21. 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
In the 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.
22. The shape of the speed-armature current characteristics in the DC shunt motor is __________
A. Hyperbolic
B. Parabola
C. Negative slope linear line
D. Ellipse
Answer: C
The shape of the speed-armature current characteristics in the DC shunt motor is a 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.
23. The shape of the speed-torque characteristics in the DC shunt motor is __________
A. Rectangular Hyperbolic
B. Whole x-y plane
C. Circle
D. Ellipse
Answer: A
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.
24. DC shunt motors are used where the high _________ is required.
A. Starting torque
B. Maximum torque
C. Minimum torque
D. Breakdown torque
Answer: A
DC series motors are used where a high starting torque is required. At starting the torque produced by DC series motor is very high.
25. The shape of the current-torque characteristics in DC shunt motor is __________
A. Rectangular Hyperbola
B. Parabola
C. Straight line
D. Ellipse
Answer: C
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.
26. 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
The torque developed in the DC shunt motor is the ratio of power developed and the angular
speed of the motor
T=EbIa÷ωm=P÷ωm
T ∝ P.
27. The value of current at time of starting is ___________
A. High
B. Low
C. Very low
D. 0
Answer: A
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.
28. Calculate the bandwidth in a 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
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.
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