Dynamics of Electrical Drives Questions and Answers

Answer: C

Correct answer: 3,924 N·m

The torque at the drum is the weight times the drum radius:

The speed does not enter, which is what makes a hoist a constant-torque load. The 9,810 answer omits the radius and gives the force rather than the torque.

Answer: D

Correct answer: 4.9 kW

Power may be taken either at the load or at the drum, and the two agree:

or equally \( T\omega = 3924 \times 0.5/0.4 \). The lifting power is what the motor must supply in the steady state, before adding the friction of the mechanism and the torque needed to accelerate the inertia at the start.

Answer: A

Correct answer: 47.5 kJ

The energy given up is the change in kinetic energy:

A speed drop of only ten per cent releases nearly a fifth of the stored energy, because the energy goes as the square - which is why a flywheel can equalise a heavy load fluctuation with only a modest change of speed.

Answer: B

Correct answer: is controllable over a wide range with high efficiency

An electric drive can be controlled continuously in speed and torque, in both directions, with quick response and high efficiency, and it can brake electrically and often return the energy. It is also clean at the point of use and quiet. What it cannot do is work without a supply, which is precisely the advantage the diesel prime mover retains and the reason it survives where no supply exists.

Answer: C

Correct answer: Independence from any external energy supply

This is the electric drive’s one great disadvantage, not an advantage: it requires a supply, whether from the mains or from a battery, and where none exists a diesel or petrol engine is used instead. The other three - high efficiency with no local exhaust, electrical braking with the possibility of regeneration, and controllability over a wide range - are its genuine merits.

Answer: D

Correct answer: where the motor and load characteristics cross

Steady speed requires \( T = T_l \), which occurs where the two torque-speed curves intersect, so the operating point is fixed by both characteristics together and by neither alone. Inertia governs how quickly the drive reaches that point but not where the point is; the modulator rating limits what the motor can do but does not by itself set the speed.

Answer: A

Correct answer: shifted so that it crosses the load curve higher up

The operating point is the intersection of the two curves, and the load curve is fixed, so the intersection can only be moved by moving the motor characteristic - by raising the armature voltage, weakening the field, or changing the supply frequency according to the machine. Making the characteristic steeper without shifting it changes the regulation but leaves the crossing point where it was.

Answer: B

Correct answer: closed-loop control with speed feedback

Open-loop control sets the motor characteristic and lets the speed fall where the load dictates, so any change in load moves the speed. Measuring the speed and correcting the modulator until it matches the demand removes that dependence, and the accuracy achieved is set by the feedback rather than by the machine. A drooping characteristic and a large inertia both make the response sluggish; neither holds the speed accurately.

Answer: C

The torque equation is a first-order differential equation in speed, with inertia as the coefficient and the torque difference as the forcing term. Nothing about efficiency or supply quality enters that relation directly.

Answer: D

Torque follows current, so the limit caps the accelerating torque and lengthens the run-up, which is the price of protecting the windings and the semiconductors. A drive that must start quickly needs a machine and converter rated for the larger current.