Special Machines Questions and Answers

Answer: C

Two stator windings in quadrature give a pair of signals from which the angle follows unambiguously over a revolution. Having no optics and no electronics at the shaft, it tolerates heat, dirt and vibration far better than an encoder.

Answer: D

Correct answer: a small permanent magnet dc generator

A dc tachogenerator produces a voltage directly proportional to speed, with a polarity that reverses with direction - both properties being exactly what a velocity loop needs. The output carries commutator ripple, so it is usually filtered lightly before use.

Answer: A

Correct answer: proportional to speed in amplitude and fixed in frequency

The drag cup generator has a reference winding excited at a fixed frequency and an output winding at right angles to it. Rotation of the thin non-magnetic cup produces an output at that same frequency whose amplitude is proportional to speed and whose phase reverses with direction. Having no commutator, it gives a much cleaner signal at low speeds.

Answer: B

Correct answer: 1, 2 and 3

All three are correct and the first two go together: because a stepper is normally run open loop, nothing detects a lost step and the position error persists until the axis is re-homed. Microstepping subdivides each full step by proportioning the phase currents, improving resolution and smoothness - though the accuracy of an individual microstep is much poorer than that of a full step.

Answer: C

Correct answer: 1-Q, 2-P, 3-R

Three quite different torque mechanisms. The switched reluctance machine develops torque from the rotor’s tendency to align with the excited stator poles. The hysteresis motor develops it from the lag of the rotor material’s magnetisation behind the revolving field, which is why it produces constant torque at any speed. And the permanent magnet synchronous machine develops it from the interaction of magnet flux with stator current.

Answer: D

Correct answer: 1-Q, 2-R, 3-P

Three transducers with three outputs. A resolver gives the sine and cosine of the shaft angle, from which the angle follows unambiguously. A tachogenerator gives a voltage proportional to speed. And a control transformer gives a voltage proportional to the angular error between its own position and the transmitted one.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Since each pulse advances the shaft by the same repeatable angle, counting the pulses issued is equivalent to knowing the position, and no measurement is needed. The qualification is that this holds only while no step is lost - which is exactly why the load must be kept within the torque available.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. The hysteresis motor develops the same torque at every speed from standstill to synchronism, so a large inertia is accelerated steadily and pulled into step without the dip in torque that an induction machine shows. The smooth, quiet running that follows is why the type was used in turntables and gyroscopes.

Answer: C

Correct answer: True, its rotor being plain shaped steel

The rotor is a stack of laminations shaped into salient poles - no winding, no magnet, no cage, no brushes. That is the machine’s principal attraction: cheap to make, mechanically robust, able to run at high speed and high temperature, and with nothing on the rotor to fail.

Answer: D

Correct answer: False, since it is designed for low inertia and a linear characteristic

Fitting an encoder to an ordinary machine gives feedback but not the dynamic response a servo application needs. A servo machine is built for the purpose - a long slender rotor for low inertia, a linear torque-speed characteristic, high peak torque and a construction that tolerates rapid reversal.