Dynamics of Electrical Drives Questions and Answers

Answer: C

Correct answer: regenerative braking

Regeneration is the case in which the braking energy is returned to the supply rather than dissipated. Dynamic braking burns it in an external resistor, and plugging dissipates it - along with further energy drawn from the supply - in the machine and its circuit. Only regeneration recovers anything, and it requires a supply able to accept power in the reverse direction.

Answer: D

With hoisting taken as positive, lowering a load reverses the speed while the torque stays in the hoisting sense, which puts the machine in the fourth quadrant, or the second if the signs are taken the other way round. Either way the machine is braking and the load is supplying the power.

Answer: A

Correct answer: reverse both its speed and its torque

The four quadrants are the four combinations of the signs of speed and torque, so a drive that can reach all of them must be able to produce torque of either sign at speed of either sign. That means motoring and braking in both directions of rotation, and it requires a converter capable of passing power both ways - a dual converter or a four-quadrant chopper on a dc drive.

Answer: B

At the intersection the two torques are equal and the acceleration is zero, so the drive can run there indefinitely. Whether it will stay there when disturbed is a separate question, answered by the stability criterion.

Answer: C

Correct answer: a net retarding torque

Stability requires the system to oppose any departure from the operating point. If a small speed increase produces a net retarding torque, the drive is pushed back towards the equilibrium; formally,

A net accelerating torque instead would carry the drive further away, which is the unstable case.

Answer: D

If the load torque rises faster with speed than the motor torque does, any speed increase produces a deficit that pulls the drive back. The inequality is evaluated at the intersection, so a drive can be stable at one intersection and unstable at another.

Answer: A

Correct answer: unstable at that operating point

With the load torque constant, \( dT_l/d\omega = 0 \), while the motor torque rises, so \( dT/d\omega > 0 \) and the stability criterion is violated. A small increase in speed raises the motor torque above the unchanged load torque, the drive accelerates further, and the departure grows. A stable intersection needs the motor characteristic to fall with speed - which is exactly what a shunt motor’s droop provides.

Answer: B

Beyond breakdown, torque falls as speed falls, so a momentary slowing reduces the torque further and the machine stalls. That is why the region between standstill and breakdown torque is not a working part of the characteristic.

Answer: C

Both the machine and the converter feeding it must be able to pass power in the reverse direction, and the supply must be able to absorb it. Where it is available it is the most efficient braking method, since the energy is recovered.

Answer: D

The machine is disconnected from the supply and made to feed a resistor bank, so it acts as a generator into a load that turns the energy into heat. It works even when the supply cannot absorb power, which is its main advantage.