DC Motor Drives Questions and Answers

Answer: C

Correct answer: four-quadrant operation

A class E chopper is a full bridge of four switches with their diodes, so the armature voltage and current can each take either sign independently. That gives motoring and regenerative braking in both directions of rotation - all four quadrants - from a single unipolar supply, which is what a reversing servo or traction drive requires.

Answer: D

Correct answer: the supply or battery to accept the returned current

Regeneration means the machine’s kinetic energy is converted to electrical energy and sent back, so something must absorb it. A battery can be charged, and a line-commutated converter can invert into the mains. If the source cannot accept it - a diode rectifier, for instance - the link voltage simply rises until something fails, and dynamic braking into a resistor must be used instead. Reversing the field or switching the chopper off achieves nothing.

Answer: A

Correct answer: disconnecting the armature and connecting it across a resistor

In dynamic or rheostatic braking the machine is separated from the supply while its field is maintained, so it continues to generate; the armature is connected across a resistor and the kinetic energy is dissipated there as heat. It is simple and needs no cooperation from the supply, but the energy is wasted. Regenerative braking returns it instead, and plugging reverses the supply connections, which is a third method again.

Answer: B

The applied voltage and the back emf now add across the armature circuit, so a large reverse current flows and a heavy braking torque results. Resistance must be inserted to limit that current to something the machine can survive.

Answer: C

Current responds far faster than speed, so a fast inner loop regulates it while a slower outer loop commands it. Since torque follows current, the inner loop is in effect a torque loop and also provides the current limit.

Answer: D

Correct answer: the current reference for the inner loop

A cascade structure has a fast inner current loop within a slower outer speed loop. The speed controller compares the measured speed with the demand and produces a current reference, which the inner loop then enforces by adjusting the firing angle. The arrangement is used because current is proportional to torque and must be limited absolutely: clamping the outer loop’s output caps the current directly, protecting the machine and the converter.

Answer: A

Because the speed controller's output is the current reference, saturating it caps the current the inner loop will call for. The drive then accelerates at constant torque until the speed error falls back into the linear region.

Answer: B

Below that point full flux gives the most torque per ampere, so there is nothing to gain by weakening the field. Once the converter can raise the voltage no further, reducing the flux is the only route to higher speed.

Answer: C

Converter-fed induction and permanent magnet machines have taken over new installations, being cheaper and needing no brush maintenance. Existing dc machines are still re-controlled rather than replaced where the mechanical plant is sound.

Answer: D

Correct answer: commutator and brushes

The commutator is a mechanical rectifier with sliding contacts, so it wears, produces carbon dust, needs periodic brush replacement and undercutting of the mica, and limits the machine’s speed and its use in dirty or explosive atmospheres. The converter and windings need little attention by comparison. This single drawback is the main reason induction and permanent magnet drives have displaced dc drives in most new installations.