Torque is defined as the vector product of displacement and force. The unit of the force is Newton(N) and the displacement is a meter (m) so the unit of torque is N-m.
22. Calculate the value of the torque when 10 N force is applied perpendicular to a 10 m length of rod fixed at the center.
A. 200 N-m
B. 300 N-m
C. 100 N-m
D. 400 N-m
Answer: B
Torque can be calculated using the relation
T = (length of rod) × (Force applied)
= r×F×sin90.
F is given as 10 N and r is 10 m then torque is 10×10 = 100 N-m. (the angle between F and r is 90 degrees)
23. What is the dimensional formula for torque?
A. [ML2T-2]
B. [MLT-2]
C. [M1L2T-3]
D. [LT-2]
Answer: A
Torque is a vector product of force and displacement.
Dimensional formula for force is [MLT-2] and displacement is [L] so dimensional formula for torque is [MLT-2] [L] = [ML2T-2].
24. Buck converter is used to _________
A. Step down the voltage
B. Step up the voltage
C. Equalize the voltage
D. Step up and step down the voltage
Answer: A
A Buck converter is used to Step down the voltage.
The output voltage of the buck converter is Vo = D×Vin.
The value of the duty cycle is less than one which makes the Vo < Vin.
The buck converter is used to step down voltage. Vin is a fixed voltage and Vo is a variable voltage.
25. If the starting torque of the motor is less than the load torque, the motor will fail to start.
A. True
B. False
Answer: A
J×d(w)÷d(t) = Motor torque – Load torque is the dynamic equation of the motor.
If starting torque (motor torque) is less than the load torque then d(w)÷d(t) <0, acceleration <0 so the motor will decelerate and fails to start.
26. Torque is a vector quantity.
A. True
B. False
Answer: A
A scalar quantity has only magnitude whereas a vector quantity has both directions and magnitude. Torque is a force applied to a body perpendicularly. As the force is a vector quantity, the torque must be treated as a vector quantity.
27. 250V, 15A, 1100 rpm separately excited dc motor with armature resistance (Ra) equal to 2 ohms. Calculate back emf developed in the motor when it operates on half of the full load. (Assume rotational losses are neglected.
A. 210V
B. 240V
C. 230V
D. 235V
Answer: D
Back emf developed in the motor can be calculated using the relation
Eb = Vt-I×Ra.
In question, it is asking for half load, but the data is given for full load so current becomes half of the full load current
= 15÷2 = 7.5 A. 250V is terminal voltage it is fixed so
Eb = 250-7.5×2 = 235V.
28. Duty cycle (D) is _______
A. Ton÷Toff
B. Ton÷(Ton+ Toff)
C. Ton÷2×(Ton+ Toff)
D. Ton÷2×Toff
Answer: B
Duty cycle (D) is defined as the ratio of time for which a system is active to the total period. It is also known as the power cycle. It has no unit.
29. A 220 V, 1000 rpm, 60 A separately-excited dc motor has an armature resistance of .5 ω. It is fed from a single-phase full converter with an ac source voltage of 230 V, 50Hz. Assuming continuous conduction, the firing angle for rated motor torque at (-400) rpm is _________
A. 122.4°
B. 117.6°
C. 130.1°
D. 102.8°
Answer: D
During rated operating conditions of the motor
Eb = Vt-Ia×Ra = 220-60×.5=190 V.
As Eb=Kmwm so
Km=190×60÷(2×3.14×1000) = 1.8152 V-s/rad.
Back emf at (-400 rpm) is
Kmwm = 1.8152×(2×3.14×(-400))÷60 = -76 V. Now Vt = -76+60×.5 = -46 V.
The average voltage of a single-phase full converter is 2×Vm×cos(α)÷3.14.
The output of the converter is connected to the input terminal of the motor so α = cos-1(-46×3.14÷2×230×1.414) = 102.8o.
30. The unit of angular acceleration in rad/s2.
A. True
B. False
Answer: A
Angular acceleration is defined as a derivate of angular velocity with respect to time. It is generally written as α. The unit of angular velocity in rad/sec and time is second so the unit of angular acceleration in rad/s2.
