This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Motors Rating and Heating – Load Equalisation”.
1. Calculate the moment of inertia of the flywheel of mass 45 kg and radius of 2 m.
a) 185 kg-m2
b) 180 kg-m2
c) 175 kg-m2
d) 170 kg-m2
Explanation: The moment of inertia of the flywheel can be calculated using the formula I=M(R2). The mass of flywheel and radius is given. I=(45)×(2)2=180 kg-m2. It depends upon the orientation of the rotational axis.
2. Electric motor and control system are the basic elements of the electric drive.
Explanation: Electric motor and control system are the basic elements of the electric drive. The electric drive uses the control system methods to control the speed and torque of the motor.
3. Capacitor start capacitor run single-phase Induction motor is used in ceiling fan.
Explanation: Capacitor start capacitor run single-phase Induction motor is used in ceiling fan. The motor operation is silent and non-vibrational. It provides a high starting torque.
4. Which of the following are the types of Longitudinal stress?
a) Tensile stress, Compressive stress
b) Tensile stress, Volume stress
c) Tensile stress, Shear stress
d) Tensile stress, Lateral stress
Explanation: Longitudinal stress is one the type of stress. Longitudinal stress is of two types. It consists of Tensile stress and Compressive stress. It has a unit of N/m2.
5. Which type of motor is used for milling and grinding operations?
c) Split phase
d) Reluctance motor
Explanation: Squirrel cage Induction motor is used for milling and grinding operations. SCIM has a rigid structure and high tolerating capability. It provides good running characteristics.
6. Calculate the work done in stretching the wire of length=2 m and the values of stress and strain are 16 N/m2 and 24.
a) 1566 J
b) 1644 J
c) 1522 J
d) 1536 J
Explanation: The work done in stretching a wire is .5×Stress×Strain×Volume. The value of the work done is .5×8×16×24=1536 J. The volume of wire is 2×2×2=8 m3.
7. Which type of force phenomenon is used in Sugar mills?
a) Centripetal force
b) Centrifugal force
c) Electrostatic force
d) Nuclear forces
Explanation: In sugar mills, a centrifugal is used to separate out sugar crystals from the syrup by the action of centrifugal forces. Centrifugal force acts away from the center.
8. The Speed control by variation of field flux results in _____________
a) Constant torque drive
b) zero speed
c) Variable torque drive
d) Variable power drive
Explanation: Field flux control method is used to achieve the speed above the base speed. It provides constant power drive and variable torque drive.
9. Calculate the compensator rating required for sin(Φ)=.8.
a) 0.9 P.U
b) 0.8 P.U
c) 0.7 P.U
d) 0.2 P.U
Explanation: The compensator rating can be calculated using the relation QP.U=√1-cos2(∅)=sin(∅)=.8. This per unit value VAR compensator is required to improve the power factor of the system.
10. Calculate the heating time constant of the machine if the thermal capacity (c) of the machine is 16 watts/℃ and heat dissipation constant value (D) is 4 watts/℃.
a) 6 sec
b) 7 sec
c) 4 sec
d) 9 sec
Explanation: The heating time constant of the machine is the ratio of the thermal capacity of the machine and heat dissipation constant value. τ = c÷D = 16÷4 = 4 sec.
11. Calculate the heat absorbed by a 20 Ω resistor when 40 V is applied across it.
a) 120 W
b) 100 W
c) 80 W
d) 110 W
Explanation: The resistor always absorbs active power and dissipates in the form of the heat. Power absorbed by a 20 Ω resistor is 40×40÷20=80 W.
12. Full form of VFD is _____________
a) Variable frequency drive
b) Variable force drive
c) Value frequency drive
d) Variable force diverter
Explanation: VFD stands for variable frequency drive. It is a controller for AC or DC motors to achieve the desired speed. It can perform the four quadrant operation.
13. What is Load Equalisation?
a) Process of smoothing the fluctuating load
b) Process of removing the load
c) Process of changing the voltage level
d) Process of changing the speed
Explanation: The Load Equalisation is the process of smoothing the fluctuating load. Load fluctuation causes a large voltage dip in the system and can damage the device.
14. Which device is used to store the energy during load equalization?
Explanation: Flywheel is used to store the energy during load equalization. It acts as a reservoir. It stores the energy during light load and utilizes the stored energy during peak load conditions.
Sanfoundry Global Education & Learning Series – Electric Drives.
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