 2 sec
B. 37 sec
C. 3 sec
D. 1 sec
Answer: A
The fundamental time period of the cosine wave is 2π.
The time period of y(t) is 2π÷π=2 sec.
The time period is independent of phase shifting and time-shifting.
