Dynamics of Electrical Drives Questions and Answers

Answer: A

Space harmonics of the stator field produce their own torque-speed curves, and a machine can settle stably on one of them at about a seventh of synchronous speed. Winding design that suppresses those harmonics is the cure.

Answer: B

Correct answer: improves the acceleration available at a given torque

A higher ratio divides the referred load inertia by the square of the ratio while dividing the referred load torque by only the first power, so the torque available per unit of inertia improves and the drive accelerates more readily. The steady speed at the load falls in the same proportion, which is the price. Matching the ratio to make the referred inertia equal the motor inertia is the classical rule for the fastest acceleration.

Answer: C

Correct answer: unstable at their intersection

Stability requires \( dT_l/d\omega - dT/d\omega > 0 \). Here the motor torque rises with speed, so \( dT/d\omega > 0 \), and the load torque falls, so \( dT_l/d\omega < 0 \); the difference is negative on both counts and the criterion is violated twice over. A small increase in speed leaves the motor torque higher and the load torque lower, so the drive runs away from the intersection rather than back to it.

Answer: D

Electrical machines are cheapest and most efficient at a few thousand revolutions a minute, while many loads want tens. Reduction trades that speed for torque and incidentally hides the load inertia behind the square of the ratio.

Answer: A

Machine tools, robots and large wind turbines all use direct drive to eliminate lost motion and maintenance. The penalty is a large low-speed machine, which is why the arrangement is confined to cases where precision or reliability justifies it.

Answer: B

Continuous, short-time, intermittent and cyclic duties all place quite different thermal demands on a machine of a given size. Choosing the rating from the duty cycle rather than the peak load is what keeps drives from being grossly oversized.

Answer: C

Heating is what limits a machine, and a short overload followed by a long cooling period may raise the temperature no further than a modest continuous load would. Rating from the thermal duty rather than the peak is the whole of the art.

Answer: D

A positive product means power leaving the machine at its shaft, and a negative product means power entering it there. That single test distinguishes the motoring quadrants from the braking ones without any further reasoning.

Answer: A

Correct answer: its shaft power is negative and returns to the supply

Regeneration means the machine is generating: the load is driving it, the shaft power is negative, and the converter passes that power back to the supply instead of drawing from it. It occurs in the second and fourth quadrants, where torque and speed have opposite signs, and it requires a supply able to accept power in the reverse direction - which a diode rectifier is not.

Answer: B

Correct answer: A mine winder

A winder hoists in one direction and lowers in the other, and in each direction it must both accelerate and brake - so all four combinations of torque and speed sign occur in a single cycle. It also has an active load, gravity, which drives the machine during the descent, so the braking is regenerative and the energy substantial. Fans, pumps and conveyors run one way at steady load and need only the first quadrant.