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The correct answer is (d) Step up and step down the voltage

To elaborate: The OUTPUT voltage of the buck-BOOST converter is Vo = D×Vin ÷ (1-D). It can step up and step down the voltage depending UPON the VALUE of the duty CYCLE. If the value of the duty cycle is less than .5 it will work as a buck converter and for duty cycle greater than .5 it will work as a boost converter.

Correct option is (b) Regenerative mode

The explanation is: The step-down CHOPPER is used in MOTORING mode but a step-up chopper can OPERATE only BRAKING mode because the CHARACTERISTICS are in the second quadrant only.

Correct OPTION is (b) -.6 rad/s^2

The best explanation: Using the dynamic EQUATION of MOTOR J*(angular acceleration) = Motor torque – Load torque: 50*(angular acceleration) = 10-40 = -30, angular acceleration=-.6 rad/s^2. The motor will decelerate and will fail to start.

Right choice is (d) 102.8°

To explain I would say: 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. Average voltage of 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) = 102.8^o.

Right answer is (d) 235V

For explanation: 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.

The CORRECT ANSWER is (B) Ton÷(Ton+ Toff)

To elaborate: DUTY CYCLE (D) is defined as the ratio of time for which system is active to the total time period. It is also known as the power cycle. It has no unit.

The correct answer is (b) False

Easy explanation: Scalar QUANTITY has only magnitude whereas VECTOR quantity has both DIRECTIONS and magnitude. Torque is a force APPLIED on a body perpendicularly. As the force is a vector quantity, the torque must be TREATED as a vector quantity.

Correct choice is (a) True

The explanation: J×d(w)÷d(t) = Motor torque – Load torque is the dynamic equation of 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.

Right answer is (a) STEP down the VOLTAGE

The explanation: 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.

Right option is (a) F = W×sin(ALPHA)

The explanation: When a body is MOVING upward on an inclined PLANE its weight can be resolved in two perpendicular components that are W×sin(alpha) and W×cos(alpha). W×cos(alpha) is the COMPONENT that is opposite to normal of the inclined plane and W×sin(alpha) is the component that opposes the upward MOTION of the body.

The correct choice is (a) [ML^2T^-2]

For explanation: 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] = [ML^2T^-2].

Right choice is (b) 300 N-m

Best explanation: 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)

Correct choice is (a) N-m

To explain: TORQUE is defined as the vector product of displacement and force. The UNIT of the force is NEWTON(N) and of the displacement is a meter (m) so the unit of torque in N-m.

The correct answer is (c) ARMATURE copper loss

To EXPLAIN: Armature copper losses are variable losses as they depend on armature current which further depends on LOAD. As load CHANGES armature current changes hence armature copper losses (I^2 * R) also changes.

The CORRECT choice is (b) 2 rad/s^2

The explanation: Using the dynamic equation of motor J*(angular ACCELERATION) = Motor torque – Load torque: 20*(angular acceleration) = 60-20=40, angular acceleration=2 rad/s^2.

Right option is (a) True

The best I can explain: In reverse motoring mode motor ROTATES in opposite to original DIRECTION as direction of motor torque changes which MAKES the motor to run in opposite direction and load torque tries to oppose the motion of motor.

Correct option is (a) MOTORING mode

Explanation: Only motoring mode is available in case of step-down chopper (Type-A chopper). Value of OUTPUT voltage (VO) is LESS than the input voltage (Vin) in case of step-down chopper.

Right ANSWER is (C) Non-Linearly rising torque CHARACTERISTICS

Explanation: Torque produced by the fan is directly proportional to square of speed throughout the range of USABLE fan SPEEDS. This type of loads exhibits non-linearly rising torque characteristics.

Correct option is (B) Second

The best EXPLANATION: Regenerative braking is only available in second QUADRANT as power from motor is fed BACK to source. Back emf generated (Eb) is more than armature terminal (VT) so it works as a generator.

Correct ANSWER is (C) J*d(w)/dt = MOTOR torque – LOAD torque

The best I can explain: J*d(w)/dt = Motor torque – Load torque is the DYNAMIC equation of the motor. Motor torque will try to aid the motion of the motor, but load torque will oppose the motion of motor that’s why it subtracts in the equation.

Right choice is (d) Separately excited dc generator connected to the resistive LOAD

The explanation is: Separately excited dc generator connected to the resistive load is an example of linearly rising load TORQUE CHARACTERISTICS as the torque INCREASES linearly with an increase in SPEED.

The correct option is (c) Load TORQUE = Motor torque

Best explanation: According to the DYNAMIC equation of motor, load torque must be equal to motor torque so that motor should RUN at a UNIFORM speed. If load torque is greater than motor torque, the motor will FAIL to start and if load torque is less than motor torque, the motor will run at a higher speed which can damage the shaft of the motor.

Right option is (a) True

The explanation: Passive torques are due to friction or shear and deformation in elastic BODIES. They ALWAYS oppose the MOTION, RESTRICTING the motion of the MACHINE.

The CORRECT choice is (b) TWO

The best explanation: LOAD torques can have two types. They are active and passive load torques. Active load torques are able to run the motor under equilibrium CONDITIONS and their sign remains the same even if the motor rotation changes but passive load torques always opposes the motion by changing their sign with the change in rotation of the motor.

The CORRECT choice is (c) Gravitational forces

The explanation: Gravitational forces are responsible for ACTIVE torques. Active torques DUE to gravitational forces can be obtained in the CASE of HOISTS, lifts or elevators and railway locomotives operating on gradients.