Special Machines Questions and Answers

Answer: C

Dispensing with feedback is the whole economic case for the machine, since a driver and a pulse train are all that is needed. Where the consequences of a lost step are serious, an encoder is added and the saving disappears.

Answer: D

Correct answer: a low rotor inertia and a linear torque-speed characteristic

A servo machine is designed to be accelerated and reversed rapidly, so its rotor is made long and slender to minimise inertia, and its torque-speed characteristic is kept linear so the control loop sees a predictable plant. Feedback is part of the system rather than part of the machine.

Answer: A

A contouring axis spends its life accelerating and decelerating, so that ratio governs how quickly it can follow a commanded path. Matching the machine inertia to the reflected load inertia is a standard part of sizing such an axis.

Answer: B

A magnet rotor gives the torque density and the low inertia, and continuous position feedback makes it behave as a controlled torque source. The older two-phase induction servo survives only in small instrument drives.

Answer: C

Correct answer: salient poles on both its stator and its rotor

The doubly salient construction is what defines the machine. The rotor is plain shaped steel with no winding, no magnet and no cage, which makes it cheap, rugged and able to run hot and fast - and it is why the machine cannot start or run without its electronic converter.

Answer: D

Correct answer: the tendency of the rotor to align with the excited stator poles

The rotor moves so as to minimise the reluctance of the magnetic circuit, bringing its poles into line with whichever stator poles are excited. Exciting the phases in sequence keeps the rotor chasing the alignment. No current is induced in the rotor and no magnet is involved.

Answer: A

Correct answer: the phase current need flow in one direction only

Torque depends on the square of the current rather than its sign, so unidirectional current suffices and each phase needs only an asymmetric half bridge rather than a full one. The converter is therefore simpler and more fault-tolerant than an inverter, since a shoot-through path between the rails does not exist.

Answer: B

Correct answer: torque ripple and acoustic noise

Torque is produced in pulses as each phase takes over from the last, so the output is inherently rippled, and the strong radial forces pulling the rotor poles towards the stator excite the stator as an acoustic structure. Both are matters of control and mechanical design rather than defects in principle, but they have limited the machine’s adoption.

Answer: C

Laying the winding out flat produces a field that travels along the track instead of revolving, and the reaction plate follows it at a slip. Baggage conveyors, sliding doors and some transit systems use the arrangement.

Answer: D

Flux has to be established in each new piece of plate as it enters and dies away as it leaves, which costs efficiency and distorts the thrust. It has no counterpart in a rotary machine, whose rotor never leaves the field.