1. Load torques can be classified into how many types?
A. Three
B. Two
C. Four
D. Five
Answer: B
Load torques can have two types. They are active and passive load torques. Active load torques can 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.
2. Rolling mills exhibit what type of load torque characteristics?
A. Constant torque characteristics
B. Linearly rising torque characteristics
C. Non-Linearly rising torque characteristics
D. Non-Linearly decreasing torque characteristics
Answer: D
Rolling mills are an example of non-linearly decreasing torque characteristics because torque and speed exhibits inversely proportional relationships and power are constant.
3. What is the relationship between torque and speed in constant type loads?
A. Torque is independent of speed
B. Torque linearly increases with an increase in speed
C. Torque non-linearly increases with an increase in speed
D. Torque non-linearly decreases with an increase in speed
Answer: A
A speed hoist is a perfect example of constant type loads in which torque variation is independent of speed.
The speed-torque characteristics of this type of load are given by T=K where K is a constant.
4. Torque inversely varies with the speed in the windage load torque component.
A. True
B. False
Answer: B
Torque varies with a square of speed in the windage load torque component whereas in the coulomb torque component torque is constant.
5. What type of force handles for active torques?
A. Strong nuclear forces
B. Weak nuclear forces
C. Gravitational forces
D. Electrostatic forces
Answer: C
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.
6. Passive torques always oppose the motion of the driven machine.
A. True
B. False
Answer: A
Passive torques are due to friction or shear and deformation in elastic bodies. They always oppose the motion, restricting the motion of the machine.
7. Among the following which one exhibits linearly rising load torque characteristics?
A. Elevators
B. Rolling Mills
C. Fan load
D. Separately excited dc generator connected to the resistive load
Answer: D
A 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.
8. What is the condition for the steady-state operation of the motor?
A. Load torque > Motor torque
B. Load torque <<<< Motor torque
C. Load torque = Motor torque
D. Load torque < Motor torque
Answer: C
According to the dynamic equation of the motor, load torque must be equal to motor torque so that the 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.
9. Choose the correct one. (× stands for multiplication, J represents the moment of inertia, w represents angular speeD..
A. J×d(w)/dt = Load torque – Motor torque
B. J×d(w)/dt = Load torque + Motor torque
C. J×d(w)/dt = Motor torque – Load torque
D. J×d(w)/dt = Load torque × Motor torque
Answer: C
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 the motor that’s why it subtracts in the equation.
10. Regenerative braking mode can be achieved in which quadrant (V-I curve)?
A. Third
B. Second
C. Fourth
D. First
Answer: B
Regenerative braking is only available in the second quadrant as power from the motor is fed back to the source. Back emf generated (EB. is more than armature terminal (Vt) so it works as a generator.
